What's the Differential Diagnosis of Polydipsia and Polyuria in Dogs and Cats?

There are many potential causes of polyuria and polydipsia.  Primary disorders of water balance (i.e. central diabetes insipidus, primary nephrogenic diabetes insipidus and primary polydipsia) although uncommon, should always be considered in the differential diagnosis of polyuria and polydipsia. In general, animals with these disorders have only one laboratory abnormality; a low urine specific gravity (SG) or osmolality.

In most instances the more common causes of polyuria and polydipsia (e.g. hyperadrenocorticism, chronic renal failure, pyelonephritis and pyometra) have other specific and obvious abnormalities on screening laboratory tests (complete blood cell count, serum biochemical profile and urinalysis). In some cases, however, a low urine SG is the only abnormality found in animals with these latter disorders.

The work-up for polyuria and polydipsia can be tedious, time-consuming, expensive, confusing and not without significant patient morbidity, especially in those dogs and cats with normal or near-normal screening test results. This chapter focuses on the diagnostic approach, especially the problems associated with testing, and the treatment of dogs and cats with disorders of water balance.

Differential diagnosis
The causes of polyuria and polydipsia can be divided into those that cause primary polydipsia (with secondary polyuria) and those that cause primary polyuria (with compensatory polydipsia). These are listed in the table below.

The major cause of primary polydipsia in dogs is psychogenic polydipsia. In contrast, the causes of primary polyuria are much more numerous and can be subdivided into the categories of: central diabetes insipidus; primary nephrogenic diabetes insipidus; secondary nephrogenic diabetes insipidus; and osmotic diuresis.

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DIFFERENTIAL DIAGNOSIS OF POLYDIPSIA AND POLYURIA

1. Primary polydipsia

• Psychogenic polydipsia (compulsive water drinking)
  
• Dipsogenic diabetes insipidus (thirst centre abnormality)
  
• Metabolic disorders (e.g. hyperthyroidism, hepatic failure)

2. Primary polyuria

Central diabetes insipidus (neurogenic, cranial, ADH-responsive)

• Idiopathic
  
• Trauma-induced
  
• Neoplastic
  
• Post-hypophysectomy

Primary nephrogenic diabetes insipidus (congenital or familial)

Secondary nephrogenic diabetes insipidus (acquired)

• Acromegaly
  
• Chronic renal disease
  
• Drug administration
  
• Liver disease
  
• Hyperadrenocorticism
  
• Hypercalcaemia
  
• Hyperthyroidism
  
• Hypoadrenocorticism
  
• Hypokalaemia
  
• Leptospirosis
  
• Postobstructive diuresis
  
• Pyelonephritis
  
• Pyometra

Osmotic diuresis (increased renal tubular solute load)

• Diabetes mellitus
  
• Primary renal glycosuria (e.g. Fanconi’s syndrome)
  
• Postobstructive diuresis
  
• Renal failure
  
• Leptospirosis

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In our next blog post, I will talk about my approach to working up the dog or cat with polyuria or polydipsia.