Companion animal autopsies at the AHL: What do we do to assist practitioners?
Margaret Stalker, Beverly McEwen, Michael Deane
As part of the AHL data contribution to the new Companion Animal Expert Network of the Ontario Animal Health Network (OAHN), canine and feline submissions to the AHL-Guelph and AHL-Kemptville for the last quarter of 2015 and the first quarter of 2016 were reviewed to determine: 1) reason for submission for autopsy examination, and 2) final diagnosis.
Cases were sorted into 8 general categories based on reason for submission for autopsy examination:
· Investigation of anesthetic-associated deaths (including death during premedication, induction, maintenance, or the immediate recovery period following anesthesia, for a variety of procedures).
· Investigation of deaths resulting from post-surgical complications.
· Investigation of deaths possibly associated with an adverse drug or vaccination reaction.
· Determination of cause of sudden unexpected death, or animal found dead.
· Determination of underlying disease process in ill animal, died or euthanized.
· Determination of underlying disease process in ill animal exhibiting neurologic signs, died or euthanized.
· Determination of cause of abortion/stillbirth.
· Medicolegal/forensic investigations.
Although investigations into cause of sudden unexpected death and cause of illness are the most common case types, medicolegal/forensic cases and investigations into anesthetic-related death also form a significant part of our companion animal autopsy caseload (Fig. 1).
An underlying cause of death for anesthetic-related deaths was determined in 4 of 10 cases in dogs (various diagnoses, including cardiomyopathy and congestive heart failure), and in 2 of 3 cases in cats (cardiomyopathy).
The leading cause of sudden death in dogs is hypovolemic shock or cardiac tamponade due to hemorrhage from a ruptured neoplasm (10 of 22 cases), usually hemangiosarcoma (7 cases).
The leading cause of sudden death in cats was cardiomyopathy (6 of 10 cases).
A wide variety of diagnoses were made in the other categories of investigation.
Figure 1. Reasons for autopsy of dogs and cats at the AHL, Oct, 2015 - March, 2016.
Update on the diagnosis of canine mast cell tumors
Since our previous newsletter article on the diagnosis of canine mast cell tumors (Cytology and surgical biopsy of canine mast cell tumors, 2010), there have been several changes in pathologists’ approach to this disease. A commonly used grading scheme for dermal mast cell tumors (MCT), developed by Patnaik in 1984, suffered from a limited concordance (64%) for the diagnosis of grade 1 and 2 tumors, and 75% for the diagnosis of grade 3 tumors, with the majority of mast cell tumors categorized as grade 2. This grading scheme was also often used in the prognostication of subcutaneous mast cell tumors, though it was not originally developed for this purpose and it has long been recognized that MCT originating in subcutis often have a better prognosis (relative to dermal MCT). Since then, 2 studies have evaluated canine cutaneous and subcutaneous MCT as independent entities with an eye to developing more objective methods of subclassifying these lesions into those more likely to behave in a benign or malignant fashion.
Kiupel et al (2010) subclassified cutaneous MCT into 2 categories, low-grade and high-grade, with a weighted agreement between pathologists of 99.3%. High-grade cutaneous MCT were characterized by 1 of the following: the presence of 7 or more mitotic figures in ten 400X fields, 3 multinucleated (3+ nuclei) cells in ten 400X fields, 3 or more bizarre nuclei in ten 400X fields, or karyomegaly. Dogs with high-grade cutaneous MCT had a median survival time <4 months, whereas dogs with low-grade dermal MCT (Fig. 1)had a median survival time >2 years. Overall, this new 2-tier grading scheme is a better predictor of survival than the traditional Patnaik grading scheme, and as such it is a valuable standard to apply to these tumors.
In addition, Thompson et al., 2011, evaluated subcutaneous MCT, and dogs with subcutaneous MCT with <4 mitotic figures in ten 400X fields had a mean survival time >891 days. Dogs with subcutaneous MCT with >4 mitotic figures in ten 400X fields had a median survival time of 212 days, reduced to 140 days with the presence of infiltration and multinucleation. Because of the prognostic difference between cutaneous and subcutaneous MCT, it is important to collect a sample that will represent this location by including skin and subcutis.
In a proportion of cases, diagnosis may be aided by histochemical (‘special’) stains (toluidine blue stain) or by immunohistochemistry (c-KIT/CD117, Ki-67). Additional external assessment is available, including c-KIT PCR and AgNOR evaluation, which may provide additional therapeutic and prognostic information for individual cases. Your surgical pathologist may order special stains on your submission to confirm the diagnosis, and may advise you on further testing recommendations.
Because the cytologic features of canine MCT are usually straightforward, cytology provides the best option for initial diagnosis prior to surgical planning (Fig. 2). A very recent study by Camus et al., 2016, has also provided a novel method for grading cutaneous MCT based on cytologic features, principally mitotic indices, presence/absence of binucleate or multinucleate cells, nuclear pleomorphism, or dramatic anisokaryosis. Cytologic grade was found to correlate well with histologic grade, and mean survival times were similar between cytologic and histologic grading categories. Further investigation and broader clinical validation is required before this can be applied as a routine procedure with cytologic examination of MCT.
Figure 1. Histopathology from a low-grade canine cutaneous mast cell tumor, 600X magnification, Wright's stain. Numerous well-granulated mast cells separated by edema, moderate numbers of eosinophils and small quantities of hemorrhage.
Figure 2. Aspiration cytology from the same low-grade canine cutaneous mast cell tumor, 600X magnification, Wright's stain. Numerous widely spaced and well-granulated mast cells, some hemorrhage, and few eosinophils.