Collection of clear fluid within the arachnoid, presumably resulting from abnormal embryological development of the subarachnoid space.

Arachnoid cysts vary in size (some are quite large), and occur in several different locations around the brain and spinal cord.

The clinical presentation depend on the size and location of the cyst, as well as the patient's age.  

Many arachnoid cysts are discovered "incidentally" (unexpected finding on CT or MR obtained to investigate a brain problem not related to the cyst).

Patients with a  suprasellar cyst often present earlier than middle cranial fossa cysts because the former is associated with hydrocephalus.

It is diagnosed by CT or MRI

Treatments include: 

Creating an opening in the cyst that allows it to drain into the subarachnoid space.

Placing a shunt in the cyst allowing it to drain into the peritoneal space (abdomen) [or other space if the peritoneum is scarred or for some other reason is unsuitable].

Arachnoid cyst - pathophysiology

Location of arachnoid cyst

     Temporal arachnoid cyst

     Intrasellar arachnoid cyst

     Spinal arachnoid cyst

Thoracic are the most common spinal arachnoid cysts.  They can occur under the dura covering the spinal cord or that forming the nerve root sheath.

Arachnoid cysts are not due to trauma or to inflammation (i.e. they do not a complication of arachnoiditis).

Spinal arachnoid cysts can be asymptomatic.  Symptoms result from spinal cord compression and include weakness and/or numbness of the extremities with or without bladder and/or bowel incontinence (loss of control).   

Surgical treatment should be considered for even mildly symptomatic spinal arachnoid cysts that are compressing the spinal cord.

Arachnoid cysts at the nerve roots are unlikely to cause enough arm, trunk, or other pain to justify operating on these lesions in most cases.

Arachnoid cyst - neuroimaging

(compare: Tarlov cyst)

(compare: epidermoid / dermoid cyst)

(compare: arachnoiditis)

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