Equine Opthalmic Exam Flashcards
Describe the unique features of the equine eye
-Prominent globes, laterally positioned, placed high on head
-horizontally elliptical pupil
-granulae iridica/corpora nigra- extension of posterior iris to protect too much UV light from entering retina
How large is the equine visual field?
350 degree overall field
-146 degree monocular field laterally, 65 degree binocular field in front of them, blind spot at forehead and tail
Losing one eye can have a major impact
What order should you perform an ophthalmic exam in horses?
1: first examine in a stall or round pen and evaluate vision and behavior (how can they navigate their surroundings)
2: examine from afar in a well lit environment for symmetry, orbital abnormalities, eyelash position, pupil size epiphora
3: Examine up close in a darkened room or stall
-utilize restraint and sedation, and regional nerve blocks
What is the normal position of the equine eyelashes? If the eyelashes are pointed up or down, what may this indicate?
Normal: eyelashes perpendicular to the eye
Pointed down: indicates enophthalmos
Pointed up: exophthalmos
How should you assess the cranial nerves during an equine ophthalmic exam?
-assess before any sedation is induced
-menace response (avoid contacting the vibrissae)
-dazzle reflex using a bright light source
-palpebral reflex on both the medial and lateral canthi
-PLRs- indirect is less prominent in horses due to decussation at the chiasm
What nerve blocks should be performed to perform a detailed ocular exam in horses?
Auricularpalpebral should be performed in almost every case as pressure applied during maipulation during examination can lead to globe damage and the orbicularis oculi muscles are extremely strong
-may add on supraorbital or frontal block to block out the sensation to the eye
Describe the auriculopalpebral block
Blocks the palpebral branch of the auricular palpebral nerve (branch of the facial)
-causes paralysis of the orbicularis oculi muscle (most of upper eyelid)
-use a 25 g needle and inject 1 mL local, lasts 1-2 hours (should apply ocular lubricants)
-landmarks are caudal to posterior ramus of mandible, dorsal to highest point of zygomatic arch, on zygomatic arch caudal to bony process of the frontal bone
Describe the supraorbital or frontal block
Blocks the ophthalmic branch of the trigeminal nerve
-causes lack of sensation to the central upper eyelid
-use 25 ga needle and inject 1 mL local, lasts 1-2 hours
-to identify-rule of thumb is to place thumb below the dorsal orbital rim and place middle finger in supraorbital fossa, then place index finger straight down, midway between the thumb and the middle finger to locate the supraorbital foramen
What is the normal tear production and intraocular pressure in a horse? What else can be performed on eye exams if indicated?
Tear production: >20 mm/min (tear deficiencies are rare)
IOP: 15-30 mm Hg. Can use rebound or applanation tonometry (requires topical anesthetic)
-can also do culture and cytology if appropriate, or fluorescein stain if suspecting ulcer (inject with syringe- diluted strip, break needle from hub)
What are the important adnexa that should be assessed in every eye exam?
Eyelids and nictitating membrane
-conjunctiva
-nasolacrimal system
Look for ectopic cilia (common in morgans), Jones test (to test patency of nasolacrimal duct)
If you see an ulcer over the middle of the eye, what is one thing that should be closely assessed?
The third eyelid- suspicious of foreign body
Describe the Jones test
Apply fluorescein stain and measure how long it takes for stain to come out of nose
- should take around 15-20 min
When should irrigation of the nasolacrimal duct be performed?
-epiphora, mucoid ocular discharge, nasal puncta discharge
Describe the techniques for flushing the nasolacrimal system
Retrograde: from distal nares openings. Easiest to perform. Use 5 or 6 polyethylene urinary catheter and 12-20 mL syringe with sterile eyewash +/- Fl stain
Normograde: from proximal eyelid puncta. Lower eyelid puncta is usually larger, use 8-20 g IV cannula
-excessive force can damage the duct
What should you observe on the ocular surface?
Cornea and sclera
-look for corneal edema, vascularization, lacerations, foreign bodies
-document everything!
T/F: you can see the iridocorneal angle (drainage angle) in a horses eye without any special equipment
True-unique to horses
What intraocular structures should be assessed on eye exam?
Anterior chamber, iris
-look for aqueous flare (tyndall effect), posterior synechia (caused by uveitis), iris hypoplasia, uveal cysts, corpora nigra atrophy, hyperpigmentation of iris
-lens- assess with direct focal illumination (transillumination), and retroillumination (light reflexted by tapetum. Dilate eye to assess this (with tropicamide, use 20 min prior to exam)
What should you look at on your fundic exam?
-optic disc: should be oval, salmon-pink and located in the nontapetal fundus
-retinal vessels: 30-60 small ones radiating from optic disc
-fibrous tapetum- color variation is common
What are the “stars of winslow”?
Black dots that can be seen on the tapetum
-choroidal vessels
What are the methods for performing your up close ophthalmic exam of fundus?
Indirect fundoscopy-gives you a wider field of view with less magnification, image is inverted and reversed. Used for general exam
Direct fundoscopy: allows you to see structures under even higher magnification, image seen is upright
Describe the anatomy of the horses orbit
Complete bony orbital rim comprised of the frontal, lacrimal, zygomatic and temporal bones
-deep internal wall composed of the sphenoid and palatine bones
What are most of the extraocular muscles innervated by?
Oculomotor nerve innervates most of them
-lateral rectus innervated by abducens nerve
-retractor oculi muscle innervated by abducens n
-dorsal oblique innervated by trochlear nerve
What orbital diseases can cause the eye to have a sunken appearance?
Enophthalmos, microphthalmos, phthisis bulbi (game over)
Describe enophthalmos and list some potential causes
Recession of the globe within the orbit
-usually occurs secondary to loss of orbital contents
Causes: orbital fractures, resorption of orbital fat, dehydration in foals (will lead to entropion), sympathetic denervation (horners- uncommon in horses)
