Leg MEPs Lost Fifteen Minutes After Aortic Cross-Clamp

The territory at risk is that of the anterior spinal artery, fed in the thoracolumbar cord largely by the great radicular artery (artery of Adamkiewicz), which most often enters between T8 and L1. The anterior spinal artery supplies the anterior two-thirds of the cord: anterior horns, spinothalamic tracts, and the corticospinal tracts that carry the descending volley recorded as a MEP. Those tracts are the most flow-dependent, so MEP loss is the expected early sign. The dorsal columns, supplied by the posterior spinal arteries, carry the PTN SSEP; isolated SSEP preservation with MEP loss is the classic anterior cord pattern. Here the PTN SSEP also fell, which argues that the ischemic insult is either extending or that a second factor is present.

The unchanged median nerve SSEP is the decisive control. Any systemic cause that degrades cortical responses globally - volatile agent, hypothermia, hypotension, a stimulator or recording problem common to all channels - would reduce the median SSEP too. Because it is unchanged, the change localizes to the lower cord or the leg-specific recording chain, and the low distal clamp pressure plus the timing makes genuine segmental ischemia the leading explanation.

1. Confirm the signal is real. Check impedances and connections on the affected leg channels, verify the stimulus is being delivered, and repeat the trial. The median SSEP control already tells you a global technical failure is unlikely, but a leg-specific hardware fault is still possible.
2. Confirm the physiology you can control. Proximal pressure, distal pressure, temperature, TOF, hematocrit, and whether the change is unilateral or bilateral.
3. Notify immediately, with numbers and a timeline, not adjectives. For example: "Fifteen minutes after cross-clamp, left PTN amplitude has dropped about 65% and left leg MEPs are absent at 500 V; right leg and both median nerves are unchanged; distal pressure is 34 mmHg."
4. Ask for intervention in the surgeon's and anesthesiologist's domain: raise proximal MAP with volume and pressors, shorten clamp time, reimplant or preserve intercostal arteries, consider a distal perfusion shunt, rewarm.
5. Repeat the studies every two to three minutes and record the clock time of every event.

The organizing principle is that spinal cord ischemia after cross-clamp is potentially reversible within a short window; the value of the monitor is not the observation, it is the speed of the report and the concrete request that follows it.

Treat the persistent MEP loss as the dominant finding, not the partial SSEP recovery. Recovery of the dorsal column response indicates that perfusion improved, but it does not clear the anterior horn and corticospinal tracts, which are the tissue that determines motor outcome. A partially recovered SSEP with persistently absent MEPs is a recognized pattern of residual anterior cord ischemia and correlates with postoperative weakness.

Before attributing everything to cord ischemia, exclude the confounding causes of MEP suppression: confirm TOF is 4/4, temperature is corrected, and anesthetic depth has not increased; a residual neuromuscular blocker or a deeper volatile concentration can keep MEPs abolished after cord perfusion is restored. Then continue the perfusion strategy - a defined MAP target agreed with anesthesia, avoidance of anemia and hypoxia, no further clamp time - and keep re-trialing MEPs at fixed intervals, including after clamp release, so that the case record shows the trajectory rather than a single point.

At case end, tell the anesthesia and surgical teams that the patient should have an immediate and repeated neurologic examination, and document the sequence: onset time, affected channels, pressures, interventions, times of partial recovery and persistent loss.