Intraocular Tumors in Horses: Diagnosis, Tumor Classification, Oncologic Assessment and Therapy

Intraocular neoplasia in horses is rare and only few case reports and small case series exist. Intraocular neoplasia has various clinical signs and includes important differential diagnoses in ocular disease. This narrative review of the current literature aims to provide a clinically relevant overview and classification of intraocular tumors in horses and adds a comparative oncological perspective concerning diagnosis, treatment and future considerations. The available clinical and imaging examination techniques allow for a reliable and differentiated investigation of the tumor, even in the standing horse, using high-frequency ultrasound or optical coherence tomography, which have gained importance in equine ophthalmology. Sectional imaging techniques, in particular computed tomography, are suitable for the examination of the peribulbar, retrobulbar and orbital structures. Differentiated diagnostics including precise tumor staging (TNM: tumor, node, metastasis) are essential for a general prognostic and therapeutic assessment. The embryologic and anatomic tissue origin of the neoplasm is the basis for clinicopathologic classification. Medulloepithelioma and uveal melanocytic neoplasia are the most common intraocular tissue formations occurring in horses. Whereas melanocytic neoplasia of the iris can be treated surgically, neuroepithelial tumors regularly lead to bulbus extirpation. Other primary intraocular neoplasms are sporadically reported, as well as intraocular metastasis of systemic neoplasia. Chemotherapy and radiation therapy are not currently used to treat intraocular neoplasia in horses and need to be further investigated, especially regarding the latest developments in human and small animal medicine. In addition, horses and dogs may serve as models for human oncologic research.