Fluoxetine blocks myotonic runs and reverts abnormal surface electromyogram pattern in patients with myotonic dystrophy type 1

Objectives: To verify the effects of a muscular injection of fluoxetine both on needle electromyogram (EMG) "myotonic runs" and on the surface EMG pattern in patients affected by myotonic dystrophy type 1. Methods: Needle EMG recording: We performed needle EMG recordings on the tibialis anterior or opponent thumb muscle in 3 patients. The resting electrical activity and the myotonic discharge were detected before and after the local injection of 100 μL of fluoxetine. Surface EMG recording: A motor point stimulation protocol was carried out on the tibialis anterior of 3 patients. Stimulation consisted of 10-second, 15-Hz pulse train, 0.1 ms in duration. A supramaximal stimulation was applied, and the surface myoelectric signal was recorded. The averaged rectified value (ARV) of the amplitude was evaluated before and after the intramuscular injection of 300 μL of fluoxetine. Results: Needle EMG: The injection of fluoxetine induced a clear-cut reduction of the basal electrical activity and made it impossible to evoke "myotonic runs" in all the patients tested. The reversibility of the effect of the drug was checked in 2 patients who exhibited a partial recovery of myotonic EMG activity 40 minutes after the administration. Surface EMG: The patients showed the typical decreasing ARV pattern before the drug administration; the fluoxetine injection consistently provoked a clear and complete recovery of the normal increasing ARV curve. Conclusions: We showed, for the first time, that the local application of fluoxetine produces functional modifications in myotonic dystrophy type 1 muscle electrical properties. The relevance of this study consists in the introduction of fluoxetine, a well-known and largely used drug, as a tool for investigating further therapeutical approaches in this disease.