The effect of use of pyridostigmine and requirement of vecuronium in patients with myasthenia gravis

Abstract

Context: Patients with myasthenia gravis receive pyridostigmine, an anticholinesterase agent, as a part of therapy. These patients demonstrate a heightened sensitivity towards non-depolarising muscle relaxants. Continuing pyridostigmine till the day of the surgery or omitting it on the night before surgery could provide variable results with regards to the effect of vecuronium.

Aims: Myographic evaluation of a dose of vecuronium in patients with myasthenia gravis on pyridostigmine therapy.

Setting and design: A randomised, double-blind, clinical study conducted in a teaching hospital.

Subjects and methods: Medically (oral pyridostigmine) well-controlled adult patients with myasthenia gravis who were posted for thymectomy, were randomly divided into two groups. Patients in Group 1 received their last dose of pyridostigmine on the night before surgery while those in Group 2 received even the morning dose of the drug on the day of surgery. Neostigmine (1-2 mg) intravenously was used as rescue medication. Vecuronium (0.01 mg/kg) was used for intubation and muscle relaxation during trans-sternal thymectomy and its effect was reversed using neostigmine and atropine.

Results: Fourteen patients (7 in each group) belonging to both sexes were enrolled in the study. The intubating dose of vecuronium showed quicker onset time (155 sec or 2.7 min approx.) and peak effect (99% T1 suppression) in patients belonging to Group 1, and 3/7 (43%) complained of respiratory discomfort while waiting for surgery. By giving the morning dose of pyridostigmine (Group 2), an identical intubating dose of vecuronium showed relative resistance (peak effect-97% T1 suppression) and delayed onset time (198 sec approx.). However, the reversal was complete at the end of surgery in both the regimens.

Conclusions: Omission of the pyridostigmine dose on the day of surgery predisposed patients with myasthenia gravis to the possibility of respiratory discomfort and sensitivity to vecuronium. Continued administration significantly prolonged the onset time of vecuronium and the patients required a higher dose of vecuronium.