An 11-year-old girl presented to the emergency room with flaccid paralysis of the lower extremities, to the point where she could neither walk nor sit up in bed.1 Symptoms had begun one hour earlier. Parental interview revealed that she had experienced similar symptoms intermittently over the past eight years. At first, the attacks had occurred about twice a week and lasted only a few minutes. Recently, the attacks had increased in frequency so that now they occurred three to four times every week and lasted two to three hours.
The attacks were characterized by weakness mainly in the lower extremities that often started upon waking in the morning. Neither potassium-rich foods, cold temperatures, nor fatigue were not known to trigger the attacks. Her mother, brother, and sister had experienced similar attacks.
The girl was admitted to the hospital for testing and observation. Two hours after hospitalization, her symptoms spontaneously remitted.
Physical examination and laboratory findings
The child’s neurological exam was notable for symmetrical decreased muscle strength and absence of deep tendon reflexes in both lower extremities. No sensory impairments, grip or percussion myotonia were noted. Her potassium levels and brain MRI scan were normal.
Genetic sequencing of the entire coding region of the SCN4A gene in all symptomatic family members and asymptomatic grandparents identified a missense mutation (p.Thr704Met) in the affected family members. The mutation was not found in unaffected grandparents, revealing that it had occurred de novo in the mother.
The diagnosis was normokalemic periodic paralysis, a rare genetic disorder inherited in an autosomal dominant fashion. Periodic paralysis causes periodic muscle weakness, often caused by variations in serum potassium levels. Primary periodic paralysis is categorized into three types: hypokalemic PP, hyperkalemic PP, and normokalemic PP.
1. Akaba Y, Takahashi S, Sasaki Y. Successful treatment of normokalemic periodic paralysis with hydrochlorothiazide. Brain Dev. 2018;40:833-836.