Paramyotonia congenita is an autosomal dominant sodium channelopathy, caused by mutations in gene coding for muscle voltage-gated sodium channel alpha subunit.We report the case of a 38-year-old man who described since childhood muscle stiffness with attacks ok weakness induced by two provocative stimuli: cold exposure and exercise. It primarily concerned eyelids and hands, occasionally limbs. Family history suggested an autosomal dominant mode of transmission. Clinical examination revealed myotonia at the thenar eminence percussion. Generalized myotonic discharges were observed on electromyography. Molecular diagnosis reported an Arg1448Cys mutation in exon 24 in gene coding for muscle voltage-gated sodium channel alpha subunit (SCN4A) in chromosome 17.Paramyotonia congenita is not evolutive. Treatment is essentially preventive. Some medications could be proposed: membrane stabilizing agents like antiarrhythmic drugs (mexiletine, tocainide), or the carbonic anhydrase inhibitor (acetazolamide). Precautions may be taken during general anaesthesia because of diaphragm myotonia risk.