HISTORY / CLINICAL SIGNS: 1 year history of excessive urination and increased water consumption. Owners report having to refill the patientโs large water bowl several times daily and letting him outside frequently to urinate large amounts at a time. No weight changes or other issues. UTD on vaccines. No current medications/supplements.
PHYSICAL EXAM FINDINGS: Hyper temperament. Otherwise, unremarkable PE.
What is on your differential diagnosis list? What diagnostic tests would you order?
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DIFFERENTIALS: Causes of Polyuria/Polydipsia (PU/PD). Daily urine production volume and amount of water consumed must be quantified to truly diagnose PU/PD. PU is defined as >50mL/kg/day of urine output while PD is defined as >100mL/kg/day of water consumption. Although this patientโs urine volume and water consumption have not been accurately measured, his ownersโ description of having to refill a large water bowl several times daily and the patientโs need to go outside quite often to urinate heavy streams fits with true PU/PD.
Differentials for PU/PD include: Renal disease/failure. Diabetes mellitus. Liver disease. Hyperadrenocorticism (Cushingโs disease): pituitary-dependent/ADH vs. adrenal-dependent/ADH. Hypoadrenocorticism (Addisonโs disease). Central diabetes insipidus (CDI). Nephrogenic diabetes insipidus (NDI). Psychogenic polydipsia. Acromegaly. Hyperthyroidism. Primary renal glucosuria. Hypokalemia. Hypercalcemia. Drugs (corticosteroids, phenobarbital).
DIAGNOSTIC TESTS & RESULTS: (Many of these tests were performed sequentially, not necessarily simultaneously.)
- Bloodwork (CBC/chemistry): Slightly elevated liver enzymes. All other values within normal reference range.
- T4: Within normal reference range.
- Urinalysis: Free-catch sample collected mid-day. USG 1.003 (hyposthenuria). Otherwise, UA unremarkable.
- Abdominal radiographs: Unremarkable.
- Abdominal ultrasound: Small hypoechoic splenic nodule (incidental finding). Otherwise, unremarkable.
- ACTH Stimulation Test: Both pre- and post-Cortrosyn cortisol values are within normal reference range.
- Synthetic Vasopressin – Desmopressin (DDAVP or ADH response test): No change in clinical signs during use. Unfortunately, serum potassium levels were not rechecked during administration.
Updated Differentials:
- Psychogenic polydipsia (with compensatory polyuria): A modified water deprivation test may be warranted to rule in/out. If this is the cause, the patientโs urine should concentrate and show no response to ADH administration during this test. This test can be risky, however, if the patient actually has another cause of PU/PD and does not have access to water, which can cause severe electrolyte imbalance. Alternatively, the USG of a first morning urine sample could be measured to see if it concentrates (assuming that the patient does not have access to water during the night).
- Nephrogenic diabetes insipidus (NDI): Nephrogenic diabetes insipidus cannot be ruled out yet. However, as it is typically an acquired condition secondary to an underlying problem (ex. pyelonephritis, Cushingโs or Addisonโs disease, hypercalcemia or hypokalemia, liver failure, hyperthyroidism, or acromegaly), NDI seems highly unlikely since the patient does not have the majority of these conditions. However, the hyposthenuria and failure to respond to desmopressin can be signs of NDI. Therefore, a modified water deprivation test may be indicated if another cause of the PU/PD cannot be found.
Based on this patientโs signalment, clinical signs, physical exam findings, and test results, what is your diagnosis, recommended treatment, and prognostic outlook?
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PRESUMPTIVE DIAGNOSIS: Psychogenic Polydipsia (with compensatory polyuria).
Hyperactive large breed dogs (including Labs, like this patient) are predisposed to this overall uncommon behavioral disorder.
The owners declined a modified water deprivation test to definitively rule out nephrogenic diabetes insipidus (NDI), but due to the lack of evidence for an underlying problem causing secondary NDI, this diagnosis is less likely.
TREATMENT: Behavioral assessment and any subsequent training are necessary. Psychogenic polydipsia may coincide with other compulsive or neurotic tendencies (separation anxiety, etc.); therefore, the behavioral root cause must be addressed and corrected. If the PU/PD continues, a biochemistry should occasionally be rechecked to monitor electrolyte levels (namely, serum sodium) as excessive water consumption can lead to medullary washout and hyponatremia. Sodium supplementation may be necessary in such cases.
PROGNOSIS: Excellent. No physical problems noted.
Any other questions, comments, or suggestions? What would you have done differently?
Unless otherwise stated, these clinical cases are real-life cases that I have managed as a veterinarian in general small animal veterinary practice or else assisted with as a student in vet school. These cases are a great real-world learning tool for vet students and other veterinary professionals. They shall be used for learning purposes and collaboration of knowledge only. These cases are not intended to diagnose or treat any disease by pet owners. – Maranda Elswick, DVM
