Cushing’s disease is one of the most complex endocrine disorders in dogs. Also known as hyperadrenocorticism, it’s caused by excessive production of cortisol – the stress hormone – typically due to a benign pituitary gland tumor. There are two main types of Cushing’s: pituitary dependent (PDH), which accounts for 80-85% of cases, and adrenal dependent (ADH).
Atypical Cushing’s falls under the minority adrenal dependent category, presenting unique diagnostic challenges. Keep reading to understand how vets diagnose this tricky form of the disease.
What is Atypical Cushing’s Disease?
With typical PDH Cushing’s, the pituitary gland in the brain secretes excess ACTH hormone, which then overstimulates the paired adrenal glands to produce too much cortisol.
In atypical ADH Cushing’s, a tumor within one of the adrenal glands itself autonomously secretes cortisol without ACTH involvement. Around 15-20% of Cushing’s cases are this atypical form.
Atypical Cushing’s usually only affects one adrenal gland, causing asymmetric enlargement detectable via ultrasound. Dogs with atypical Cushing’s tend to exhibit more severe symptoms like vomiting, lethargy, and flank alopecia (hair loss).
Challenges in Diagnosing Atypical Cushing’s
The complex feedback loop between the pituitary, adrenal glands, and end organs makes diagnosing atypical Cushing’s tricky. Tests often yield confusing rather than definitive results.
Pituitary Tumors Can Mimic Adrenal Disease – In 10% of dogs, a macroadenoma pituitary tumor can physically compress the adjacent adrenal glands, also leading to excess cortisol. Clinically these dogs will present like atypical Cushing’s, but respond best to pituitary-directed treatment.
Cyclical Hormone Production – Cortisol levels in dogs with atypical Cushing’s frequently fluctuate, yielding normal results on some tests. Repeated testing is often needed to capture cycles of cortisol excess.
Concurrent Health Conditions – The stress of illness raises cortisol naturally, obscuring Cushing’s testing in dogs with concurrent infections, diabetes, organ disease, etc. These must be stabilized first.
Due to these challenges, vets use a combination of diagnostic tests to definitively diagnose atypical Cushing’s disease.
Key Diagnostic Tests for Atypical Cushing’s
Confirming adrenal dependent Cushing’s relies on clinical signs, physical exam, and multiple lab tests. Key diagnostic steps include:
Clinical Signs & History
- Increased thirst/urination
- Ravenous appetite
- Potbelly & muscle wasting
- Recurring UTIs or skin infections
- Lethargy, panting, vomiting
- Alopecia on flanks or tail base
- Thinning/fragile skin
Physical Exam
- Obesity, especially above rib cage
- Potbelly & distended abdomen
- Muscle wasting over shoulders & hips
- Excessive panting
- Thin, wrinkled skin with poor healing
Complete Blood Count (CBC)
- High white blood cell count indicates infection
- Low red blood cells suggest anemia
- High liver enzymes may indicate liver disease
- Electrolyte imbalances from cortisol excess
Urinalysis
- Low urine specific gravity indicates impaired kidney concentrating ability from cortisol
- Presence of infection requires antibiotic treatment
Adrenal Gland Ultrasound
- Detects unilateral adrenal gland enlargement in atypical Cushing’s
- Helps distinguish adrenal vs pituitary tumors
- Guides surgical treatment if adrenalectomy elected
Low-Dose Dexamethasone Suppression Test (LDDST)
- Failure to suppress cortisol after dexamethasone administration confirms Cushing’s
ACTH Stimulation Test
- Excessive cortisol response to ACTH injection indicates an adrenal tumor source
High-Dose Dexamethasone Suppression Test
- 8-hour cortisol suppression indicates pituitary origin while lack of suppression points to adrenal tumor
Endogenous ACTH Levels
- Low levels confirm adrenal tumor as the cortisol source
Through a combination of clinical signs, imaging, hormonal testing, and response to medications like dexamethasone, vets determine a diagnosis of atypical Cushing’s disease. Proper diagnosis guides appropriate treatment to relieve the symptoms of this complex endocrine disorder.
Frequently Asked Questions
What causes atypical Cushing’s disease?
Atypical Cushing’s is caused by a non-cancerous tumor in one adrenal gland secreting excessive amounts of the hormone cortisol. The specific trigger for adrenal tumor formation is unknown.
Is atypical Cushing’s in dogs fatal?
If left untreated, the detrimental effects of excess cortisol can eventually be fatal. However, there are effective treatment options for managing atypical Cushing’s to extend a dog’s life.
Is surgery an option for atypical Cushing’s?
Yes, surgical removal of the affected adrenal gland (adrenalectomy) may cure dogs with a solitary adrenal tumor and is performed at some specialty centers.
What medications treat atypical Cushing’s?
Medications like Lysodren and Trilostane help reduce circulating cortisol levels. Proper dosing tailored to an individual dog is key for safety and efficacy.
What is the long-term prognosis with atypical Cushing’s?
Early diagnosis and appropriate treatment helps stabilize most dogs with atypical Cushing’s for months to years depending on the case severity and dog’s age. Regular monitoring and medication adjustments are critical for optimal outcomes long-term.
Conclusion
Atypical, adrenal dependent Cushing’s disease presents unique diagnostic hurdles due to fluctuating hormones and crossover of clinical signs with other conditions. Through a combination approach assessing clinical presentation, imaging, and select lab work, veterinarians can discern atypical Cushing’s from other diseases. Accurate diagnosis guides appropriate treatment to manage this challenging endocrine disorder and improve quality of life for affected dogs.
