A thorough ophthalmic examination in the field

Approximately 10 % of equine emergencies involve the ocular region. In general, it is thought that more than 50 % of horses have some kind of ophthalmic lesion. Therefore, a thorough ophthalmic examination is extremely important for the equine practitioner. The knowledge of anatomy (and the range of normal appearances of ocular and periocular structures) and physiology/pathophysiology are the basis for an ophthalmic examination, but also the how to assess ocular function and perform ocular diagnostic tests.

Before starting the clinical exam obtaining a thorough general and specific history and noticing the signalment of the patient will help to look for certain problems, like insidious ERU (equine recurrent uveitis) in horses with leopard complex (like Appaloosas or Knabstruppers) or squamous cell carcinoma in Haflingers. Young horses or foals are more likely to present with congenital defects and geriatric horses might present with senescent ocular changes or ocular problems related to a concurrent disease, like PPID (pituitary pars intermedia dysfunction). Regarding the history it has to be noted, that most eye diseases have already been treated and it is paramount to get information on applied medication, etc.

The first assessment of the patient should include the observation from a distance with regards to general condition, behaviour, comfort, and visual deficits. Before starting the ophthalmic examination, a clinical examination must be performed, not only to look for comorbidities but also because many systemic diseases have ocular manifestations. Furthermore, the majority of horses, especially when there is a painful eye condition, need to be sedated for a complete eye examination. In some performing periocular nerve blocks and using topical ophthalmic anaesthetic and a mydriatic agent can further facilitate ocular examination.

First, facial and ocular symmetry should be assessed, followed by an evaluation of cranial nerve function and vision testing (for example palpebral reflex, menace response, dazzle reflex and pupillary light reflexes) and palpation of the orbital rim as well as digital tonometry to subjectively assess ocular pressure. Following a standardised approach minimises the risk to miss lesions. Always both eyes must be examined. Detailed examination of ocular structures should be performed starting from the outside and proceeding to the inside of the eye using a bright light source and a direct ophthalmoscope: specifically, eyelids, third eyelid, conjunctiva, sclera, cornea, anterior chamber, iris, lens, vitreous and fundus. An additional test, which is easy to perform, is the fluorescein stain for corneal assessment and nasolacrimal patency. Additional examinations, which can be performed in the field, also depending on equipment and complaint are tonometry, further stains (like Rose Bengal) or a Schirmer tear test (which has to be performed before manipulation of the eye), taking samples for cytology and culture and very importantly, in case of a “cloudy eye”, which prevents evaluation of deeper structures, ocular ultrasound.

Summarising all detected ocular abnormalities and test results should lead to an ophthalmic diagnosis and a decision, if further complementary tests need to be performed or if referral is needed as well as the selection of the most appropriate (initial) treatment.