Absent Lower-Limb SSEPs in a Patient Who Cannot Lie Flat
A 62-year-old with 3 weeks of progressive ascending numbness to the knees and new urinary urgency is referred for tibial somatosensory evoked potentials. He becomes dyspneic when supine and cannot tolerate the standard position; he can lie at 30 degrees of head elevation. He has a pacemaker (bipolar, implanted 2 years ago) and bilateral knee replacements. Stimulation of the right tibial nerve at the ankle at motor threshold plus 2 produces no reproducible cortical response at Cz'-Fz after 200 averages, though a clear lumbar potential is seen at 21 ms. Left tibial stimulation shows a small, poorly formed P37 at 48 ms. Height is 183 cm.
1. The study is technically limited. Identify the non-physiologic reasons the cortical response may be absent on the right, and state what you would change before concluding the pathway is abnormal.
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Absence of a cortical response with a preserved lumbar potential must be treated as a technical problem until proven otherwise:
- Position: head elevation at 30 degrees does not abolish SSEPs, but it changes the montage's referential field and can reduce P37 amplitude; document the position and keep it identical between limbs.
- Averaging: 200 sweeps is often insufficient for a low-amplitude tibial cortical response, especially in a tall patient; repeat with 500-1000 sweeps and confirm reproducibility in two separate runs.
- Stimulus: verify supramaximal intensity (motor threshold plus 2 may be submaximal if the threshold was set poorly), check electrode impedance, and confirm the stimulator is actually delivering current (knee replacements do not block stimulation at the ankle, but poor contact does).
- Montage: Cz'-Fz is standard, but a contralateral parietal reference or additional channels can recover a response buried in noise.
- Relaxation and artifact: muscle tension and 60 Hz noise degrade cortical averages; check the patient is relaxed and use appropriate filters (typically 30-3000 Hz).
Only after a technically adequate, reproducible study should an absent cortical response with a normal lumbar potential be interpreted as a central or proximal conduction abnormality.
2. Address the pacemaker and the interpretation of the left-sided P37 at 48 ms given the patient's height.
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Pacemaker: surface electrical stimulation can interact with a cardiac device. The safe approach is to consult the device clinic or cardiologist, confirm the device is bipolar and functioning, avoid stimulation directly over the device or leads, keep current at the minimum needed, and have monitoring and resuscitation equipment available. If the device team does not clear the study, defer SSEPs and document the decision.
Latency interpretation: P37 latency depends strongly on limb length. For a 183 cm patient, an absolute P37 of 48 ms is not automatically abnormal; the appropriate reference is the height-adjusted or limb-length-adjusted norm, and the more robust internal control is the side-to-side difference and the lumbar-to-cortical interval. With a lumbar potential at 21 ms on the right, a normal central conduction time would leave a cortical latency in the mid-40s to low-50s range for this height. The left P37 at 48 ms with a small amplitude may therefore be within the height-adjusted range rather than clearly delayed.
Conclusion: report the study as technically limited, repeat with better averaging and a consistent posture, and interpret latencies against height-adjusted norms and the lumbar-to-cortical interval rather than the raw absolute value.