The Effect of use of Pyridostigmine and Requirement of Vecuronium in Patients with Myasthenia Gravis

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 (0.01mg/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 2.7min 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. Myasthenia gravis is an autoimmune disease, characterised by muscle weakness. It is related to the reduction in the functional acetylcholine (Ach) receptor population at the neuromuscular junction due to the presence of Ach-receptor antibodies. These patients have a variable response to muscle relaxants and are very sensitive towards non-depolarising muscle relaxants. It has been reported that vecuronium bromide in reduced doses is safe and effective and that larger the preoperative pyridostigmine dose, greater is the patient's sensitivity to the non-depolarizing agents.[1] It has also been suggested that the sensitivity to vecuronium in patients with myasthenia gravis may be influenced by the anti-cholinesterase therapy.[2],[3] The continued use of anti-cholinesterase therapy prior to thymectomy in patients with myasthenia gravis has been a matter of ongoing debate. The aim of this prospective clinical study was to compare the effect of two different regimens of pyridostigmine in patients with myasthenia gravis undergoing thymectomy under balanced general anaesthesia.