Electromyography in myasthenia gravis

Electromyographic responses in myasthenia gravis are usually measured from the abductor digiti minimi muscle following repetitive supramaximal stimulation of the ulnar nerve at the wrist at 3Hz. There is a gradual diminution of the compound muscle potential. Check that the patient is not undertaking anticholinesterase medication.

Note - single fibre electromyography records action potential (AP) from individual muscle fibres, and allows to record the AP simultaneously from two muscle fibres innervated by a single motor neuron. The difference between the time of onset of these two action potentials is called the ‘jitter’. In myasthenia gravis (MG) this ‘jitter’ will increase because of the reduced NMJ transmission. This is the most sensitive among the diagnostic tests for MG.

Reference

1. Jayam Trouth A et al. Myasthenia gravis: a review. Autoimmune Dis. 2012;2012:874680