Sub-clinical neuromuscular dysfunction after envenoming by Merrem’s hump-nosed pit viper (Hypnale hypnale)

Abstract Envenoming by hump-nosed pit vipers (genus Hypnale) causes local effects, mild coagulopathy, and rarely, acute kidney injury. Neuromuscular dysfunction has not been clinically reported but, in vitro studies show a weak, reversible, neurotoxic effect on the neuromuscular junction. We investigated the neuromuscular dysfunction in H. hypnale envenoming. Eighteen patients with authenticated H. hypnale envenoming were included. All 18 had serial clinical and neurological examinations. We evaluated neuromuscular dysfunction using stimulated concentric needle single-fibre electromyography (sfEMG) of orbicularis oculi in the patients within two days of the bite and compared to 29 normal subjects. Patients with abnormal jitter were reviewed six weeks later. All 18 patients had local effects and one-third had non-specific systemic symptoms. No patient had clinical features of neuromuscular paralysis. Median jitter values of patients were statistically significantly higher than normal subjects (19.6 ± 13.0 µs compared with 15.6 ± 7.4 µs.). None of the patients or normal subjects had neuromuscular blocks. Three patients had median jitter values higher than the maximum in normal subjects, which returned to normal by six weeks. H. hypnale envenoming leads to sub-clinical neuromuscular dysfunction, despite patients not showing any clinically detectable neurotoxicity.