Histopathology of brain AVMs part I: microhemorrhages and changes in the nidal vessels

Background Arteriovenous malformations of the brain (bAVM) may rupture from aneurysms or ectasias of the feeding, draining, or nidal vessels. Moreover, they may rupture from the immature, fragile nidal vessels that are characteristic to bAVMs. How the histopathological changes of the nidal vessels associate with clinical presentation and hemorrhage of the lesion is not well known. Materials and methods We investigated tissue samples from surgically treated bAVMs (n = 85) using standard histological and immunohistochemical stainings. Histological features were compared with the clinical presentation of the patient. Results Microhemorrhages from nidal vessels were found both in bAVMs with a history of clinically evident rupture and in bAVMs considered unruptured. These microhemorrhages were associated with presence of immature, pathological nidal vessels (p = 0.010) and perivascular inflammation of these vessels (p = 0.001), especially with adhesion of neutrophils (p < 0.001). In multivariate analysis, perivascular inflammation (OR = 19, 95% CI 1.6 to 230), neutrophil infiltration of the vessel wall (OR = 13, 95% CI 1.9 to 94), and rupture status (OR = 0.13, 95% CI 0.017 to 0.92) were significantly associated with microhemorrhages. Conclusions Clinically silent microhemorrhages from nidal vessels seem to be very common in bAVMs, and associate with perivascular inflammation and neutrophil infiltration. Further studies on the role of perivascular inflammation in the clinical course of bAVMs are indicated.