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False negatives, muscle relaxants, and motor-evoked potentials.

To the Editor: Irie et al have carried out a clinically interesting and useful study. However, we are concerned that those with less experience with neurophysiologic monitoring may be mislead in the clinical application of this study. First, why were somatosensoryevoked potentials (SEPs) not used as well? The primary goal of neuromonitoring during brain aneurysm surgery is to detect the impairment of cortical perfusion. SEPs can often predict motor outcome because in most instances ischemia affects sensory and motor pathways simultaneously. However, SEPs may fail to indicate motor ischemia in instances of isolated motor pathway injury, such as perforating vessel occlusion and subcortical infarcts, particularly those occurring in the internal capsule, which cause hemiparesis in monitored patients. Therefore, motor-evoked potential (MEP) monitoring is used as a supplementary technique to detect perforating vessel compromise, which may lead to motor impairment not reflected by SEPs. Although MEPs are superior to SEPs in reflecting impending motor impairment from subcortical ischemia, SEPs are useful in monitoring cortical perfusion, such as middle cerebral arteries and anterior cerebral artery territory ischemia, and have fewer limitations of application in microsurgery. MEP is added to SEP monitoring to predict subcortical ischemia causing pure motor deficit, but MEP should not replace SEP monitoring during aneurysm surgery. Some MEP-negative cases with corona radiate ischemia reported by Irie et al might have been predicted by SEP monitoring. Second, why were the patients kept partially paralyzed? This required much higher stimulation intensity that is i.e. 500V. Different parts of the corticospinal tract can be activated depending on the intensity of the stimulus. It is postulated that there are 3 favorable points that are susceptible to depolarization on the corticospinal tract: cortex/subcortex with a low stimulation threshold; internal capsule with a moderate threshold; and brainstem/foramen magnum with a high stimulation threshold. If activation occurs at the internal capsule, motor cortex and superficial subcortical motor pathway ischemia, such as corona radiate infarcts as reported by Irie et al, can go undetected. Therefore, one must be aware that high-intensity stimulation may bypass a cortical lesion by stimulating the motor tract fibers distal to the lesion thereby giving rise to false-negative results. Stimulation close to threshold may prevent excitation of corticospinal tract fibers deep within the white matter. It is possible that corticospinal tracts were stimulated at the internal capsule level at 500V stimulation, which may explain the reason why MEPs did not pick up corona radiate ischemia. It has been recommended that one uses the lowest stimulation intensity possible to avoid deeper activation and this also reduces patient movement. MEPs can be recorded from the muscles of the upper extremities only and this can usually be achieved with <150V. This can be done without patient movement in our experience and therefore does not necessitate paralyzing the patient to avoid movement.