A 24-year-old Pakistani woman presented one day after returning from a 6-week holiday in Pakistan with a 3-day history of generalised weakness, difficulty in walking and left flank pain. She was known to have mixed connective tissue disease (MCTD). Clinically she was pyrexial and had generalised muscular weakness (power 3/5), and hypotonia. Biochemically, she was found to have hypokalaemia (1.8 mmol/l), raised serum urea (8.1 mmol/l), and creatinine (160 μmol/l), high ESR (106 mm/h), hypocalcaemia (1.96 mmol/l) and normal serum sodium. She had a metabolic acidosis (pH 7.14, HCO3 11.2 mmol/l and a base deficit −16.8) with a normal lactate and pCO2 and a normal anion gap of 15.5 mmol/l (1220 mmol/l). Urine pH was 7.1. A diagnosis of distal renal tubular acidosis (RTA) was made in the view of hypokalaemic acidosis and an inability to acidify the urine. Additionally she was polyuric. The urine osmolality was 135 mOsm/kg and plasma osmolality 296 mOsm/kg. After DDAVP, urine osmolality was 185 and plasma osmolality 305. A diagnosis of nephrogenic diabetes insipidus possibly secondary to the potassium and calcium abnormalities was made. Further questioning revealed a history of intermittent diarrhoea, and further investigations showed vitamin D deficiency (3.8 μg/l, normal >20 μg/l) and a duodenal biopsy showed histological evidence of coeliac disease. The patient improved with antibiotics and electrolyte/ fluid replacement, initially in a high dependency unit (HDU). The patient was started on gluten-free diet, KHCO3, Sando-K and ergocalciferol.
MCTD can be complicated by distal RTA, in turn resulting in the development of hypokalaemia and nephrogenic diabetes insipidus. The autoimmune disease was also associated with coeliac disease which may have contributed to vitamin D deficiency and hypocalcaemia.
This case highlights that multiple metabolic complications can develop in the context of autoimmune disease. Such complications can be life-threatening and precipitate acute hospital admission.