Diabetes insipidus and polyuria-polydipsia — OSCE

Station brief (candidate)

• Explain the diagnosis in plain language: the surgery to remove the tumour was very close to the gland that makes a water-control hormone (vasopressin), and that gland is no longer making enough, so his body cannot hold on to water — he passes a lot of dilute urine and gets very thirsty. This is called central diabetes insipidus. [1]
• Reassure the parents that it is treatable: a medicine called desmopressin replaces the missing hormone, tells the kidneys to hold water, and brings the urine volume and thirst back to normal. [4]
• Explain the key safety rule simply: the medicine is powerful, so we let the urine "break through" a little before each dose. If he is kept holding water all the time and keeps drinking, the salt in his blood can drop too low and make him unwell — so we do not over-treat, and we watch the blood tests. [4]
• Warn gently about the early post-surgery pattern: in the first couple of weeks the water balance can swing — a phase of too much urine, then a phase of holding too much water, then settling. This is why the blood sodium is checked often and the dose is adjusted, not fixed. [4]
• Explain the broader plan: the same surgery can affect the other pituitary hormones (steroid, thyroid, growth), so he will be checked for those and given replacements if needed. Coordinated endocrine and neurosurgical follow-up will be arranged. [1]
• Give concrete safety-netting: keep water available, watch for signs of too much or too little water (drowsiness, headache, or on the other hand very high urine output and intense thirst), have a sick-day plan, and contact the team early if he is vomiting or not drinking. [11]

Role-player notes

You are the parents of a 6-year-old who has just been through brain surgery for a tumour. You are relieved the tumour is out but exhausted and worried. You have noticed he cannot stop drinking and is soaking through the sheets at night, and you are frightened this means the tumour is back or the surgery went wrong. You do not understand what "diabetes insipidus" means and are alarmed by the word "diabetes" because a relative has diabetes and needs insulin. You want to know: is this the same as sugar diabetes? Will he need this medicine forever? Is it dangerous? You engage well when the doctor explains clearly that this is a water problem, not a sugar problem, reassures you it is treatable, explains the safety of the medicine, and gives you a concrete plan with contact points.

Expected candidate performance

1. Opening and reassurance: "The constant drinking and the large amounts of pale urine are because the surgery affected a small gland that makes a hormone controlling water. Without it, his body cannot hold water, so he passes a lot and gets thirsty. This is not sugar diabetes — his sugar is fine. It has a confusing name, diabetes insipidus, but it is a water problem, and it is very treatable." [1]
2. Explaining the treatment: "We give a medicine called desmopressin that replaces the missing hormone. It tells the kidneys to hold onto water, so the urine volume and the thirst come back to normal. It comes as a tablet, a melt, a nasal spray, or an injection, and we choose and adjust the dose for him." [4]
3. The safety rule: "The medicine is powerful, so there is one important rule: we let his urine break through a little before each dose. If he holds water all the time and keeps drinking, the salt in his blood can fall too low and make him unwell. So we deliberately avoid over-treating and we check the blood tests, especially at first." [4]
4. The early swing and monitoring: "In these first weeks the water balance can swing — too much urine, then a phase of holding too much water, then settling. That is expected after this surgery, and it is why we check the blood sodium often and adjust the dose rather than fixing it." [4]
5. The broader plan and safety-netting: "This surgery can also affect the other pituitary hormones — steroid, thyroid, growth — so we will check those and replace any that are low. Keep water available, watch for drowsiness or headache (too much water) or very heavy urine output and intense thirst (too little medicine), have a sick-day plan, and call us early if he is vomiting or not drinking. We will arrange coordinated endocrine and neurosurgical follow-up, and, if the thirst sense is affected, a fixed daily fluid plan." [11]

Marking domains

• Clinical reasoning: correctly frames post-surgical central diabetes insipidus, distinguishes it from diabetes mellitus, and anticipates the triphasic swing and the wider pituitary deficits. [4]
• Communication: uses plain language, separates "water diabetes" from "sugar diabetes," and addresses the parents' fear of tumour recurrence. [1]
• Treatment and safety: explains desmopressin, the daily antidiuretic break, and the risk of hyponatraemia from over-treatment. [4]
• Shared decision-making and safety-netting: concrete monitoring, sick-day plan, escalation advice, and coordinated multidisciplinary follow-up. [11]

Common fails

• Using jargon ("vasopressin", "antidiuretic", "hyponatraemia", "triphasic") without translation. [1]
• Not clearly separating diabetes insipidus from diabetes mellitus, leaving the parents fearing insulin and sugar disease. [4]
• Omitting the desmopressin safety rule (the daily break) and the risk of water intoxication from over-treatment. [4]
• Not warning about the early post-surgical swing in water balance and the need for frequent sodium checks. [4]
• Forgetting the broader pituitary assessment (cortisol, thyroid, growth) after this surgery. [1]
• Not addressing the risk of adipsic diabetes insipidus if thirst is affected, and the need for a scheduled fluid plan. [11]