“EMG disease” is characterized by diffusely increased insertional activity on needle electromyography (EMG) without the presence of any neuromuscular diseases. Familial aggregation in many cases suggests an underlying muscular channelopathy of genetic origin such as via the CLCN1 gene mutation; however, the involvement of the CLCN1 gene was found not to be the case. 

We present the case of a 51-year-old female who was referred to the EMG lab for possible ALS. An EMG had been previously performed elsewhere for the evaluation of possible lumbosacral radiculopathy. There were no neuromuscular complaints other than those associated with possible root disease. Needle examination of multiple muscles showed runs of positive sharp waves with waning amplitude and frequency, of several seconds duration, and only triggered by needle displacements. Clinical examination was normal and, specifically, no percussion myotonia or any abnormal muscle response to percussion was present. Short and long exercise tests were normal. Family history was negative for neuromuscular disorders. There was no exposure to drugs associated abnormal muscle excitability or toxicity.

Genetic testing identified a missense sequence variation in the SCN4A gene, predicted to be deleterious by in silico models. Additional genes evaluated included DMPK, CNBP, CLCN1, ATP2A1, CACNA1S, CAV3, GLRA1, HINT1, and PTRF. SCN4A related mutations have previously been linked in other cases to neuromuscular disorders such as paramyotonia congenita, hypokalemic periodic paralysis and hyperkalemic periodic paralysis. 

To our knowledge, this is the first reported case in whom asymptomatic, increased insertional activity is associated with a SCN4A sequence variation. Based on our research we hypothesize that EMG disease be classified as a non-dystrophic myotonic disorder that is a part of the family of skeletal muscle sodium channelopathies.