Intraoperative Cortical Function Localization Techniques

This article describes two functional techniques for localizing eloquent cortical areas during surgery. The motivation for their use is derived from the inherent variability in cortical surface blood vessel and sulci pattern between patients; the distortion of the cortical surface from cortical masses; limited craniectomy size, which alters the perspective gained from surrounding cortical anatomy; and cortical plasticity of function, in which some functions relocate within the cortex when invaded by pathologic processes. The advantage of this approach is that maximum surgical removal of diseased tissue can occur while minimizing the risk of incurring new neurologic deficits. The first technique involves discrete electrical stimulation of the cortical surface. This approach has been used for localizing sensory, motor, and cortical areas important for speech function. The technique in most cases requires that the neurosurgical procedure be carried out on sedated patients. The second technique can be used to localize the central sulcus in both anesthetized and awake patients. It involves a morphologic analysis of the cortical surface somatosensory evoked potentials acquired when electrically stimulating contralateral peripheral nerves. The neurophysiologic basis for locating the central sulcus by finding the cortical surface where a phase reversal occurs in the distributed somatosensory evoked potentials is presented. Finally, practical aspects important for the successful use of both techniques are reviewed.