Severity, type, and distribution of myotonic discharges are different in type 1 and type 2 myotonic dystrophy

Abstract

To characterize and compare electrical myotonia in myotonic dystrophy type 1 (DM1) and type 2 (DM2), 16 patients with genetically confirmed DM1 and 17 patients with DM2 underwent standardized concentric needle electromyography of deltoid, biceps, extensor digitorum communis, first dorsal interosseous, tensor fascia lata (TFL), vastus lateralis (VL), tibialis anterior, and thoracic paraspinal muscles. Eight needle insertions per muscle were made by electromyographers blinded to DM type who recorded the presence and type of myotonia (e.g., classic waxing-waning or less specific waning discharges). Manual muscle testing was performed by a physical therapist. Overall, myotonia was more elicitable in DM1 than DM2; only in VL and TFL was myotonia more elicitable in DM2 than DM1. The major type of myotonia was waxing-waning in DM1, and waning in DM2. Four DM2 (24%), but no DM1 patients had only waning myotonia. In the arms, myotonia was distally predominant in both DM1 and DM2. In the legs, it was distally predominant in DM1, but both proximal and distal in DM2. The severity of myotonia was positively correlated with muscle weakness and with the presence of waxing and waning discharges in DM1, but with neither in DM2. Thus, myotonia is qualitatively and quantitatively different in DM1 than DM2. Except for proximal leg muscles, myotonia is more evocable in DM1 than DM2. It tends to be waxing-waning in DM1 but waning in DM2, thus making electrodiagnosis of DM2 more challenging. Its severity correlates with muscle weakness and the presence of waxing-waning discharges in DM1 but not DM2.

Myotonic dystrophy type 2 with focal asymmetric muscle weakness and no electrical myotonia

“…The absence of waxing amplitude with waning of only the frequency in DM2 makes the "electrodiagnosis of DM2 more challenging." 6 Finding only "fibrillation potentials" is the ultimate example of this problem.…”

Clinical Reasoning: Persistent respiratory failure following cardiac arrest

“…2 DM1 demonstrates more electrical myotonia (EM) than DM2. 4 The EM is the result of decreased expression of muscle-specific chloride channel type 1 due to abnormal RNA processing from toxic gain of function of transcribed DMPK repeat protein. 5 This results in membrane hyperexcitability, which can be curtailed by altering sodium channel opening frequency or duration, which is the basis for mexiletine therapy for myotonia.…”

Teaching Video Neuro Images : Trapezius myotonia percussion sign in myotonic dystrophy type 2

“…1 In comparison with myotonic dystrophy type 1 (DM1), myotonia is less symptomatic, more proximal, and harder to detect during clinical and electrodiagnostic testing. 2 Here we document the presence of trapezius myotonia in patients with DM2 (video on the Neurology ® Web site at www.neurology.org). In our experience, similar proximal percussion does not produce as marked a response in DM1 or nondystrophic myotonic disorders.…”