Unobtainable Lower-Limb MEPs at Baseline in Cervical Myelopathy
A 68-year-old man with cervical spondylotic myelopathy undergoes C3-C5 posterior laminectomy and fusion. Preoperatively he has 4/5 deltoid and biceps strength, 4-/5 hip flexion bilaterally, hyperreflexia, and no history of neuromuscular disease. Anesthesia is total intravenous with propofol and remifentanil, TOF 4 of 4, MAP 80 mmHg, temperature 36.6 C. Baseline transcranial MEPs are robust from deltoid, biceps, and abductor pollicis brevis bilaterally, but no lower-limb MEP can be elicited at 500 V from tibialis anterior or abductor hallucis. Posterior tibial SSEPs are present but low amplitude, 0.3 microV bilaterally with N30 at 33 ms; median SSEPs are normal. The surgeon says, "If we can't monitor the legs, we might as well not monitor at all - let's just start."
1. How do you answer the surgeon's statement? What does an absent baseline MEP mean and not mean here?
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An absent baseline MEP means there is no dynamic range on that channel: a response that is not there cannot be shown to deteriorate, so it cannot serve as an injury detector. That is a limitation on those four channels, not a verdict on monitoring.
It does not mean the cord is unmonitored. Posterior tibial SSEPs are recordable and give a continuous read on the dorsal column at and below the operative level. Median SSEPs give a rostral reference. The upper-limb MEPs from deltoid, biceps, and abductor pollicis brevis are actually the channels that matter most at C3-C5, because the segments being decompressed and instrumented supply the C5 and C6 myotomes, and C5 palsy is one of the specific deficits this operation is monitored to catch. Free-run and triggered EMG on the cervical nerve roots adds root-level coverage that no MEP provides.
The interpretation to give the surgeon is: the lower-limb MEP absence is most consistent with the pre-existing myelopathy - the corticospinal tract cannot generate a recordable descending volley into a chronically compressed cord at maximal output, with neuromuscular blockade and anesthetic effects excluded - and it is a baseline finding, not an event. Monitoring should proceed, with the plan redefined around the channels that work.
2. What monitoring plan and alarm criteria do you propose, and what must be documented before incision?
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The plan restates the coverage honestly: upper-limb MEPs (deltoid, biceps, triceps, abductor pollicis brevis) as the primary motor channel for the operative levels, posterior tibial and median SSEPs as the ascending pathway, spontaneous and stimulus-triggered EMG for nerve root irritation, and periodic retrials of the lower-limb MEPs after decompression and after any change, since a chronically compressed cord occasionally generates a response once decompressed or once stimulation parameters are adjusted.
Agree explicit alarm criteria in advance rather than improvising: for example, a 50% or greater amplitude drop in any upper-limb MEP or a reproducible increase in stimulus threshold, a 50% amplitude loss or 1 ms latency increase in SSEPs, and sustained neurotonic EMG. State that the lower-limb channels are non-informative at baseline and will be reported as such.
Documentation before incision should include the baseline values by channel; the stimulation parameters and the maximum output used; the evidence that the absent responses are not technical or pharmacologic (impedances, TOF 4 of 4, no volatile agent, temperature and pressure); the pre-existing clinical deficit; the conversation with the surgeon about the limitation and the agreed plan; and the agreed alarm criteria. That record is what makes the interpretation defensible at any later point in the case.
3. Two hours later, deltoid MEP amplitude falls from 400 to 210 microV on the left only, with normal SSEPs. Write the report language and the handoff you would give.
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Report it as a change against a defined reference with the relevant context attached, and avoid both over- and under-claiming: "At 14:32, left deltoid MEP amplitude decreased from a baseline of 400 microV to 210 microV (approximately 48%) at unchanged stimulus intensity; biceps, triceps, and abductor pollicis brevis on the left and all right-sided upper-limb channels unchanged; median and posterior tibial SSEPs unchanged; lower-limb MEPs remained unobtainable at baseline as previously documented; TOF 4 of 4, MAP 78 mmHg, temperature 36.5 C, no volatile agent." Then the actions and outcome with times: who was notified, what was requested, what was done, what the signals did afterward. If it recovers, say when and to what amplitude; if it does not, say that.
For handoff, lead with what the receiving clinician can act on: the deficit at baseline, which channels were informative and which were not, the event and its trajectory, and the current status. Explicitly flag that lower-limb motor monitoring was never available, so the absence of a leg MEP change carries no reassuring information.
Keep interpretation separate from observation in the wording. "Amplitude fell 48% and returned to 360 microV after retractor release" is defensible; "cord was ischemic for four minutes" is not.