Diabetes Insipidus Flashcards
What is diabetes insipidus characterised by
Hypotonic polyuria (i.e. urine output of more than 3L per day with osmolality
What is DI classified into
central DI
Nephrogenic DI
dipsognic DI
What is central DI due to
Deficiency of the posterior pituitary hormone ADH
What is nephrogenic DI due to
Renal resistance to ADH
What is dipsogenic DI to
excessive drinking
What sex is more commonly affected by DI
both are affected equally
What are some of the underlying causes of central DI
Idiopathic Head trauma/ neurosurgery pituitary tumours granulomatous diseases (sarcoidosis, histiocytosis X) Less common include: Sheehan's syndrome CNS infections hypoxic encephalopathy
What might nephrogenic DI be secondary to
X linked mutations in the gene encoding the ADH receptor V2
persistent hypercalcaemia
severe hypokalaemia
Drugs: lithium, antifungals
How does gestational DI arise
It is due to increased placental vasporessinase activity and degradation of vasopressin
Does gestational DI resolve ?
Yes - after delivery
What is primary polydipsia due to
excess flid intake resulting in an inhibition of ADH release
What are the initial tests to confirm DI
plasma osmolality and sodium are high and urine osmolality is low
Serum sodium is either normal or slightly high but in primary polydipsia, low due to water overload
What other test can be done to differentiate between the different types of DI
water deprivation test
How is the water deprivation test carried out
1st stage: No water for 8 hours
2nd Stage: Desmopressin is given to patients who have not concentrated their urine adequately and hourly urine volumes and osmolality are measured for 4 hours. Patients are allowed to drink freely during this stage
Describe the results of a negative water deprivation test
ADH would be stimulated
Urine is therefore concentrated
plasma osmolality remains low
Describe the results of a positive water deprivation test
reduced ADH release/ action inhibits urine concentration (urine osmolality remains low)
plasma osmolality rises
Describe the different effects of desmopressin in patients with cranial DI
They can then concentrate their urine
Describe the different effects of desmopressin in patients with nephrogenic DI
cannot concentrate their urine
Why is it that in patients with primary polydipsia, their urine is concentrated to a lesser degree
CHronic polyuria washes out the medullary interstitial solutes and impairs the urine-concentrating ability
Chronic overhydration causes a suppression of ADH release
What type of history may favour primary polydipsia if there is an uncertainty about the results of the water deprivation test
A history of psychiatric illness and a gradual onset
What should be assessed in patients diagnosed with cranial DI on water deprivation test
Hypothalamic pituitary anatomy using MRI
What is the treatment for Central DI
Desmopressin
In what ways can desmopressin be administered
Intranasal
Oral
Subcutaneous
Is desmopressin safe during pregnancy
Yes
What should be monitored in a patient who is taking desmopressin
Sodium - patients could become hyponatraemic
plasma osmolality and clinical response should also be measured
What is the treatment for Nephrogenic DI
Treat the underlying cause (hypokalaemia or hypercalcaemia) Stop causative drug
low dossier diet
thiazides and NSAIDs
How do thiazides work in nephrogenic DI
they induce mild volume depletion, resulting in creased proximal tubular sodium sodium and water reabsorption leading to decreased water delivery to the collecting tubules and reduces urine output
How do NSAIDs work in nephrogenic DI
They inhibit renal prostaglandin synthesis and augment ADH action
What is the treatment for polydipsia
Treat the underlying psychiatric disorder
