The exercise test in Andersen syndrome

Background: Andersen syndrome is a rare form of periodic paralysis (PP) associated with dysmorphic features and potentially fatal cardiac dysrhythmias. To date, no electrodiagnostic abnormalities have been reported that can be used to confirm the presence of PP in this condition. Objectives: To determine if the exercise test could be used to confirm the diagnosis of PP in Andersen syndrome. To evaluate the exercise test as a means to assess neuromuscular status during treatment. Methods: We performed the exercise test on 2 patients with Andersen syndrome. In 1 patient, we used a modified version of the test to document responsiveness to treatment with tocainide. Results: Studies in both patients demonstrated a progressive decline in the compound muscle action potential amplitude after exercise that was characteristic of the phenomenon seen in other forms of PP. In 1 patient, improvement in interattack strength and a reduction in the number of attacks of weakness correlated with improvement in the test results. Conclusions: Our cases demonstrate that the exercise test can confirm the diagnosis of PP in Andersen syndrome. A modified version of exercise testing may also be considered as an objective method for documenting treatment responses in PP.