Pathology of Cerebral Arteriovenous Malformations in Children

Pathophysiology

• AVMs common vascular lesion: AVMs are the most common symptomatic intracranial vascular abnormality (2).
• Vessel dilation possible mechanism of enlargement: The mechanism of expansion of AVM has been debated. One possibility is “mechanical” dilatation as the result of increased flow through poorly differentiated vessels and recruitment of collateral arterial feeders. Ischemia and surrounding micro-hemorrhages with resultant gliosis may promote enlargement of the AVM by destruction of the surrounding parenchyma. 
• Growth possible mechanism for enlargement: Expansion of an AVM may also result from growth of new blood vessels, i.e., angiogenesis – a complex process regulated by a wide range of proteins, including metalloproteinases and related growth factors such as VEGF (14–20). 
• VEGF and growth of lesions: Immunochemical studies on resected cerebral AVMs have shown upregulation of VEGF, more prominently after preoperative embolization, thereby supporting the concept that postnatal influences can affect the development and evolution of these lesions (14, 18, 20, 21).

Molecular/Genetic Pathology

• VEGF and TGFα expressed: AVMs and cerebral cavernous malformations express VEGF and increased structural proteins in the endothelium and subendothelium and TGFα in endothelial and perivascular layers (16).
• RASA1 mutations: The RASA1 mutation, resulting in familial AVMs and/or cutaneous capillary malformations, has been associated with symptomatic cerebral AVMs in a small number of families (13).
• HHT and AVMs: HHT is a genetic condition that predisposes affected individuals to AVMs.  Transgenic mice with one deletion of endoglin (the gene that causes HHT showed progressive destruction of the capillary bed related to endothelial cell nitric oxide synthetase deficiency (22).

Histopathology

• Artery to venous connection without capillaries: AVM consists of direct arterial-to-venous connections without intervening capillaries. They occur in the cerebral hemispheres, brainstem, and spinal cord.  Functional neural tissue does not reside in the lesion (1).