The Field Eye Exam Toolkit: Essential Techniques for Equine Ophthalmology
Practical guidance for systematic ocular evaluation in equine practice
Equine ophthalmology becomes manageable with the right approach. Dr. Rachel Allbaugh, DACVO, emphasizes that early recognition and proper examination techniques are critical for preserving vision in horses.¹
Setting Yourself Up for Success
Moving horses to a dark environment whenever possible will make your life dramatically easier, as bright sunlight causes pupils to constrict, limiting your view of intraocular structures.¹ If indoor facilities aren’t available, consider using a lean-to, forest area, or creating a tent-like environment with a blanket over your head and the horse’s head (after sedation).¹
Sedation is invaluable for thorough examinations, particularly with painful eyes or uncooperative patients.¹ Standard alpha-2 agonist protocols provide adequate restraint, though a shoulder or nose twitch may be necessary for additional control.¹
The Auricular Palpebral Nerve Block: Your Best Friend
The auricular palpebral nerve block is essential for safe, thorough eye examinations in horses with painful eyes. This technique denervates the orbicularis oculi muscle, preventing squinting and blinking.¹
Technique: Locate the nerve by running your finger along the zygomatic arch where you’ll feel a “linguini-like slip” as the nerve passes over the bony rim.¹ Tent the skin and insert a 25-gauge needle, then inject 1-2 cc of mepivacaine (preferred over lidocaine as it’s less irritating).¹ Always aspirate before injection.¹
Duration: The nerve block typically provides 1-2 hours of motor blockade.¹
The frontal (supraorbital) nerve block provides additional motor denervation as an adjunct.¹ Locate the supraorbital foramen by pinching the dorsal orbital rim at its narrowest point with your middle finger and thumb, then rotating your index finger down to find the depression.¹
Systematic Examination Approach
Use a consistent approach starting with distance examination and progressing to detailed assessment:¹
Distance assessment (retroillumination): Stand several feet away and shine a bright light toward the horse’s face, bouncing it back from the fundus. This reveals pupil size and symmetry while highlighting any opacities.¹ Shadows that move with eye movement indicate anterior pathology (cornea, anterior chamber, or front of lens), while shadows moving opposite to eye movement suggest posterior lesions.¹
Cranial nerve testing: Assess pupillary light reflexes (both direct and indirect), remembering that PLRs are much slower in horses than in dogs or cats.¹ Sometimes it’s easier to watch for rebound dilation when assessing indirect responses.¹ Perform menace response testing while covering the opposite eye.¹
Structural examination: Progress from orbit to adnexal tissues (eyelids, third eyelid) to ocular structures, moving from outside to inside.¹ Always retropulse the globe to evaluate the third eyelid, particularly important in breeds predisposed to squamous cell carcinoma.¹
Essential Diagnostic Tests
Fluorescein staining should be performed on every horse presenting with an eye concern.¹ This water-soluble dye adheres to exposed corneal stroma when epithelium is missing, making it essential for diagnosing corneal ulcers.¹ Apply by touching the impregnated strip to the ventral lacrimal lake.¹
Schirmer tear testing is rarely needed in horses but becomes important in cases of facial nerve paralysis or chronic non-healing ulcers.¹ Normal values in horses are typically over 12mm, often ranging 25-30mm.¹
Tonometry (intraocular pressure measurement) helps identify uveitis (pressure typically <15 mmHg) or glaucoma (pressure >30 mmHg).¹ Normal equine IOP ranges from 15-30 mmHg.¹ The Tono-Vet is ideal for horses as it has equine corneal calibration, while Tono-Pen devices may underestimate high pressures.¹
If tonometry equipment isn’t available, digital tonometry using alternating pressure with both index fingers over the upper eyelid can help identify abnormally firm (glaucomatous) or soft (uveitic) eyes.¹
Ocular ultrasound provides valuable information when the visual axis is compromised.¹ A normal equine eye measures approximately 4 cm in diameter. Use 7.5 MHz for orbital evaluation and 10 MHz for detailed ocular assessment.¹ The transcorneal approach provides better anterior segment visualization, while transpalpebral approach may be necessary for painful, swollen eyes.¹
Using Your Direct Ophthalmoscope Beyond Fundoscopy
The slit beam assesses corneal curvature, helping differentiate superficial ulcers (normal curvature) from deeper stromal involvement (flattening or indentation).¹ The small circle beam is excellent for detecting aqueous flare by creating a focal beam of light through the anterior chamber while observing from the side for the “headlight in fog” effect of suspended protein.¹
When to Refer
Key indicators for ophthalmologic referral include:¹
- Non-responding infected or melting ulcers
- Deep corneal ulcers (50% depth or greater)
- Ruptured eyes requiring surgical intervention
- Deep corneal stromal abscesses unresponsive to medical therapy
- Suspected immune-mediated corneal conditions
- Extensive orbital or ocular neoplasia
Many seemingly severe conditions, including corneal perforations with iris prolapse, can heal with aggressive medical management if the eye retains light perception.¹
Successful equine ophthalmology comes down to systematic approach, proper technique, and early recognition. With these tools and techniques, you can provide comprehensive eye care for your equine patients.
This content is based on lectures by Dr. Rachel Allbaugh, DACVO, presented at VetOnIt CE.
Continue Learning: Explore Advanced Equine Ophthalmology Techniques
Tags: Ophthalmology, Equine, Large Animal
References
- Allbaugh, R. Introduction to Equine Ophthalmology: Examination Using Your Eyes, Instruments and Ultrasound. VetOnIt CE Webinar, 2025.