The size, location, and venous drainage characteristics of a cerebral AVM for surgical planning, Spetzler and Martin devised a widely used grading scale. The summed score gives guidance as to the difficulty in surgical removal and has a less certain relationship to the clinical behavior of the lesion. Lesions 1 to 3 mm are considered small, and give 1 point; 3 to 6 mm are medium sized and 2 points; over 6 mm are large and assigned 3 points; location in an eloquent site gives 1 point and venous drainage to the deep veins gives another point (the summed score is between 1 and 5). The use of this scale to plan treatment is discussed in the next section.
It is estimated that the risk of bleeding from a known AVM is approximately 3 percent per year but the rate varies with the location of the lesion and whether there has been bleeding in the past. Features that have been related to risk of bleeding in some series have been a deep or brainstem location of the AVM or deep venous drainage channels, and a previous hemorrhage summarized by Stapf and colleagues. The natural history of AVMs has been studied by Ondra and colleagues, who presented a large and comprehensive series of untreated malformations in Finland over a 30-year period, and another similar series has been reported by Crawford and coworkers in Great Britain. As commented, the rate of rebleeding in most series has been 2 to 4 percent per year over decades but may be as high as 6 to 9 percent in the year after a first hemorrhage. In a series of 1,000 patients referred mainly for proton-beam radiation of an AVM and studied by our colleague
R.D. Adams, 464 had a hemorrhage as the first manifestation and 218 had a seizure (mainly with frontal and frontoparietal lesions). In those few AVMs that came to attention as a result of a progressive neurologic deficit, most were situated in the posterior fossa or axially in the cerebrum. The combination of a prolonged history of headaches, seizures, and a progressive deficit in Adams’ series almost always indicated a large malformation. The matter of an increased risk of AVM rupture during pregnancy has been disputed. The weight of evidence suggests that the risk is not greatly raised by pregnancy alone.
Fully 90 percent of AVMs are disclosed by CT if performed with contrast infusion, and an even larger number by MRI (Fig. 33-30). Magnetic-susceptibility MRI shows small areas of previous bleeding around AVMs. Arteriography is usually necessary to establish the diagnosis with certainty and will demonstrate AVMs larger than 5 mm in diameter; MRI may fail to reveal smaller lesions. Another value of arteriography, particularly if performed with rapid sequential and delayed images is to define all of the feeding arteries, the presence of an associated aneurysm and the channels of venous drainage, all of which inform the expectations of future bleeding and the most advisable methods of obliterating the lesion. The decision to obtain imaging to detect an AVM in cases of typical cerebral hemorrhage of the type discussed in earlier sections is based on factors such as young age (childhood and adolescence onset is particularly suggestive), a history of an unusual unilateral headache syndrome, a focal seizure disorder, the absence of other apparent cause (e.g., coagulopathy, chronic hypertension, metastatic tumor), but most of all, recurrent bleeding in one location of the brain.