A 47-year-old man presented with a longstanding history of recurrent transient episodes of muscular weakness that had started in childhood. Attacks occurred monthly, lasted about one week, and appeared to be triggered by prolonged exercise. Between attacks, the patient was asymptomatic. A review of his medical history revealed cardiac arrythmia of unknown cause that was currently being worked up. Family history was unremarkable.
Physical examination and laboratory findings
His physical exam was notable for a broad forehead, retrognathia (small, posterior positioned mandible), and syndactyly (fusion) of the second and third toes. Blood tests, including thyroid function, potassium and creatine kinase, were all within normal limits. ECG was notable for prominent U waves, with sinus rhythm and a normal QT interval. Genetic screening identified a heterozygous mutation in the KCNJ2 gene.
The patient returned during an attack for nerve conduction and needle EMG studies. Results were notable for marked asymmetric weakness of the tibialis anterior muscle, worse on the left than on the right. Compound muscle action potential (CMAP) amplitude was reduced in the extensor digitorum brevis muscle (also worse on the left than on the right), and the right ulnar nerve. The latter was unchanged after brief exercise but showed a major reduction lasting 30 minutes in response to long exercise testing. Motor unit potential (MUP) was myopathic in the left tibialis anterior, but normal in first interosseous and right tibialis anterior. Neurological exam was notable for weak but intact deep tendon reflexes.
Two months later, the patient returned for follow-up neurophysiological testing. At this time, he was not experiencing an attack, and results showed normal CMAP amplitude of the right ulnar nerve, which was unchanged after long exercise testing. The MUP of the left tibilais anterior had also normalized.
What is the most likely diagnosis?
A. Hypokalemic periodic paralysis
B. Hyperkalemic periodic paralysis
C. Normokalemic periodic paralaysis
D. Andersen-Tawil syndrome
1. Falcão de Campos C, de Carvalho M. Neurophysiologic characterization of periodic paralysis episode in a patient with Andersen-Tawil syndrome. Clin Neurophysiol. 2018;129:558-559.