Keywords

Cancer; Myokymia; Neuromyotonia; Radiation; Tongue

Introduction

Myokymia is a clinical phenomenon presenting as continuous involuntary undulation or rippling of the muscles with electrophysiological evidence of myokymic discharges. Radiation therapy can affect both central and peripheral nervous system, with the latter being much more common, often presenting as brachial or lumbosacral plexopathy with myokymia. Myokymia secondary to post-irradiation cranial neuropathies is much less common, with only few published reports. We report a case of delayed post-irradiation myokymia and neuromyotonia, confirmed electrodiagnostically, in patient with history of tonsillar cancer.

Case Report

A 62-year-old male presented with a two-year-history of progressive dysarthria, dysphagia, and limited motion of the tongue in all directions. He received radiation therapy for tonsillar cancer 10 years prior. Neurological exam showed dysarthria and lingual atrophy, especially on the right, with continuous irregular undulating movements in the right side of his tongue. Fasciculations were also noted in the left upper trapezius muscle. The rest of the neurological examination was unremarkable. Needle electromyography (EMG) of the tongue revealed myokymic and neuromyotonic discharges, but no fibrillations, positive sharp waves, or fasciculations. Needle EMG of the right upper and lower limb muscles was normal. Neck and head MRI were also unremarkable, ruling out recurrence of the cancer.

Discussion and Conclusion

Myokymia is a spontaneous repetitive discharge of one or a few motor units at regular intervals. It presents clinically as a continuous and irregular wave-like rippling of the skin or mucosa overlying the affected muscles. Neuromyotonia is a spontaneous burst of continuous electrical discharge firing at a frequency up to 300 Hz, lasting up to seconds, rhythmical, and with an abrupt onset and offset. Post-irradiation myokymia and neuromyotonia of the tongue is extremely rare and seldom reported in literature [1-3]. As reported in this case, patient presented with dysarthria, dysphagia, and tongue atrophy with continuous undulating movements of the tongue (Figure 1) (Video 1).

Figure 1: Tongue atrophy (A) Most prominent on the right (B) Patient attempting to protrude his tongue forward (C) To the right and (D) To the left.

Some patients display facial myokymia [2]. Myokymia and neuromyotonia may present several months or years after irradiation with interval between 10 months [4] to 14 years [5]. Earlier onset of symptoms may be related to more direct radiation exposure or to higher dose of radiation applied. In 4 of the 6 reported cases, myokymia and neuromyotonia followed the treatment for nasopharyngeal carcinoma [1-5], in one other case – radiation therapy for cervical melanoma [6] and in another – irradiation of the middle ear chemodectoma [7]. In all cases, including current report, needle EMG showed myokymia and neuromyotonic discharges, but no fibrillations or positive sharp waves (Video 2). Fasciculations were reported in one case [8]. Brain MRI with contrast did not show any evidence of tumor recurrence, suggesting irradiation as the most probable etiology. Radiation therapy may injure the hypoglossal nerve and the myokymia and neuromyotonia may result from motor neuron hyperexcitability with ectopic discharges, which reflects the nonspecific neuronal response to injury. Both myokymia and neuromyotonia are thought to originate along segments of the motor axon and are presumed to derive from spontaneous depolarization or ephaptic transmission along segments of demyelinated nerve, secondary to altered membrane excitability [9]. Damage of the peripheral nerve voltage gated K+ channels was also suggested [8]. The post-irradiation hypoglossal nerve damage is probably a process similar to that described in post-irradiation brachial plexopathy [1]. Delayed radiation induced nerve damage is thought to be due to either direct damage to the nerve cells, or alternatively – injury to the vascular endothelium with resultant ischemia, fibrosis and secondary compromise of the neural tissue [10]. Myokymic discharges after radiation therapy indicate radiation-induced neuropathy rather than tumor recurrence or metastatic spread, which makes electrodiagnostic studies valuable in differential diagnosis [3]. Other conditions to consider in differential diagnosis for bulbar myokymia are listed in Table 1. Myokymia sometimes responds to anticonvulsants such as Carbamazepine [4]. To our knowledge, this is the first case of post-irradiation tongue myokymia and neuromyotonia after treatment of tonsillar cancer.

Table 1: Etiology of Bulbar Myokymia.

References

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