Serum potassium exercise test in the diagnosis of familial periodic paralysis.

We studied the serum potassium rise during a 30-min 100 W exercise test in 5 controls, 3 familial periodic paralysis (FPP) patients and their 2 symptom-less brothers. The controls showed a steep and high exercise response (the 10-min rise was 0.59 +/- 0.13 mmol/l and the maximal rise was 0.71 +/- 0.11 mmol/l). The FPP patients showed a slow and flat exercise response (the 10-min rise was 0.08 +/- 0.11 mmol/l and the maximal rise was 0.17 +/- 0.12 mmol/l, P less than 0.001 for both values). The 2 symptom-less brothers showed a similar divergence. The values of 0.05 mmol/l (10-min) and 0.25 mmol/l (maximal) of one brother suggest the diagnosis of FPP, whereas the values of 0.55 (10-min) and 0.65 mmol/l (maximal) of the other brother suggest the exclusion of the disease. We regard the serum potassium exercise test to be worthy of further study as a diagnostic test of FPP.