Meningeal cyst

Meningeal cyst Wall of dura, arachnoid syncytial cells Collagen (S) EMA (S) Spinal epidural surface... 

A cyst that is located in the posterior or lateral epidural space in the spinal canal and that is lined only by fibrous tissue resembling dura and lacking arachnoid membrane is a meningeal cyst or diverticulum (see Table 20.12). A subdural or subarachnoid cyst that has a thinner wall than the epidural cyst and that protrudes toward brain or spinal cord is an arachnoid cyst. Reactivity for vimen-tin, progesterone receptors, and EMA is common. This inununoreactivity resembles that of arachnoid granulations and meningiomas. Other cysts have variable thickness and are more difficult to categorize. 

Important characteristics of carcinoma as it relates to the CNS and meninges are its distinctively epithelial structure (Fig. 20.58A see Table 20.6) and the overwhelming predominance of metastatic over primary carcinomas. Metastatic carcinomas are described in detail in Chapter 8, Immunohistology of Metastatic Carcinomas of Unknown Primary. Rare primary brain carcinomas occur in the choroid plexus, from germ cell tumors of the pineal and suprasellar regions, and from cysts. 725 section emphasizes how to distinguish between carcinomas (see Fig. 20.58B to D) and various primary intracranial tumors (see Box 20.2). 

Both the neuroepithelial cyst and ependymal cyst have an epithelioid surface that is positive for S-100 protein and GFAP, resting on a fibrillary glial base that is also positive for these two antibodies. These cysts often occur near a ventricle (Table 20.13). They rarely cause aseptic meningitis. 

Dermoid cysts are frequently midline cysts, possibly arising from embryonic inclusions of skin at the time of closure of the neural groove (see Table 20.13). They occur between the cerebellar hemispheres, in the fourth ventricle, in the lumbosacral region of the cord, and in the skull. These cysts may involve CNS, meninges, or both. Ruptured dermoid cysts can cause sterile meningitis and inflammation resembling an abscess. Identification of squamous epithelial cells with CK or cholesterol clefts within the inflammation are clues to its true cause. 

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