Hyponatraemia and hypernatraemia — OSCE

Candidate brief

You are the paediatric registrar. Your patient is 6 and has just had surgery for a brain tumour near his pituitary. The surgery has left him with central diabetes insipidus: his body no longer makes the hormone that tells the kidney to hold onto water, so he passes large volumes of dilute urine and drinks constantly to keep up. If he cannot drink enough, his sodium rises dangerously. He has been started on a medication called desmopressin that replaces the missing hormone. The parents want to understand what diabetes insipidus is, why he drinks so much, what the medication does, whether it is permanent, and what to do when he is sick or cannot drink. Explain clearly, check understanding, and agree a safety-net plan. [2]

Marking domains

Clinical knowledge and accuracy (3). Explains that central diabetes insipidus is the loss of arginine vasopressin (ADH) after pituitary surgery, so the kidney cannot concentrate urine and loses too much water; that desmopressin replaces the missing hormone and is effective in the central form; that the risk is hypernatraemia if water intake cannot match the urine losses; and that whether it is permanent depends on whether the pituitary recovers, which is monitored over weeks to months. [2][5]

Communication and plain language (3). Avoids jargon or defines it; uses an analogy (the kidney is like a tap for water, and the hormone is the washer that turns it off); paces the information; pauses to check understanding; invites questions. Conveys that the surgery was necessary and that this is a manageable consequence, not a failure. [2]

Management plan and safety-net (3). Explains that the child must always have access to water and must never be fluid-restricted; that desmopressin is given regularly and the dose is adjusted to control the urine output; that on sick days he must be encouraged to drink, and if he cannot keep fluids down or becomes drowsy he needs urgent medical attention for intravenous fluids; that a MedicAlert or equivalent is essential; and that the school and any carer must know the diagnosis and the plan. [5]

Empathy and partnership (1). Acknowledges the strain of a recent brain-tumour diagnosis and surgery, validates the family's concern, and frames the diabetes insipidus as a known, treatable consequence with a clear plan, while being honest that it may be permanent. [1]

Examiner notes

Strong candidates explain WHY water access is non-negotiable (the child can become dangerously hypernatraemic within hours without it), name desmopressin correctly and distinguish the central from the nephrogenic form, and give a concrete safety-net (sick-day rules, MedicAlert, school plan) rather than vague reassurance. Weak candidates confuse diabetes insipidus with diabetes mellitus, fail to mention the danger of water restriction, or offer a cure. [2][5]