Cerebral arteriovenous malformation (AVM) is a congenital disorder of the blood vessels characterized by tangle(s) of veins and arteries located in various parts of the brain. The cause of AVMs remains unknown. Whereas, approximately 70% of cases with cerebral AVM present with symptoms caused by sudden bleeding due to the fragility of the abnormally structured brain vessels, some patients may remain asymptomatic or have minor complaints due to the local effects of the tangle of vessels. Interestingly small AVMs are more likely to bleed. Additionally, once an AVM bleeds, the probability of rebleeding increases substantially.
AVMs that do not bleed may cause symptoms by either directly compressing the brain tissue or decreasing the blood flow to the neighbouring tissues and thus causing ischemia (lack of blood flow that is important for the vitality of the cells). Both mechanical and ischemic factors cause a permanent and continuous loss of nerve cells (neurons). In case of a bleeding, the blood may penetrate either into the brain tissue (cerebral hemorrhage) or into the subarachnoid space, which is located between the sheaths (meninges) surrounding the brain (subarachnoid hemorrhage).
The most frequently observed problems related with the mechanical and ischemic effects of AVMs are headache and seizures. Moreover, AVMs with certain critical positions may stop the circulation of the cerebrospinal fluid and cause accumulation of the fluid within the skull, thus giving rise to a clinical condition named as hydrocephalus. Symptoms due to bleeding include loss of consciousness, sudden and severe headache, nausea and vomiting, blurred vision and stiff neck (usually as a result of increased pressure within the skull and irritation of the meninges) and focal deficits related with local brain tissue damage on the bleeding site (e.g. focal seizures, hemiparesis, loss of touch sensation on one side of the body).
The diagnosis is established by neuroimaging studies (a head CT scan examination is usually performed to discover the site of bleeding, magnetic resonance imaging and cerebral angiography [visualisation of brain vessels by using radioactive reagents injected into the vessels] methods are superior to CT examination in demonstration of AVMs) or a spinal tap (lumbar puncture) to examine the red blood cells, the existence of which is indicative of leakage of blood from the bleeding vessels into the subarachnoid space.
The treatment in case of a sudden bleeding includes the restoration of vital functions (including anticonvulsant medications for seizure and intracranial pressure-lowering medications) and surgical intervention to prevent a repeat bleeding. Although the surgical intervention (either removal of AVM or clipping its neck) is the preferred type of action, embolization (occlusion of blood vessels) and beam radiation methods may also be suggested. However, the feasibility and efficiency of latter two methods are still under investigation.