ACVP 2017: Going Back to Basics to Interpret Pathologic Findings

According to Roger Kelly, BVSc, MVSc, PhD, registered veterinary specialist (pathology) and honorary research consultant at the University of Queensland School of Veterinary Science in Australia, diagnosticians should always consider basic knowledge of the principles of disease during the diagnostic process.

During an interactive session at the American College of Veterinary Pathologists 2017 Annual Meeting in Vancouver, British Columbia, Canada, Dr. Kelly used material from several diagnostic pathology cases to illustrate how veterinarians can use this knowledge to apply deductive logic to interpret pathologic changes in diagnostic reasoning.

There are really only 4 pathologic processes to consider, he stressed:

• Developmental (encompassing congenital and developmental anomalies)
• Inflammatory (acute, subacute, or chronic)
• Degenerative (including necrosis and intoxications)
• Proliferative (including neoplasia and hyperplasia)

However, he reminded the audience that diseases may include more than 1 of these 4 categories. One such example is a necrotizing malignancy, said Dr. Kelly, in which neoplasia, inflammation, and degeneration might be present. “So, it’s not always a clear-cut distinction,” he said, “but it gives you something to start with.”

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When faced with a diagnostic case, Dr. Kelly advised veterinarians to consider these 4 disease categories and, depending on the lesions present in the animal, use a process of elimination to determine which category most likely fits the case.

He shared digital images of lesions from postmortem examinations, along with case histories, to illustrate this process.

One case involved a dog with a history of progressive lethargy and weakness for several weeks before euthanasia. The most prominent gross finding at postmortem examination was diffuse yellow discoloration of the tissues, consistent with jaundice (icterus).

Dr. Kelly deduced from this finding that the developmental, inflammatory, and proliferative disease categories could be ruled out in this case, thus leaving degenerative disease as the most likely cause of the jaundice, he said.

Next, Dr. Kelly encouraged the audience to consider the 3 causes (pre-hepatic, hepatic, and post-hepatic) of jaundice. He noted that the dog in this case also had a diffusely enlarged spleen, suggesting that red blood cell destruction has occurred, he explained. If red cells were being destroyed in the circulation (intravascular hemolysis), Dr. Kelly added that evidence of hemoglobinuria would also be found in the form of dark red-brown discoloration of the kidneys and urinary bladder.

In this particular dog, however, the kidneys and urinary bladder were not discolored, thus suggesting extravascular hemolysis as the underlying mechanism involved. Dr. Kelly, therefore, deduced that immune-mediated hemolytic anemia was the most likely cause of the jaundice.

He contrasted this case with images of postmortem findings from a sheep with jaundice caused by chronic copper toxicity. This animal had an acute hemolytic crisis, Dr. Kelly said. The liver was abnormally pale orange in color. And the kidneys and urine were dark red-brown stained by hemoglobin because of the massive intravascular hemolysis characteristic of this condition. Signs of chronic copper toxicity arise when the capacity of the liver to store copper is exceeded, resulting in the sudden release of copper into the circulation. The characteristic combination of liver damage and intravascular hemolysis led to jaundice in this sheep.

Dr. Kelly concluded that gross postmortem findings can play a significant role in diagnostic reasoning, and thus urged the audience not to forget the deductive process when performing a postmortem examination. After opening the body cavities, and before removing tissues, diagnosticians should “stop, stand back, look, and think” about the gross lesions present, he emphasized, “because everything you do after that will disrupt the scene,” making it likely that some findings might be missed.