endocrinology (diabetus insipidus) Flashcards
describe the pathophysiology of diabetes insipidus
diabetus insipidus is a disease characterised by severe PU/PD with the production of hyposthenuric urine and an inability of the kidneys to concentrate urine through either:
- an absolute deficiency of arginine vasopressin synthesis and secretion from the pituitary (central or neurogenic DI) - or a lack of responsiveness to vasopressin at the level of the renal tubules (renal or nephrogenic DI)
these defects may be partial or complete
what is the frequency of diabetes insipidus in cats
diabetes insipidus is a rare disease in cats
what are the causes of central DI
central DI can be either:
- primary (idiopathic and congenital)
- or secondary (usually as a result of head trauma or pituitary neoplasia)
what are the causes of nephrogenic DI
nephrogenic DI can be either:
- primary (has not been reported in cats)
- or secondary (relatively common) to renal disorders (e.g., renal failure), metabolic disorders (e.g., hypercalcemia, hypokalemia, hepatic disease, hyperthyroidism) and drug therapy
what is the epidemiology of diabetes insipidus
primary central DI usually occurs in kittens (usually apparent at 2-6 months of age)
the onset of disease in older cats usually indicates secondary causes such as head trauma or pituitary neoplasia
psychogenic polydipsia has not been reliably reported in cats
what is the typical clinical presentation
the primary clinical sign is severe PU/PD, usually more than 100 ml/kg/24 hours
- the severity will depent on whether the defect is partial or complete
if there is secondary inappetence, there may be some weight loss, and dehydration may occur with the obligate polyuria
the combination of diabetes insipidus and mydriasis is suggestive of pituitary pathology
how would you investigate a case of DI
routine blood work is usually normal or reflects dehydration (i.e., pre-renal azotemia, hypernatremia, elevated PCV)
urinalysis will reveal persistent hyposthenuria to isosthenuria depending on the severity of the DI
- USG usually in the range of 1.003-1.012
the diagnosis of DI is usually achieved firstly by ruling out the other principal differential diagnoses, with subsequent confirmation of the inability to concentrate urine being met with a water deprivation test
explain the protocol for a water deprivation test
hospitalise the cat and withold food and water
weigh the cat at initiation and then every 1-2 hours
the end-point is when body weight has reduced by 5% or if physical/neurological deterioration occurs due to dehydration or if periodic urine sampling indicates a USG greater than 1.030
a USG > 1.030 is regarded as an adequate renal response, but if not achieved, in the absence of any other identified cause, this is suggestive/diagnostic of DI
explain the vasopressin response test protocol
immediately following the water deprivation test, a vasopressin-response test is performed to categorise DI as central or nephrogenic
urine concentration is determined and the bladder is emptied of urine
aqueous vasopressin is given by IM injection (0.5 U/kg), or by using 1-2 drops of the intranasal solution in the conjunctival sac for 2-3 days
urine concentration is assessed 1-2 hours post-injection or after a few days on the intranasal drops
a considerable increase in USG (>1.015) following administration of vasopressine is indicative of central DI, failure to concentrate suggests nephrogenic DI
what are the treatment options for diabetes insipidus
therapy for DI can take the form of hormone replacement (desmopressin) or the use of thiazide diuretics, the latter being the only form of treatment suitable for nephrogenic cases
thiazide diuretics can reduce the polyuria by 50% through increasing proximal tubular sodium and water resorption and thus reduce the volume of urine reaching the distal tubule
- doses have to be titrated (initial dose of 1-2 mg/kg BID) and monitoring is required to ensure hypokalemia does not develop
desmopressin (DDAVP) is a synthetic analog of AVP with a longer half-life
- it can be given by injection, conjunctival drops or orally
- usually administered at a dose of 2-5 mg/cat SQ once or twice a day
