Presumed primary muscular lymphoma with ocular presentation in a cat

Case summary: Lymphoma is a prevalent malignancy in cats, commonly involving lymphatic or gastrointestinal tissues; however, primary muscular lymphoma is an exceedingly rare condition in veterinary medicine. This report describes a 7-year-old spayed female domestic shorthair cat that was presented for a 3-day history of progressive change in iris colour in the right eye. After rapid deterioration, the affected eye was enucleated and the cat was diagnosed with large T-cell lymphoma based on ocular histopathology and immunohistochemistry. The initial clinical signs were ocular; however, subsequent diagnostic evaluation revealed muscular involvement. Lymphoma was suspected to have originated in the quadriceps femoris muscle and subsequently metastasised to the eye. Relevance and novel information: This case underscores the diagnostic complexity of lymphoma and highlights the importance of considering systemic disease in cases of unexplained uveitis. It was not possible to determine whether the muscle or the eye was the primary site; clinical signs were first ocular, but this does not confirm the eye as the origin. The association between primary muscular lymphoma and ocular presentation has not been previously reported. Early detection of lymphoma can improve clinical management, but diagnostic challenges often arise because of atypical presentations and the absence of detectable masses.