• The number of attacks/week over the last 8 weeks of the initial 9-week study phase. [ Time Frame: 8 weeks ] [ Designated as safety issue: Yes ]
  

• Efficacy: severity-weighted attack rate; muscle strength and mass measures; changes in health-related quality-of-life; intolerable increase in attack frequency or severity necessitating withdrawal from the 9-week phase (HOP trial only). [ Time Frame: 8 to 61 weeks ] [ Designated as safety issue: No ]
  

Periodic paralysis is a relatively rare, life-long disorder characterized by intermittent bouts of paralysis, progressive weakness, and diminished quality of life. Two drugs, acetazolamide and dichlorphenamide, have been prescribed to treat the disorder, however, dichlorphenamide is no longer available.

In this multi-center, parallel, randomized trial researchers will compare the effects of dichlorphenamide vs. placebo in patients with hyperkalemic (HYP) and hypokalemic (HOP) periodic paralysis.

The trial consists of two 9-week studies—one study will enroll persons with hyperkalemic periodic paralysis and the other study will enroll persons with hypokalemic periodic paralysis. Participants will be randomly assigned to one of two treatment groups: dichlorphenamide or placebo (an inactive substance). During the studies, participants will be asked to keep a daily computer diary to record the time, length, and severity of each episode of weakness. The study coordinator will contact participants weekly to review the diary information.

The 9-week phase will be followed by a 1-year open-label dichlorphenamide extension without placebo to determine the long-term effects of dichlorphenamide on the course of the disease and on inter-attack weakness.

Duration of the trial for participants is approximately 65 weeks, including a screening phase to determine eligibility, the first 9-week treatment phase, and the one-year open-label extension phase.