Polyuria and polydipsia — viva

Opening (must-hit)

"This is a water diuresis after pituitary surgery — the normal glucose excludes an osmotic diuresis, and the urine osmolality of 120 with a serum osmolality that is clearly raised confirms a water diuresis. Two weeks after craniopharyngioma surgery this is central diabetes insipidus until proven otherwise, complicated now by hypernatraemia at 152. My immediate priority is the sodium: he needs intravascular volume restoration with isotonic saline, then slow free-water correction at no more than 0.5 mmol per litre per hour, max 10 to 12 over 24 hours, to avoid cerebral oedema. Once the sodium is controlled and his cortisol status is confirmed, I would confirm central disease and start desmopressin, recognising that after pituitary surgery the picture can be complicated by the triphasic response." [4][12]

Examiner: "Why check the cortisol before judging his vasopressin?"

"Cortisol deficiency impairs free-water excretion and can mask diabetes insipidus, so a child who appears to have resolved DI may simply have untreated cortisol deficiency holding the water in. Craniopharyngioma surgery can damage the whole pituitary, so I would measure a morning cortisol and replace glucocorticoids before judging vasopressin action. If he is cortisol-deficient and I give desmopressin without correcting it first, I misread the picture; if I correct cortisol and he was depending on it to excrete water, I can unmask or worsen his diuresis, so I monitor the sodium and urine output through the transition." [4][1]

Examiner: "Tell me about the triphasic response and how it changes your management."

"After pituitary surgery the posterior pituitary can pass through three phases. The first is transient diabetes insipidus from shock to the neurohypophysis, lasting hours to days, with dilute urine and a rising sodium. The second is an antidiuretic phase from uncontrolled release of stored vasopressin as the cells degenerate, during which the urine concentrates and the sodium can fall — so I withhold desmopressin to avoid hyponatraemia. The third is permanent diabetes insipidus if the vasopressin neurons are destroyed. Because I cannot predict which phase he is in or whether he will recover, I manage by the measured serum sodium and urine output, not a fixed schedule, and I only treat the sodium and the water balance that are in front of me." [4][10]

Examiner: "How would you confirm central versus nephrogenic disease if the picture were not post-surgical?"

"I confirm a water diuresis first — a urine osmolality below 300 with a raised or high-normal serum osmolality. Then I use the serum sodium as the pointer: high or high-normal favours true diabetes insipidus, low or low-normal favours primary polydipsia. The dynamic test is the supervised water deprivation test, withholding fluids and monitoring weight, urine and serum osmolality, stopping at a serum osmolality above 300 with a urine plateau below 300, or at a weight loss over 3 to 5 percent; I then give desmopressin and remeasure the urine osmolality. A rise of more than 50 percent means central disease; no response means nephrogenic. Where copeptin is available, a hypertonic-saline-stimulated copeptin below 21.5 pmol per litre identifies arginine vasopressin deficiency and avoids the indirect deprivation test, and a baseline copeptin below 4 to 5 strongly suggests central disease." [7][1]

Examiner: "What if the child had a low sodium instead — what would you be worried about?"

"A low or low-normal serum sodium in a polyuric child points to primary polydipsia, because the axis is intact and merely suppressed by excess intake. The danger is reaching for desmopressin: because the axis is working, desmopressin forces the kidney to retain the ingested water, and the child develops acute water intoxication with severe hyponatraemia and seizures. So a low sodium in a polyuric child is a red flag against desmopressin until primary polydipsia is excluded. I would confirm primary polydipsia by showing that the urine concentrates appropriately with deprivation and a normal stimulated copeptin, and then I would treat the cause — psychiatric or behavioural — rather than the kidney." [10][1]

Examiner: "How would your management differ if this were an infant with congenital nephrogenic disease?"

"In an infant the danger is that they cannot report thirst or reach for water, so uncontrolled water loss drives hypernatraemic dehydration within hours, and the presentation is failure to thrive, irritability and fever rather than polydipsia. Nephrogenic disease cannot be treated with desmopressin because the defect is downstream of the V2 receptor. The mainstay is adequate free water — frequent feeds, a nasogastric drip overnight — and a low-solute diet to reduce obligate water loss. I add a thiazide, which paradoxically reduces urine volume through mild volume depletion and increased proximal reabsorption, combined with amiloride or indometacin to enhance concentrating ability. I would arrange AVPR2 and AQP2 genetic testing, because the X-linked AVPR2 form accounts for about 90 percent of inherited cases and is relevant for family counselling." [9][3]

Closing summary

"In summary: polyuria-polydipsia is glucose first, osmolality second, sodium third, desmopressin response fourth. This post-surgical boy has central diabetes insipidus with hypernatraemia — I correct the sodium slowly, confirm cortisol status, and manage the triphasic response by the measured sodium and urine output. The four things never to forget are slow sodium correction to avoid cerebral oedema, cortisol before vasopressin, never give desmopressin to primary polydipsia, and never escalate desmopressin in nephrogenic disease." [1][12]