In this article, we'll discuss warning signs for both the typical and atypical forms, best diagnostic practices, and treatment methods to improve patient outcomes.

What is Addison's Disease?

Clinical signs include:

• Lethargy and weakness: Additionally, dogs with Addison's disease may also be hesitant to exercise.
• Gastrointestinal (GI) issues: This can include vomiting, diarrhea, or weight loss, which may occur intermittently, adding to the challenge of diagnosis.
• Polyuria and polydipsia (PU/PD): Electrolyte imbalances typically affect the kidneys, leading to this condition.
• Hypothermia: Acute cases can lead to a lower body temperature; clinically, this looks like lethargy, shaking, stiff muscles, and, in some cases, discoloration of gums.
• Bradycardia: Electrolyte imbalances in sodium and potassium levels create this low heart rate.

In severe cases, patients with these signs can present with a potentially life-threatening health crisis. Early diagnosis is essential for effective treatment and preventing these emergencies. However, identifying Addison's disease can be challenging because the clinical signs are often vague or nonspecific.

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Classic vs. Atypical Addison's Disease

Addison's disease presents in two ways: classic and atypical. Below, we'll differentiate between the two, offering insight as to why atypical Addison's can be more difficult to detect.

Identifying Classic Addison's Disease

A classic Addison's disease canine patient is deficient in both glucocorticoids and mineralocorticoids, leading to significant electrolyte imbalances. On a chemistry panel, veterinarians will notice hyponatremia (low sodium levels), hyperkalemia (high potassium levels), and a low sodium-to-potassium ratio, which is typically less than 27:1.

The sodium-to-potassium ratio is often a key diagnostic indicator, as a lack of aldosterone causes sodium loss and potassium retention.

Identifying Atypical Addison's Disease

Conversely, atypical Addison's disease mainly involves a deficiency in glucocorticoids (cortisol), while mineralocorticoid function might remain normal. As a result, patients often have normal electrolyte levels, which makes diagnosis more challenging. Instead, atypical Addison's usually presents with nonspecific clinical signs, like fatigue and gastrointestinal problems, that do not improve with standard treatments.

Due to these challenges, veterinarians should consider Addison's as differential when patients present with the following clinical scenarios:

• Non-responsive signs: Canine patients who present with persistent nonspecific signs, including vomiting, diarrhea, lethargy, weight loss, or PU/PD, that don't respond to standard treatment.
• Breed susceptibility: Some breeds are genetically predisposed to have Addison's, including Standard Poodles, Rottweilers, and Great Danes.
• Stress-related flare-ups: Intermittent clinical signs are a warning sign, but even more so when they intensify after periods of stress, including infection, trauma, boarding, or surgery.

Screening for Addison's Disease with Baseline Cortisol

One of the most common tests used for initial screening of Addison's disease is the baseline cortisol test. This blood test measures dogs' baseline cortisol levels under normal conditions. The primary purpose of this test is to rule out Addison's disease when cortisol levels are above a clinical decision point.

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A thorough understanding of a test's specificity will enable veterinary teams to appreciate its clinical usefulness. Specificity refers to the test's ability to correctly identify dogs that do not have Addison's disease. Baseline cortisol testing only has a high specificity of 91% when results are < 1 μg/dl, but specificity drops to 63% if results are 1-2 μg/dl. However, this test has 100% sensitivity, meaning that if a patient has a resting cortisol greater than 2 μg/dL, the patient is highly unlikely to have Addison's disease, and further testing is rarely required.

While baseline cortisol is a practical screening test, it is not a definitive measure. A screening showing low baseline cortisol indicates potential adrenal insufficiency and requires further testing with an ACTH stimulation test—the gold standard for diagnosing Addison's disease.

With more recent advancements in diagnostic technology, real-time cortisol testing is available for veterinary practices. This allows for almost immediate results during an exam, enabling faster decision-making in suspected cases and for earlier diagnosis in emergencies.

Screening for Addison's Disease in Chronic GI Workups

Veterinarians should always consider Addison's disease when evaluating chronic gastrointestinal signs, especially in dogs experiencing recurrent vomiting, diarrhea, weight loss, or a lack of appetite.

In these cases, a chronic GI workup should include a baseline cortisol test. If the baseline cortisol level is low or borderline, indicating possible Addison's, additional testing with an ACTH stimulation test is necessary.

Early Identification Improves Patient Outcomes

Vague and common clinical signs make diagnosing Addison's disease challenging. Veterinary teams often overlook signs such as chronic GI distress, stress-related flare-ups, lethargy, and PU/PD when performing tests. However, by maintaining a high index of suspicion and using strategic diagnostics, veterinarians can detect this disease early, transforming a crisis into a manageable condition.

As veterinarians identify more cases earlier, patient outcomes will improve, and veterinary teams will gain confidence in recognizing and treating "the great pretender."

References:

1. Kintzer, P. P., & Peterson, M. E. "Diagnosis of canine Addison's disease: Sensitivity of different diagnostic tests." Journal of the American Veterinary Medical Association, vol. 250, no. 1, 2017, pp. 56-62.
2. Peterson, M. E., & Kintzer, P. P. "Diagnosis of Addison's disease in dogs: ACTH stimulation test revisited." Journal of the American Veterinary Medical Association, vol. 246, no. 8, 2015, pp. 900-904.
3. Kooistra, H. S., & van der Meer, F. "Hypoadrenocorticism: Pathophysiology and clinical diagnosis." Journal of Endocrinology and Metabolism, vol. 15, no. 4, 2018, pp. 228-236.
4. McAlister, E. "Addison's disease in dogs." Veterinary Practice News, 2017, Retrieved from https://www.veterinarypracticenews.com.
5. Simpson, K. W., & Rishniw, M. "Canine hypoadrenocorticism." Veterinary Clinics of North America: Small Animal Practice, vol. 49, no 3, 2019, pp. 463-475.
6. Bovens, C., et al. "Basal serum cortisol concentration as a screening test for hypoadrenocorticism in dogs." Journal of Veterinary Internal Medicine, vol. 28, no. 5, 2014, pp. 1541–1545.

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