Intraoperative neurophysiological monitoring is an implicit aid, used to prevent unwanted neurological complications in spine deformity correction procedures. From the available armamentarium, Tc MEP and SSEP are most sensitive and most specific techniques respectively1. Loss of intraoperative signals is alarming and indicates either a temporary or a permanent neurologic injury2. Here we present a case of a young female with adolescent idiopathic scoliosis who developed unilateral intraoperative SSEP and Tc MEP signal loss but had intact post-operative neurology.

Lateral; Pedicle; Screw; Neuro-Monitoring; Signal; Loss

Jain N, Rawall S, Saini P (2020) A Rare Loss of Intraoperative Neuro-Monitoring Signals with Laterally Misplaced Pedicle Screw: Case Report. JSM Clin Case Rep 8(2): 1177.

Tc MEP: Trans Cranial Motor Evoked Potentials; SSEP: Somatosensory Evoked Potentials; ROM: Range of Motion; GA: General Anaesthesia; QUAD: Quadriceps muscle; TA: Tibialis Anterior musclem; AH: Abductor; Hallucis muscle; APB: Abductor Pollicis Brevis; CMAP: Compound Muscle Action Potential

A 13 year old female presented to our spine department with complaints of increasing back deformity for last 6 months. The patient was a full term caesarean section delivered child with no antenatal, perinatal or post-natal complications. The cognition and milestones were appropriate for her age. Family tree was not significant for any spine or neurological disorder.

Physical examination revealed full and painless ROM of spine. Right shoulder was higher. Right rib hump was prominent. PSIS, popliteal fossa and medial malleoli were at same levels. There were no neuro cutaneous markers, swelling/ tuft of hair/ scar mark/ sinus/ discoloration over back.

Neurological examination revealed intact motor, sensory and reflex functions in all four limbs. Planters were flexor and bowel/ bladder control was intact. The patient was diagnosed as Adolescent Idiopathic Scoliosis, Lenke Type 1 A N with D5-L2 = 47 degrees, curve to right and Nash Moe rotation = grade 2 at apex (D10).

Patient underwent Posterior instrumentation D5-L2 + deformity correction under Tc MEP and SSEP monitoring under GA. The neuro-monitoring machine used was Natus Xltek® Protector 32 IOM.

The neuro-monitoring signals were taken thrice before skin incision: after intubation, after placing patient prone and after painting/ draping (Figure 1-4).

Figure 1 Neuro-monitoring graph showing green lines as baseline CMAP of right and left QUAD, TA and AH. APB was kept as control. Immediately after putting left L2 screw neuro-monitoring signals in left QUAD, TA and AH were lost as depicted by flat coloured lines.

Figure 2 Fluoroscopy images showing laterally misplaced left L2 pedicle screw.

Figure 3 Fluoroscopy images showing corrected left L2 pedicle screw.

Figure 4 Post op MRI axial images showing intact spinal cord and screw trajectory.

First screw was placed in left L2 pedicle. The signals repeated immediately after, presented as lost in left lower limb. Fluoroscopy showed screw had slipped lateral to the pedicle.

The vitals were: BP: 120/70 mmHg; mean BP: 96.5 mmHg; SPO2: 100 %; Pa CO2: 34 mmHg; room temperature: 18° C. Warming blankets and warmed intravenous fluids maintained the core body temperature. Anaesthetic drugs used were fentanyl, sevoflurane with MAC of 0.3 and propofol/ dexmedetomidine infusions.

The screw was repositioned into the correct tract under fluoroscopic guidance.

But signals did not return. Therefore screw was removed. Signals repeated after screw removal also showed no improvement. Stagnara wake-up test result was indeterminate. Distal pulses in both lower limbs were intact. After a unanimous decision, the procedure was abandoned and wound was closed. Total duration from induction to extubating was 4 hours. Total blood loss was 150 ml and urine output was 150 ml.

The patient was extubated. The motor function was tested in left lower limb and was found present. The patient was shifted to ICU. After patient was fully conscious/ oriented detailed motor and sensory testing was performed. It revealed intact power, reflexes, light touch, proprioception, crude touch and pain functions in all 4 limbs. Urgent post-op dorso-lumbar spine MRI was performed. It revealed uninjured spinal cord and the left L2 screw tract.

The Neuro monitoring machine was returned to the company for testing.

After thorough discussion with parents, patient was replanned for surgery. A different neuro-monitoring machine was procured (NIM-Eclipse® IOM system) and the surgery was done 4 days later. Posterior instrumentation D5 –L2 + deformity correction was performed successfully without any adverse events. Screws were placed bilaterally in D5, D6, D8, D10, L 1 & L2, right only in D12 and left only in D9 & D11 vertebrae. A good curve correction was achieved. Post-op neurology was intact. The patient was mobilised out of bed on day 3 and discharged to home on day 5. At the latest follow up the patient was healthy and had joined back to school

TcMEP and SSEP combined, provide real time highest safety in deformity correction surgeries at present [3]. Cord injuries due to implant, vasospasm, ischemia, stretch, hypotension, hypothermia, hypoxia, anaesthetics and positioning can lead to intraoperative signal loss [2].

We placed the patient prone with all precautions. During instrumentation there was never any medialslip. Throughout, the position of the patient and limbs was unchanged. The electrodes were always intact, connected and in place. Impedance and CMAP from control (APB) in neuro-monitoring machine was always normal. The results were negative for any technical faults from the company. The vital parameters were maintained. Femoral and iliac artery occlusions can lead to signal loss but in our case, the distal pulses in both lower limbs were intact and the extremities were warm[4,5].

Arguably vessel occlusion stands as the most probable cause for signal loss. Localised vasospasm can lead to loss of intraoperative signals which recover when timely action is taken [6,7]. Segmental spinal arteries run in the valleys of vertebral bodies and branch at vertebral foramen, to anastomose with the spinal cord arcade [8]. Artery of Adamkiewicz is an important feeder to the watershed zone. It can be found anywhere between D5-L2 level on the left side of the spinal column[9]. Compression of these segmental vessels, leads to reduced blood supply leading to cord ischemia, paraplegia and signal loss, but is completely reversed when the pressure is released in a timely manner [10]. It is possible that the laterally misplaced left L2 pedicle screw compressed a major segmental vessels causing ipsilateral lower limb signal loss. After the screw was removed, the blood supply to the spinal cord was restored presenting as intact neurology post-op.

We present a rare case where pedicle instrumentation led to signal loss in the ipsilateral limb without directly injuring the spinal cord. We assume the cause to be temporary vascular occlusion and believe it as the first ever case reported of its kind.

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