Abducens neuromyotonia: a delayed neuro-ophthalmic complication of cranial radiation

Demystifying the spontaneous phenomena of motor hyperexcitability

Possessing a discrete functional repertoire, the anterior horn cell can be in one of two electrophysiological states: on or off. Usually under tight regulatory control by the central nervous system, a hierarchical network of these specialist neurons ensures muscular strength is coordinated, gradated and adaptable. However, spontaneous activation of these cells and their axons can result in abnormal muscular twitching. The muscular twitch is the common building block of several distinct clinical patterns, namely fasciculation, myokymia and neuromyotonia. When attempting to distinguish these entities electromyographically, their unique temporal and morphological profiles must be appreciated. Detection and quantification of burst duration, firing frequency, multiplet patterns and amplitude are informative. A common feature is their persistence during sleep. In this review, we explain the accepted terminology used to describe the spontaneous phenomena of motor hyperexcitability, highlighting potential pitfalls amidst a bemusing and complex collection of overlapping terms. We outline the relevance of these findings within the context of disease, principally amyotrophic lateral sclerosis, Isaacs syndrome and Morvan syndrome. In addition, we highlight the use of high-density surface electromyography, suggesting that more widespread use of this non-invasive technique is likely to provide an enhanced understanding of these motor hyperexcitability syndromes.

Surgery option in the management of delayed diplopia after radiation therapy for nasopharyngeal carcinoma

PURPOSE

Radiation therapy is a standard treatment for nasopharyngeal carcinoma. Diplopia due to radiation damage to the sixth nerve significantly erodes the patient's quality of life. This study investigated the effectiveness of extraocular surgery in the treatment of delayed diplopia caused by radiation therapy.

METHODS

A retrospective case series of 16 patients (7 men and 9 women) with delayed diplopia after radiation therapy for nasopharyngeal carcinoma was enrolled in the study. Unilateral lateral rectus resection was performed under topical anesthesia. Follow-up time was more than 12 months. Outcome measures were prism diopter and self-reported symptoms.

RESULTS

All patients diagnosed with sixth nerve palsy reported elimination of symptoms on postoperative day 1 without complications. One patient required a second procedure due to recurrence of symptoms. At 12-month follow-up, no patient reported recurrence of symptoms. The absolute horizontal deviation significantly decreased from a preoperative value of 16 prism diopter to a value of 1.5 prism diopter postoperatively (p < 0.001).

CONCLUSION

These results suggest that unilateral lateral rectus resection under topical anesthesia is an effective treatment for delayed diplopia after radiation therapy for nasopharyngeal carcinoma. A large randomized prospective study to confirm these findings is warranted.