Vascular malformations composed mainly of clusters of thin-walled veins without important arterial feeders and with little or no intervening nervous tissue make up a significant group, some 7 to 8 percent of cerebral AVMs. Conventional subdivisions of this group into cavernous, venous, and telangiectatic types have not proven useful. Most clinicians roughly designated them all as cavernous malformations and several attributes set them apart from other vascular malformations. Their tendency to bleed is probably no less than that of the more common AVMs but far more often, the hemorrhages are small and clinically silent. The incidence of bleeding is uncertain but is estimated to be less than 1 percent per year per lesion but quite often they are multiple lesions so that the cumulative risk in any one patient is higher. Flemming and colleagues, in a survey of 292 patients followed for an average of almost 10 years, gave the rate as 0.3 percent annually for asymptomatic lesions and found that individuals who had a previous episode of bleeding or had more than one lesion were 2.5 times more likely to have another hemorrhage. Of course, the location of the bleeding, especially if in the brainstem, can have major clinical consequences.
As mentioned, approximately 10 percent of these lesions are multiple and 5 percent are familial. In one family we followed, there were 29 affected members in three generations and inheritance followed an autosomal dominant pattern. At the present, several genes have been identified, particularly KRIT1, as possibly causative in individual families. One interesting characteristic of this group, as pointed out by Labauge and colleagues, is the appearance over time of new lesions in one-third of patients. The followup of some of our patients has affirmed this.
The diagnosis is based on clinical manifestations and MRI, which discloses a cluster of vessels surrounded by a zone of hypodense ferritin in the T1-weighted images (Fig. 33-32), the product of previous small episodes of bleeding. An uncertain number is associated with adjacent deep venous anomaly visualized by imaging studies and these are discussed in a separate section below.
About one-half of all cavernous angiomas lie in the brainstem, and in the past (before the availability of MRI), many of them were misdiagnosed as multiple sclerosis because of the stepwise accumulation of neurologic deficits with each hemorrhage. A large series of cavernous malformations of the brainstem, most in the pons, has been described by Porter and colleagues. They describe a higher rate of bleeding than had been reported for similar malformations in the cerebral hemispheres, frequent adjacent venous anomalies, and good results from surgical ablation. They estimated the rate of bleeding to be 5 percent per year and the rate of rebleeding, close to 30 percent per year.
Treatment Cavernous angiomas on the surface of the brain, within reach of the neurosurgeon, even those in the brainstem, can be plucked out like blackberries, with low morbidity and mortality. Kjellberg and colleagues treated 89 deeply situated cavernous angiomas with low-dose proton radiation, but our impression is that these vascular malformations, like hemangioblastomas, respond poorly to radiation and are not amenable to treatment by endovascular techniques.
Lesions that cause recurrent bleeding and are surgically accessible with little risk are often removed but incidentally discovered angiomas, even if they have caused a small hemorrhage, and those that are inaccessible may be left alone. Although this conservative approach is usually taken, there are not adequate data on the rate and risk of bleeding to determine the proper course of action.