Primary Diagnosis

Multiple spinal ‘arachnoid’ cysts following aneurysmal SAH.

Secondary Diagnosis

N/A

Diagnosis Discussion

Cysts may develop after hemorrhage, trauma & inflammation. Pre-existing or de novo? may have hemosiderin. Composed of single layer of meningothelial cells. May produce back pain/myelopathy that may be intermittent (syrinx). Arachnoid cysts also occur within the spinal canal, in which 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 arachnoid 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. Arachnoid cysts often are an incidental finding on imaging and, usually, patients are asymptomatic even if the cyst is quite large. The most commonly associated clinical features are headache, calvarial bulging, and seizures, with focal neurologic signs occurring less frequently. Controversy surrounds the treatment of arachnoid cysts. Some clinicians advocate treating only patients with symptomatic cysts while others believe that even in asymptomatic patients, 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 two types of cysts differs along the neural axis. The cysts located along the cerebral convexity and in the spinal cord are mostly arachnoid while 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, with hyaline change at times. No epithelial lining is present. The outer wall of the cyst adheres loosely to the dura. The cyst wall is devoid of blood vessels, and changes of inflammation or hemorrhage seldom occur. Arachnoid cysts usually occur in association with normal arachnoid cisterns, and 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. Most likely, arachnoid cysts expand when hydrodynamic pulse waves of cerebrospinal fluid (CSF) become entrapped in arachnoid locations. 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 comparable to CSF. A few cysts may contain elevated protein content. Gross appearances of glioependymal cysts usually are indistinguishable from arachnoid cysts; however, their microscopic appearances vary. Glioependymal cysts are epithelial lined and may bear cilia, but only glial tissue is seen lining the lesion if the epithelial lining becomes dehiscent or lost. 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, neonatal infections and can 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 of 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 become space occupying 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. Recently, cine MRI has shown abnormal fluid flow in intradural spinal arachnoid cysts presenting with cord compression. A common localization is in the floor of the middle cranial fossa, in particular, in the sylvian fissure and the parasagittal location in the interhemispheric fissure. Most cysts are unilateral, smoothly rounded, and adhere loosely to the dura. source: e-medicine Ali Nawaz Khan, MBBS, FRCP, FRCP.

General Anatomic Area:

Other

Secondary Anatomic Area:

subarachnoid space, intradural space, spinal canal

Disease Category:

Toxic

Contributor:

Maurice Castillo, MD, W. Kucharczyk MD

Date:

06/01/2005

Difficulty Level:

Difficult

Pathology Confirmed?:

No
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ACR Category Number:

N/A

Keywords:

intraspinal cysts