Type I extradural meningeal cysts without spinal nerve root fibers

IA “extradural meningeal/arachnoid cyst”

IB (occult) “sacral meningocele”

Type II extradural meningeal cysts with spinal nerve root fibers (“Tarlov perineural cyst,” “spinal nerve root diverticulum”)

Type III spinal intradural meningeal cysts (“intradural arachnoid cyst”)

The classification of spinal meningeal cysts (MCs) in the literature is indistinct, confusing, and in certain categories histologically misleading. Based on a series of 22 cases, Nabors et al. propose a classification comprising three categories: spinal extradural MCs without spinal nerve root fibers (Type I); spinal extradural MCs with spinal nerve root fibers (Type II); and spinal intradural MCs (Type III). Although water-soluble myelography may disclose a filling defect for all three categories, computerized tomographic myelography (CTM) is essential to reveal communication between the cyst and the subarachnoid space. Communication demonstrated by CTM allows accurate diagnosis of a spinal MC and rules out other mass lesions. Magnetic resonance imaging appears useful as an initial study to identify an intraspinal cystic mass. Final characterization is based on operative inspection and histological examination for all three categories ¹⁾.

Spinal arachnoid cyst may be either intradural (type III meningeal cyst) or extradural (type IA meningeal cyst).