<unk> s, are type II innervated meningeal<unk> s, cerebrospinal-fluid-filled (CSF) sacs most frequently located in the spinal canal of the sacral region of the spinal cord (S1–S5) and much less often in the cervical, thoracic or lumbar spine. They can be distinguished from other meningeal<unk> s by their nerve-fiber-filled walls.<unk> s are defined as<unk> s formed within the nerve-root sheath at the dorsal root ganglion. The etiology of these<unk> s is not well understood; some current theories explaining this phenomenon have not yet been tested or challenged but include increased pressure in CSF, filling of congenital<unk> s with one-way valves, inflammation in response to trauma and disease. They are named for American neurosurgeon Isadore<unk> , who described them in 1938.<unk> s are relatively uncommon when compared to other neurological<unk> s. Initially, Isadore<unk>  believed them to be asymptomatic, however as his research progressed,<unk>  found them to be symptomatic in a number of patients. These<unk> s are often detected incidentally during MRI or CT scans for other medical conditions. They are also observed using magnetic resonance neurography with communicating subarachnoid<unk> s of the spinal meninges.<unk> s with diameters of 1 cm or larger are more likely to be symptomatic; although<unk> s of any size may be symptomatic dependent on location and etiology. Some 40% of patients with symptomatic<unk> s can associate a history of trauma or childbirth. Current treatment options include CSF aspiration, fibrin-glue therapy, laminectomy with wrapping of the<unk> , among other surgical treatment approaches. Interventional treatment of<unk> s is the only means by which symptoms might permanently be resolved due to the fact that the<unk> s often refill after aspiration.<unk> s often enlarge over time, especially if the sac has a check valve type opening. They are differentiated from other meningeal and arachnoid<unk> s because they are innervated and diagnosis can in cases be demonstrated with subarachnoid communication.<unk>  perineural<unk> s have occasionally been observed in patients with Marfan syndrome, Ehlers–Danlos syndrome, and Loeys–Dietz syndrome.
Tarlov cyst
Tornwaldt cyst
Dentigerous cyst
Colloid cyst
Epidermoid cyst
Choristoma
Ganglion cyst
Gartner's duct cyst