"type I: extradural meningeal cyst without neural tissue"
"type Ia: extradural spinal arachnoid cyst"
"type Ib: sacral meningocele"
"type II: extradural meningeal cyst containing neural tissue, e.g. Tarlov cyst"
"type III: intradural spinal arachnoid cyst"
"type I: extradural meningeal cyst without neural tissue"