Pathological findings of angiographically occult vascular malformation.

Forty cases with angiographically occult vascular malformations (AOVMs), in which the diagnosis was confirmed pathologically, are analysed. Pathology of the AOVMs were classified into 22 AVMs (20 thrombosed, two cryptic), 16 cavernous malformations and two capillary telangiectasias. The location of AOVMs were in the cerebral hemisphere (32), cerebellum (3), thalamus (2), lateral ventricle (1), brain stem (1) and optic nerve (1). Common characteristic histopathological features between thrombosed AVMs and cavernous malformations were that the malformations had subclinically bled in the past and apparently had proceeded to undergo various degree of thrombosis, fibrosis and calcification. However, in addition to these findings, the thrombosed AVMs showed a contribution of arterial components, hypertrophied vascular wall and intervening gliotic tissue. On the other hand, the cavernous malformations showed closely packed abnormal vessels with surrounding collagen band. All cryptic AVMs presented with haemorrhagic episodes and small nidus within clot were proven in histological specimens. Common histopathological findings between thrombosed AVMs and cavernous malformations are discussed.