All Four Leg MEP Channels Disappear on a Single Sweep

Work backwards from the pattern. The upper-limb MEPs are produced by the same transcranial stimulus, through the same stimulating electrodes and the same return, as the lower-limb MEPs. Because the arms still respond at the same intensity, the stimulator, the corkscrews, and the stimulation path are functioning. That removes a large class of common-mode failures. Likewise, a systemic cause - hypotension, hypocapnia, hypothermia, deepening anesthesia, residual blockade - would not spare the arms, so physiology is a weaker explanation than it first appears, though it cannot be dismissed without checking.

What the failed channels share is the recording side for the lower limbs: a separate headbox or preamplifier, a separate cable bundle, and a common reference or ground electrode for the legs. A fault in any of those shared elements abolishes all four leg channels at once, exactly as seen. The 48 kOhm left tibialis anterior impedance is a real problem but cannot by itself explain loss in the right leg, so it is either one contributor or a red herring.

The sequence of reasoning is: pattern defines the candidate shared element, then the examination tests that element rather than randomly swapping leads.

Immediately, before certainty: "We have lost all four leg MEP channels on one sweep; upper limbs and SSEPs are unchanged. Hold for two minutes while we sort out whether this is real." Announcing early costs little; a delayed report of a genuine ischemic or mechanical event costs a great deal.

Then, in order and with the clock running:

1. Inspect the shared recording elements - leg preamplifier seated, cable bundle intact at the headbox and at the drape edge, leg reference electrode on and conducting.
2. Fix the 48 kOhm electrode; re-gel or replace. Recheck all impedances.
3. Move the leg channels to different headbox inputs or swap in a known-good cable to isolate the box versus the leads.
4. Run an upper-limb trial to confirm the stimulus is unchanged, then a leg trial.
5. Only if the hardware is proven sound, examine physiology - pressure, temperature, TOF, hematocrit, ventilation - and then treat the finding as a true cord event and notify the surgeon to inspect the operative site.

Keep the surgeon informed of which branch you are on. A five-minute silence followed by "it was a bad electrode" trains the team to ignore you; a running commentary trains them to trust the monitor.

Document the whole episode as a discrete event rather than letting it dissolve: time of loss, which channels were affected, the fact that upper-limb MEPs and all SSEPs were unchanged at that moment, the specific impedance values including the 48 kOhm channel, each troubleshooting action with its time, and the time and manner of signal return. State the working explanation factually - "loss of lower-extremity MEPs coincident with a failed lower-extremity reference electrode; signals returned immediately after electrode replacement; SSEPs and upper-extremity MEPs remained stable throughout" - and avoid a causal claim you cannot support, such as asserting that the cord was never at risk during those minutes.

Tell the team three things: the signals are back and are interpretable, there was an approximately X-minute window in which the leg MEPs could not be relied on, and the SSEPs - which were unaffected - provided partial coverage of that window. That is honest about the gap in surveillance without implying a neurologic event occurred.

Then use it: flag the electrode to the setup routine (secure the leg reference, re-gel before draping), because a repeat episode later in a long fusion, when the surgical risk is at its peak, is the outcome worth preventing.