Ipsilateral motor activation in patients with cerebral gliomas.

Objective: The aim of this study is to provide neurophysiologic evidence of ipsilateral hemispheric activation in patients affected by intracerebral gliomas via the use of transcranial magnetic stimulation.

Background: The mechanisms involved in such ipsilateral activation have yet to be established, but they may involve preexisting routes that either are suppressed or undetected in the normal brain. Ipsilateral pathways may act in reserve, activated by the impairment of contralateral control. This hypothesis is suggested by the fact that the considerable size of the tumors in our patients is not matched by a proportionate loss of motor performance in the limbs contralateral to the affected hemisphere. However, it remains possible that ipsilateral motor-evoked potentials (iMEPs) may reflect reorganizational changes without significant functional effects.

Methods: The effects of such activation were investigated using both focal and nonfocal coils stimulating cortical motor areas, with MEPs recorded from both left and right thenar muscles. Fifteen healthy control subjects and seven patients were examined.

Results: iMEPs were generally absent in normal subjects, but in contrast they were obtained in the patients by stimulating the healthy hemisphere using both round and figure-of-eight coils. Distinct from contralateral MEPs, iMEPs are obtained with higher thresholds (range, 60 to 80% of stimulator output) and display longer latencies (20.9 msec versus 19.4 msec).

Conclusions: Taken in conjunction with recent research using functional imaging brain exploration and a variety of clinical, anatomic, and neurophysiologic studies, our results reflect a growing awareness of ipsilateral motor control and its potential compensatory role when contralateral routes are damaged.