eMedicine Specialties > Radiology > Brain/Spine

Arachnoid Cyst

Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR, LRCP, Chairman of Medical Imaging, Professor of Radiology, NGHA, King Fahad National Guard Hospital, King Abdulaziz Medical City, Riyadh, Saudi Arabia
Ian Turnbull, MB, ChB, MD, DMRD, FRCR, Lecturer, Department of Radiology, University of Manchester; Consulting Neuroradiologist, Hope Hospital, Salford, Manchester and North Manchester General Hospital, UK; Riyadh Al-Okaili, MBBS, Interventional/Therapeutic and Diagnostic Neuro-Radiologist, King Abdulaziz Medical City; Sumaira MacDonald, MBChB, PhD, MRCP, FRCR, Lecturer, Sheffield University Medical School; Endovascular Fellow, Sheffield Vascular Institute; Khalid Mahmood, MBBS, FCPS, Locum Appointment Training Specialist Registrar, Department of Radiology - Paediatric, Royal Liverpool (Alder Hey) Children's Hospital
Contributor Information and Disclosures

Updated: Dec 9, 2008

Introduction

Background

Arachnoid cysts are benign cysts that occur in the cerebrospinal axis in relation to the arachnoid membrane and that do not communicate with the ventricular system. They usually contain clear, colorless fluid that is most likely normal cerebrospinal fluid; rarely, they contain xanthochromic fluid. Most are developmental anomalies. A small number of arachnoid cysts are acquired, such as those occurring in association with neoplasms or those resulting from adhesions occurring in association with leptomeningitis, hemorrhage, or surgery. They constitute approximately 1% of intracranial masses; 50-60% occur in the middle cranial fossa. Cysts in the middle cranial fossa are found more frequently in males than in females; they occur predominantly on the left side. Most arise as developmental anomalies. A small number of arachnoid cysts are associated with neoplasms.

Arachnoid cysts also occur within the spinal canal; in such cases, arachnoid cysts or arachnoid diverticula may be located subdurally or in the epidural space, respectively. Spinal arachnoid cysts are commonly located dorsal to the cord in the thoracic region. A cyst in this location is usually secondary to a congenital or acquired defect and is situated in an extradural location. Intradural spinal arachnoid cysts are secondary to a congenital deficiency within the arachnoidal trabecula, especially in the septum posticum, or are the result of adhesions resulting from previous infection or trauma. Microscopic examination shows that their walls are formed from a splitting of the arachnoid membrane, with an inner and outer leaflet surrounding the cyst cavity.¹

Arachnoid cysts often are an incidental finding on imaging. Usually, arachnoid cysts are asymptomatic; this is true even of cysts that are quite large. The most commonly associated clinical features are headache, calvarial bulging, and seizures; focal neurologic signs occur less frequently. Controversy surrounds the treatment of arachnoid cysts. Some clinicians advocate treating only patients with symptomatic cysts, whereas others believe that even asymptomatic cysts should be decompressed to avoid future complications. The most effective surgical treatment appears to be excision of the outer cyst membrane and cystoperitoneal shunting.

Pathophysiology

Cystic lesions localized within the arachnoid membrane may be classified according to the location along the neural axis or by the histologic composition of the cyst wall, which is either arachnoid connective tissue or glioependymal tissue. Distribution of these 2 types of cysts differs along the neural axis. The cysts located along the cerebral convexity and in the spinal cord are mostly arachnoid, whereas cysts found in the supracollicular or retrocerebellar region may be either arachnoid or glioependymal cysts.

Microscopic examination of arachnoid cysts shows that the walls are formed from a splitting of the arachnoid membrane, with an inner and outer leaflet surrounding the cyst cavity. The cyst wall consists of fibrous connective tissue slightly denser than normal arachnoid tissue; occasionally, hyaline changes occur. No epithelial lining is present. The outer wall of the cyst adheres loosely to the dura. The cyst wall is devoid of blood vessels; inflammation or hemorrhage seldom occurs.

Arachnoid cysts usually occur in association with normal arachnoid cisterns; such cysts are congenital, arising from arachnoid clefts and arachnoid duplications. Glioependymal cysts are rare; only a few instances of interhemispheric glioependymal cysts are known. Glioependymal cysts may be associated with agenesis of the corpus callosum, heterotopia, and other dysplasias.

Arachnoid cyst expansion most likely occurs when intracranial pulsation pushes CSF through defects (which behave like valves); CSF then becomes entrapped in arachnoid locations. Less likely, expansion occurs when the cyst wall excretes fluid, perhaps along an osmotic gradient. The cysts may be unilocular or loculated by septations. The wall of the cyst is usually smooth. Most cysts are filled with clear, colorless fluid of low protein content that is comparable to that of CSF. In a few cysts, the protein content is elevated.

In gross appearance, glioependymal cysts are usually indistinguishable from arachnoid cysts; however, their microscopic appearances vary. Glioependymal cysts have epithelial lining and may bear cilia. When the epithelial lining is lacking, glial tissue is seen to line the lesion. Glioependymal cysts are believed to derive from displaced neuroectodermal tissue. Histologic classification is only of systematic interest and has little bearing on prognosis.

Acquired arachnoid cysts may develop following surgery, trauma, subarachnoid hemorrhage, or neonatal infections; they occasionally occur in association with extra-axial neoplasm. Arachnoid cysts associated with tumors develop as a consequence of CSF loculation surrounded by arachnoid scarring, with expansion resulting from osmotic filtration or via a ball-valve mechanism. These acquired arachnoid cysts have been described variably as acquired, secondary, or leptomeningeal cysts. The reason arachnoid cysts grow and come to occupy space is far from clear. No inner lining is present through which active transport can take place. Neurosurgeons have observed ostia with pulsating fluid in exposed cysts, suggesting a hydrodynamic flap-valve or ball-valve mechanism. Cine MRI has shown abnormal fluid flow in intradural spinal arachnoid cysts presenting with cord compression.

A common location is in the floor of the middle cranial fossa, particularly anteriorly and parasagittally in the interhemispheric fissure. Most cysts are unilateral, smoothly rounded, and adhere loosely to the dura. Arachnoid cysts may indent deeply into the hemisphere or invaginate into major fissures, displacing and flattening the underlying cortex. Compression severe enough to cause tissue necrosis is exceptional. Other locations of arachnoid cysts include the suprasellar/chiasmatic site (cysts in this location may cause endocrinopathy); the cerebellopontine angle (11%); the quadrigeminal plate cistern (10%); in relationship with the vermis (9%); and in the prepontine/interpeduncular cistern (3%). Patients with spinal arachnoid cysts may become symptomatic because of local cord displacement/cord compression. Typically, this occurs at the midthoracic level; less frequently, it occurs at the lumbosacral or sacral level.

In juveniles, epidural arachnoid cysts often are associated with kyphoscoliosis. Spinal arachnoid cysts form elongated membranous sacs of CSF dorsal to the cord. Some cysts have no connection to the dura, whereas others communicate with the subarachnoid space via a thin stalk penetrating the dura. Arachnoid cysts are associated with myelodysplasia in cases of spinal dysraphism.

Presentation

Demographics

• Arachnoid cysts constitute 1% of intracranial masses; 50-60% occur in the middle cranial fossa.
• Morbidity and mortality depend on the location of the arachnoid cyst and on the occurrence of complications, such as acute mass effect by intracystic hemorrhage or the development of a subdural hygroma/hematoma. The exact incidence of mortality and morbidity from an arachnoid cyst is not known. Arachnoid cysts may cause occlusive hydrocephalus by compression of the fourth ventricle.^{2, 3}
• The male-to-female ratio is 4:1.
• Patients of any age may present with a symptomatic arachnoid cyst.^{4, 5}

Anderson and Landing¹⁰ and Aicardi and Bauman⁴ described the clinicopathologic features of supratentorial cysts in infancy. Neither a history of antecedent trauma nor the clinical features of preceding cranial trauma was noted in the infants. The infants had a tense fontanelle and widened cranial sutures. Craniomegaly was present either diffusely or with localized protrusion, with transillumination and thinning of the calvarium, as noted on skull radiographs.

In symptomatic patients, clinical features depend on the location of the arachnoid cyst. Cysts of the middle cranial fossa (50%) may compress the tip of the temporal lobe, displacing it in the occipital direction. Cysts of the middle cranial fossa have been linked to ipsilateral chronic subdural hematomas. Rarely, such cysts may communicate with the subdural space, forming a slitlike extension over the hemispheric surface.

Arachnoid cysts are found in several syndromes, but data are not sufficient to indicate whether the association is typical or fortuitous. Arachnoid cysts may be associated with Cockayne syndrome and Menkes syndrome. Ependymal cysts are common in oral-facial-digital syndromes. Patients with spinal arachnoid cysts may develop symptoms associated with local cord displacement or cord compression. Typically, spinal arachnoid cysts occur at the midthoracic level; less frequently, they occur at the lumbosacral or sacral level. Epidural arachnoid cysts often are associated with kyphoscoliosis in juveniles. Arachnoid cysts also are associated with myelodysplasia in cases of spinal dysraphic lesions. Pain produced by intraspinal arachnoid cysts typically is aggravated by the Valsalva maneuver, which increases pressure within the cyst.

Spinal arachnoid cysts are generally misdiagnosed, because symptoms are often nonspecific. Often, the cysts are an incidental finding on MRI. Asymptomatic cysts do not require intervention, regardless of their size. Cysts that cause symptoms from cord compression are best evaluated with MRI; they should be surgically excised, if possible.^{11, 12, 13}

Preferred Examination

MRI is the diagnostic procedure of choice because of its ability to demonstrate the exact location, extent, and relationship of the arachnoid cyst to adjacent brain or spinal cord. Myelography and CT myelography remain of diagnostic value, especially for cases that are not definitive on MRI.

Plain radiographic findings are nonspecific and have little to offer in the diagnosis of arachnoid cysts, although changes in skull contour may be detected on skull radiographs performed for other indications, such as trauma.

Cranial ultrasonography is an important diagnostic tool during the first year of life. Although symptomatic arachnoid cysts are comparatively rare in infants, ultrasound is useful as a noninvasive imaging technique with high yield in the detection and characterization of cystic masses.

Although angiography may show associated anomalies of venous drainage and the relationship of the lesion to normal vasculature, which may be useful for surgical planning, in practice, angiography is rarely performed, because CT angiography or magnetic resonance angiography provides the same information noninvasively.

Every effort must be made to reliably detect arachnoid cysts, because most arachnoid cysts are an incidental finding and most patients are asymptomatic. Arachnoid cysts must be differentiated from the more serious cystic intracranial and intraspinal tumors. In cases involving larger arachnoid cysts, consideration should be given to the use of serial scans, because such cysts may enlarge over time; patients with such cysts may become candidates for surgery.

Limitations of Techniques

Skull radiographic findings cannot be relied upon because the signs are nonspecific. Arachnoid cysts may be confused with several intracranial cysts of various etiologies (see Differentials, below). The confusion is much more likely to occur with CT. CT attenuation values and signal intensities of arachnoid cysts parallel those of CSF, and difficulties may be encountered in cases of hemorrhagic cysts.

The most important differentiation to make is between arachnoid cysts and epidermoid cysts; MRI diffusion-weighted images (DWIs) make differentiating the 2 masses easier. Some arachnoid cysts contain proteinaceous fluid or blood; in such cases, signal loss on DWIs may not be marked, which may pose diagnostic problems. Also, tissue contrast with fluid-attenuated inversion recovery (FLAIR) imaging is similar to that with a T2-weighted image, but FLAIR shows no signal arising from the CSF. Thus, unlike with epidermoid cysts, arachnoid cysts containing CSF demonstrate a suppressed (low) signal on FLAIR.

Differential Diagnoses

Epidermoid, Brain
Hydatid cyst

Other Problems to Be Considered

Epidermoid cyst
Dermoid cyst
Supracollicular cysts — All causes of hydrocephalus
Cerebellopontine angle cysts — Acoustic neuroma and epidermoid cysts
Intracerebral cysts — Cerebral abscess, cystic tumors, and parasitic cysts
Basal midline cysts — Midline cysts associated with agenesis of corpus callosum and other causes of hydrocephalus
Posterior fossa cysts — Dorsal cyst associated with holoprosencephaly, Dandy-Walker cyst, and vein of Galen aneurysm

Deep invagination of an arachnoid cyst into the cerebral hemisphere may simulate porencephaly to such an extent that it has been termed pseudoporencephaly. However, the inferior aspect of the arachnoid cyst shows a displaced but otherwise normal cerebral cortex, whereas in porencephaly, the surrounding cortex and white matter are abnormal.

Chronic subdural hygromas usually are located subdurally rather than in the subarachnoid space; they are often bilateral and are flat or lentiform in profile. Hygromas compress but do not invaginate sulci or fissures.

Intraspinal cysts

• Cystic loculations in adhesive arachnoiditis (often multiple; other features of arachnoiditis may be evident)
• Epidural acquired arachnoid cysts secondary to penetrating trauma or surgery
• Meningoceles (most common in the lumbosacral region; readily identified in association with spina bifida)
• Cysts of dorsal nerve root ganglia (observed in 8-24% of autopsies, although not in children)
• Extradural ganglion cysts (presumably arising from bursae of the vertebral joints; seen in elderly patients)
• Tarlov cysts (perineural cysts of sacral or coccygeal nerve roots arising between the layers of connective tissue covering the nerve roots; communication between the cyst and subarachnoid space may be present)

References

• Epidermoid, Brain
• Hydatid cyst