Unilateral SSEP Change During Scoliosis Surgery
A 14-year-old girl with adolescent idiopathic scoliosis is undergoing posterior spinal fusion from T4 to L1. Baseline posterior tibial SSEPs are present bilaterally with P37 latencies of 38.2 ms (left) and 38.6 ms (right) and amplitudes of 1.8 µV and 2.0 µV. Twenty minutes after rod derotation, the left P37 amplitude falls to 0.7 µV with a 2.4 ms latency increase; the right is unchanged. Blood pressure is 84/48 mmHg, core temperature 34.9 °C, and the surgeon has just requested a propofol bolus for the patient's movement. The anesthesia record shows a 40-minute period of controlled hypotension at 80/50 mmHg. Stimulating electrodes are on the posterior tibial nerves at the ankles; recording is at Cz-Fz with a popliteal reference.
1. What are your immediate priorities when the left SSEP changes?
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First confirm the change is real and not technical: check the left tibial stimulating electrodes, the recording electrodes at Cz-Fz and the peripheral channel, impedances, and connections. Repeat the average. Check limb temperature and position, because a cold or compressed leg will slow conduction. Then notify the surgeon and anesthesiologist immediately: the change is unilateral, the blood pressure is low, and the temperature is low. Ask anesthesia to raise the mean arterial pressure and correct hypothermia, and ask them to avoid the requested propofol bolus if possible, because an anesthetic bolus can further suppress SSEPs. Time matters: spinal cord ischemia is reversible if corrected early. Document the time of change, the interventions, and the response.
2. How do you separate a technical cause from a physiologic cause?
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Technical causes usually affect one channel or one limb and can be seen in the peripheral or cervical channel as well as the cortical channel. Check the stimulus artifact, the pop, and the impedance. If the peripheral response is stable but the cortical response falls, the problem is more likely central. Physiologic causes include hypotension, hypothermia, cord compression or ischemia, and anesthetic effects. A unilateral change after rod derotation with low blood pressure strongly suggests cord ischemia, but the propofol request and the 34.9 °C temperature are confounders. Compare with the right side, review the trend, and look at the raw sweeps. Do not assume technical simply because the change is sudden.
3. The change persists for 10 minutes. The surgeon asks whether to proceed. What is your role?
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My role is to provide reliable monitoring data and facilitate communication, not to make the surgical decision. I would state clearly that the left cortical SSEP has remained reduced by more than 50% with a latency increase for 10 minutes despite corrective attempts, and that this is a warning sign. I would present the technical checks I performed and the physiologic variables. I would continue monitoring and document the timeline. The surgeon and anesthesiologist decide whether to release the rods, raise blood pressure further, or wake the patient. I should not tell the surgeon to proceed or stop; I should make sure the data are understood and that the record reflects the discussion.