Periodic muscle weakness usually involving the axial musculature is the result of potassium metabolic aberrations. Epidemiological studies of various families reflect an autosomal dominant pattern of inheritance. However, sporadic cases are not infrequent. Sudden universal muscle weakness may also be encountered in akinetic epilepsy and cataplexy. But other features i.e. myoclonus, EEG changes, narcolepsy are sufficiently constant markers to differentiate these problems from hyper/hypokalemic period paralysis.