Anaesthetic management of scoliosis surgery with IONM in a patient with CMT disease

Sir,

Charcot Marie Tooth (CMT) disease is the most common neuromuscular disorder.[1] CMT is a sensorineural peripheral polyneuropathy. Anaesthetic management of patients with CMT disease undergoing scoliosis correction surgery is complicated by the neurophysiological abnormalities which cause hindrance in obtaining reproducible intraoperative neuromonitoring (IONM) responses.

A 16-year-old boy, weighing 37 kg, who was a known case of CMT disease Type 1B was posted for correction of thoracolumbar scoliosis. He had pes cavus and wasting of small muscles of the foot. Cobb’s angle was 55°. Posterior instrumentation and fusion of T₆—L₄ vertebrae was planned. Electrocardiogram, echocardiography, and pulmonary function tests were normal. His laboratory parameters were essentially normal. For pre-emptive analgesia, gabapentin and paracetamol tablets were given before shifting to operating room. Under standard monitoring, general anaesthesia was induced with intravenous fentanyl 3 μg/kg and titrated boluses of propofol. After ensuring adequate depth, laryngoscopy was done, vocal cords were sprayed with 4% lignocaine and 6.5 mm cuffed endotracheal tube was inserted without muscle relaxant. He was ventilated with a mixture of air and oxygen. A central venous and radial arterial catheters were inserted. Pressure points were padded and bite block was placed.

For the purpose of performing IONM total intravenous anaesthesia with propofol (3-5 mg/kg/h), ketamine (0.1-0.3 mg/kg/h) and dexmedetomidine (0.3-0.5 μg/kg/h) were employed. Bispectral index was maintained between 40-60 by titrating the infusions. IONM was performed using Xltek-protektor-32 (Xltek, a division of Natus, Middleton, WI).

Subdermal needle electrodes for recording Transcranial motor evoked potential (TcMEP) were placed in quadriceps femoris, tibialis anterior, medial gastrocnemius, and abductor hallucis muscles in the lower limbs. Trains of six to eight square-wave stimuli (0.5 msec duration) were applied with intensities ranging from 300 to 400 V. Baseline TcMEP recordings were done in the supine position. No TcMEP responses were obtained with a sweep speed of 100 msec. Sweep speed was gradually increased to 700 msec and reproducible responses were obtained. [Figure 1] We failed to elicit any reproducible SSEP recordings even after altering the parameters and it was decided to proceed without monitoring SSEP. [Figure 2] Patient was positioned prone and post-positioning potentials remained unaltered. Surgery proceeded with successful monitoring of TcMEP throughout the surgery. He remained haemodynamically stable and body temperature was maintained at 35-36⁰. No additional analgesics were given intraoperatively. At the end of surgery, patient was extubated uneventfully. For postoperative pain management, both ketamine and dexmedetomedine infusions were continued along with intravenous paracetamol. He made an uneventful recovery and was discharged on fifth postoperative day.

Figure 1.

Lower extremity prepositioning baseline Transcranial motor evoked responses

Figure 2.

Non-reproducible Somatosensory evoked potential responses

CMT disease has a clinically and genetically heterogeneous spectrum of presentation. CMT1 has a demyelinating pathology, whereas CMT2 is characterized by axonal loss. The motor nerve conduction velocities in patients with CMT1B is slower than in patients with CMT1A, especially in patients with early onset disease.[1]

The anaesthetic management is complicated by the unpredictable response to neuromuscular blockers, so even the intubating dose of muscle relaxant was avoided in our patient as the prolonged action would interfere with the recording of TcMEP. Nitrous oxide can cause neurotoxicity in these patients as well as interfere with IONM. Cardiac conduction abnormalities are also common and drugs enhancing conduction blockade should be avoided.[2]

Performing scoliosis correction without IONM increases the chance of neurological complications. Stagnara wake up test is not always possible in these patients with preoperative weakness. We were able to achieve successful monitoring of TcMEP by modifying the parameters of the IONM and by avoiding all anaesthetic influences which could hamper the responses. The recommended approach for successful monitoring of TcMEP is to avoid inhalational agents. Propofol decreases the amplitude while latency is unaffected. Ketamine enhances the potentials.[3,4] Care must be taken to avoid facial and lip injuries when TcMEP monitoring is done.[5]

Anaesthetic management in cases requiring IONM has to be carefully planned and close interaction with the neurophysiologist is essential for successful IONM especially in patients with neuromuscular diseases.

Declaration of patient consent

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Conflicts of interest

There are no conflicts of interest.