Compare and contrast central and nephrogenic diabetes insipidus.
Please tabulate your answer under the following headings: definition, etiology, clinical features, investigations, and specific management.
Aim: To explore the clinical entities of Diabetes Insipidus.
Key sources include: Paper 2015.1 Q29, CanMEDS Medical Expert.
Discussion: This question is similar in content to the previously published SAQ. Candidates did well if they were able to demonstrate the pathophysiology particularly in nephrogenic DI.
There was a knowledge gap for many candidates. Management of sodium issues, the requirement for paired serum and urine samples and a discussion of correction strategies of water/sodium balance were contained in the better answers.
Generic answers that did not address the headings asked or omitted specific details of the investigations and rationale were unsuccessful. To improve the answers, familiarity with the Glossary of terms using compare and contrast to state how the aetiology and clinical features are different between the two pathologies.
Question 29 from the first paper of 2015 was actually about SIADH and cerebral salt wasting, rather than diabetes insipidus. Still, it's all sodium, innit. The historical SAQs interrogating the trainee's understanding of DI were Question 5.2 from the first paper of 2018 and Question 17 from the first paper of 2018, in both of which investigations and management were expected. This "compare and contrast" question does work best as a table, and the examiners specifically asked you to tabulate your answer and gave you the headings, so there's probably no excuse for failing to use that structure:
|Heading||Central DI||Nephrogenic DI|
Diagnosis of DI:
Discrimination between different causes of DI:
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Christ-Crain, Mirjam, et al. "Diabetes insipidus." Nature reviews Disease primers 5.1 (2019): 54.
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Khositseth, Sookkasem, et al. "Hypercalcemia induces targeted autophagic degradation of aquaporin-2 at the onset of nephrogenic diabetes insipidus." Kidney international 91.5 (2017): 1070-1087.
Khositseth, Sookkasem, et al. "Autophagic degradation of aquaporin-2 is an early event in hypokalemia-induced nephrogenic diabetes insipidus." Scientific reports 5.1 (2015): 18311.