Channelopathies occur due to mutations in genes coding for voltage-gated ion channels, leading to abnormalities in skeletal muscle excitability. Here, we report two cases of hypokalemic periodic paralyses (HPP), one hereditary form and another secondary to diabetes mellitus.
Case 1. A 26-year old male presented with both upper and lower extremities weakness. While receiving conservative treatment for acute pancreatitis after binge drinking, sudden dyspnea occurred and he was diagnosed of metabolic acidosis due to uncontrolled diabetes. He received intravenous insulin and bicarbonate supply therapy in the intensive care unit. After he was transferred to the general ward, upper and lower extremities weakness was observed with MRC scale of 2/5 in upper and lower extremities. Sensory was intact, but deep tendon reflexes were hypoactive. Motor conduction study revealed delayed latencies, low amplitudes and decreased conduction velocities but sensory conduction study was normal. No motor units were noticeable in the left deltoid, but normal motor units were noted in all the other muscles examined. The electrophysiologic findings did not satisfy the criteria for diabetic polyneuropathy, but together with clinical findings, the results were suspicious of HPP (Table 1).
Case 2. A 63-year old male presented with both upper and lower extremities weakness of abrupt onset. On initial admission, MRC scale was 2-3/5 in upper and lower extremities, but no sensory impairments were detected. Initial potassium level was reported 2.1 mmol/L. After intravenous potassium replacement, potassium level recovered and motor strength was restored to normal. He had a family history of hypokalemic periodic paralysis: his son had previously been admitted for hypokalemia. Electrodiagnostic study was performed when his potassium level was normal. Motor and sensory conduction study was normal. Needle EMG revealed small polyphasic motor unit potentials with reduced recruitment patterns in the left deltoid and biceps brachii muscles, but normal motor unit potentials were noted in all the other muscles examined (Table 2). Because his clinical findings were highly suggestive of HPP, long exercise test was additionally performed. 14.2% increment and 34.8% decrement of amplitude, and 32.2% increment and 57.5% decrement of area was noted. Thus, with supportive long exercise test, the patient was diagnosed with probable HPP (Fig. 1).
The timing of electrodiagnostic study is important because if the patient recovered from hypokalemia and no longer complains of motor weakness, the routine electrodiagnostic findings can be normal. Exercise test is a technique to indirectly measure the progressive loss of muscle fiber membrane instability, and the sensitivity increases in symptomatic patients or those with recent symptoms. If routine electrophysiologic findings do not confirm HPP while the clinical findings highly do suggest so, long exercise test is useful in the diagnosis. |
