Diabetes Insipidus in Children
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Direct answer
Diabetes insipidus is a disorder of water balance producing large volumes of dilute urine and compensatory thirst, caused either by antidiuretic hormone deficiency (central) or by kidney resistance to it (nephrogenic). The water deprivation test separates diabetes insipidus from primary polydipsia, and the desmopressin response then separates the two types, because a central defect corrects with desmopressin while a nephrogenic one does not.
What you must remember
- Presentation: polyuria, polydipsia, nocturnal enuresis in a previously dry child, and hypernatraemia when water intake is inadequate.
- Urine osmolality: persistently dilute urine, below about 300 mOsm/kg, with rising plasma osmolality during deprivation is the key biochemical pattern.
- Central causes: craniopharyngioma, germinoma, Langerhans cell histiocytosis, head injury, neurosurgery, and idiopathic or autoimmune forms.
- Nephrogenic causes: AVPR2 and AQP2 mutations, lithium, hypercalcaemia, hypokalaemia and chronic kidney disease.
- Primary polydipsia: excessive water intake with intact ADH, giving urine that concentrates during deprivation; this is the main mimic.
- Management: desmopressin for central disease, while nephrogenic disease needs thiazides, salt restriction and adequate water intake.
A supervised pathway to the diagnosis
The water deprivation test is performed under close supervision, because dehydration and hypernatraemia can develop quickly in young children.
- Confirm the polyuria: measure 24-hour urine volume, urine osmolality and serum sodium, and exclude osmotic diuresis from glycosuria.
- Restrict fluids under supervision: monitor weight, urine output and urine osmolality at protocol-defined intervals.
- Stop at the endpoint: end the test when weight falls about 3 to 5 per cent, plasma osmolality rises to the protocol limit, or urine osmolality plateaus.
- Interpret the response: urine that becomes more concentrated than plasma suggests primary polydipsia, while persistently dilute urine indicates diabetes insipidus.
- Give desmopressin: a rise in urine osmolality of more than 50 per cent indicates central diabetes insipidus, whereas no response suggests nephrogenic disease.
- Image and replace: MRI of the hypothalamic-pituitary region for central causes, followed by long-term replacement with sodium and fluid monitoring. Results can be confounded by renal impairment and by medicines, so the test is interpreted alongside the clinical picture and imaging. Repeated measurements are often needed before a diagnosis is considered settled.
Where the exam tries to catch you
Diabetes insipidus vignettes turn on the difference between polydipsia and true ADH deficiency.
- Trap: diagnosing diabetes insipidus from polyuria alone Correct: primary polydipsia and osmotic diuresis also cause polyuria, so concentrating ability must be measured.
- Trap: running fluid restriction without supervision in a young child Correct: the test is done in hospital with monitoring, because hypernatraemia can develop rapidly.
- Trap: giving desmopressin for nephrogenic diabetes insipidus Correct: the kidney cannot respond, so thiazides, salt restriction and water management are used.
- Trap: missing a central lesion in a child with new diabetes insipidus Correct: craniopharyngioma and germinoma must be excluded with MRI.
Frequently asked questions
What is the water deprivation test used for?
The water deprivation test distinguishes diabetes insipidus from primary polydipsia and shows whether the defect is central or nephrogenic. It is performed under supervision with careful monitoring of weight, sodium and urine osmolality.
How does desmopressin help distinguish central from nephrogenic diabetes insipidus?
Central diabetes insipidus shows a rise in urine osmolality of more than 50 per cent after desmopressin, because the kidney can respond to the hormone. Nephrogenic diabetes insipidus does not respond, since the renal tubules are resistant to antidiuretic hormone.
What are the common causes of central diabetes insipidus in children?
Central diabetes insipidus may be idiopathic or caused by tumours such as craniopharyngioma, germinoma or Langerhans cell histiocytosis. MRI is essential to look for a structural lesion, especially in new or unexplained cases.
Why is primary polydipsia an important mimic of diabetes insipidus?
Primary polydipsia causes large fluid intake with intact ADH, so urine becomes concentrated when water intake is limited. It is distinguished from diabetes insipidus by the response to the water deprivation test.
What is the main treatment for nephrogenic diabetes insipidus?
Nephrogenic diabetes insipidus is treated with thiazide diuretics, which paradoxically reduce urine volume, along with a low-salt diet and adequate water intake. Desmopressin is ineffective because the kidney cannot respond to it.