Contemporary understanding and treatment of intracranial vascular malformations has evolved considerably over the past three decades as significant advancements in neuroimaging, endovascular therapy, microsurgical technique and stereotactic radiosurgery have occurred.
Vascular malformations may be divided into four distinct categories each with distinctive anatomical, pathological, radiological and clinical features. The four traditional subtypes include: arteriovenous malformations (AVMs), cavernous malformations, venous malformations and capillary telangiectasias. The two most common and clinically significant are the arteriovenous and cavernous malformation and will be discussed below. Patients typically present between the second and fourth decades with these lesions (and thus are generally younger at presentation than those suffering from ruptured intracranial aneurysms).
AVMs are approximately one seventh to one tenth as common as cerebral aneurysms with between 2,500 to 3,000 new cases presenting each year. An estimated 280,000 patients are afflicted with AVMs in the United States.
The natural history of AVMs is incompletely understood. The annual risk of hemorrhage is noted to be 4% and 2% for symptomatic and asymptomatic AVMs, respectively. Each hemorrhagic episode appears to be associated with a 20% risk of major neurologic morbidity and a 10% mortality. There is an increased risk of rupture with smaller malformations, AVMs with associated aneurysms, a single draining vein, deep venous drainage or venous stenosis.
AVMs most frequently present as a consequence of hemorrhage in patients in their third and fourth decades. Unlike with rupture of cerebral aneurysms, early rebleeding (within two weeks) and cerebral vasospasm is rare in association with rupture of AVMs because hemorrhage secondary to AVMs is predominantly located intraparenchymally, rather than within the subarachnoid space. Seizures are the second most common presentation of patients with AVMs being noted in 11% – 33% of cases.
Patients may present with the new onset of headache (from stretching of the dura, elevated venous pressure or from hydrocephalus) or progressive neurological deficit or cognitive decline (from arterial steal or venous hypertension). Cerebral angiography remains the gold standard for diagnostic evaluation of putative AVMs. It provides detailed information regarding the configuration and vascular dynamic properties (such as flow rate, arterial steal/venous hypertension and collateral flow). Magnetic resonance (MR) imaging and computed tomographic (CT) imaging are also useful adjuncts.
Comprehensive management of patients harboring AVMs involves three main therapeutic modalities: endovascular therapy, microsurgery and stereotactic radiosurgery. Endovascular therapy, applying catheter administered materials for embolization, is a useful adjunct to microsurgery and stereotactic radiosurgery to diminish the degree of arterial shunt. Embolization is rarely curative alone, however. In younger patients possessing superficial lesion in noneloquent areas, microsurgery is clearly the treatment of choice. Microsurgery is also more effective than radiosurgery in ameliorating symptoms of patients with intractable epilepsy and headaches. Stereotactic radiosurgery is reserved for patients with small (< 3 cm), unruptured AVMs in eloquent brain substance with deep venous drainage.
Stereotactic radiosurgery invokes vascular injury and produces delayed thrombosis after months to years. Thus, protection from hemorrhage imparted from radiosurgery is delayed unlike with surgery. Benefits of radiosurgery diminish and adverse effects (damage to adjacent parenchyma) increase as the size of the lesion increases. Radiosurgery may be a viable option for the treatment of deep, residual AVMs after attempted microsurgical resection. At the University of Virgnia, the treatment of AVMs is individualized on the clinical and radiologic characteristics of the single case and incorporates all of these treatment modalities alone or in various combinations.
Cavernous malformations are benign tumors of blood vessels and represent a lobulated collection of dilated endothelial-lined sinusoidal spaces. A familial tendency has been noted in certain instances. Clinically significant hemorrhage is thought to have an annual incidence of 1% to 4.5% in asymptomatic and symptomatic lesions, respectively. Seizures are the most common manifestation of supratentorial cavernous malformations. Surrounding hemosiderin and gliosis is thought to account for the epileptogenicity. Symptomatic hemorrhage is the next most common presenting scenario followed by headaches.
Additionally, cavernous malformations are frequent incidental findings on radiographic studies for other indications. Cavernous malformations are angiographically occult lesions and are best seen with MR imaging. With MR imaging, cavernous malformations appear as a central focus of mixed signal intensity representing hemorrhage of various stages (popcorn-like) surrounded by a hypointense rim of hemosiderin from multiple micro-hemorrhages.
Cavernous malformations that are asymptomatic are not generally treated because of their low risk of hemorrhage (~ 1 % / year). Microsurgical resection is indicated for supratentorial lesions that present with hemorrhage or that are associated with medically intractable epilepsy or headaches. Gamma Knife radiosurgery can reduce the risk of hemorrhage and epilepsy in deep seated and eloquently located cavernomas. Brainstem cavernous malformations are considered for surgical resection when there is repeat hemorrhage, progressive neurological deficit and superficial location.