Arteriovenous Malformation (cont.)
What Are the Health Consequences of AVMs?
The greatest potential danger posed by AVMs is
hemorrhage. Researchers believe that each year between 2 and 4 percent of all
AVMs hemorrhage. Most episodes of bleeding remain undetected at the time they
occur because they are not severe enough to cause significant neurological
damage. But massive, even fatal, bleeding episodes do occur. The present state
of knowledge does not permit doctors to predict whether or not any particular
person with an AVM will suffer an extensive hemorrhage. The lesions can remain
stable or can suddenly begin to grow. In a few cases, they have been observed to
regress spontaneously. Whenever an AVM is detected, the individual should be
carefully and consistently monitored for any signs of instability that may
indicate an increased risk of
hemorrhage.
A few physical characteristics appear to indicate a greater-than-usual
likelihood of clinically significant hemorrhage. Smaller AVMs have a greater
likelihood of bleeding than do larger ones. Impaired drainage by unusually
narrow or deeply situated veins also increases the chances of hemorrhage.
Pregnancy also appears to increase the likelihood of clinically significant
hemorrhage, mainly because of increases in blood pressure and blood volume.
Finally, AVMs that have hemorrhaged once are about nine times more likely to
bleed again during the first year after the initial hemorrhage than are lesions
that have never bled.
The damaging effects of a hemorrhage are related to lesion location. Bleeding
from AVMs located deep inside the interior tissues, or parenchyma, of the brain
typically causes more severe neurological damage than does hemorrhage by lesions
that have formed in the dural or pial membranes or on the surface of the brain
or spinal cord. (Deeply located bleeding is usually referred to as an
intracerebral or parenchymal
hemorrhage; bleeding within the membranes or on the surface of the brain is
known as subdural or subarachnoid hemorrhage.) Thus,
location is an important factor to consider when weighing the relative risks of
surgical versus non-surgical treatment of AVMs.
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