Diabetes Insipidus

Scope 

This section will cover nephrogenic and central diabetes insipidus

Exam Cram

• Diabetes insipidus is caused by lack of ADH (anti-duiretic hormone) effect on the kidney, this can either be an issue with the pituitary ADH secretion (cranial) or kidney's response to ADH (nephrogenic)

• Presentation: polydispsia, polyuria; often lithium use

• Diagnosis: low blood osmolality (concentrated) post water deprivation test

• Management: localise cranial vs nephrogenic by response to ADH

Outline

Assessment

History:

• Polyuria, polydispia 4-6L/d

• Recent trauma (cranial)

• Bipolar on lithium (nephrogenic)

Examination:

• Dehydrated

• Postural drop

Investigations

• High serum osmolarity (dehydrated)

• Low serum osmolality (dilute, inappropriately)

Diagnosis

Diagnostic criteria: 

• Water deprivation test: urine osmolarity < serum osomolarity

Differentials

• Psychogenic Polydipsia: serum osmolarity will be low (well hydrated) and urine high (concentrated)

Classification (site):

• Cranial

  • Diagnosis: good response to ADH/ desmopression

• Nephrogenic

  • Diagnosis: no response to ADH/ desmopression

  • Caused: lithium (bipolar)

Management

Nephrogenic

1. Stop offending drug if relevant: lithium/ ofloxacin

2. Low salt, low protein diet

3. Thiazides

4. Amiloride (potassium sparing)

5. (desmopression unlikely to have large benefit)

Cranial 

1. If from head injury, likely transient: observation

2. If persistent: Desmopressin

Deeper Dive

Osmolality 

• Osmolarity = concencration of solute per litre 

• Osmolality = number of osmoles in one kilogram of solvent. 

• Density of water = 1, hence 1L = 1kg. Therefore if water is the solvent, osmolarity = osmolality!

Causes

• • Cranial Diabetes insipidus: Amyloidosis 

  • Nephrogenic DI: lithium

References and Resources

Page written in 2024.