A 10-year-old neutered male Miniature Poodle presents wit...

Thank you for presenting this case. A 10-year-old Miniature Poodle with classic clinical signs of PU/PD and a pot-bellied appearance, coupled with bilateral adrenal hyperplasia on ultrasound, certainly points strongly towards hyperadrenocorticism (HAC), most likely pituitary-dependent hyperadrenocorticism (PDH). The "equivocal" low-dose dexamethasone suppression (LDDS) test, however, means we need to dig a bit deeper.

First, to clarify what "equivocal" means in this context: Was there some suppression at the 4-hour or 8-hour mark, but not enough to definitively rule out HAC? Or was there no suppression at all, but the baseline cortisol was not sufficiently elevated to definitively diagnose HAC? Understanding the precise cortisol values at each time point is crucial for interpreting the test. Stress during the test can also lead to equivocal results by elevating cortisol.

Next Diagnostic Step to Confirm or Rule Out Hyperadrenocorticism

Given the strong clinical suspicion and imaging findings, my primary goal now is to definitively confirm the diagnosis of hyperadrenocorticism and concurrently differentiate between pituitary-dependent hyperadrenocorticism (PDH) and adrenal-dependent hyperadrenocorticism (ADH), as this guides our treatment strategy.

Here are the most appropriate next steps, ranked by diagnostic utility in this scenario:

Endogenous ACTH (eACTH) Measurement:

* This is often my preferred next step in cases of equivocal LDDS, especially when adrenal hyperplasia is noted.
* Rationale: eACTH levels are the gold standard for differentiating PDH from ADH. In PDH, the pituitary is overproducing ACTH, so eACTH levels will be normal to high. In ADH, the adrenal tumor is autonomously producing cortisol, suppressing pituitary ACTH production, so eACTH levels will be low or undetectable.
* Confirmation: While primarily for differentiation, a high or normal eACTH in a dog with strong clinical signs and an equivocal LDDS helps confirm PDH.
* Logistics: Requires a specific collection tube (EDTA, chilled immediately, plasma separated and frozen quickly). Ensure the lab can perform this assay reliably.

High-Dose Dexamethasone Suppression Test (HDDS):

* Rationale: The HDDS uses a higher dose of dexamethasone (0.1 mg/kg IV) to assess whether the pituitary can still be suppressed. Approximately 75% of dogs with PDH will suppress their cortisol levels (typically <50% of baseline or <1.4 µg/dL at 4 or 8 hours post-dexamethasone), while dogs with ADH typically will not suppress.
* Confirmation: If suppression occurs, it confirms PDH. If no suppression occurs, it could be either ADH or a subset of PDH that doesn't suppress with high-dose dexamethasone (about 25% of PDH cases).
* Utility: This test can both help confirm HAC (if suppression occurs, it's HAC; if no suppression, it's still HAC, but ADH is more likely) and aid in differentiation.
* Consideration: If the LDDS was equivocal due to stress or technical issues, repeating a carefully performed LDDS with an HDDS might be considered, but eACTH is often more definitive for differentiation.

While an ACTH stimulation test is excellent for monitoring treatment, it is less sensitive than the LDDS for diagnosing HAC. If the LDDS was equivocal, an ACTH stim might give a false negative if the adrenal glands are not maximally responsive to exogenous ACTH, or if the dog has atypical HAC. Therefore, I would prioritize eACTH or HDDS for definitive diagnosis and differentiation in this case.

Preferred Initial Treatment for Pituitary-Dependent Hyperadrenocorticism

Once pituitary-dependent hyperadrenocorticism (PDH) is confirmed, the preferred initial treatment for medical management is Trilostane (Vetoryl).

Trilostane Protocol:

* Mechanism: Trilostane is a competitive inhibitor of 3-beta hydroxysteroid dehydrogenase, an enzyme involved in cortisol synthesis in the adrenal cortex. It reversibly inhibits the production of cortisol (and aldosterone).
* Dose: I recommend starting at the lower end of the dosing range, especially in a Miniature Poodle, as they can sometimes be more sensitive. The initial dose is 1-2 mg/kg orally BID with food. For a 10-year-old Miniature Poodle, I would typically start at 1 mg/kg PO BID.
* Administration: It is crucial to administer trilostane with food, as this significantly improves its bioavailability and absorption.
* Monitoring Schedule:
1. Initial Recheck (2 weeks post-initiation): Perform an ACTH stimulation test 4-6 hours post-pill. This timing is critical to assess the drug's efficacy at its peak activity. Also, check electrolytes (Na, K) and renal values (BUN, creatinine, SDMA) to monitor for potential Addisonian crisis or renal decompensation.
2. Second Recheck (4 weeks post-initiation): If the 2-week ACTH stim was within the target range (post-stim cortisol 1.4-5.4 µg/dL), repeat the ACTH stim test 4-6 hours post-pill, along with electrolytes and renal values. If the dog is still symptomatic or the post-stim cortisol is too high, a dose adjustment may be warranted.
3. Ongoing Monitoring (every 3 months thereafter, or sooner if clinical signs recur): Continue with ACTH stim tests 4-6 hours post-pill, electrolytes, and renal values. Clinical signs (PU/PD, polyphagia) should also be closely monitored, as these often guide dose adjustments.

Important Considerations for Clients:

* Addisonian Crisis: Educate owners on signs of hypocortisolemia (lethargy, vomiting, diarrhea, anorexia, weakness) and to discontinue trilostane immediately and contact you if these signs occur.
* Lifelong Treatment: Trilostane therapy is typically lifelong for PDH.
* Cost: Discuss the ongoing cost of medication and monitoring tests.

My confidence in this diagnostic and treatment plan is High confidence. We have a strong clinical picture, and the proposed steps are standard for resolving equivocal results and initiating appropriate therapy.