Classification:

1. True intramedullary AVM
   = nidus of abnormal intermediary arteriovenous structure with multiple shunts
   Age: 2^nd–3^rd decade
   Cx: subarachnoid hemorrhage, paraplegia
   Prognosis: poor (especially in midthoracic location)
2. Intradural arteriovenous fistula
   = single shunt between one / several medullary arteries + single perimedullary vein
3. Dural arteriovenous fistula
   = single shunt between meningeal arteries + intradural vein
4. Metameric angiomatosis