A Case of Myotonic Dystrophy with Electrolyte Imbalance
A 42-yr-old Korean male visited our hospital on August 1, 2011 because of intensified myalgia and muscle weakness. At the initial visit, he was 171.2 cm tall, weighed 65 kg, and had a body mass index (BMI) of 22.5 kg/m2, blood pressure of 120/80 mmHg, pulse rate of 80 beats/min, respiration rate of 20 breaths/min, and a temperature of 36.0℃. He was clearly conscious, but showed signs of overall tiredness and chronic illness. The patient exhibited symptoms of whole-body hair loss, and there were also observations of ptosis bilaterally. There were no indications of edema in the peripheral limbs; sensation in the upper and lower limbs was normal according to the neurologic examination, but his motor ability was slightly decreased. So an electromyogram (EMG) was performed. Myotonic discharge was observed, where small EMG waves (within 10 ms) continued to increase and attenuate in several muscles. This result is a characteristically observed in myotonic dystrophy, so the relevant DNA tests were performed. A PCT-Southern analysis was performed with a biotin-(CTG) 10 probe to investigate the abnormal expansion of (CTG)n. The results showed that the patient had abnormal amplification of 1,200 bp PCR, which indicated that the (CTG)n of the DMPK gene had been amplified to an approximate 300 recurrence rate. Based on these results, the patient was diagnosed with a classic form of type 1 mytonic dystrophy with more than 100 repeated sequence numbers.\nA peripheral blood test revealed a hemoglobin level of 13.5 g/dL, a white blood cell count of 5,220/µL, a platelets count of 177,000/µL. Results of a biochemical serum test except muscle enzymes were in the normal range, but showed evidence of both hypernatremia and hyperkalemia, with 148 mEq/L sodium, 5.8 mEq/L potassium. In the urinalysis, sodium excretion was 107 mM/L, potassium excretion was 10 mM/L, plasma osmolality was 288 mOsm/kg, and urine osmolality was 711 mOsm/kg, yielding a TTKG of 0.7. The patient's urine quantity was less than 1 L/d, and there was no water loss in the form of insensible loss. And it was only cured by normal saline hydration. So the hypernatremia was considered to be caused by primary hypodipsia.\nHyperkalemia is diagnosed mostly as a result of laboratory error, pseudohyperkalemia, acidosis, an increase in diet, hemolysis, and renal failure. Other than these causes, renin and aldosterone secretion, potassium secretion of the renal tubule and abnormalities in potassium absorption by extra-renal organs, can occasionally induce hyperkalemia (). The patient's TTKG was less than 5, which indicated a decrease in potassium excretion in the urine. A baseline morning endocrine profile revealed a normal fasting glucose, a renin activity of 5.6 ng/mL/h (normal 0.65-3.2 ng/mL/h), an aldosterone of 56.7 pg/mL (normal 50-194 pg/mL), a cortisol level of 23.6 pg/mL (normal 7-25 µg/dL). Renin activity in the blood was considerably elevated, indicating an endocrine abnormality. The plasma renin activity increased from 5.6 ng/mL/h before the stimulus to 7.9 ng/mL/h two hours after intravenous administration of furosemide (20 mg), while aldosterone showed a weak response from 239 pg/mL to 319 pg/mL. In the rapid ACTH stimulation test, both cortisol and aldosterone showed no response 30 and 60 min after a stimulus was applied. Especially aldosterone was decreased rather. Perturbation of the renin-angiotensin-aldosterone system resulted in appropriately enhanced renin activity but with a subnormal aldosterone response, which appeared to be due to adrenal hyporesponsiveness. And it has been corrected by hydrocortisone. So the cause of hyperkalemia was thought to be the selective hyperreninemic hypoaldosteronism.\nAfter hospitalization, the patient's myalgia improved as the potassium in the patient's blood returned to normal with the administration of diuretics, fluid treatment, and hydrocortisone. Thus, we determined that the myalgia and muscle weakness from DM1 had appeared as a form of secondary hyperkalemia through periodic paralysis due to selective hypoaldosteronism. Hypernatremia was considered to be caused by primary hypodipsia also treated by fluid therapy.