[Hypokalemic periodic paralysis: exercise test]. / Paralisis periódica hipopotasémica: prueba de ejercicio.

INTRODUCTION: Periodic paralyses are illnesses related with the dysfunction of the ionic pathways of the muscle fibre membrane. Neurophysiological diagnosis by means of a conventional electromyogram (EMG) is rather unsatisfactory when exploring patients in the asymptomatic phase, since they often display unspecific changes. Furthermore, it does not identify the alterations in muscular excitability. The exercise test consists in determining the variation in the area or the amplitude of the muscular action potential, which is obtained through electrical stimuli that are repeated periodically for several minutes after having carried out local exercise with the explored muscles. It is a simple technique that enables us to achieve a significant reduction in the area or amplitude of the action potential (more than 40% or 50% with regard to the basal value) in certain diseases produced by ionic pathway dysfunction, while the patient is in the asymptomatic phase. CLINICAL CASES: We report the cases of three patients diagnosed as suffering from hypokalemic periodic paralysis. During the asymptomatic phase, the EMG revealed characteristic myopathic changes in only one case and unspecific alterations in the other two. Nevertheless, the exercise test that was also performed during the asymptomatic phase showed a characteristic pathological response in the three patients. CONCLUSION: The exercise test is a technique that indirectly shows up the progressive loss of muscle fibre membrane excitability and helps diagnosis in asymptomatic patients.