Equine Opthalmic Exam

Question 1

Describe the unique features of the equine eye

Answer 1

-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

Question 2

How large is the equine visual field?

Answer 2

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

Question 3

What order should you perform an ophthalmic exam in horses?

Answer 3

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

Question 4

What is the normal position of the equine eyelashes? If the eyelashes are pointed up or down, what may this indicate?

Answer 4

Normal: eyelashes perpendicular to the eye
Pointed down: indicates enophthalmos
Pointed up: exophthalmos

Question 5

How should you assess the cranial nerves during an equine ophthalmic exam?

Answer 5

-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

Question 6

What nerve blocks should be performed to perform a detailed ocular exam in horses?

Answer 6

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

Question 7

Describe the auriculopalpebral block

Answer 7

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

Question 8

Describe the supraorbital or frontal block

Answer 8

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

Question 9

What is the normal tear production and intraocular pressure in a horse? What else can be performed on eye exams if indicated?

Answer 9

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)

Question 10

What are the important adnexa that should be assessed in every eye exam?

Answer 10

Eyelids and nictitating membrane
-conjunctiva
-nasolacrimal system

Look for ectopic cilia (common in morgans), Jones test (to test patency of nasolacrimal duct)

Question 11

If you see an ulcer over the middle of the eye, what is one thing that should be closely assessed?

Answer 11

The third eyelid- suspicious of foreign body

Question 12

Describe the Jones test

Answer 12

Apply fluorescein stain and measure how long it takes for stain to come out of nose
- should take around 15-20 min

Question 13

When should irrigation of the nasolacrimal duct be performed?

Answer 13

-epiphora, mucoid ocular discharge, nasal puncta discharge

Question 14

Describe the techniques for flushing the nasolacrimal system

Answer 14

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

Question 15

What should you observe on the ocular surface?

Answer 15

Cornea and sclera
-look for corneal edema, vascularization, lacerations, foreign bodies
-document everything!

Question 16

T/F: you can see the iridocorneal angle (drainage angle) in a horses eye without any special equipment

Answer 16

True-unique to horses

Question 17

What intraocular structures should be assessed on eye exam?

Answer 17

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)

Question 18

What should you look at on your fundic exam?

Answer 18

-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

Question 19

What are the “stars of winslow”?

Answer 19

Black dots that can be seen on the tapetum
-choroidal vessels

Question 20

What are the methods for performing your up close ophthalmic exam of fundus?

Answer 20

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

Question 21

Describe the anatomy of the horses orbit

Answer 21

Complete bony orbital rim comprised of the frontal, lacrimal, zygomatic and temporal bones
-deep internal wall composed of the sphenoid and palatine bones

Question 22

What are most of the extraocular muscles innervated by?

Answer 22

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

Question 23

What orbital diseases can cause the eye to have a sunken appearance?

Answer 23

Enophthalmos, microphthalmos, phthisis bulbi (game over)

Question 24

Describe enophthalmos and list some potential causes

Answer 24

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)

Question 25

What is microphthalmia?

Answer 25

A congenital anomaly resulting in a small globe
-can see this with other ophthalmic abnormalities

Question 26

Describe phthisis bulbi

Answer 26

A nonvisual eye that is gradually shrinking over time due to chronic inflammation and low IOP
-damage to the CB results in decreased aqueous humor production
-should be enucleated if it is uncomfortable

Question 27

What can cause a bulging appearance to the eye?

Answer 27

Buphthalmos: caused by increase IOP or mass. Usually secondary to glaucoma (also will see corneal edema and haabs striae, vision may be present, reduced or absent)-> poor prognosis for vision
Exophthalmos: anterior displacement of normal sized globe. Can be the result of a retrobulbar mass, orbital cellulitis/abscess or trauma
-if pain on digital retropulsion, likely inflammatory. if not painful likely a neoplasia or cystic mass

Question 28

Where on the orbit are fractures most likely to occur?

Answer 28

At the dorsal rim and zygomatic arch due to prominent location on skull
-clinically may see facial asymmetry, blepharaedema, periocular lacerations, conjunctival hyperemia, chemosis, and SQ emphysema
-may be caused by rearing in confined spaces, kicks to the face or colliding with stationary objects
-can be accompanied by displaced bony fragments and there may be impingement on orbital structures (globe, optic nerve, extraocular muscles

Question 29

What are some of the treatment options for orbital fractures?

Answer 29

-ensure adequate corneal protection and lubrication
-check visual status (CN exam)
-eyelid and skin lacerations should be clean with good apposition
-minor non-displaced fractures may not require surgical treatment
-displaced fractures may require reposition
-comminuted fractures may require repositioning of fragments and interosseous wiring techniques or bone grafts

Question 30

What is orbital fat prolapse?

Answer 30

When the orbital fat herniates through weakened episcleral fascia- often a result from trauma
-diagnose through fine needle aspirate or biopsy
-treat by resecting the “mass” and suturing closed the conjunctival surface over the exposed area

Question 31

What is the clinical appearance of orbital cellulitis?

Answer 31

-exopthalmos
-belpharaedema or blepharitis
-severe conjunctival swelling
-elevated third eyelid
-mucoid ocular discharge
-IOP can be normal or elevated (due to compressing the globe)

Question 32

What are the causes of orbital cellulitis? How can you diagnose? Treat?

Answer 32

Causes: direct trauma, foreign body, seeding septic emboli, uncontrolled septic endophthalmitis

Diagnosis: palpation, sinus percussion, radiographs/US/MRI/CT/FNA, evaluation of teeth

Treatment: systemic antimicrobials, aggressive NSAIDs, topical lubricants, drainage of abscess, removal of FB, enucleation

Question 33

What cancers can affect the eye of horses?

Answer 33

Not common
-can be due to neuroendocrine tumors, extra-adrenal paranglioma, nasal and orbital adenocarcinoma, sarcoma, lymphoma, SCC
-clinical appearance: Exophthalmos, TE elevation, strabismus, anisocoria, blindness, chemosis, epistaxis

Diagnosis: Advanced imaging, ocular ultrasound, FNA or biopsy

Treatment: exenteration, radiation, chemotherapy, euthanasia. Prognosis is often grave