PMC8080406_01
Peri-operative O-arm/CT imaging of the thoracic spine. (A) Pre-operative CT imaging of the thoracic spine shows a 3-column fracture through an ankylosed T9-T10 disc space and posterior elements. (B) Forty minutes after flipping the patient to prone position, transcranial motor evoked potentials were significantly decreased in the bilateral lower extremities. Intraoperative O-arm scan confirmed enlarged distraction of the T9-T10 fracture. (C) Immediate post-operative O-arm scan demonstrated satisfactory instrumentation with restoration of the normal spinal alignment.
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T9-T10 disc space and posterior elements (Fig. 1A). The T9-T10 fracture was considered to be unstable with disruption of all three spinal columns, typical of trauma in the setting of ankylosing spondylitis. Immediate post-operative O-arm scan demonstrated satisfactory instrumentation with restoration of the normal spinal alignment (Fig. 1C).
Stack of transcranial motor evoked potentials (tcMEPs). Upper panel shows the tcMEPs of left-side upper and lower extremities. Lower panel shows the tcMEPs of right-side upper and lower extremities. Second column indicates the set stimulation voltage and actual delivered current. Constant-voltage stimulation was used to elicit tcMEPs following optimization of stimulation parameters at baseline. Blue arrows highlight the pre- and post-flip baseline, which did not raise any concerns regarding potential neurologic injuries from the flipping. Forty minutes after flipping, a significant decrement of tcMEPs in bilateral lower extremities were reported (marked by red arrows). Response of right first dorsal interosseous also showed a >50% decrease (orange arrow), which was resolved by re-positioning the right arm. After decompression and fusion, tcMEPs in the lower extremities improved with reproducible responses in distal muscles. However, tcMEPs in upper legs were still diminished. FDI: first dorsal interosseous; IL: iliopsoas; AD: adductor longus; QD: vastus lateralis; TA: tibialis anterior; GS: gastrocnemius; AH: abductor hallucis. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Immediate post-flip SSEPs and tcMEPs were unchanged from pre-flip baselines (Fig. 2, marked by blue arrows). However, 40 min after positioning the patient prone, we noticed and reported a significant decrease in the tcMEP responses of the bilateral lower extremities (Fig. 2, marked by red arrows; Fig. 3A), although the lower-extremity SSEPs remained stable bilaterally (Fig. 4). The amplitude reduction in right-side FDI muscle (Fig. 2, marked by orange arrow; Fig. 3B) might have resulted from the unsatisfactory positioning of the arm, which was consistent with the changes in right-side ulnar nerve SSEPs (Fig. 5). After decompression, tcMEP responses in the distal lower extremities improved, i.e., bilateral abductor hallucis muscles (Fig. 2). Final tcMEP responses were present in the bilateral feet and left lower leg (Fig. 2, Fig. 3C).
Traces of transcranial motor evoked potentials (tcMEPs) at three critical time points. (A) At 18:25:06, tcMEPs showed unexpected significant decreases in bilateral lower extremities, with absent responses in right side. (B) At 18:38:24, tcMEPs of left lower extremity deteriorated. Right hand motor response also reduced in amplitude by more than 50%, which was positioning-related. (C) At 22:52:43, tcMEPs of bilateral lower extremities improved after open decompression and instrumentation. Black: traces at critical time points; Red: baseline tcMEP responses. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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At this point, based on the observation that bilateral lower extremities' tcMEPs continued deteriorating (Fig. 3B), an intraoperative O-arm/CT scan was obtained and revealed that there was new distraction of the T9-T10 fracture with loss of thoracic kyphosis, and significant narrowing of the spinal canal concerning for spinal cord compression (Fig. 1B).
Partial stack of posterior tibial nerve (PTN) somatosensory evoked potentials (SSEPs). When changes in tcMEPs were reported, there was no significant change noted in the PTN SSEPs. However, there were some subtle increments of latency (~2 ms, marked with right arrows) in right-side PTN SSEPs after the changes in tcMEPs being noted, which did not meet the alarm criteria.
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However, 40 min after positioning the patient prone, we noticed and reported a significant decrease in the tcMEP responses of the bilateral lower extremities (Fig. 2, marked by red arrows; Fig. 3A), although the lower-extremity SSEPs remained stable bilaterally (Fig. 4). Of note, despite not meeting the alarm criteria, there were some subtle increases in latency (~2 ms, Fig. 4) of the right posterior tibial nerve SSEPs.
Partial stack of ulnar nerve somatosensory evoked potentials (SSEPs). The patient was flipped to the prone position. The patient's arms were placed on arm boards in the prone "superman" position, with sufficient padding around the elbows and axilla areas. However, we still noticed a significant decrement in the right-side ulnar nerve SSEPs (highlighted in red rectangle) during the case. The SSEPs returned to baseline and remained stable after re-positioning the right arm and adjusting the padding around the elbow. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Meanwhile, the right-side ulnar nerve SSEPs started to show reduction in amplitude (>50%) both in the cortical and subcortical channels and almost became absent subsequently, despite increasing the stimulation current from 30 mA to 35 mA (Fig. 5, highlighted in red). The amplitude reduction in right-side FDI muscle (Fig. 2, marked by orange arrow; Fig. 3B) might have resulted from the unsatisfactory positioning of the arm, which was consistent with the changes in right-side ulnar nerve SSEPs (Fig. 5).