PU/PD in Small Animals Flashcards
What is the main hormone that governs urine production?
Where is it made, and where it is stored?
Where does it take its effect?
Anti-Diuretic Hormone (aka Vasopressin)
Made: Supraoptic and paraventricular nuclei of the hypothalamus
Stored: In the posterior pituitary
Effects: the Kidneys (distal renal tubules and collecting ducts) and blood vessels
What causes ADH to be released from the posterior pituitary?
- Changes in plasma osmolality
- Blood volume depletion
- Decreased blood pressure
- RAAS activation
Where and what is the effect of ADH on the kidneys?
- ADH affects the distal tubules and the collecting ducts, leading to resorption of water via binding to its receptors, leading to an increased expression of aquaporin-2 channels
- This will decrease urine output, and increase urine concentration
Where and what is the effect of ADH on the blood vessels?
- Binds to Endothelial receptors of blood vessels to release vWF
- Increases vasoconstriction + BP
- Increases platelet aggregation
- Causes an increase in CRH (corticotropin-releasing hormone) and thus increases ACTH secretion
Does ADH have any effect on water loss from the feces, evaporation/ sweating, saliva etc.?
No!
What is needed in order to allow for ADH to have its effects?
- Intact hypothalamus + posterior pituitary gland
- 1/3 of functioning renal system
- Normal binding and stimulation of the ADH-receptors in the kidneys
- Medullary concentration gradient must be sufficient: which is dependent on urea and Na+ concentrations to allow for the resorption of water
What is a normal water intake in dogs and cats?
20 - 90 ml/kg/day
Note: this is highly dependent on many factors such as diet, environment, exercise, productivity status (lactating or not) etc.
What is a normal urine output in dogs and cats?
20 - 45 ml/kg/day
What is defined as Primary polydipsia?
Excess drinking causing excess urination
What goes in, must come out
What is defined as Primary polyuria?
Excess urination with compensatory polydipsia
What is lost must be replaced
What is more common, primary polydipsia or polyuria?
Primary Polyuria, leading to a compensatory polydipsia
Primary polydipsia is extremely RARE!
What are the DDx for Primary Polydipsia?
- Behavioural: environmental or emotional cause
- Dipsogenic Diabetes Insipidus: hypothalamic lesions: idiopathic or infections, or neoplastic leading to increased thirst response
- Intestinal disease, particularly diarrhea
What are the DDx for Primary Polyuria?
- ADH deficiency: due to central diabetes insipidus (DI)
- Primary nephrogenic DI
- Acquired nephrogenic DI
- Osmotic diuresis
What are the causes of Central Diabetes Insipidus, leading to Primary Polyuria?
- Idiopathic: most common cause as we never find an actual cause
- Trauma: to the head leading to the pituitary stalk separation
- Neoplasia
- Surgery: Hypophysectomy
- Developmental defects in structure
- Infection
- Inflammation
- Cysts
These will all lead to an ADH deficiency
What is Diabetes Insipidus?
Diabetes insipidus (DI) is a condition characterized by large amounts of dilute urine and increased thirst.
More common in dogs than cats
What are the causes for Primary nephrogenic DI, leading to Primary Polyuria?
- Genetic mutations of the ADH-receptor or aquaporin channel
- Usually inherited, and breeds such as Huskies are commonly affected
What are the causes for Acquired nephrogenic DI, leading to Primary Polyuria?
- Kidney disease
- Liver disease
- Pyelonephritis
- Pyometra/ E.coli endotoxemia
- Hypercalcemia
- Hypokalemia
- Hyperthyroidism
- Hyperadrenocorticism
- Hypoadrenocorticism
- Drugs/ diet
- Hyperviscosity Syndrome: overproduction of erythropoietin leading to erythrocytosis
What are the causes for Osmotic diuresis, leading to Primary Polyuria?
- Post-obstructive diuresis: urine blockage that we have unblocked, and for a short time after urea is excreted acting as an osmotic substance, pulling water out with it and causing diuresis
- Glucosuria: due to Primary renal glycosuria, Diabetes mellitus or Fanconi syndrome (Basenji’s predisposed)
- Drugs: Mannitol
What are the unusual causes of PU/PD in a patient where the more normal causes have been ruled out?
- Polyuric phase of Acute Kidney Injury from Raisin ingestion
- Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
- Phaeochromocytoma
- Leptospirosis
- Splenic hemangiosarcoma
- Intestinal leiomyosarcoma
What are the most common causes of PU/PD in dogs? (8)
- Hyperadrenocorticism
- Diabetes mellitus
- Chronic kidney disease
- Pyelonephritis
- Pyometra
- Hypercalcemia
- Liver disease
- Diabetes insipidus
What are the most common causes of PU/PD in cats? (3)
- Chronic renal failure
- Diabetes mellitus
- Hyperthyroidism
How can you confirm the presence of PU/PD in a patient?
Urine analysis
- Presence of glucose
- Specific Gravity: would expect it to be low. Urine that is appropriately concentrated or higher means the animal is NOT polyuric
You are presented with a patient that the owners claim is PU/PD, you take a urine sample and note Hyposthenuria
What DDx list can now be made?
- Dipsogenic/ Psycogenic
- Central DI
- Hyperadrenocorticism
- Hypercalcemia
- Liver disease
- Hypoadrenocorticism
- Infections such as pyelonephritis/ pyometra
You suspect primary polydipsia in a dog even though it is rare, plasma osmolality may be helpful is ruling it in or out
How do you calculate plasma osmolality?
2[Na] + [glucose] + [urea]
Reference range = 290 -310/330 mOsm
A dog with primary polydipsia will have a plasma osmolality that is at the lower end of this range or below
e.g.
Na+ = 142.1 mmol/L
Glucose = 6.0 mmol/L
Urea = 4.6 IU/L
2(142.1) + 6 + 4.6 = 294.8
294.8 is at the low end of the Reference Range
