Intraoperative Monitoring Modalities (SSEP, MEP, BAEP, EMG, EEG)
What each modality answers
SSEP - stimulate a mixed nerve (median or ulnar at the wrist, tibial at the ankle) and record peripherally (Erb's point), cervically, and over cortex (N20, P37). It answers "is the somatosensory pathway conducting?" and is robust to anesthetics. MEP - transcranial electrical stimulation delivered as a train of several pulses (temporal summation at the anterior horn cell is required) with recording from limb muscles or, for D-waves, from the epidural space. It answers "is the motor pathway conducting?" and is exquisitely anesthetic-sensitive. BAEP - click stimulation with waves I–V; wave V is the most robust and is used for monitoring, while I–V interpeak latency prolongation suggests retrocochlear pathology. EMG - free-running (spontaneous neurotonic discharges signaling nerve irritation or traction), triggered by surgical manipulation, and electrically stimulated for nerve identification. EEG - detects ischemia (as in carotid endarterectomy), symmetry, burst suppression, and seizure activity.
Alarm criteria
A widely used convention is a ≥50% drop in amplitude or ≥10% increase in latency from baseline.
Traps: applying the 50/10 rule to a modality where it does not fit; treating a single averaged trial as a change; forgetting that MEP loss can be purely anesthetic; mistaking stimulus artifact for a response.
Worked example: during a cervical decompression, the left tibial cortical amplitude falls 60% while the right side and both median SSEPs are unchanged - the change is regional and lateralized, so it is treated as a genuine surgical/positional event and reported immediately, not attributed to anesthesia.