Diabetes Insipidus Flashcards
What is diabetes insipidus?
Diabetes insipidus is a rare condition causing polyuria and polydipsia. Diabetes insipidus is not related to diabetes but share common signs and symptoms.
Describe the two types of DI?
Cranial: Impaired ability to secrete enough AVP
Nephrogenic: Normal AVP production, but nephrons cant respond to it
Describe the primary and acquired aetiology of central DI
Primary causes include genetic, e.g. DIDMOAD syndrome), or developmental syndromes (septo-opticdysplasia).
The acquired causes include tumours (germ cell tumours), trauma, inflammatory, vascular (aneurysm)
Describe the primary and acquired causes
of nephrogenic
central DI
The primary causes are typically genetic, e.g. X-linked recessive V2-R
The acquired causes include CKD, metabolic cause (hypercalcaemia, hypokalaemia), drug induced, osmotic diuresis, systemic disorder (amyloidosis)
What is the first step to diagnosing DI?
The first is to establish the presence of polydopsia and poluria using a 24hr urine collection. Patient testing positives will show >40mL/Kg per 24hr in adults, and >100mL/Kg per 24h in infants
What blood and urine tests will help diagnose DI?
They will be used to rule out more common causes of polydipsia and polyuria, using serum calcium, plasma glucose, serum potassium. Best test is to perform a random paired serum and urine osmolality test
What paired serum and urine osmolality test results indicate DI?
A patient presenting with a urine osmolality of >750mOsm/kg is unlikely to have DI, as the patient can appropriately concentrate their urine. Whereas a raised serum osmolality accompanied with a low urine osmolality indicate strong clinical suspicion for DI.
Diagnosis of DI using water deprivation test
If urine osmolality is still <750mosm/kg after 8h fluid restriction a desmopressin test (artificial AVP) will be carried out. Nephrogenic DI will show no response whilst cranial achieves normal concentration of urine
What direct testing for DI is available?
This can be done using AVP/copeptin measurements. Hypertonic saline infusion given till serum sodium reaches 150mmol/L. Serum plasma samples taken for AVP/copeptin measurement under osmotic stimulus. Central DI patients will present a sub-normal (flat response) AVP/copeptin response to osmotic stimulus. Nephrogenic DI will present a high basal AVP/copeptin or exaggerated response
Why is copeptin a good measure for diagnosing DI?
It is derived from the pre-pro-hormone of AVP. Thus AVP and copeptin are produced equimolarly. It is an easier measure than AVP and has a quicker turnaround
How can DI be differentiated?
Using copeptin. Copeptin of <5pmol/L is likely to be CDI. Whereas copeptin of >5pmol/L shows CDI to be unlikely. Copeptin usually markedly elevated in NDI as the AVP is still secreted, but due to nephron resistance can’t act on it.
Treatment of DI
Central DI is treated using hormone replacement, such as ddAVP in tablets or as a nasal spray. Whereas nephrogenic DI is more complicated and is dependent on the underlying cause but can be treated by withdrawing medication if medication is related, thiazide diuretics/NSAIDs, ensure adequate hydration
