Clinical, histopathological and immunohistochemical characterization of a novel equine ocular disorder: heterochromic iridocyclitis with secondary keratitis in adult horses.

This research explored a novel equine ocular disorder, known as heterochromic iridocyclitis with secondary keratitis (HIK), characterizing its clinical presentation and the corresponding pathological and immunological features. The study found that horses with HIK show signs of anterior uveitis and corneal endothelial inflammation along with iris pigment dispersion and retrocorneal fibrous membrane formation, with some needing enucleation to alleviate discomfort.

Study Methods

• The researchers carried out a retrospective study on sixteen adult horses, aged between 9 and 25 years, showing evidence of the disease. They collected information from medical records including the horses’ signalment, clinical signs, duration and response to previous treatments, findings from ophthalmic examinations, postreferral treatment, response, and outcome.
• The first signs observed in the horses affected by HIK were blepharospasm (uncontrollable twitching of the eyelid), epiphora (excessive tearing), and/or corneal opacification. Common findings at the time of referral comprised pigmented keratic precipitates (KPs), corneal edema, iris depigmentation, and retrocorneal membrane formation.
• The horses received treatment in the form of topical and/or systemic anti-inflammatories and antibiotics, with varying degrees of success. An observed trend was that reducing or stopping anti-inflammatory therapy led to worsening of clinical signs and disease progression. This led to the need for enucleation (surgical removal of the eyeball) in eight of the affected eyes.

Key Findings

• The researchers documented histopathology changes such as loss and dispersion of iris pigment, retrocorneal fibrous membrane formation, and keratitis (inflammation of the cornea).
• The team also noted varying degrees of lymphoplasmacytic inflammation characterized by T-cell dominance within the corneal stroma, retrocorneal fibrous membrane, iris, and ciliary body with occasional presence of multinucleated giant cells.
• In conclusion, the research characterized heterochromic iridocyclitis with secondary keratitis by uveal inflammation along with pigment dispersion and suspected corneal endothelial dysfunction. It underscored the importance of rigorous and frequent follow-up examinations as well as intensive local immune suppression to manage the disease. Despite best efforts in treatment, the study highlighted that HIK may not respond to therapy and enucleation may ultimately be needed to maintain the horse’s comfort.