First, inadvertent bolus injections of anesthetics or muscle relaxation must be excluded, as well as a drop of blood pressure and body temperature, which all may significantly affect MEP amplitudes. In particular, a slow, gradual decrease of blood pressure and body temperature must be taken into account. The MEP parameters are not linearly related to cerebral perfusion but can change abruptly in a more stepwise fashion. When individual threshold values are encountered, MEP amplitude may have a sudden deterioration at an unpredictable point in time. For example, there is no absolute blood pressure threshold value. However, any mean arterial pressure below 70 mmHg may be critical, and significant drops in blood pressure must be avoided and reported when they do occur. Body temperature should be maintained above 36 °C by air-warming systems, if necessary. After this check for nonsurgical causes, a warning must be issued to the surgeon. Typically, resection or dissection is halted at this point. At the same time, inadvertent decreases of blood pressure or body temperature are reversed and these measures should be communicated to the surgeon. The surgeon must exclude technical causes for MEP changes such as displacement of stimulation electrodes; poor contact of an electrode with the result of high impedance (subdural irrigation and wet cottonoids on top of the electrode are helpful); subdural air collection; or a shift of the motor cortex away from the stimulation electrodes after removal of a mass lesion.

With the possibility of a surgically related cause for the MEP change , the surgeon’s attention must be directed at specific surgical conditions that may have caused the monitoring event. Obvious causes may be detected such as resection and electrocoagulation in close vicinity or within the motor tract as revealed by neuronavigation or anatomic criteria. The intervening activity is halted and may be resumed only after MEP changes have stabilized or recovered, or if further resection appears to be possible and safe according to anatomic or other external criteria. Frequently, no specific reason can be found for MEP deteriorations. However, a temporary halt of dissection and readjustment of the brain retraction is often sufficient to enable MEP recovery and further safe resection. However, the previous surgical course of the procedure must be taken into account at this point. Extensive manipulation of remote blood vessels supplying the motor tract at some previous step of dissection is a typical cause of inexplicable MEP deterioration. It may be useful to place pieces of gelfoam or cottonoids soaked with papaverine or nimodipine at sites of (previous) vascular manipulation.

Clinical case series have shown that MEP deterioration occurs at stages when motor damage is imminent but still reversible. The clinical correlation of MEP recordings—motor function—is not assessable at the time of monitoring unless an awake craniotomy is being performed. Thus postoperative motor outcome is the best surrogate parameter. In large case series, the following correlation has been repeatedly confirmed: If MEP amplitudes recover or there is partial recovery as a result of surgical intervention, there is no deficit or only transient/minor new motor deficits postoperatively. Fortunately, MEP deterioration is reversible after surgical intervention in the majority of cases. In many of those cases, diffusion-weighted MRI reveals ischemic lesions but not definite stroke affecting the corticospinal tract [15].

If there is an irreversible amplitude decrease and in particular an irreversible MEP loss, there is a high probability of permanent new paresis, frequently associated with a stroke comprising the corticospinal tract. Conversely, stable MEP recordings point to a favorable motor outcome and allow for safe completion of critical steps of the procedure. Therefore, there are three reasons for the use of monitoring: (1) prevention of new permanent deficits; (2) safe completion of critical procedures in order to achieve maximal tumor cytoreduction; and (3) an educational reason, which is to steepen the surgeon’s individual learning curve and to improve the surgical skills for future cases. Monitored cases seem to have both a lower incidence of new postoperative deficits and better surgical resections, which lead to favorable results [15, 31, 32].

Resection of supratentorial mass lesions is associated with considerable functional morbidity, particularly when the lesions are located near blood vessels or near the eloquent cortices (e.g., motor cortex). New paresis is of particular concern due to the extended course of the motor system in the brain and the lack of clearly identifiable primary corticospinal pathways. New functional deficits are frequently caused by ischemic lesions that occur during tumor resection, and not very often by cutting into functional cortex and fiber tracts. This applies in particular with insular and other deeply seated tumors as in the present case. Therefore, motor preservation requires both mapping of the motor cortex (cortical stimulation, SSEP phase reversal) and continuous monitoring using MEP recordings, which can be performed with the patient under general anesthesia. Other functions such as language, vision, and somatosensory perception may be mapped and monitored in awake procedures or by other neurophysiologic and imaging methods. The causes of MEP changes may include nonsurgical conditions such as technical, physiological, pharmacological, and positional causes that need to be identified and excluded. Stable MEP recordings allow for safe completion of surgery whereas deterioration due to surgical causes should lead to early surgical intervention. Restoration of the MEP signals may prevent the occurrence of permanent new deficits.