Disclosure
Background: In 1891, Hans von Chiari described certain hindbrain abnormalities as postmortem findings in infants. These came to be known as Chiari malformations. Four types of Chiari malformations are described in the literature: types I, II, III, and IV. (Chiari malformation types II, III, and IV are distinct from type I and are not discussed in this article.) Chiari I malformation (CMI) is characterized by herniation of the cerebellar tonsils through the foramen magnum into the cervical spinal canal. The cerebellar tonsils often are elongated and peglike. Mild caudal displacement and flattening or kinking of the medulla may be present. The vermis cerebelli and the fourth ventricle are normal or only minimally deformed. CMI is not directly associated with other congenital brain malformations, specifically myelomeningocele. Myelomeningocele is a feature of Chiari II malformation. However, craniovertebral malformations are common in patients with CMI. Distention with cerebrospinal fluid (CSF) of the central canal of spinal cord (ie, hydromyelia) or paracentral cavities (ie, syringomyelia) is present in approximately 25% of patients with CMI. The cervical cord is the most common site of syringohydromyelia. Pathophysiology: Evidence suggests that CMI may in fact be a mesodermal disorder. Underdevelopment of the occipital somites of the para-axial mesoderm produces a diminutive, overcrowded posterior cranial fossa (PCF). Tonsillar herniation occurs secondarily as a result of mechanical factors. A small PCF was identified in morphometric studies of patients with CMI, compared with the supratentorial volume in the same patients and the PCF volume in control subjects. A diminutive PCF underlies the development of acquired CMI in patients with craniosynostosis, hypophosphatemic rickets, achondroplasia, Paget disease, and acromegaly. The association of CMI with cranial and vertebral anomalies is well known. CMI is not directly associated with other neuroectodermal abnormalities. When present, neural abnormalities are attributable to the effects of tonsillar herniation. CMI may have a genetic basis, as suggested by (1) its association with known genetic disorders such as achondroplasia, Hadju-Cheney syndrome, and Klippel-Feil syndrome and (2) reports of familial aggregation and concordance among monozygotic twins and triplets. Findings from pedigree analyses in some families suggest an autosomal dominant inheritance pattern with reduced penetrance or an autosomal recessive inheritance pattern. Acquired CMI is reported to develop after lumboperitoneal or ventriculoperitoneal shunt placement, in addition to the conditions mentioned above. Radiologic findings in patients with acquired CMI cannot be distinguished from those in congenital form of the malformation. Altered CSF flow Altered CSF dynamics may play a pathophysiologic role. CSF flow is characterized by systolic and diastolic CSF displacements related to the phases of the cardiac cycle. An increase in the cerebral blood volume with the systolic arterial pulse produces a wave of CSF displacement that descends into the cervical subarachnoid space from the basilar cisterns (craniocaudal displacement). Net cerebral venous outflow and the elastic recoil of the spinal dura during diastole cause CSF displacement in the reverse direction (caudocranial). Respiration also affects CSF flow pulsation. During inspiration, the increased venous return causes the spinal epidural veins to collapse and promotes craniocaudal CSF flow; during expiration, distention of the epidural veins promotes caudocranial flow. As the herniated tonsils fill the foramen magnum in the setting of CMI, CSF flow is reduced at the craniovertebral junction, and a compensatory pulsatile descent of the cerebellar tonsils is observed during systole. This combination can effectively plug the CSF pathway at the foramen magnum. Anterior indentation of the medulla and cord secondary to basilar invagination may further contribute to the obstruction of CSF flow at the foramen magnum. This effect exaggerates the pulsatile systolic wave in the spinal subarachnoid space. During diastole, rapid recoil of the brain stem and tonsils disimpacts the foramen magnum and allows normal CSF diastolic pulsation. Given the small PCF-CSF space, decreased CSF compliance is expected. Along with altered CSF dynamics, this effect can play a role in causing headaches and other symptoms that mimic an orbital pseudotumor, Ménière disease, bulbar compression, or syringomyelia. Patients with CMI remain asymptomatic for prolonged periods. Arachnoidal scarring and adhesions are suggested to build up at the foramen magnum as the cerebellar tonsils rub against bone over the years. These arachnoidal adhesions may increase the compression of the hindbrain and spinal cord and further interfere with CSF flow at the foramen magnum and thus cause symptoms. These adhesions are often discovered at the time of surgery; however, many patients remain symptomatic despite adequate surgical decompression of the foramen magnum. Many patients report trauma, such as whiplash injuries and direct blows to the head and neck, as the precipitating event for symptom onset. Such trauma may accentuate tonsillar impaction or result in subarachnoid hemorrhage that destabilizes a marginally compensated CSF system. Syringohydromyelia Syringohydromyelia probably is secondary to pathologic CSF dynamics. The exaggerated pulsatile systolic wave in the spinal subarachnoid space drives the CSF through anatomically continuous perivascular and interstitial spaces into the central canal of the spinal cord. More severe obstruction occasionally can cause hydrocephalus. Syringohydromyelia has been suggested to result from pulsatile flow directed down the central canal of spinal cord from the fourth ventricle with obstructed channels. However, most syringes are separated from the fourth ventricle by an occluded or stenotic segment of the central canal. CSF flow abnormalities improve after decompressive neurosurgery; the symptoms resolve, and the syrinx cavity collapses. Syringomyelia was observed in only 14% of patients in a pediatric case series. Spinal MRI results were available in only a minority of patients; therefore, this figure may represent an underestimation of the presence of syringomyelia. In addition, sufficient time may not have elapsed for syringomyelia to develop. Frequency:
Mortality/Morbidity: See Clinical Details. Race: To the author's knowledge, no data specifically address the racial distribution of CMI. Sex: A female predominance was reported in some large case series, with a male-to-female ratio of 2:3. Age: CMI is predominantly a congenital abnormality. Prior to MRI, CMI was observed almost exclusively in those aged 10-30 years, hence the term adult-type Chiari malformation. In recent studies, patients were aged 10 months to 65 years. In one series, the mean patient age at diagnosis was 39 years (range, 6-60 y). In another series, the mean patient age at the time of symptom onset was 24.9 ± 15.8 years. Patients with syringomyelia presented at a slightly younger age (24.7 ± 16.6 y). Among the patients, 37% reported lifelong symptoms such as headaches and clumsiness, and 24% reported that trauma was the precipitating event. Anatomy: The anatomic landmarks of this disorder are depicted in Image 1. A discussion of the precise anatomy may be found in the sections about osteology and neuroanatomy in Gray's Anatomy. Clinical Details: A variety of symptoms are reported in the literature, including the following:
Patients who have CMI and a syrinx almost always present with symptoms referable to the syrinx. If the syrinx extends into the medulla, bulbar symptoms predominate. In pediatric patients, the most common symptoms are headache, neck pain, and ataxia. Usually, the degree of tonsillar ectopia and the presence and severity of neurologic symptoms and signs are directly correlated. Posttraumatic neurologic complications in patients with CMI are well described. Minor trauma can result in significant symptoms in the setting of pediatric CMI.
The natural history of CMI is not understood clearly. Many patients are asymptomatic and probably remain so all of their lives. The age of patients at symptom onset is variable. In symptomatic patients, whether the malformation is causative or incidental is not always clear. Many patients have serious clinical sequelae. Rarely, the malformation resolves spontaneously.
Usually, patients with clinical signs of CMI have greater mean tonsillar herniation (ie, 12.3 mm) compared with that of patients with incidentally discovered CMI (ie, 7.3 mm). Herniations greater than 12 mm are almost always symptomatic.
Tonsillar ectopia does not explain the entire clinical picture. The extent of tonsillar ectopia is not correlated with the patient's age at onset, presence of syringomyelia, or symptoms such as headache. As many as 31% of patients with a tonsillar herniation of 5 mm or greater may be asymptomatic. In one series, tonsillar ectopia of 25 mm was discovered incidentally. Tonsillar herniation may be asymmetric without corresponding lateralizing symptoms or signs. The extent of tonsillar descent is not correlated with the Karnofsky score or the incidence of syringomyelia. Asymptomatic children with tonsillar ectopia of greater magnitudes may be at higher risk for neurologic symptoms later in life. Preferred Examination: MRI is the imaging modality of choice except in patients in whom MRI is contraindicated. MRI demonstrates the abnormal CSF flow and configuration and position of the brain and spinal cord. Limitations of Techniques: MRIs may not reliably demonstrate abnormal findings of the skeleton associated with Chiari malformations. X-ray–based imaging modalities such as conventional radiography or CT are preferred for this indication.
Radiologic findings in patients with acquired CMI cannot be distinguished from those in congenital form of the malformation.
Findings: Skeletal anomalies are easily delineated with radiography. Osseous anomalies of the skull base and skeletal system are observed in 25-50% of patients with CMI and include the following, with the frequency of association in parentheses:
Other associated skeletal abnormalities include a reduced height of the supraocciput and clivus. Degree of Confidence: Radiography is reliable in the demonstration of skeletal abnormalities. |
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Findings: On CT scans, the following findings may be observed:
Rarely, spinal CT may show syringomyelia. CT can be used to assess associated bony abnormalities of the skull base and vertebral column (see X-Ray section above). In the past, CT cisternography and/or myelography, supplemented by image reconstruction in nonaxial planes, was used to assess tonsillar position and configuration. CT myelograms do not demonstrate the lower brainstem and bulbomedullary junction in sufficient detail. Associated syringomyelia is often missed. CT may be of value in patients in whom MRI is absolutely contraindicated. Degree of Confidence: CT is reliable in detecting osseous abnormalities.
Findings: MRI has revolutionized the diagnostic evaluation for CMI. It can be used to detect CMI that previously remained unrecognized or was misdiagnosed. Tonsillar position, tonsillar configuration, and many associated abnormalities are depicted on sagittal and axial T1- and T2-weighted MRIs. On MRIs, the following findings may be observed:
Tonsillar ectopia The degree of tonsillar ectopia is expressed as the number of millimeters that the tonsillar tips extend below a line connecting the basion with the opisthion. Perform all measurements by using sagittal T1-weighted images. Use the signal intensity of the cortical bone, not that of the marrow, to define the anatomic landmarks. Tonsillar tips that extend less than 3 mm below the landmark are normal. Tonsillar herniation should be primary and not secondary to an intracranial mass lesion (eg, brain tumor, cerebral edema) to meet the criteria for congenital CMI. The most reliable criterion is herniation of at least 1 cerebellar tonsil that is 5 mm or more below the plane of the foramen magnum, as defined above. Asymmetric tonsillar herniation may be observed. Tonsillar herniation of less than 5 mm does not exclude the diagnosis. Herniation of both tonsils that are 3-5 mm below the foramen magnum, accompanied by certain other features, may suggest CMI. These other features include a syrinx, cervicomedullary kinking, elongation of the fourth ventricle, and a pointed or peglike appearance of the tonsils. Cerebellar tonsils ascend with age. Some authorities suggest the following criteria for tonsillar ectopia: (1) herniation of 6 mm in those aged 0-10 years, (2) herniation of 5 mm in those aged 10-30 years, (3) herniation of 4 mm in those aged 30-80 years, and (4) herniation of 3 mm in those aged 80-90 years. Other findings Narrowing or obliteration of the retrocerebellar CSF spaces is observed in association with a meniscus sign at the lower pole of the cerebellar tonsils. The height of supraocciput is reduced, and the slope of tentorium is increased. The PCF volume, in absolute terms and expressed as a ratio of supratentorial volume (posterior fossa ratio), is significantly smaller; however, mean brain volumes did not differ in patients and control subjects. The cervical subarachnoid space below the level of the C2-3 disks is markedly narrowed in patients with syringomyelia as a result of spinal cord expansion. The posterior subarachnoid space below the tip of the cerebellar tonsils may be completely obliterated. Other findings include anterior displacement of the cerebellum, kinking of the medulla, compression of the fourth ventricle, hydrocephalus (mild or moderate), and an empty sella. The cerebral aqueduct is frequently elongated and narrowed; however, no significant descent of the latter structure or the brainstem is observed. Syringohydromyelia is most commonly observed between the C4-6 levels. Holocord hydromyelic cavities may be present. Cervical/upper-thoracic and bulbar/cervical syringes also are observed. Isolated thoracic syringes are not described. The level of widest syrinx diameter most frequently occurs at the C2-3 level. Asymmetric or multiple axial syringes are described. CSF flow abnormalities Several investigators have studied CSF flow abnormalities in CMI. All patients had narrowing of the CSF pathways at the foramen magnum, at the C2-3 disk level, and in the posterior subarachnoid space below the tip of the cerebellar tonsils. The cardiac cycle and respiration-related CSF flow pulsatility are altered. These effects are detectable with motion-sensitive MRI sequences, and they can be gated to the cardiac cycle. A prolongation of CSF systole is observed in the area above the foramen magnum. In the anterior subarachnoid space below the foramen magnum and in the posterior subarachnoid space immediately below the tips of the cerebellar tonsils, systolic velocities are reduced, and the duration of CSF systole and the ratio of systolic-to-diastolic CSF displacement are decreased. These findings indicate impaired CSF systolic (craniocaudal) pulsations. Diastolic flow is unimpaired. A reduction of CSF flow can be observed in the subarachnoid space of the PCF (cisterna magna; retrocerebellar, premedullary, and prepontine cisterns), along with a compensatory pulsatile downward motion of the cerebellar tonsils. These flow abnormalities have been shown to revert to normal levels after cranial decompression. A relationship exists between CSF flow abnormalities detected on MRIs and syringomyelia. However, CSF flow abnormalities are not correlated with the degree of tonsillar ectopia or the presence of clinical symptoms or their severity. Cine MRI may be helpful in demonstrating a disturbance of CSF velocity and/or flow at the foramen magnum in patients with tonsillar ectopia of less than 5 mm. The usefulness of MRI flow studies in the management of CMI remains uncertain. Currently, to the author's knowledge, guidelines for the prediction of serious complications based on abnormalities seen on MRI flow studies have not been established. Incidental CMI is more common than previously recognized. A number of patients who underwent imaging for reasons unrelated to CMI were found to have CMI. In the absence of a syrinx or clinical symptoms and signs, some authors consider follow-up imaging unjustifiable. Careful clinical assessment remains the cornerstone of proper diagnosis and management. Degree of Confidence: Tonsillar ectopia of 5 mm is 100% specific and 92% sensitive for CMI.
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