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Brain & Spine Disorders

Arachnoid Cyst

A spinal arachnoid or meningeal cyst is an expanding diverticulum of the subarachnoid space that freely communicates with the subarachnoid space through a relatively narrow opening and is thought to enlarged through a "valve-like" mechanism. Recently, a new classification has been proposed in an attempt to help clarify the multiple terms and postulated etiologies previously used to describe these entities. This classification describes three types: 
  • Type 1 is an extradural meningeal cyst without nerve root fibers that is further subdivided into: (a) an extradural meningeal cyst (extradural arachnoid cyst, pouch, diverticulum), and (b) a sacral meningocele (occult sacral meningocele).
  • Type 2 are extradural meningeal cysts with nerve root fibers (Tarlov's perineural cyst, spinal nerve root diverticulum, meningeal diverticula).
  • Type 3 are intradural meningeal cysts (intradural arachnoid, arachnoid diverticula).
Previously, Tarlov's cysts were distinguished from root diverticula by the existence of communication of the subarachnoid space. It is now felt that all three types communicate and are demonstrated on immediate and sometimes delayed water soluble post myelographic CT (CTM). Plain myelography may disclose a filling defect alone without filling, and therefore CTM is essential in the diagnosis and rules out other mass lesions.

Though the exact etiology is uncertain, some authors state that all meningeal cysts are congenital in origin, with a valve-like mechanism explaining their enlargement. They further state that this "valve" might further be compromised following trauma or arachnoiditis from other causes, which would explain the development of acute symptoms. Other postulated etiologies include previous surgery (laminectomy, discectomy, multiple lumbar punctures), closed spinal trauma and protrusion through a congenital defect in the dura. Recent reports suggest a possible association of type III cysts with syringomyelia.

Although another term for extradural arachnoid cysts is "arachnoid cysts," the inner arachnoid single cell lining is frequently absent on histologic examination.

In contradiction, intradural meningeal cysts (type 3) always have an arachnoid lining and occur anywhere within the posterior spinal canal, but are most commonly located in the thoracic spine. Anterior cysts, though much less common, do occur, and discrete clefts in the septum posticum are a sole postulated etiology. Like type 2 cysts, they are often multiple and asymptomatic. When symptoms arise, they are often secondary to cord and/or root compression like other intradural lesions. They usually present after the second decade and affect both sexes equally. The postulated etiology causing the "valve" mechanism is a proliferation of arachnoid trabeculae. Bone erosion and a widened spinal canal do occur but are infrequently seen on plain x-ray. Plain myelograms frequently show an extramedullary/intradural defect that will fill with contrast on CTM, though at times, delayed CTM imaging is necessary.

MRI is useful as a screening tool for all meningeal cysts with signal characteristics equivalent to CSF. All sequences should help to exclude most other mass lesions. Type 3 cysts may be difficult to detect since they have thin walls and are only recognizable by virtue of their mass effect. Therefore, definitive diagnosis is made by MR. The signal intensity within the cyst cavity may vary slightly when compared with that of the subarachnoid space on T2WI. This is explained by the differing CSF motion within the canal and/or cyst.

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