In a recent substack post we discussed myasthenia gravis and pointed out that this disease can cause fade in the train-of-four in the absence of neuromuscular blocking drugs, depending upon the severity of disease, the time since an anti-cholinesterase drug such as pyridostigmine was taken and various unknowable factors.
In this example, a patient with myasthenia was anesthetized for a robotic port access surgery. The baseline train-of-four ratio, following induction of anesthesia but prior to administration of rocuronium, was abnormal, in the range of 0.85 to 0.90 (not shown in the figure). Fade in the baseline train-of-four ratio is not unexpected in patients with myasthenia gravis, as noted in our previous post. Rocuronium 20 mg was administered to facilitate intubation. Subsequently several 5 mg incremental doses of rocuronium were administered to maintain deep to profound block.
Near the end of the procedure, with a post tetanic count of 9, sugammadex 200 mg was administered (designated by the first yellow arrow at approximately -30 minutes). Within a few minutes the train-of-four ratio was about 0.5 but showed no further improvement. Subsequently 2 additional 200 mg doses of sugammadex were administered (designated by the second and third yellow arrows at approximately -20 and -16 minutes). However the train-of-four ratio actually declined to approximately 0.3.
At that time, the anesthesia team concluded that the rocuronium had most likely been reversed by the 600 mg of sugammadex, but the patient’s myasthenia gravis was manifesting as fade in the train-of-four ratio. Therefore neostigmine (and glycopyrrolate) were administered (designated by the fourth yellow arrow at approximately -9 minutes). The purpose of the neostigmine in this instance was not to antagonize rocuronium, but to specifically treat the myasthenia gravis by making more acetylcholine available at the neuromuscular junction. This resulted in prompt improvement in the train-of-four ratio which reached 0.91 prior to discontinuation of monitoring. The patient was extubated uneventfully soon thereafter.