Clinical Reasoning: A 73-Year-Old Woman With Episodic Dysarthria and Horizontal Binocular Diplopia

A 73-year-old woman presented with transient episodes of dysarthria and horizontal diplopia. She had stereotactic radiosurgery 18 years prior for a retroclival meningioma. Neurologic examination was notable for right-sided tongue deviation, tongue fasciculations, and intermittent impaired abduction of the right eye. MRI ruled out recurrence or progression of the retroclival meningioma. EEG failed to reveal electrographic seizures. EMG showed spontaneous depolarizations in bursts that sounded like "marching soldiers" in the right hemitongue, consistent with myokymia. Focal myokymia is an unusual EMG finding that is usually seen in demyelinating disorders, after radiation, or in neoplastic/inflammatory conditions. The clinical presentation and EMG findings were most consistent with delayed radiation-induced myokymia. Similar cases of transient dysarthria and tongue myokymia from radiation have been infrequently reported in the literature; however, this case uniquely exhibited additional episodes of transient horizontal diplopia, which was possibly from ocular myokymia or neuromyotonia. Although there are limited data, sodium channel inhibitors (e.g., carbamazepine, oxcarbazepine, and lacosamide) have shown some success to provide symptomatic relief, most likely secondary to their ability to inhibit underlying peripheral nerve hyperexcitability. Our patient was started on lacosamide 50 mg twice a day with a notable decrease in symptom frequency. This case illustrates the importance of detailed clinical and electrodiagnostic studies in making the diagnosis of delayed radiation-induced myokymia with episodic dysarthria and provides guidance on potential therapeutics.