Polyuria
Polyuria can be defined as the passage of abnormally large amounts of urine. This may be a normal response when excessive fluid, electrolytes, or both are presented to the tubules of a healthy kidney.
It may also occur with renal failure when tubular function is impaired or when sufficient nephron loss overwhelms the remaining nephrons with excess fluid and/or solute. Polyuria is also present in central or neurogenic diabetes insipidus (DI) (caused by insufficient secretion of antidiuretic hormone [ADH]), nephrogenic DI (caused by diminished effect of ADH on receptors in the kidney), renal medullary washout (caused by an insufficient interstitial concentration gradient), excessive drinking (polydipsia), liver failure, and certain electrolyte abnormalities. A phenomenon termed postobstructive diuresis results in polyuria after relief of urinary tract obstruction. Common causes of polyuria are listed in Box 10.5.Approach to Diagnosis of Polyuria
Evaluation should begin with careful inquiry about any history of recent disease, drug administration (e.g., diuretics, corticosteroids, xylazine), fluid therapy, change in diet, change Causes of Anuria, Oliguria, and Polyuria in Horses and Ruminants
Anuria and Oliguria
Acute or chronic renal failure (multiple causes) Blood loss
Cardiac failure
Dehydration
Rupture of the urethra, bladder, or ureter(s) Septic or toxic shock
Urinary tract obstruction (e.g., urinary calculi)
Apparent Anuria or Oliguria
Bladder paralysis or atony
Botulism
Dysautonomia (grass sickness) (E)
Ectopic ureter(s)
Encephalopathies
Equine herpesvirus myeloencephalopathy (E) Neonatal maladjustment syndrome
Painful diseases of the abdomen or abdominal wall
Painful diseases of the thorax or thoracic wall
Patent urachus
Prolonged recumbency
Ruminal acidosis/grain overload
Severe spinal cord disease
Polyuria
Acute or chronic renal failure (multiple causes) Cantharidin intoxication
Diabetes insipidus (central or nephrogenic) Diabetes mellitus
Drugs: diuretics, xylazine, corticosteroids Encephalopathies
Excessive intravenous fluid therapy
Excessive water or salt ingestion
Hepatic failure
Hypercalcemia (including iatrogenic)
Hyperglycemia (including iatrogenic)
Medullary washout
Organophosphate intoxication
Pituitary pars intermedia dysfunction (PPID) (E) Postobstructive diuresis
Psychogenic polydipsia
Renal amyloidosis
Salt deficiency
Vitamin D intoxication
E, Equine.
in water quality or availability (e.g., drought, frozen water), or known laboratory evidence of renal disease. The next step is to collect a urine sample to measure the osmolality or specific gravity (UspG). If the osmolality is close to the isosthenuric range (UspG of 1.008 to 1.014) in the face of dehydration, primary renal disease should be considered. This is typically confirmed by measuring the serum creatinine and blood urea nitrogen (BUN) concentrations, although radionuclide clearance studies and/or 12- or 24-hour urinary creatinine clearance can also be performed. If evidence of renal failure is not present on these tests (see Polyuria and Polydipsia in the Equine Renal System section, Chapter 34), a water deprivation test may be necessary to determine the tubules' ability to concentrate urine. This should be performed in a normovolemic patient with careful monitoring of patient status because the mild dehydration induced during this test may exacerbate preexisting, occult renal disease.
As an alternative to the water deprivation test used to identify tubular disease and dysfunction as the cause of polyuria, the fractional clearance (fractional excretion) of sodium in the urine can be measured. This test is performed by collecting urine and plasma samples at exactly the same time, then measuring the creatinine and sodium in both samples. The fractional clearance of sodium (FcNa [%]) is then determined using the following formula:
FcNa(%) -
x[Cpl ?100 [ Nap ] [Cru ]
where Nau is the urine sodium concentration, Nap is the plasma sodium concentration, Cru is the urine creatinine concentration, and Crp is the plasma creatinine concentration. A fractional sodium clearance value above 1% in adult horses is suggestive of primary tubular disease, particularly if the animal’s diet and physiologic state are such that avid sodium conservation is expected.28 Fractional clearance of sodium values of up to 4% have been measured in healthy, lactating dairy cattle.29 For this test to be valid, salt intake must be normal and the animal must not be given diuretics, parenteral fluids, or fluids administered orally other than what is drunk voluntarily.
If the UspG or osmolality is less than that of the plasma (1.007), DI, psychogenic polydipsia, and renal medullary washout should be considered. Although rare, DI has been reported in the horse and may be the result of inadequate secretion of vasopressin (neurogenic DI) or inadequate response to vasopressin in the kidney (nephrogenic DI).30,31 However, the patient may simply be ingesting large amounts of water in response to exercise or a high ambient temperature, so interpretation of low UspG requires due consideration of the animal’s physiologic state.