Polyuria and polydipsia — formative SAQs

SAQ 1 (10)

A 4-month-old boy presents with poor weight gain, irritability, intermittent fever and constipation. His mother reports he drinks voraciously and has soaked through nappies throughout the day and night. A maternal uncle had the same problem in infancy. On examination his weight is below the third centile and he is clinically dehydrated. A venous blood gas shows a serum sodium of 158 mmol per litre with a serum osmolality of 325 mOsm per kg; a urine osmolality is 110 mOsm per kg. His blood glucose is normal. ^[9][3]

1. Define polyuria and explain why the blood glucose and urine osmolality place this child on the water-diuresis rather than the osmotic-diuresis side of the diagnostic fork. (3) ^[1][10]
2. What is the most likely diagnosis, and why is the family history and sex of the child relevant? (3) ^[9][3]
3. Outline the immediate and definitive management, including the rate of sodium correction and the drug regimen. (4) ^[12][9]

Model answer

Definition and diagnostic fork. Polyuria is a urine output over 2 litres per square metre per day or over 40 to 50 mL per kg per day, and the polyuria-polydipsia syndrome pairs it with compensatory thirst. The first branch of the algorithm separates osmotic from water diuresis: this child's blood glucose is normal, excluding the osmotic diuresis of diabetes mellitus (the commonest cause of polyuria in childhood). His urine osmolality is 110 mOsm per kg — well below the 300 threshold that defines a water diuresis — paired with a raised serum osmolality of 325, confirming that the kidney cannot concentrate urine despite a hypertonic serum. That places him firmly on the water-diuresis side of the fork, opening the diabetes insipidus pathway. ^[1][10]

Diagnosis and relevance. The most likely diagnosis is congenital nephrogenic diabetes insipidus (arginine vasopressin resistance). The combination of an infant boy, onset in the first months of life, failure to thrive, hypernatraemic dehydration and a maximally dilute urine with a maternal family history is the classic presentation of the X-linked AVPR2 defect, which accounts for about 90 percent of inherited nephrogenic disease and therefore affects mostly boys; the autosomal AQP2 form makes up roughly the remaining 10 percent. The intact axis is being stimulated by the high serum osmolality but the kidney cannot respond, so the urine stays dilute — distinguishing it from central disease, where vasopressin is absent. ^[9][3]

Management. Immediate management is resuscitation of the hypernatraemic dehydration: secure the airway, restore intravascular volume with isotonic saline boluses of 10 to 20 mL per kg, then replace the free-water deficit slowly — no faster than 0.5 mmol per litre per hour, to a maximum of 10 to 12 mmol per litre over 24 hours — to avoid cerebral oedema, with frequent sodium monitoring and millilitre-for-millilitre replacement of ongoing urinary losses. Definitive management of nephrogenic disease is a low-solute (low-sodium, low-protein) diet to reduce obligate water loss, plus a thiazide diuretic — which paradoxically reduces urine volume through mild volume depletion and increased proximal reabsorption — combined with amiloride or indometacin to further enhance concentrating ability. Desmopressin has no role, because the defect is downstream of the V2 receptor. Genetic confirmation with AVPR2 and AQP2 testing guides counselling, and the family is taught to ensure adequate free water and to seek help early with intercurrent illness. ^[12][9]