Extreme hypokalaemia in familial periodic paralysis – case report

Summary

Periodic paralysis (PP) is a rare neuromuscular disorder characterized by episodic muscle weakness with abnormal levels of potassium in the blood.

We present a case report of a 17-year-old boy with a familial form of hypokalaemic periodic paralysis. The disease manifested as caudocranially progressive symmetrical muscle weakness of the limbs with rapid onset in the early morning hours shortly after awakening. Three days prior, the boy had noticed increased tiredness of the lower limbs. His past medical history was unremarkable. Extremely low potassium was observed in the laboratory (1.1 mmol/l; normal 3.3–4.7 mmol/l) with concomitant marked hypophosphatemia (0.43 mmol/l; normal 0.94–1.55 mmol/l), normal magnesaemia (0.80 mmol/l; normal 0.62–0.91 mmol/l), normal natremia and chloridaemia (Na 139 mmol/l; Cl 108 mmol/l). The parameters of the acid-base balance were undisturbed. The markers of muscle metabolism (creatine kinase and transaminases) were normal, myoglobin was slightly increased. With the therapeutically achieved normalization of potassium, the above-mentioned clinical symptoms resolved within three hours.

Upon exclusion of the secondary causes of hypokalaemia (especially thyrotoxicosis and renal tubular disorders) the diagnosis of hypokalaemic periodic paralysis was suspected. Molecular genetic testing detected a heterozygous point mutation in SCN4A gene located on chromosome 17 (17q23), encoding the alpha-subunit of the sodium channel, thus confirming the diagnosis of familial hypokalemic periodic paralysis of type 2. The identical pathogenic variant was found in the asymptomatic father of the patient.

Conclusion: The diagnosis of primary PP is based on the characteristic clinical presentation and confirmed by genetic testing. The absence of previous spontaneously receding episodes of muscle weakness in the family or personal history does not exclude the primary (familial) form of PP.

Keywords:

hypokalaemia – muscle weakness – periodic paralysis