eMedicine - Spinal Dysraphism/Myelomeningocele : Article by Ali Nawaz Khan

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Spinal Dysraphism/Myelomeningocele

Article Last Updated: Feb 20, 2007

Background

Spinal dysraphism, or neural tube defect (NTD), is a broad term encompassing a heterogeneous group of congenital spinal anomalies, which result from defective closure of the neural tube early in fetal life and anomalous development of the caudal cell mass.

Some forms of spinal dysraphism can cause progressive neurologic deterioration. The anatomic features common to the entire group is an anomaly in the midline structures of the back, especially the absence of some of the neural arches, and defects of the skin, filum terminale, nerves, and spinal cord.

Spinal dysraphism can be classified as closed forms or open forms, which include myelocele, meningocele, and myelomeningocele. These open forms are often associated with hydrocephalus and Arnold-Chiari malformation type II and may be classified as spina bifida aperta. Von Recklinghausen (1886) gave a detailed account of spina bifida cystica (aperta). Spina bifida is described in the medieval literature, although the condition was recognized even earlier. Indeed, the association of foot deformities with lumbar or lumbosacral hypertrichosis may be the origin of the mythological figure of the satyr.

The closed form of spina bifida is termed spina bifida occulta. 5-10% of the general population may have bony spina bifida occulta with intact overlying skin. Most of these cases are found incidentally.

Open NTDs (ONTDs) represents a serious congenital anomaly. If the neural tube fails to fuse at the skull, the result may be that of anencephalus or encephalocele. If the tube fails to fuse along the spine, the resulting defect is an open NTD such as meningomyelocele. Infants with NTDs frequently have additional serious neurologic, musculoskeletal, genitourinary, and bowel anomalies.

Spina bifida occulta is characterized by variable absence of several neural arches and various cutaneous abnormalities, such as lipoma, hemangioma, cutis aplasia, dermal sinus, or hairy patch, and it is often associated with a low-lying conus and other spinal cord anomalies. Whenever the conus lies below the L2-3 interspace in an infant, cord tethering should be considered. The term tethered cord implies that the cord may be attached to vertebral column or subcutaneous tissues by a thickened filum terminale, fibrous band, dermal sinus tract, diastematomyelia, or a lipoma (lipomyelomeningocele). Patients with spina bifida occulta may present with scoliosis in later years.

Approximately 95% of couples that have a fetus affected with ONTD have a negative family history. Most ONTDs are caused by multifactorial inheritance, including genetic and environmental factors.

Pathophysiology

The defect of spinal dysraphism occurs in the first 8.5 weeks of fetal life. The neural tube develops from ectodermal cells, while mesoderm forms the bony elements, meninges, and muscle. The skin is separated from the neural tube by the mesoderm. Incomplete separation of ectoderm from the neural tube results in cord tethering, diastematomyelia, or a dermal sinus. Premature separation of the cutaneous ectoderm from the neural tube results in incorporation of mesenchymal elements between the neural tube and skin, which may result in the development of lipomas. If the neural tube fails to fuse in the midline posterior spinal abnormalities such as myelomeningoceles occur.

The disease process may be classified in terms of the deformity of the spine or cord. The widely accepted classification is based on the type of spinal defect. Spina bifida occulta consists of a defect in the vertebral arches not associated with an externally visible sac on the back. These lesions differ from spina bifida cystica in which the vertebral defect is associated with a cystic mass on the back.

Spina bifida cystica (aperta) is further subdivided into the following groups: (1) a meningocele without cord tissue within the sac; (2) myelomeningocele in which spinal neural tissue forms part of the sac; and (3) rachischisis, which is the most severe form and involves a widely patent dorsal opening of the spine with or without residual cord tissue. Rachischisis is usually associated with anencephaly. Anomalies of the cord may be classified as amyelia, nonfusion of the dorsal cord, diastematomyelia, hydromyelia, tethered cord, overgrowth of the cord or nonspecific dysplasia. Either type of classification can cause confusion if the mesodermal or neuroectodermal lesions are emphasized to a degree at which sight is lost of their mutual relationship (Friede, 1989). The accompanying flow chart makes a fresh attempt at a simplified classification.

Myelomeningocele ONTD is classified as a teratogenic process that results in a failed closure and abnormal differentiation of the embryonic neural tube during the first 4 weeks of gestation. Abnormal development of the posterior caudal neural tube produces a myelodysplasia. The defect can occur anywhere from the base of the skull to sacrum. The anatomic level of the spinal cord lesion roughly correlates with the patient's neurologic deficit.

Developmental abnormality of the cephalic anterior tube gives rise to several CNS anomalies, including Arnold-Chiari type II malformation characterized by cerebellar hypoplasia and varying degrees of caudal displacement of the hindbrain into the upper cervical canal through a widened foramen magnum. Arnold-Chiari type II malformation may impede CSF flow and cause hydrocephalus, which occurs in more than 90% of infants with myelomeningocele(particularly after treatment of the myelomeningocele). Cerebral cortex dysplasia, including cerebral heterotopias, polymicrogyria, abnormal lamination, fused thalami, and corpus callosum abnormalities, are other anomalies often associated with Arnold-Chiari type II malformations.

Myelomeningocele often occurs with multisystemic congenital anomalies. Mesodermal elements surrounding the neural tube, such as the vertebra and ribs, also may be malformed, leading to congenital or early onset kyphosis and scoliosis (only if the myelomeningocele is crossing the thoracolumbar junction). Other anomalies associated with myelomeningoceles are facial clefts, congenital heart disease, and genitourinary tract anomalies. Anomalies of the urinary tract, such as a solitary kidney or ureteric abnormalities, may further contribute to increased morbidity in the presence of neurogenic bladder dysfunction.

In a myelocele, a midline plaque of exposed neural tissue is flush with the skin surface. This reflects a failure of dysjunction. Myelomeningoceles (meningomyeloceles) represent a similar herniation of spinal contents through a bone defect in the posterior spinal arches except that the neural herniation is elevated above the skin surface by a posterior expansion of the subarachnoid space. In both conditions, the cord is tethered low. An associated Chiari II malformation is present.

Posterior meningoceles, lipomyelomeningoceles, and myelocystocele are closed defects and may occur with a skin-covered back mass. A posterior meningocele is a skin-covered, CSF-filled mass that is continuous with the CSF in the spinal canal and associated with a tethered cord or hydromyelia. A myelocystocele is a herniated, skin-covered, cystic dilatation of the central canal of the terminal portion of a low-lying cord. A lipomyelocele/lipomyelomeningocele occurs when a lipoma extends from the subcutaneous tissues to the dorsal aspect of the cord and tethering the cord inferiorly. This process reflects a premature separation of the cutaneous ectoderm during the process of neurulation that allows mesenchyme to enter the unclosed neural tube and differentiate into fat.

A dorsal dermal sinus is a long, thin, epithelium-lined sinus that extends from the skin surface for a variable distance. This sinus may become symptomatic due to an infection if it has a connection within the theca or compression of neural tissue if associated with a dermoid/epidermoid. An infected dermal sinus may cause a localized abscess or meningitis. Microscopic examination of a dermoid sinus tract shows a single or multiple channels lined with cornifying squamous epithelium. The lumen may contain desquamated keratin and hair. Often, the surrounding connective tissue shows follicles and skin appendages.

An intradural lipoma has no anatomic connection with the subcutaneous fat and lies wholly within the dural space in embryologic terms. This is similar to a lipomyelomeningocele except that the neural tube closes after the mesenchyme has entered.

A split notochord results from abnormal splitting of the notochord, which is usually associated with a persistent tract between the gut and the dorsal skin. The intervening tract may become obliterated at any point to create an enteric cyst, diverticulum, fistula, sinus, neurenteric cyst, or enteric duplication cyst.

Diastematomyelia refers to a complete or partial clefting of the spinal cord in a sagittal plane into 2 symmetric or asymmetric hemicords. The cord usually reunites more distally. Each of the hemicords has its own central canal and dorsal and ventral nerve roots. They are divided into 2 types: In type A (50-60%), the hemicords are contained within a single dural and arachnoid lining without a dividing spur. In type B, each hemicord has its own dural and arachnoid sac with a spur or septum separating the 2. The septum may be bony, cartilaginous, or fibrous. The most common site for this anomaly is the thoracolumbar region. Associated cutaneous stigmata at the site of the anomaly, cord tethering, and syringohydromyelia are common and therefore must be looked for in diastematomyelia.

In 1953, Garceau first described a tethered cord (filum terminal syndrome). Tethering implies that the anchoring lesion is formed by dense fibrous connective tissue. Normal spinal nerve roots do not anchor the cord but the dentate ligaments do. Tethered cord is associated with low conus below the mid L2 level and a short, thick filum. Garceau's concept that anchoring of the cord by the filum causes Chiari malformation is open to criticism, but his observations on the clinical manifestations of tethering were confirmed in many subsequent reports.

In filum terminal syndrome, a thickened filum is attached to the dura, or an extradural band may anchor the cord. The filum usually retracts dorsally upon transection. In some patients, the cord is bound down by lipomatous tumors or fibrous tissue, the sacral roots ascending. Lipomas may attach dorsally to the conus and be sessile or pedunculated, while other lipomas occupy the distal end of the conus, elongating the latter and terminating in a small lipoma with attached nerve roots. The dorsal and caudal lipomas may occur in combination. Filar lipomas occupy an enlarged filum terminale.

Lipomyelomeningoceles are more extensive lesions and represent a combination of a splayed cord fused with a lipomatous mass, which in turn fuses with the subcutaneous adipose tissue. The neurologic deficits of cord tethering are probably caused by impaired circulation in the stretched cord, as evident by its reduced oxidative metabolism or to abnormal development as the cause of the neurologic deficits as in myelomeningocele (Yamada, 1981).

Hydromyelia refers to an abnormal dilatation of the central canal lined by ependyma, usually in the lumbosacral region. Myelomeningocele or diastematomyelia is commonly associated with this condition. The central canal dilatation varies from oval to irregular or slit-shaped, projecting either bilaterally or dorsally. A hydromyelia must be differentiated from a syringomyelia, which is a fluid-filled cavity within the spinal cord, not lined by ependyma. Syringomyelia is rare in children and usually related to trauma, inflammation, or tumor.

A myelocystocele is a variant of hydromyelic dilatation of the central canal, the cystic cavity being within the cord and the spinal roots originating at the ventral and dorsal outer surface of the cyst wall. A myelocystocele is often associated with defects of the vertebral bodies or intestinal fistula. These types of lesions are often located in the cervical or upper thoracic cord at the level the underlying bony defect. Heterotopic cerebellar tissue has been described within the myelocystocele.

Multiple factors are implicated in the causation of a myelomeningocele, including genetic, racial, and environmental factors. Exposure to a risk factor may alter hyaluronate metabolism, which results in the failure of vertebral architecture to develop normally. Most infants with myelomeningocele are born to mothers with no previously affected children. Other offspring in a family with one affected child are at increased risk of NTD than children without affected siblings.

The risk is 1 in 20-30 for subsequent pregnancies, and if 2 children are affected, the risk becomes 1 in 2. As many as 10% of fetuses with an NTD detected in early gestation have an associated chromosome abnormality. Associated chromosome abnormalities include trisomies 13 and 18, triploidy, and single gene mutations (13%).

Maternal risk factors include insulin-dependent diabetes mellitus and hypothermia. Intrauterine drug exposures to valproate, carbamazepine, and drugs to induce ovulation are identified risk factors (1.5%).

Studies in the 1980s showed that correction of folic acid deficiency as an effective means of primary and recurrent prevention. In the early 1990s, studies showed that 50% of NTDs are related to a nutritional deficiency of folic acid; thus, they are preventable.

Frequency

United States

Statistics on the frequency of spina bifida occulta vary, and as much as 2-24% of the population is affected. This variation appears to be age related. Spina bifida occulta of the first sacral vertebral body is found in 51.6% of the children aged 7-8 years and in 26% of adults. The fifth lumbar vertebral body is affected in 16.1% of children aged 7-8 years and in 2.2% of adults (Satow, 1956).

In 1972, James and Lassman examined routine radiographs of 1172 consecutive autopsies and found an incidence of 5% of spina bifida occulta in adults.

In the United States, the incidence of NTDs declined 50% between 1970 and 1989 (1.3 cases per 1000 to 0.6 case per 1000 live births). In the same period, the proportion of spina bifida cases increased relative to anencephaly.

An ONTD is the most common major birth defect. The incidence at birth of ONTD is reported to be 4.4-4.6 cases per 10,000 live births in 1983-1990. The highest rates of ONTD in the United States have been reported in Appalachia. The incidence is higher in the eastern United States than on the West Coast.

International

NTDs are among the most common birth defects with substantial racial and geographic variations. The incidence rates of ONTD vary widely not only among countries but also among regions within countries. Rates are significantly higher in areas with a low socioeconomic status. Over the last 50 years, epidemics of ONTDs have reported in Boston, MA; Rochester, NY; Dublin, Ireland; the Republic of China; and Jamaica.

The disease is 2.5 times more frequent in whites than blacks; it is particularly common in Belfast, Liverpool, and Dublin and uncommon in Japan, based on the statistics of 1970s and 1980s. The incidence remains higher in Great Britain than in Japan. The incidence in the United Kingdom and in mainland Europe is decreasing.

Currently, the highest reported incidence is in Northern China (3.7 cases per 1000 live births). Indian and Eastern Mediterranean populations (with the exception of Israeli Jews) also have relatively high incidences of NTDs. However, unlike the Western white populations, these populations more commonly have anencephaly than spina bifida.

Mortality/Morbidity

Developmental spinal defects in the lumbosacral region are a major source of disability among children and adults. An overlying intact skin masks the underlying abnormality of the lower neural axis in spina bifida occulta and the cutaneous signs of spinal dysraphism may be subtle. Early recognition is important because symptoms may not be obvious until later in childhood or early adult life, when an irreversible neurologic deficit may already have occurred. Because the disease is progressive, prophylactic surgery is indicated in most cases.

Urodynamic studies in infants with established spinal dysraphism may be normal as opposed to older children. This finding underscores the importance of early diagnosis so that appropriate surgery is offered to the patient. The most common warning sign of deterioration in adults is the appearance and exponential increase of pain, particularly in the lower back and legs, and the further tightening of feet and leg tendons.

Over 90% of patients experience pain relief after surgery, but bladder control is improved in few. The argument to perform detethering surgery as soon as possible to maintain continence although supported by some remains controversial. If the dysraphism and associated cord tethering is neglected, the risk of urologic and neurologic damage may be irreversible and permanent (Satar, 1995).

In Europe and the United States, myelomeningocele is the leading identified cause of infant death from congenital birth defects. Mortality rates of 90-100% have been reported in untreated myelomeningoceles. Most affected infants die within the first year of life if untreated. Hydrocephalus or meningitis is leading causes of death in untreated infants in the first 2 years of life. The likelihood that a 2-month-old infant untreated for myelomeningocele lives to be 7 years is only 28%.

Recent advances in clinical measures have dramatically improved survival rates for infants with myelomeningocele, primarily as a result of effective antibiotics and developments of neurosurgical techniques in the management of hydrocephalus. In Europe and the United States, antibiotics, sac closure, and ventriculoperitoneal shunt placement are instituted in 93-95% affected infants in the perinatal period. Early death in both treated and untreated patients occurs as a result of advanced hydrocephalus and associated congenital anomalies.

Neurologic consequences of myelodysplasia include paraplegia and functional bladder and bowel problems, which may be present at birth. A variety of chromosomal and nonchromosomal anomalies associated with myelomeningoceles may cause further neurologic deficits that appear acutely or chronically at birth or later. These deficits can be related to hydrocephalus, Arnold-Chiari II malformation, or a variety of other intraspinal anomalies.

Seizures occur in 10-30% of affected children and adolescents. Seizures can be related to brain malformation, or shunt malfunction or infection. Renal compromise is common and a leading cause of death after the first year of life as a consequence of a neurogenic bladder. Neurologic deficits may cause musculoskeletal disease, resulting in progressive bony and joint deformities, pathologic fractures, and muscle atrophy.

Difficult childbirth may occur with subsequent traumatic birth, leading to complications such as cerebral palsy from ischemic brain injury, permanent disability, with weakness or paralysis of the legs, loss of bowel or bladder control, frequent urinary tract infections, and meningitis.

At least 75% of children born with a myelomeningocele can be expected to reach their early adult life. Late deterioration is common. One of the greatest challenges in medicine today is establishing a network of care for these adults with spina bifida (Bowman, 2001).

Complications may follow surgery for NTDs and include wound infection, CNS infection, delayed wound healing, CSF leakage, additional damage to the cauda equina, and acute hydrocephalus. Long-term complications include cord tethering and progressive hydrocephalus.

Approximately 25-35% or more of all children with myelomeningocele have hydrocephalus at birth. Furthermore, an additional 60-70% of patients with myelomeningocele develop hydrocephalus after the spinal defect is closed. Hydrocephalus can cause a reduction in cerebral cortical mantle and is associated with a considerable decline in intellect.

Complications from shunt surgery for hydrocephalus include intracerebral and/or intraventricular hemorrhage, bowel perforation, and infection. Long-term complications include infection, overdrainage or underdrainage, and obstruction of the shunt system. The literature describes a significant incidence of complications related to CSF diversionary shunts, including mechanical failure, occlusion, fracture, and the development of loculated CSF collections around the ventricular or distal limbs of the shunt tube. A fibrous sheath may encase the catheter tip, or the catheter may migrate out of the peritoneal cavity.

Secondary tethering of the spinal cord in patients with myelomeningocele is almost universal, but the incidence of secondary tethered cord syndrome is relatively low (Brezner, 1999).

Derotation osteotomy with plate fixation is performed for internal and external tibial torsion that are commonly associated with myelomeningoceles. However, postoperative complications, such as nonunion, delayed union, and infection, are common. Patients are usually immobilized for 6-8 weeks after surgery.

Deformities of the foot and ankle may cause skin ulceration, prevent the patient from wearing shoes and compromising his or her ambulation.

Regarding open spina bifida, 13% of cases are associated with aneuploid fetuses. The likelihood of an associated anatomic abnormality has been found in 40%, though in 1 study, only 22% of these were detectable on sonograms. If the spina bifida is an isolated abnormality on sonograms, a 4% risk of aneuploidy still persists. Prenatal sonography can help to predict karyotypically abnormal fetuses with spina bifida. However, 20% are missed if sonography is used alone in the setting of a prenatally detected spina bifida. Some authors believe that cytogenetic analysis is justified in these instances.

Race

In studies that predate the introduction of prenatal screening and prophylactic use of folic acid, the birth incidences of both spina bifida and anencephaly were reported to be higher in the European white population than in the black population.

Sex

Anencephaly has a female preponderance, especially among premature births, with a female-to-male ratio of 3:1. The birth prevalence rate of myelomeningocele is slightly higher in girls than boys (with a ratio of 1.2:1), as reflected in data from both state and national surveillance systems in 1983-1990.

Age

Spinal dysraphism is a congenital anomaly the most severe form of the disease such as a myelomeningocele is present at birth, but spina bifida occulta may not be obvious until later in childhood or early adult life.

Anatomy

An understanding of the normal developmental anatomy is essential in the interpretation of imaging of congenital spinal anomalies. Although the exact embryology of spinal dysraphic lesions is not known, dysraphism most likely originates early in embryogenesis beginning in the third and fourth week of gestation.

Two processes form the CNS. The first is primary neurulation, which refers to the formation of the neural structures into a tube, thereby forming the brain and spinal cord. Secondary neurulation refers to the formation of the lower spinal cord, which gives rise to the lumbar and sacral elements. During this period, the notochord induces formation of the neural plate from the surface ectoderm of the embryonic disk. The neural plate forms in the midline on the dorsal aspect of the embryonic disk.

At about the fourth gestational week, the neural plate invaginates along a midline longitudinal axis to form the neural groove. The thicker neural folds form on either side of the neural groove. As the invagination progresses, the neural folds come together in the midline and fuse forming the neural tube.

Any disruption at gestational days 17-23, when the neural plate begins its first fold and fuses to form the neural tube, can cause craniorachischisis, the most severe form of NTD. At days 23-26 is when closure of the rostral neuropore occurs. Failure at this point results in anencephaly. Myelomeningocele is a result of disruption at days 26-30, closure of the caudal neuropore. Beyond day 26, a disruption is unlikely to be able to cause an NTD such as myelomeningocele.

Studies of mice embryos have shed light on the pathogenesis of associated anomalies seen with NTDs, such as hydrocephalus and the hindbrain Chiari II malformation. In 1992, McLone and Naidich postulated that both the hindbrain and the spinal cord anomalies are related to the initial event of failure of the neural folds to completely close, leaving a dorsal defect or myeloschisis. This permits the CSF to leak from the ventricles through the central canal and into the amniotic fluid and creates a collapse of the primitive ventricular system.

Leakage of CSF from the primitive ventricular system leads to a failure of the ventricles to increase in size and volume, due to both downward and upward herniation of the small cerebellum. In addition, the posterior fossa does not develop to its full size, and the neuroblasts do not migrate outward at a normal rate from the ventricles into the cortex. Therefore, the entire posterior fossa anomalies are related to a single initial inciting event. The process of disjunction involves the separation of surface ectoderm from the lateral margins of the neuroectoderm. This process occurs simultaneously with neurulation. After separation, the surface ectoderm grows across the midline and fuses and eventually forms skin covering the dorsal spine.

The filum terminale, conus medullaris, and distal central canal develop through the process of retrogressive differentiation. This involves the formation of the distal neural tube through canalization, but much of the neural tube thus formed subsequently involutes and form the filum terminale. The remaining portion forms the conus medullaris and distal central canal. Skin-covered spinal defects, spinal lipomas, and a dorsal dermal sinus originate in failure of neurulation, as well as in disjunction.

Premature disjunction before tube closure allows primitive mesenchyme to migrate into the lumen of the developing neural tube, giving rise to lipomatous tissue preventing tube closure. Failure of disjunction is said to be the cause of the formation of dorsal dermal sinus. The genesis of diastematomyelia, myelocystocele, thickened filum, and a simple meningocele is not understood. Thickened filum, a simple meningocele terminal myelocystoceles, and lipomas of the filum occur most probably result from abnormalities of differentiation of the caudal cell mass.

Clinical Details

Bony spina bifida occulta at the L5-S1 level is a common incidental finding on radiographs, both in children and adults, and it is usually not associated with symptoms or signs. If no signs are present, no further imaging workup is required.

The signs and symptoms of occult spinal dysraphism with a tethered cord are not usually obvious at birth, except in lipomyelomeningocele. Newborns also rarely exhibit neurologic deficits, but as the child grows, the consequences of the dysraphism may become apparent.

Whether tethered cords in babies should be released before they become symptomatic is controversial, but babies and younger children are at a greater risk of recurrent tethering. Therefore, surgery is tailored to individual circumstances. The overall management of dysraphism may become an ongoing, lifelong process, and its treatment is personalized depending on the type, severity, and progression of the disease.

Several cutaneous abnormalities indicative of underlying spinal dysraphism are situated in or near the midline, usually in the lumbosacral region:

Spinal dysraphism associated with a tethered cord can first become symptomatic or symptoms can worsen due to trauma, disk herniation, or exaggerated movement of the spinal column. The last mechanism may stretch the cord or nerve roots emanating from the cord.

Signs and symptoms of occult spinal dysraphism may appear as the child grows, as more tension is put on the tethered spinal cord. Initially, there is no pain, although neurologic and musculoskeletal deficit may be evident; these include pes cavus or scoliosis. Eventually, however, neurologic symptoms, such as pain and paresthesia, may affect the legs. Subsequently, the patient may even develop neuropathic symptoms, such as numbness of the toes and feet. The pain is sciatica-like and often unilateral, affecting the buttocks and back of the thigh muscles.

Depending on the site and type of tethering, neurologic deficit may also involve the bladder and bowel function. The first sign of bowel and bladder dysfunction in a baby may be extended toilet training and difficulty in keeping the baby dry. Incomplete bladder voiding may lead to recurrent urinary tract infections.

Orthopedic problems include a new or progressive scoliosis or kyphosis, unequal size of feet and legs, numbness and paresthesia of the legs and feet, unilateral or bilateral cavovarus deformity, claw toes, rigidity of the ankles associated with absent reflexes, pes cavus, and gait disturbances. Muscle weakness and gait disturbance usually becomes obvious around 2 years of age, when a child begins to walk.

With routine antenatal ultrasonography, most myelomeningoceles are diagnosed in utero. At birth, a midline defect in the posterior elements of the vertebrae associated with a protrusion of the meninges and cord elements through an external dural sac is noted.

Paraplegia from spinal cord malformation is the most common manifestation of a myelomeningocele. The neurosegmental anatomic level of the lesion is tested to assess for neurologic deficit. However, the functional motor level does not always correspond to the anatomic level of the lesion. Serial neurologic assessment of the level of the lesion allows for the early detection of a progressive neurologic deficit.

Thoracic myelomeningocele (25%) results in variable innervation of the neck and upper trunk musculature, but usually no voluntary movements in the lower limbs are present. Patients with thoracic myelomeningocele tend to have more severe involvement of the CNS and associated cognitive deficits.

Lumbosacral myelomeningoceles (75%) can be divided into a high, low lumbar, and sacral groups. Patients in the high-lumbar group tend to retain variable hip flexor and hip adductor movement, but movements of hip extensor and abductor muscles and all knee and ankle movements are absent. The low-lumbar group retains movement of hip flexor, adductor, medial hamstring, and quadriceps muscles, but movement retained in the lateral hamstrings, hip abductors, and ankle dorsiflexors is variable. Plantar flexion movement is usually absent. The sacral group retains movement in the hips and knees, but ankle plantar flexor movement is variable. Anesthesia is present in the perineal area.

The muscle tone in any of these groups is usually flaccid; however, as many as two thirds of patients have upper motor neuronal signs, and only 9% have a true spastic paraparesis. Impaired upper limb coordination is not uncommon, particularly in patients with associated hydrocephalus. Arnold-Chiari II malformation may also contribute to impaired coordination of the upper limbs, motor-learning difficulty, and/or delayed development of hand dominance. Spinal and lower-limb deformities and joint contractures are commonly associated with a myelomeningocele. Arnold-Chiari II malformation may also be associated with signs of cranial nerve dysfunction, such as ocular muscle palsies, swallowing and eating difficulties, and problems with phonation.

Scoliosis associated with a myelomeningocele can be congenital or acquired. Approximately one half the children with a myelomeningocele have an acquired form of scoliosis related to muscle imbalances. Increased lumbar lordosis and kyphosis involving the entire spine or the lumbar spine alone is frequently seen. Spinal deformities are seen more frequently in patients with higher spinal myelomeningoceles.

During the first trimester or early second trimester, women can undergo triple screening for spina bifida, Down syndrome, and other congenital diseases in the fetus. Estimation of maternal serum alpha-fetoprotein (AFP) level has been used since the late 1970s. AFP levels are elevated in 85% of cases of fetuses with spina bifida. Because false-positive results have many causes, a presumptive diagnosis based on maternal serum AFP levels is confirmed with amniocentesis and assay of the amniotic fluid for AFP and for acetylcholinesterase (AChE), a neurospecific enzyme. Targeted prenatal sonography (level II sonography) is a reliable method for diagnosing spina bifida. A myelomeningocele can be detected in 99% of affected fetuses with the combined use of these tests.

Results of urinalysis, urine culture, and serum urea nitrogen and creatinine tests are estimated at birth to provide baseline values for the evaluation of renal function. A base line renal sonogram may also be performed at this stage. Regular urine cultures may be required for children who have vesicoureteral reflux or signs and symptoms of urinary tract infection. Urodynamic studies are helpful in the evaluation of urologic function because they help in characterizing abnormalities of detrusor control and sphincter function.

Some reviews of spina bifida include lateral thoracic meningocele in patients with neurofibromatosis or kyphoscoliosis. These are saccular meningeal herniations through the intervertebral foramina, extending anteriorly between the ribs into the thoracic cavity. Most of these lesions are located in the thorax and occur exceptionally in relationship to the lumbar spine. These lateral intrathoracic meningoceles are more closely related to arachnoid cysts and bear little relationship to spinal dysraphic lesions.

Cranial dysraphism includes incomplete raphe closure resulting in cranial bifidum. The bony midline cranial defect allows formation of a cephalocele, which is the congenital herniation of intracranial contents in which the meninges or brain protrudes via a scalp defect. A cranial meningocele is a herniation of meninges and CSF. An encephalocele is herniation of brain through skull defect. Anencephaly may be complete when the calvarium and brain are complete, as in most cases, or incomplete, when calvarium covering brain is absent.

Preferred Examination

Choosing the most appropriate imaging technology for imaging congenital malformation of the spine depends on many factors. Imaging of the bony spine requires different methods than imaging of the spinal canal and its contents. The age of the patient and the plane of imaging required influence the choice of modality. The best way to image skeletal anomalies is by means of plain radiography, possibly combined with conventional tomography, though this has now been more or less replaced with CT.

Plain images may suffice from the orthopedic point of view, but they provide little information of the associated malformations of the spinal cord and its coverings. When spinal malformations are suspected, investigation of the spinal canal and its contents are best performed by MRI.

Skeletal scintigraphy with technetium-99m diphosphonates has high sensitivity but low specificity. Bone scintigraphy is a useful procedure in children with backache of unknown origin. Minor vertebral anomalies may show increased radionuclide uptake because of abnormal stresses and reactive changes. Further imaging can be restricted when an abnormality is localized.

MRI is a noninvasive investigative technique that provides more information than myelography or CT in defining spinal cord anatomy in spinal dysraphism. Sagittal MRIs are superior to CT myelograms in demonstrating the lipoma-cord interface. Individual nerve roots are less well seen with MRI.

In the evaluation of the spinal canal, ultrasonography is limited to the neonatal period, though a spinal defect covered with soft tissue can be imaged well into adult life. Fetal ultrasonography is increasingly used as a primary screening tool for NTDs, usually at about 18 weeks' gestational age. This trend reflects the increasing confidence in fetal ultrasonography. Ultrasonography helps in avoiding the calculated 1% risk of miscarriage associated with diagnostic amniocentesis.

In myelocele or myelomeningocele/meningomyelocele, detailed imaging prior to closure is usually not required, but after repair or spontaneous reepithelialization, imaging may be performed to depict other associated pathology, such as diastematomyelia or lipoma at other levels and to evaluate intracranial abnormalities.

Deterioration in neurologic function later in the course of disease or subsequent to surgery suggests a complication and is an indication for imaging to look for a surgically remediable cause, such as retethering of the cord due to adhesions or increase in size of an inclusion lipoma, epidermoid or hydromyelia. Normally, the cord should be low after repair, but it should not be tethered unless scars extend into the closure site.

Limitations of Techniques

The amount of radiation involved in spinal plain radiographs and CT is particularly important in examining infants; children; and young, fertile women. Plain radiography, as well as CT of the lower spine, delivers a high dose to the gonads, particularly in female patients.

Ultrasonography remains operator dependent, and accurate diagnosis depends on the skill and experience of the operator and on the quality of the equipment.

Transaxial CT images may be difficult to interpret because of the complex anatomy of the vertebral bodies, the presence of segmentation anomalies, and the presence of spinal curvature abnormalities. With modern CT scanners, this limitation may not be such as a disadvantage as sagittal and coronal reconstruction can now provide exquisite images of the spine.

In parts of the developing world availability of MRI remains limited and there are some relative contraindications, such as the presence of claustrophobia and some implanted devices. Children may require sedation.

Other Problems to be Considered

Findings

With spinal dysraphism, radiographs may show structural vertebral anomalies such as hemivertebra, butterfly vertebra, or incomplete fusion of posterior elements but does not allow imaging of the spinal cord. Radiographs of the vertebrae provide information for early evaluation when an infant is born with myelomeningocele. Congenital spinal deformities need to be tracked closely. Paralytic spinal deformities require imaging based on clinical examination findings; these deformities should be followed up frequently during times of rapid growth. Plain radiographs of patients with myelomeningocele demonstrate incomplete fusion of posterior elements and increased interpedicular distance.

Tubbs and associates have shown that a horizontal sacrum, as seen on plain radiographs after closure of a myelomeningocele, is an indicator of a tethered cord. Most of the patients had a lumbosacral angle that was greater than that expected in patients with late and decreased ambulatory abilities. They also observed that the lumbosacral angle was often inappropriately increased at the time. Many patients presented with symptoms indicative of a tethered cord. They postulated that, in this group of children, the tethered cord alters the lumbosacral angle and that the horizontal nature of the sacrum may predate the clinically appreciable symptoms of a tethered spinal cord.

Early accurate assessment and subsequent frequent reassessments of neurologic status are necessary with both anatomic and physiologic evaluation techniques in the investigation of the urinary system. Anatomic information may be obtained with a voiding cystogram that assesses the lower urinary tract, including bladder capacity and the presence of vesicoureteral reflux.

Posterior neural arch defects and associated interpedicular widening is seen in most cases of lumbosacral lipoma. In addition to dural ectasia and low-lying conus, myelography may demonstrate contrast filling and associated meningocele.

Myelography shows the tethered spinal cord to be posteriorly located, sometimes tenting the dorsal thecal sac. The filum is thickened and there is lack of cord movement in various positions.

Intraspinal lipomas can produce posterior scalloping of vertebral bodies and flattening of the pedicles. In diastematomyelia plain radiographs may demonstrate a bony spur when present. Frequently associated anomalies, such as spina bifida, interpedicular widening, hemivertebrae, and fusion of the vertebral bodies and laminae, are well depicted on plain radiographs.

Degree of Confidence

The radiation dose from plain radiographs of the spine is a major limiting factor in examining infants; children; and young, fertile women. Plain radiography of the lower spine delivers a high dose to the gonads, particularly in female patients. Plain images may be sufficient for assessing myelomeningocele before early surgery to assess the extent of the bony defect, though this is not always required.

Although plain radiographs are sufficient from the orthopedic point of view, they provide little information of the associated malformations of the spinal cord and its coverings. When spinal malformations are suspected, investigation of the spinal canal and its contents are best performed by MRI. If MRI is unavailable or contraindicated, myelography combined with CT can be used.

False Positives/Negatives

With spinal dysraphism, radiographs may show structural vertebral anomalies, such as hemivertebra, butterfly vertebra, and incomplete fusion of posterior elements, but radiography does not allow imaging of the spinal cord.

Posterior scalloping of vertebral bodies and flattening of the pedicles are nonspecific findings and may occur with intraspinal tumors; neurofibromatosis; acromegaly; achondroplasia; communicating hydrocephalus; syringomyelia; and a number of congenital syndromes, including Ehlers-Danlos, Marfan, Hurler, Morquio, and osteogenesis imperfecta syndromes.

Findings

CT myelography demonstrates splitting of the cord and, in some cases, splitting of the meningeal sheath. In addition, other bony anomalies, such as an intervertebral septum, and aberrant fibroneural bands may be depicted. CT myelography allows better definition of cord expansion or deformity than can be achieved by conventional myelography. In the case of intrinsic cord tumors, repeat CT after 24 hours reveals intramedullary contrast enhancement if associated syringomyelia is present.

Spinal lipomas, with their fatty tissue contents, are identifiable both on CT scans and on MRIs. On CT scans, fatty tissue has a strongly hypoattenuating appearance that may best be appreciated in comparison with CSF on soft tissue windows and in comparison with air on bone windows. On MRIs, fatty tissue is strongly hyperintense on images obtained with both short and long repetition times. Newer techniques of fat suppression, such as a short-tau inversion recovery (STIR) imaging, may resolve any doubts. The 2 techniques are complementary: On the one hand, CT better shows osseous abnormalities associated with the lipomas, and on the other, MRI is preferred because it allows better depiction of detail and contrast resolution on soft tissues.

At CT and MRI, epidermoid cysts usually have attenuation and signal intensity values that roughly parallel those of CSF. Occasionally, the cyst contents may have slightly negative attenuation values at CT and signal intensity slightly greater than that of CSF at T1-weighted MRI. Unlike dermoids, epidermoid cysts do not usually have the low negative attenuation values (-60 to -90 HU) of true fatty tissue, though the actual characteristics of an epidermoid may vary with its ratio of keratin to cholesterol. The squamous epithelial lining of an epidermoid cyst is usually too thin to be discerned on CT and only rarely shows calcification or enhancement with the administration of contrast material.

In comparison, dermoid inclusion cysts may have a more complex imaging appearance, though they are still unilocular. Their imaging characteristics depend on their contents and lining. The walls in these cysts are thick because of dermal adnexa, and they may be radiologically visible on CT scans and MRIs. This thick lining can become calcified and may enhance with the use of contrast material. The lipid material in a dermoid has attenuation and signal intensity characteristics of fat on both CT scans and MRIs. Occasionally, dermoids do not have these radiologic features; in such cases, they may be difficult to distinguish from epidermoids.

The appearance of a unilocular, midline, cystic mass, especially one with attenuation and signal intensity characteristics similar to those of fat or with a fat-fluid level, is suggestive of a dermoid inclusion cyst. A suspected dermoid inclusion cyst should be carefully evaluated for any associated skeletal dysraphism, fibrous band, or sinus tract leading to the surface of the skin.

Accidental or iatrogenic rupture of a dermoid can be diagnosed when characteristic lipid droplets are seen in the subarachnoid spaces of the sulci and cisterns.

Degree of Confidence

CT myelography allows better definition of cord expansion or deformity than that achieved by means of conventional myelography. MRI is the imaging procedure of choice.

False Positives/Negatives

Occasionally, dermoids may be difficult to differentiate from epidermoids on CT scans on the basis of attenuation characteristics. It is not always possible to differentiate intrinsic from extrinsic conus tumors on CT myelograms.

Findings

In 2000, Mangels and associates showed that fetal MRI is an effective, noninvasive means of assessing of fetal CNS anatomy. Its ability to resolve posterior fossa anatomy is superior to that of ultrasonography, though with reference to hydrocephalus and the level and nature of spinal anomalies, it may be equivalent or inferior to sonography.

MRI of the spine is seldom required before surgery. MRI findings include absent posterior bony elements at the site of the defect. The soft tissue sac containing CSF may be obvious, and the conus is invariably low, coursing dorsally within a capacious dural sac. Inclusion dermal tissue is generally evident as a rounded mass with variable signal intensity often greater than that of the surrounding CSF. Postoperative scarring may obliterate the CSF space, and obliteration of the CSF distal to the site of repair site suggests additional scarring responsible for the tethering. Scarring is generally hypointense on images obtained with all sequences, and it may be enhancing after the administration of gadolinium-based contrast material.

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have recently been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. As of late December 2006, the FDA had received reports of 90 such cases. Worldwide, over 200 cases have been reported, according to the FDA. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble movingor straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.

Arnold-Chiari II malformations are noted in nearly all cases of myelomeningocele. These malformations are best assessed with MRI, though before the age of 6 months, cranial ultrasonography is as effective at MRI. The small posterior fossa, low insertion of the tentorium, and downward displacement of cerebellar tissue and medulla through a widened foramen magnum are obvious. When the medullary migration is not vertical, the characteristic cervicomedullary kink is noted.

Syringomyelia of the cervical cord or syringobulbia and progressive dilatation of the fourth ventricle may account for worsening neurological deficit. Additional findings in Chiari II malformation include stenogyria, partial agenesis of corpus callosum, large massa intermedia, and a beaked tectum. Rarely, a Chiari I malformation is associated with myelomeningoceles in which only the tonsils herniate below the foramen magnum.

A tethered cord manifests itself as a low conus and associated spinal lesions. By the age of 2 months, a conus below L2-L3 is considered abnormal. Axial T1-weighted images are most accurate in determining the conus level. In symptomatic patients a conus below L2-L3 may be regarded as abnormal and other supporting evidence, such as dorsal displacement of the cord associated with obliteration of the surrounding CSF spaces may support the clinical suspicion. A low cord invariably occurs with a myelomeningocele and retethering may occur after repair.

Posterior neural arch defects and an increase interpedicular distance are often associated with a lumbosacral lipoma. T1-weighted images have high sensitivity in the detection of lipomas because of the short relaxation time of fat. The fat content may also be confirmed by using fat-saturation techniques.

The diagnosis of a thickened filum is made when the filum measures more than 2 mm at the L5-S1 disk space. In infants and young children, the filum is not discretely visualized on multiple axial images below the tip of the conus. Thus visualization of the filum in this age group should be considered abnormal even if the thickness is less than 2 mm and warrants further investigation.

In diastematomyelia, MRI is used to evaluate the extent of cord clefting, a low position of the conus, scoliosis, other bony anomalies, and the commonly associated syringohydromyelia. The bony spur is well seen on T1-weighted MRIs when fatty marrow is present. Less well-developed spurs are seen as low-signal-intensity splitting the high-signal-intensity thecal sac on T2-weighted images.

Dermoids and epidermoids account for about 1-2% of all spinal tumors at all ages but about 10% of tumors in patients younger than 15 years. They may be associated with a dermal sinus or occur in isolation. When not associated with dermal sinuses, they may occur with progressive compressive myelopathy or acute onset chemical meningitis due to the rupture of the cyst and the spread of cholesterol crystals in the CSF. The thoracic lumbar and sacral spine are affected, with a slight increase in incidence in the craniocaudal direction. CT and MRI characteristics are similar to those seen in epidermoids at other sites. Diffusion MRI is promising as an alternative to CT after an intrathecal injection of contrast medium to detect and delineate the limits of the lesion.

Lipomas in the dura occur mainly in the caudal portion of the spine. Those in the region of the filum terminale are associated with congenital spinal anomalies. The lipoma is hyperintense on T1-weighted MRIs. Spinal lipomas are usually found in the extradural space in the thoracic region. The radiological and MRI signal intensity characteristics are similar to those of intradural lipomas.

Degree of Confidence

MRI is an excellent imaging modality for visualizing the spinal cord at all ages and the imaging modality of choice for defining complex spinal dysraphism. CT myelography is now used infrequently.

Brophy and associates performed MRI in 42 patients with spinal dysraphism. Scoliosis and a progressive neurologic deficit were the primary indications. Spinal cord anomalies included hydromyelia, diastematomyelia, lipoma, thickened filum terminal, and spinal cord atrophy. All patients but 1 had an Arnold-Chiari malformation. Twenty-two of the 42 patients had CT scans, pyelograms, or surgical findings that corroborated the 41 MRI findings. Three false-positive MRI findings of hydromyelia and no false-negative studies were observed.

Sutton and associates compared MRIs of lipomyelomeningocele and tethered cord with operative findings in 25 patients with the diagnosis of lipomyelomeningocele and/or tethered cord. Postoperative MRIs in 8 patients, 5 of whom in stable condition, were also compared with the preoperative studies. This review revealed 1 false-negative MRI and 4 MRIs on which the relationship of the lipoma to the conus or filum was not demonstrated accurately. In 6 patients, incidental intramedullary cystic lesions at the conus were identified on MRIs. All 8 postoperative MRIs (obtained at 1 mo to 2 y) demonstrated no change in the level of the conus, as compared with the preoperative study.

MRI is an accurate screening modality in the initial diagnosis of occult spinal dysraphism. MRI was not useful in the postoperative evaluation of lipomyelomeningocele and the tethered cord because the caudal, posterior displacement of the conus was unchanged in all studies.

False Positives/Negatives

Normal nerve roots of the cauda equina may simulate low conus on sagittal MRIs. MRI accurately depicts the extent of syringohydromyelia, which may clinically mimic a tethered cord. Despite surgical repair postoperative ascent of the conus medullaris is seldom seen on subsequent MRIs.

Findings

Spinal ultrasonography is made possible in the newborn owing to the lack of ossification of the predominantly cartilaginous posterior elements of the spine. Spinal sonography is usually not possible after the age of 6 months, except in cases of a persistent posterior spinal defect, in which sonography can be performed at any age.

In the neonate, the cord appears as a tubular hypoechoic structure with hyperechoic walls. The central canal is hyperechoic. The cord position is dependent. The subarachnoid space around the cord is echo poor. In the neonate, the conus medullaris is smooth and tapering and lies above the middle of L2 with a range varying from D10 to L2-L3. The cauda equina is seen as echogenic lines surrounding a hyperechoic filum terminale with dependent positioning.

The normal filum terminale is 1.0-1.5 mm in diameter. The vertebral bodies are seen as echogenic segmental structures lying anterior to the spinal cord. In the normal infant the cord lays a third to half way between the anterior and posterior walls of the spinal canal. There is normal pulsatile movement of the cord. When the cord is viewed axially, it appears round to oval and is surrounded by the fluid-filled subarachnoid space. The cord is fixed within the spinal canal by the dentate ligaments, which pass laterally from the cord. Below L2 the echogenic nerve roots are identified with a vertical or oblique orientation.

The spinal cord can be depicted throughout its length, allowing visualization of the conus and free movement of the nerve roots. Absence of normal transmitted pulsations and lack of free movement of nerve roots on sonograms suggest a tethered cord. In a low tethered cord, the conus is low, the spinal cord is displaced dorsally. There is lack of normal cord pulsatility and the filum terminale is thickened to over 2 mm. The thickened filum terminale may be fibrous or lipomatous. An abnormal cord may lie in a dorsal position rather than being dependent. The clinical significance of a low cord without tethering is unknown.

In a newborn term infant, the normal conus usually lies above the level of the mid L2. With a skin-covered back mass the contents of the mass can be characterized. Axial spinal sonograms readily show the 2 hemicords and the echogenic spur in diastematomyelia. In diastematomyelia, sonography may show the spur. A dorsal dermal sinus may be depicted as an echogenic tract deep to a hole in the skin.

With sonography, it may be difficult to be confident if the tract communicates with the spinal canal. However, a low-lying cord suggests tethering by intraspinal extension of the sinus, and sonograms may show abnormal echogenicity at the depth of the tract, suggesting lipoma or dermoid. Alternatively, images may show matted nerve roots due to arachnoiditis. No conus may be identified if cord terminates in a lipoma.

Anterior sacral meningocele is an isolated anomaly or an association with neurofibromatosis or Marfan syndrome. Images show herniation of the dural sac containing CSF through a defect through the anterior surface of the sacrum or coccyx. Patients may be asymptomatic or present with a pelvic mass or bowel or bladder dysfunction. Ultrasonography shows a unilocular or multicystic pelvic mass anterior to the sacrum. A plain radiograph may confirm an anterior sacral defect, and MRI may demonstrate the CSF content, which communicates with the spinal canal. Associated anomalies such as cord tethering and lipomata should be sought.

Meningocele and myelomeningocele represent the most common forms of spinal dysraphism, which arise as a result of a localized failure of fusion of the dorsal neural fold. A meningocele represents a herniation of distended meninges only, whereas in a myelomeningocele, part of the spinal cord and or nerve roots lie within the herniated sac. Ultrasonography shows an anechoic lobulated sac that is continuous with a low-lying tethered cord.

Myelocystocele is a variant of a meningocele in which the central spinal canal dilates and herniates through a posterior spinal defect, appearing as a subcutaneous mass. Hydromyelia is invariably associated. On sonograms, the CSF-filled central canal appears to funnel into the larger anechoic subcutaneous cystic mass.

Spinal lipomas can be classified into 3 groups: (1) lipomyelocele/lipomyelomeningocele, which represent 84% of the tumors, (2) fibrolipoma of the filum terminale (12%), and (3) intradural lipoma (4%). Spinal lipomas in association with spinal dysraphism have been reported in 20-50% of patients. Spinal lipomas are commonly located in the cervical and dorsal spine and may be large enough to cause spinal cord compression. Lipomas are highly echogenic and easily identified on sonograms.

Myelocele is an exposed open spinal defect, usually in the lumbar region. Surgery is usually performed on an urgent basis, and sonography is typically avoided because of the risk of infection.

A dermal sinus may show as an echogenic sinus tract deep to a hole in the skin. With sonography, it may be difficult to be confident if the tract communicates with the spinal canal. However, a low-lying cord suggests tethering by intraspinal extension of the sinus, and sonograms may show abnormal echogenicity at the depth of the tract, suggesting lipoma or epidermoid or dermoid, or they may show matted nerve roots due to arachnoiditis.

Axial spinal sonograms readily depict the 2 hemicords and the echogenic spur in diastematomyelia.

Ultrasonographic evaluation for spina bifida should include both spinal and cranial imaging. Findings in spina bifida include widening of the posterolateral spinal ossification centers, absence of the continuity of the skin over the spine, and a bulging sac past the dorsal skin line. Cranial findings suggestive include the presence of ventriculomegaly, small head size, and elongated cerebellum with obliteration of cisterna magna from a Chiari II malformation, the banana sign, and scalloping of the frontal skull region, the lemon sign.

The lemon sign is best obtained superior to the plane used to measure the biparietal diameter (BPD) at level of lateral ventricles. The lemon sign usually resolves by the third trimester because of increasing ossification of the calvarium. Therefore, this sign is sensitive for an NTD only in the second trimester. The absence of a lemon sign at 14-24 weeks' gestation makes spina bifida unlikely.

In Chiari II malformation, the cerebellum takes on a crescent or banana shape, and the cisterna magna becomes obliterated. In the second trimester, this banana sign is both sensitive and specific for ONTDs without the false-positive rate of the lemon sign. In the third trimester, the obliterated cisterna magma may be easier to see and more helpful than the banana sign. Hydrocephalus is obvious on antenatal sonograms. Hydrocephalus is associated with Arnold-Chiari II malformation in as many as 90% of these cases, and conversely, one third of fetuses with hydrocephalus have a spinal defect.

Degree of Confidence

Ultrasonography is a useful initial screening test in the infant with suspected spinal abnormality. Spinal sonography is a useful technique in the detection of the various types of spinal dysraphism, spinal tumors, arteriovenous malformations, and spinal trauma particularly in the neonate.

The sensitivity of sonography in the detection of spinal dysraphism in the neonate has been reported be equal to that of MRI with an added advantage that no sedation or general anesthesia is required. Moreover, pulsating artifact and CSF flow does not affect the sensitivity of sonography.

The examination is performed by using a 7.5- to 10-MHz linear probe in both the sagittal and axial planes along the whole of the spine, preferably with the patient in a prone position. In many centers around the world, fetal sonography is used as a primary screening tool for NTDs, usually at approximately 18 weeks' gestational age. This trend reflects the increasing confidence in fetal ultrasonography.

Ultrasonography helps in avoiding the calculated 1% risk of miscarriage associated diagnostic amniocentesis. Cesarean delivery before rupture of the amniotic membranes and onset of labor reduces the degree of neurologic deficit in fetuses with myelomeningocele.

Ultrasonography improves the detection and accuracy of determining the defect level and thus helps in identifying mothers that are most likely to benefit from an elective cesarean delivery. However, ultrasonography remains operator dependent, and accurate diagnosis depends on the skill and experience of the operator and the quality of the equipment.

False Positives/Negatives

The lemon sign is scalloping of the fetal frontal bones in axial view at the level of BPD. When seen with meningomyelocele, it is usually associated with an Arnold-Chiari type II malformation with an abnormal cerebellar position. The positive predictive value of the lemon sign in a low-risk population is 6%. The lemon sign is also described in the normal fetus, the incidence is 0.66-1.3%. No associated intracranial abnormality is apparent; the ventricle is normal sized, and the cerebellum is normal. The spine is also normal. An encephalocele is often associated with type III Chiari malformation. Most of these are occipital. In rare cases, they occur in the frontal region, where they may mimic a lemon sign.

Currently, ultrasonography is not sensitive enough to enable reliable and accurate detection of the level of the spinal defect. After confirmation of fetal myelomeningocele, clinicians at most tertiary care centers perform weekly ultrasonographic examinations to observe the growth and development of the fetus.

A bifid sacrum artifact is a skewed representation of normal anatomy and should not be interpreted as a true anomaly. It is produced by a steeply angled parasagittal scanning plane that intersects normal structures. Sonograms of the distal spine can be misleading and deceptive. The normal spine is constructed of multifaceted anatomic structures that change in appearance and relative position throughout gestation. The complex structures can be seriously misinterpreted if scanning planes are skewed or rotated off axis.

A false-positive sonographic diagnosis of spina bifida in a fetus with triploidy has been reported. A patient with elevated maternal serum AFP level presented for a fetal ultrasonographic examination. Findings on the scan included a lemon sign, a banana sign, an effaced cisterna magna, and splayed lumbar vertebrae. After termination of the pregnancy, no spinal abnormality was detected on autopsy. Radiographs of the fetal spine demonstrated narrowing in the thoracic spine. The karyotype of the fetus was 69,XXY.

Findings

CSF flow dynamics can be assessed by injecting radionuclides into the subarachnoid space via a lumber puncture or directly via a ventricular injection. The technique of radionuclide cisternography with radioiodine-labeled serum albumen was first introduced by DiChiro in 1964 and has been used extensively to investigate CSF dynamics.

In recent years, the introduction of CT and MRI has resulted in a reduction in the use of radionuclide cisternography. Currently, radionuclide techniques are used in conjunction with CT and/or MRI to assess CSF circulation in hydrocephalus and to determine CSF shunt patency and CSF leaks.

With the widespread application of various CSF shunts to treat hydrocephalus, a need has arisen to assess shunt patency in shunt malfunction. At the moment, several methods are in use, including the assessment of responses to digital compression, the injection of contrast media, radionuclide methods, ultrasonographic flow measurements, and indirect infusion methods. Indium-111 diethylenetriamine pentaacetic acid (DTPA) and technetium-99m DTPA have been successfully used in evaluating CSF shunt malfunction and were found to be particularly useful in children who present with nonspecific clinical features, such as headaches, malaise, vomiting, and irritability. In conjunction with CT and/or MRI a normal shunt study may prevent unnecessary surgical intervention.

Radionuclide may be injected directly into the shunt reservoir under strict aseptic conditions to assess shunt patency. In patients with a normal-functioning shunt, the tracer passes freely into the peritoneum via the shunt tube when a ventriculoperitoneal shunt is present. In patients with a partially blocked shunt or a completely blocked shunt, the tracer is not seen in the peritoneum and passes up the subarachnoid space into the basal cisterns and may reflux into the ventricles.

Degree of Confidence

Radionuclide methods allow for the study of CSF flow dynamics without altering them, as only a small volume of the radionuclide is injected into the CSF space. In recent years, use of radionuclide cisternographic methods has declined as a result of increasing reliance on CT and MRI. However, these are used primarily to image anatomy, whereas radionuclide methods provide functional information. Special sequences and gated MRI may enable assessment of CSF flow and may replace radionuclide methods.

False Positives/Negatives

Depending on the type of shunt used (ventriculoperitoneal, ventriculoatrial, or ventriculopleural), reflux into the ventricles is often seen even in patent shunts. If ventricular reflux does not occur, inference cannot always be made that the ventricular limb of the shunt is occluded. This is particularly true with ventriculopleural shunts in which negative pressure in the pleural cavity draws the CSF through the tube reducing ventricular reflux.

In small babies with a large ventricle, CSF flow through the shunt may be slower than in older patients. This finding is said to be related to the larger volume of the ventricles and a comparatively low intraventricular pressure.

Studies in the 1980s and 1990s showed that folic acid deficiency was an important cause of myelomeningocele and that 50% of cases of NTDs are preventable. Therefore, in 1992, the US Public Health Service (USPHS) recommended that all women anticipating pregnancy should take folic acid 0.4 mg/d. In 1996, the USPHS made the fortification of cereal grain with folic acid mandatory.

Intrauterine repair of the myelomeningocele is now possible, and early studies suggest that this may decrease development of significant hydrocephalus without changing the motor outcome, despite decreased exposure injury to the dysplastic cord.

Surgery for NTDs is performed as a matter of urgency immediately after birth if CSF leakage is present or generally within the first 24-48 hours in the absence of CSF leakage.

Occasionally, hydrocephalus associated with a myelomeningocele arrests spontaneously. However, in most cases, shunt surgery is required. Shunt techniques include ventriculoperitoneal, ventriculoatrial, and ventriculopleural procedures. Ventriculoperitoneal shunting is the preferred modality.

Orthopedic surgery may be required for a variety of musculoskeletal complications associated with myelomeningocele. Progressive spinal kyphosis or scoliosis are common and may result in a variety of neurologic and cardiopulmonary compromises. Spinal stabilization may be necessary to correct kyphosis. Paralytic scoliosis develops in approximately one half of children as a result of asymmetric muscle forces, pelvic obliquity, unilateral hip dislocation, or progressive neurologic process such as syringohydromyelia. Some of these orthopedic complications can initially be managed conservatively, but they may eventually require corrective surgery such as surgical fusion.

Lower-limb deformities, such as knee flexion, may be managed by means of simple tenotomy of the knee flexor tendons. Extension contractures are managed with extensor tendon release when not amenable to orthotic measures.

Internal and external tibial torsion are commonly associated with myelomeningoceles. If no improvement occurs with growth or conservative measures by the age of 6 years, derotation osteotomy with plate fixation is the preferred surgical technique.

A clubfoot associated with myelomeningocele usually requires surgical correction in the first year of life and generally requires multiple release procedures with tendon excisions. Muscle tendon transfer may be required in older children to correct anomalies such as calcaneovarus and calcaneovalgus.

To manage urinary bladder drainage problems, clean intermittent catheterization is preferred to long-term indwelling catheters. Catheterization may be used from birth to reduce bladder pressures and to establish social continence at a developmentally appropriate time.

Urologic surgical procedures, such as a temporary vesicostomy, augmentation cystoplasty, and urinary diversion, are surgical techniques available to manage urinary bladder drainage problems.

Intravesical transurethral bladder stimulation has been shown to improve bladder functional bladder capacity, sensation, and compliance. However, the technique has not met with success in achieving voluntary bladder control.

Fetal therapy is a rapidly advancing specialty and an integral part of interventional sonography. Open hysterotomy has been performed for the repair of myelomeningocele, resection of sacrococcygeal teratoma in fetuses with nonimmune hydrops, and treatment of an enlarging congenital cystic adenomatoid malformation that is not amenable to thoracoamniotic shunting (Coleman, 2002).

To date, more than 100 women and their fetuses have undergone in utero surgery to repair spinal defects. No fetus has been cured, and published reports indicate no significant improvement in the level of paralysis compared with optimal postnatal care. However, approximately one third of the fetuses may show improvement in Chiari malformation, decreasing the need for shunt surgery (Farmer, 2003).

Serious complications, including uterine rupture, maternal bleeding, fetal death, and prematurity, have been reported with fetal surgery. Moreover, the long-term effects of fetal surgery are not known. A multicentric, randomized, controlled trial of fetal surgery for myelomeningocele sponsored by the National Institutes of Health is underway, and its results are keenly awaited.

Spinal Dysraphism/Myelomeningocele

AUTHOR AND EDITOR INFORMATION

INTRODUCTION

Background

Pathophysiology

Frequency

United States

International

Mortality/Morbidity

Race

Sex

Age

Anatomy

Clinical Details

Preferred Examination

Limitations of Techniques

DIFFERENTIALS

Other Problems to be Considered

RADIOGRAPH

Findings

Degree of Confidence

False Positives/Negatives

CT SCAN

Findings

Degree of Confidence

False Positives/Negatives

MRI

Findings

Degree of Confidence

False Positives/Negatives

ULTRASOUND

Findings

Degree of Confidence

False Positives/Negatives

NUCLEAR MEDICINE

Findings

Degree of Confidence

False Positives/Negatives

INTERVENTION

Medical/Legal Pitfalls

Special Concerns