Neuromuscular monitoring in myasthenic syndrome

Abstract

SummaryWe describe the anaesthetic management of a 72-year-old man with myasthenic syndrome. Pre-operatively, he was treated with 3,4-diaminopyridine and showed a strong hand grip. During general anaesthesia with nitrous oxide and sevoflurane in oxygen, a mechanomyograph and two accelerographs were set up for the hands and left foot to monitor neuromuscular function. Insufficient force and acceleration of contraction with 1 Hz stimulation was observed in the hands. In the foot, the twitches produced by 1 Hz and train-of-four stimulation could barely be detected using the accelerograph, and the train-of-four ratio fluctuated between 70 and 100%. No neuromuscular blocking drugs were used during surgery. After discontinuation of sevoflurane, responses to train-offour stimulation remained small, but a strong response to tetanic stimulation was observed, with posttetanic facilitation. Extubation was successful, and recovery from anaesthesia was uneventful. Tetanic stimulation and post-tetanic facilitation are important in monitoring neuromuscular function in patients with myasthenic syndrome whose train-of-four responses are insufficient. Myasthenic syndrome (Eaton2Lambert or Lambert±Eaton syndrome), characterised by weakness of proximal limb muscles, may show abnormal sensitivity to both depolarising and non-depolarising neuromuscular blocking drugs [1]. As a result, administration of these agents can cause prolonged neuromuscular blockade [2±5]. Although monitoring of neuromuscular function during anaesthesia is considered necessary for patients with myasthenic syndrome, little information is available concerning the details of such monitoring. We describe a case of myasthenic syndrome in which monitoring of neuromuscular function during general anaesthesia was particularly difficult. Case historyA 72-year-old man with myasthenic syndrome was scheduled for thoracotomy for excisional biopsy of a suspected small-cell cancer of the lung. A coin lesion had been discovered in the left lung on a chest radiograph 2 months after the onset of neuromuscular symptoms. Electromyography (EMG) showed marked diminution of amplitude of the initial compound muscle action potential evoked by sural nerve stimulation. A decremental response was seen with lowfrequency repetitive stimulation (2 Hz), but up to 300% facilitation occurred with high-frequency repetitive stimulation (50 Hz). The patient was diagnosed with myasthenic syndrome based on these characteristic findings, absence of detectable antibodies to the acetylcholine receptor and a negative edrophonium test. At pre-operative assessment, the patient was 175 cm tall and weighed 63 kg. Weakness of the lower limbs precluded walking without assistance. Although he complained of weakness of the upper limbs, dynamometry showed a strong hand grip (. 30 kg). Other pre-operative investigations, including blood haematology and biochemistry screening, respiratory function tests and blood gas analysis, were unremarkable.The patient had taken 3,4-diaminopyridine 60 mg daily beginning 1 we...