Figure 1: AVM lesion mapping, using fMRI, of a patient performing left and then right hand motor tasks. 1. A left hemisphere area of the brain that typically controls right-sided motor function. 2. AVM lesion 3. Motor area in the right hemisphere 4. Sensory area in the right hemisphere
The use of functional magnetic resonance imaging (fMRI) for pre-surgical lesion mapping is an exciting field. Until now this mapping has been done during the actual surgical procedure and is limited to a hemisphere. Furthermore, multiple “tasks” spanning different domains of function can be tested easily.
Additionally, under previous mapping procedures, the most important information – assessment of the contra-lesional hemisphere of the brain – was unavailable. The contra-lesional hemisphere was never exposed during surgery and thus was not tested. With fMRI, the entire brain can be studied.
We have studied a number of patients who have an arteriovenous malformation (AVM). An AVM is caused when high-pressure arterial blood is shunted directly into the low-pressure venous system. These patients are at high risk for a vascular rupture. The abnormal blood flow also has an impact on the local brain function. Since these lesions are believed to be congenital, the brain has the ability to “rewire” the circuitry to account for the presence of the lesion.
We have found that in a number of patients with AVMs the contra-lesional hemisphere is involved in the performance of a task that should be controlled by the hemisphere containing the AVM. This can potentially have a positive impact on the surgical procedure. In fact, it could make it possible for somebody who is not eligible for treatment to actually obtain it.
For example, (see Figure 1) when a patient with an AVM lesion performs a left hand motor task, focal activation in the motor and sensory areas is seen in the right hemisphere, indicating a consolidated use of the cortex. When the same patient performs a right hand motor task, very weak activation in the left hemisphere is observed. Typically, the left hemisphere controls right sided motor function. However, there is strong activation in the right hemisphere. The presence of the AVM may be disturbing the flow making it difficult to detect activation around the AVM, and the right hemisphere activation may represent cortical re-mapping.