Somatosensory Evoked Potentials
Core concepts
Somatosensory evoked potentials (SSEPs) test the dorsal column–medial lemniscus pathway from a peripheral nerve to cortex. For upper limb median nerve stimulation, typical recording sites include Erb's point (N9), the cervical spine (N13), and scalp over the contralateral somatosensory cortex (N20). For lower limb tibial nerve stimulation, look for the lumbar potential, then the cortical P37/N37. Interpret latency, amplitude, and interpeak intervals (e.g., N9–N20 central conduction time).
Understand the physiology: large myelinated Aβ fibers carry the signal; the pathway crosses in the medulla. SSEPs are relatively resistant to anesthesia and are used intraoperatively to monitor dorsal column integrity. Recognize that SSEPs assess sensory pathways, not motor.
Traps
Questions confuse N20 (cortical) with N13 (cervical) and misattribute which segment a prolonged interpeak interval implicates. Remember that a peripheral abnormality can prolong all later peaks; use interpeak intervals to localize. Do not claim SSEPs directly measure motor tract function.
Worked example / rule of thumb
If N9 is normal but N9–N20 is prolonged, the lesion is central (between Erb's point and cortex). Rule of thumb: localize by comparing interpeak intervals, not absolute latencies alone, because a slow peripheral nerve shifts every downstream peak.