Endocrine Abstracts

A 6 month boy with chronic vomiting and severe weight faltering (birth weight 50th to 75th centile dropped to 0.4th centile) originally attributed to gastro-oesophageal reflux was admitted after a period of poor urine output and found to have severe hypernatraemia (Na 168 mmol/l, K 4.1 mmol/l, Urea 16.2 mmol/l, Creatinine 54 umol/l) with high plasma osmolality (330 mosm/kg) and inappropriately low urine osmolality (130 mOsm/Kg). Renal USS was normal with slightly small kidneys.

Thyroid function tests were TSH 2.7 miu/l & free T4 9.2 pmol/l suggesting a pituitary problem and so hydrocortisone, thyroxine and DDAVP were commenced. Urine output was high at up to 8 ml/kg per h even after DDAVP introduction with eventual rise in Sodium to 187 mmo/l.

The child was transferred to the regional centre for assessment.

An MRI Brain showed loss of posterior pituitary bright signal with prominent sulci (felt to be secondary to marked dehydration) supporting a diagnosis of central DI. DDAVP was continued, as the initial poor response to DDAVP was blamed on the acute renal insult. Subsequently electrolytes improved, this was felt to be due to fluid management rather than DDAVP response as polyuria persisted.

DDAVP trial: PRE-DDAVP osmolalities: Urine: 143 mOsmol/kg, Serum: 301 mOsmol/kg.

Post DDAVP urine osmolality: 142 mOsmol/kg.

A Standard Synacthen test showed normal peak Cortisol response (789 nmol/l) and repeat Thyroid function tests were TSH 3.39 miu/l & free T4 17.4 pmol/l hence Hydrocortisone and Levothyroxine were stopped.

A diagnosis of Nephrogenic DI was made and DDAVP was stopped. Chlorothiazide and Amiloride have produced gradual improvement of polyuria and weight on low Renal solute formula alongside liberal intake of water (180 mls/kg per day). Indomethacin was discontinued because of vomiting.

Clinical lesson: the loss of posterior pituitary bright spot is not always indicative of cranial DI as it can occur after exhaustion of vasopressin reserves in a child with Nephrogenic DI.