Magnetic Resonance Imaging Characteristics of Hereditary Polymyositis in the Dutch Kooiker Dog

Background: Hereditary immune-mediated polymyositis has been reported in the Kooiker dog breed, associated with a 39 kb deletion and low penetrance. Approximately 10–20 percent of homozygous dogs and 0.5–2 percent of heterozygous dogs develop polymyositis. This study examines whether magnetic resonance imaging (MRI) can assist in diagnosing polymyositis in this breed. Methods: All dogs in this prospective case study were purebred Kooiker dogs referred for clinical examination to assess them for polymyositis. A dataset was compiled, including sex, neuter status, and, if applicable, age of onset, clinical signs, CK activity, electromyogram, and histopathological findings. MRI was performed using a 1.5 Tesla MRI scanner, with T1-weighted, T2-weighted, T2W fat-suppressed short tau inversion recovery (STIR), and T1-weighted post-contrast sequences. Results: Five Kooiker dogs were included in the study. Four dogs exhibited clinical signs compatible with polymyositis (one heterozygous and three homozygous for the 39 kb deletion), while one dog was homozygous for the 39 kb deletion but showed no clinical signs. The clinically affected dogs exhibited T2-weighted, STIR, and T1-weighted post-contrast muscular hyperintensity, and the diagnosis was confirmed with histopathology. The asymptomatic dog displayed no MRI abnormalities. Conclusions: MRI has proven to be a valuable tool in assisting with the diagnosis of Kooiker dogs carrying the 39 kb deletion. MRI can act as a screening tool for dogs with the 39 kb deletion, eliminating the need for an initial biopsy. A muscle biopsy, following a confirmatory MRI, is still the preferred method for diagnosing polymyositis.