Differential diagnosis of periodic paralysis aided by in vitro myography

In vitro twitch tests were performed on excised muscle bundles from 30 periodic paralysis (PP) patients in an attempt to verify the somatic origin of PP, and to differentiate between the hypokalemic (HypoPP) and the hyperkalemic forms (HyperPP). Seventeen PP patients with a definite diagnosis of familial HypoPP, familial HyperPP (subsequently confirmed by SCN4A mutations), or thyrotoxic PP entered the study, as well as 13 patients with a history of attacks of weakness but with negative clinical provocation tests and therefore ambiguous diagnosis; 15 normal subjects served as controls. In contrast to control, bundles from patients with clear diagnosis went into sustained paralysis on exposure to Cl-free solution. Exposure to K⁺ channel activators induced a large increase in force. Specifically for HypoPP muscle, low extracellular [K⁺] decreased twitch force which was further reduced by addition of insulin or adrenaline, whereas HyperPP bundles responded with an irreversible decrease in twitch force when extracellular [K⁺] was elevated. Out of the 13 patients with unclear diagnosis, the in vitro studies made it possible to classify 10 as HypoPP and one as HyperPP (later confirmed by a M1592V mutation). In the remaining two patients who claimed to suffer from paralytic attacks, all in vitro tests were normal, questioning the occurrence of dyskalemic PP. The results demonstrate that in vitro tests can be used to ensure the proper diagnosis to a high percentage when clinical provocative tests have failed.