## 3 Results
### Myotonic Disorders  
Myotonic Disorders (MD) belong to muscle membrane excitability diseases. Patients report muscle stiffness, pain and muscle weakness, which may be permanent, as in those with dystrophic muscle biopsy changes, or intermittent, as in those without. Muscle membrane excitability is regulated by ion conductance during resting and action potential. Alteration in sodium, chloride and potassium net flow will result in a muscle membrane hyperexcitable state resulting in delayed muscle relaxation after contraction or percussion [6]. In dystrophic MD, type 1 and 2, muscle weakness is constant and progressive. In non-dystrophic MD, weakness is periodic, precipitated by cold, fasting, and exercise or rest after exercise. Decremental responses in low frequency RNS are encountered in congenital non-dystrophic MDs (i.e., Thomsen and Becker myotonia congenita, a chloride channelopathy, caused by mutations in the CLCN1 gene on chromosome 7q), reflecting fatigue occurring after exercise. Short and long exercise tests also result in decline in CMAP amplitude [27]. Sodium channelopathies, like paramyotonia congenita and hyperkalemic periodic paralysis (_SCN4A_ gene defect on chromosome 17q), also show decrement in CMAP amplitude after long exercise tests [5] and after 5 Hz repetitive stimulation [28]. In these cases, muscle weakness may not be evident until a provocative factor occurs, and exercise in one of them. In contrast to patients with dystrophic muscular dystrophies, where persistent muscle weakness precludes participation in competitive sports, in cases where paralysis is periodic, participation in sports may reveal an underlying muscle disease that was not clinically apparent until then. We have identified four publications reporting on five patients, and we have also assessed an additional one. All patients were referred for muscle stiffness and/or periodic weakness occurring during sports activity. The first study [14] reports two cases of competitive hockey players, 16 and 20 years old, who complained of muscle stiffness but not of muscle weakness. The first patient showed abundant myotonic discharges in Electromyography (EMG), while the other did not. Neither RNS nor the exercise test was described. Consultation from a muscle specialist was sought. The 16-year-old was diagnosed with myotonia congenita (Thomsen) and the 20-year-old with Myotonia fluctuans. They both received Mexiletine and were able to continue hockey at a high competence level. The second case [15] describes a 15-year-old athlete who played lacrosse, football, and basketball and who complained of stiffness in the neck and upper extremities. Symptoms developed insidiously during brief periods of rest after 10 to 20 min of activity. EMG revealed myotonic discharges in all limbs. The five-minute exercise test was normal. Clinical diagnosis was compatible with myotonia congenita, that was later confirmed by DNA testing. The patient was treated with carbamazepine and quinine that alleviated his symptoms and he was encouraged to continue with his sports activity, which he did, without any limitations. The fourth patient was a 19-year-old who described painful cramps and stiffness when he ran or participated in sports [16]. Symptoms were triggered after bursts of movements and lessened as activity continued. EMG revealed myotonic discharges. RNS or exercise test results were not described. The patient denied genetic testing. A diagnosis of myotonia congenita was established based on clinical and neurophysiological characteristics. He received carbamazepine and his symptoms resolved. The last published case [17] was about a 18-year-old runner, who insidiously developed painful cramps and muscle tightness that developed after strenuous exercise (over 20 min), aggravated when he continued the activity and resolved with rest. Symptoms were also provoked by cold. Myotonic discharges were evident in all limbs. RNS or exercise test results were not mentioned. He was treated with phenytoin and advised not to run over 50 miles per week.