A woman with generalised weakness, hypokalaemia, and metabolic acidosis

A 31 year old woman presented with generalised weakness and muscle pain a week after having a flu-like illness, which had resolved completely. She had no diarrhoea, vomiting, urinary symptoms, joint pains, rashes, dry eyes, or dry mouth. Her medical history was unremarkable. Physical examination showed that she was well hydrated. Blood pressure was 102/55 mm Hg, pulse rate 88 beats/min. Muscle strength was 4/5 and 0/5 in the flexor muscle groups and 1/5 and 4/5 in the extensor muscle groups of the upper limbs and lower limbs, respectively. Deep tendon reflexes were brisk in the upper and lower limbs. No sensory deficit was found and the Babinski reflex test was negative. The rest of the physical examination was unremarkable.

Initial blood results were sodium 141 mmol/L (normal range 135-145), potassium 1.7 mmol/L (3.5-5.0), chloride 118 mmol/L (98-110), carbon dioxide 14 mmol/L (21-30), anion gap 11 mmol/L (10-20), urea 5.4 mmol/L (3.0-7.1), creatinine 69 µmol/L (60-130), creatine kinase 320 U/L (30-135). Urine results were pH 7.00 (5.0-8.0), sodium 47 mmol/L, potassium 13.2 mmol/L, chloride 52 mmol/L, calcium 1.50 mmol/L, and creatinine 3.9 mmol/L. Arterial blood gases on room air showed pH 7.31 (7.35-7.45), partial pressure of carbon dioxide 26 mm Hg (33-45), partial pressure of oxygen 111 mm Hg (75-100), bicarbonate 15 mmol/L (22-26), and base excess −12.1 mmol/L (−2.5 to 2.5). Erythrocyte sedimentation rate was 106 mm in the first hour (0-20). Complete blood count, thyroid function tests, and liver panel were normal.

She had a polyclonal increase in gammaglobulins 24.5g (6.0-18.0), IgG of 27.90 g …