Pathological heterogeneity of angiographically occult vascular malformations of the brain

There is considerable confusion in the literature regarding the pathological substrates of angiographically occult vascular malformations (AOVMs) of the brain and their clinical significance. We retrospectively reviewed the cases of 34 consecutive patients with AOVMs undergoing surgical excision at a single institution during a 10-year period. Pathological specimens were reexamined, and the lesions were classified according to strict histopathological criteria. There were 21 cavernous malformations, 3 arteriovenous malformations, 3 venous malformations, 2 capillary malformations, and 5 mixed (pathologically heterogeneous) lesions. The initial pathological diagnostic report had been imprecise or had misidentified the lesion type in 18 of the 34 cases (53%), most commonly labeling a cavernous malformation as an arteriovenous malformation or not recognizing mixed features within the same lesion. Clinical presentation (including hemorrhage) and outcome were not significantly different among the various lesion types. Preoperative diagnostic imaging included a variety of modalities that were introduced or evolved during the period of the study and generally suggested a suspected vascular malformation but did not predict pathological subtypes. Acute hematomas in this surgical series made the identification of underlying vascular malformations highly speculative. We conclude that the majority of AOVMs requiring surgical intervention are cavernous malformations, although there was a notable pathological heterogeneity of the remaining lesions. Histopathological subtypes of AOVMs are not associated with unique clinical or radiographic features.