B80-1-04 Myotonia Congenita: A Rare Chloride Channelopathy Leading to Neuromuscular Ventilatory Dysfunction

Abstract Introduction Skeletal muscle channelopathies are a group of rare genetic neuromuscular diseases that have underrecognized respiratory implications. Mutation of the gene that encodes the chloride channel found in skeletal muscle leads to a condition known as myotonia congenita: a type of non-dystrophic myotonia in which there is excessive muscle stiffness and delayed relaxation after muscle contraction. This report presents a rare case of this chloride channel neuromuscular disease manifesting as dyspnea. Case Presentation A 58-year-old male of Scandinavian descent presents with muscular hypertrophy and chronic muscle fatigue with no increase in physical activity. He experiences frequent neck spasms, ocular muscle tension, and excessive teeth grinding. Laughter triggers a sensation of a valve closing in his airway leading to a feeling of breathlessness and near syncope. His symptoms are mildly alleviated by movement. He has frequent flares that are inadequately controlled with corticosteroids. Pulmonary function testing (PFT) shows low forced vital capacity (FVC) out of proportion to total lung capacity (TLC), low maximum voluntary ventilation (MVV), low expiratory reserve volume (ERV), and elevated residual volume (RV). This respiratory physiology pattern was suspicious for underlying neuromuscular weakness as a cause for his chronic shortness of breath. PFT also revealed flattening of the inspiratory limb of the flow volume loop, which raised concern for airway obstruction due to vocal cord dysfunction. Otolaryngologic evaluation revealed normal anatomical vocal cords with paradoxical movement during respiration. Additional investigation of possible underlying neuromuscular disease was completed, and muscle biopsy and genetic testing revealed mutation of the chloride channel protein 1 (CLCN1) seen in myotonia congenita. Discussion/Conclusion Genetic channelopathies are a rare subset of neuromuscular disease that result in motor dysfunction and impaired ventilation. The extrapulmonary symptoms of muscle fatigue and spasms relieved with movement (“warm up phenomenon”) are a known characteristic of myotonia congenita. However, the pulmonary symptoms of this rare condition were relatively unknown and prior to this case presentation were not well-characterized. This novel report highlights the respiratory symptoms of chronic dyspnea demonstrated by neuromuscular disease pattern on pulmonary function testing in addition to upper airway obstruction confirmed by visualization of paradoxical vocal cord movement. Knowledge of the respiratory mechanics seen in myotonia congenita can lead to expedited pulmonary evaluation and management. Current pharmacological management of channelopathies involves the use of ion channel blockers and anti-epileptic medications for symptom control; further research is needed for targeted gene therapies. This abstract is funded by: None