Tentative Diagnosis and Differentials: The tentative diagnosis is Diabetes Insipidus (DI). The clinical presentation of severe PU/PD in an otherwise healthy animal with persistently hyposthenuric urine is classic for this condition. The initial diagnostic workup successfully ruled out the most common differential diagnoses for severe PU/PD, including:
Diabetes Mellitus: Ruled out by normal blood glucose and negative urine glucose.
Chronic Kidney Disease: Ruled out by normal creatinine, BUN, and other chemistry values. Furthermore, the urine is hyposthenuric (actively diluted by the kidneys), not isosthenuric (passively unconcentrated) as would be expected with renal failure.
Hyperadrenocorticism (Cushing's Disease): Less likely given the lack of other clinical signs (e.g., pot belly, alopecia, skin changes) and the normal biochemistry profile.
Hypercalcemia & Liver Disease: Ruled out by the normal biochemistry profile.
The main remaining differential is Primary Polydipsia (Psychogenic Drinking), a behavioral condition. However, the presence of hypernatremia makes this less likely, as these patients tend to have normal to low sodium levels from overhydration. A definitive test is required for confirmation.
Further Diagnostic Test(s): The definitive test to confirm Diabetes Insipidus and to differentiate between its central and nephrogenic forms is the Modified Water Deprivation Test (MWDT), followed by an ADH response test.
Test Protocol and Results:
Preparation: Kodiak was admitted to the hospital. His initial weight was recorded, the bladder was emptied, and all water was withdrawn. He was placed under constant observation in the ICU.
Phase 1 (Deprivation): His body weight, urine specific gravity, and plasma osmolality were monitored hourly.
Hour 4: Kodiak lost 5% of his initial body weight, a clinical sign of significant dehydration. His plasma osmolality had increased, confirming dehydration.
Urine Concentration Check: Despite the clear evidence of dehydration, his USG remained profoundly dilute at 1.006.
Conclusion of Phase 1: A normal animal or one with psychogenic polydipsia would have concentrated its urine to a USG > 1.030 by this point. The failure to do so confirms a true concentrating defect and rules out psychogenic polydipsia. The diagnosis is now confirmed as Diabetes Insipidus. The next step is to determine the type.
Phase 2 (ADH Response): An aqueous formulation of desmopressin acetate (DDAVP), an analog of ADH, was administered subcutaneously.
1 Hour Post-DDAVP: USG increased to 1.018.
2 Hours Post-DDAVP: USG increased dramatically to 1.035.
Conclusion of Phase 2: The dramatic and rapid increase in urine concentration after the administration of DDAVP demonstrates that the kidneys are perfectly capable of responding to ADH; they were simply not receiving the hormonal signal from the brain.
Definitive Diagnosis: Central Diabetes Insipidus (CDI). The underlying cause is likely idiopathic, as is common in many cases.
Treatment Plan: The treatment for CDI involves lifelong hormone replacement therapy.
Medication: The patient was prescribed Desmopressin Acetate (DDAVP) 0.1 mg/mL ophthalmic drops.
Administration and Dosing: The owner was instructed to instill one drop into the conjunctival sac of one eye every 12 hours. They were shown how to do this gently, without touching the bottle tip to the eye.
Monitoring and Titration: The owner will monitor the response at home by measuring Kodiak's daily water intake. The goal is to find the lowest dose and frequency (it may be possible to dose only once every 24 hours) that controls the clinical signs of PU/PD without causing over-concentration of urine or hyponatremia. The owner was instructed to keep a detailed log for the first two weeks.
Client Education: It was heavily emphasized that Kodiak must ALWAYS have free access to fresh water. If a dose is missed, he will temporarily become polyuric again, and restricting water at that time would be dangerous. The prognosis is excellent for a normal quality and duration of life with consistent medication and monitoring.
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