Single fibre electromyography as a predictor of intermediate syndrome

Chanika Alahakoon, Tharaka Dassanayake, Indika Gawarammana, Michael Sedgwick, Vajira Weerasinghe

Objectives: Deliberate self poisoning for suicidal purposes using organophosphorus insecticides (OPI) is common problem in Sri Lanka. The clinical sequence of OPI poisoning includes initial cholinergic crisis, intermediate syndrome (IMS) and delayed polyneuropathy. IMS is thought to be mediated via prolonged nicotinic receptor inhibition at the neuromuscular junction (NMJ) which affects respiratory muscles leading to respiratory failure. The aim of the present study was to investigate whether NMJ activity as measured electrophysiologically by single fibre electromyography (sfEMG) within first 24 hours following exposure can predict IMS which occurs later in the course of the illness.

Methods: A prospective observational cohort study was conducted in Teaching Hospital Peradeniya, Sri Lanka from September 2014 to September 2016 on 121 patients after written informed consent. Patients were subjected to sfEMG every other day during hospitalization. Exposure was confirmed based on the history and red blood cell acetylcholine esterase assays. IMS was diagnosed based on the presence of at least three out of the following four criteria between 24-96 hours post ingestion; proximal muscle weakness, bulbar muscle weakness, neck muscle weakness, respiratory paralysis.

Results: Of the 121 patients, 64 were clinically diagnosed with IMS. 74/121 patients were intubated following respiratory failure which includes all IMS patients. 96/121 patients were subjected to repeated SfEMG testing. 67 patients were subjected to SfEMG within the first 24 hours following ingestion of the poison. 29/67 were clinically diagnosed with IMS and of those, 25 had jitter values above 33.4 μseconds, the normal upper limit. Of the 38 patients that were not diagnosed with IMS, 16 had higher jitter values. Therefore the odds of developing IMS is 8.94 (CI=2.49-29.60) times higher in the patients with increased jitter values within the first 24 hours following ingestion of an OPI with a sensitivity of 86.21% and a specificity of 57.89%.

Conclusion: SfEMG is a neurophysiological marker with a moderate sensitivity of 86.21% in diagnosing IMS in deliberate OPI ingested patients.

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