Equine Ophthalmic Exam & Orbital Diseases Flashcards
Horses are prey animals, what characteristics of their eyes support this?
- prominent globes that are laterally positioned and high on their head
- horizontal elliptical pupil
- granulae iridica/corpora nigra
- visual field = 350 degrees
what are the 2 blindspots of the horse?
below nose and forehead and behind the tail.
In what order should you do your initial ophthalmic exam?
- examine from afar (look at symmetry, orbit, eyelash position, pupil size, and any epiphora)
- examine in the stall/roundpen, etc. (eval vision and behavior)
- examine up-close with restraint, sedation, +/- local block
What do the following observations usually indicate?
- Eyelashes point downward
- Eyelashes point upward
- Eyelashes point downward = enophthalmos
- Eyelashes point upward = exophthalmos
normal eyelash position is perpendicular to cornea (straight out)
T/F: you should perform your cranial nerve evaluation after sedating the horse in order to get accurate results
false – perform this exam BEFORE any sedation.
T/F: Indirect PLRs in horses are less prominent because decussation at the chiasm is only 85%
true
Which muscle of the eye is really strong in horses and makes it necessary for us to place a local block to achieve a thorough ocular examination?
orbicularis oculi muscle
Which local block blocks motor of the orbicularis oculi muscle (mostly the upper eyelid)?
auriculopalpebral block
which local block blocks sensory input to the central-upper eyelid?
supraorbital or frontal block
What nerve is blocked with the auriculopalpebral block?
palpebral branch of the auricular palpebral nerve (which is a branch of the facial nerve)
what needle size and amount of local block is used for both the auriculopalpebral and supraorbital/frontal blocks?
25g needle
1 mL of lido/bupiva/mepiv
lasts for 1-2 hours
The landmarks described below are for which local block?
- caudal to the posterior ramus of the mandible
- dorsal to the highest point of the zygomatic arch
- on the zygomatic arch caudal to the bony process of the frontal bone
auriculopalpebral block
Which nerve is blocked by the supraorbital or frontal block?
ophthalmic branch of the trigeminal nerve
this blocks sensation to the central upper eyelid.
Which of the following blocks is described by the landmarks below?
- place thumb below dorsal orbital rim
- place middle finger in supraorbital fossa
- place index finger straight down (midway between thumb and middle finger) to locate the supraorbital foramen
supraorbital or frontal block
T/F: tear deficiencies are a common cause of KCS in horses
false – tear deficiencies are rare in horses.
Tear production tests are rarely done for this reason and are indicated only for CN V or VII dysfunction.
Normal STT in horses is <20 mm/min
What is normal intraocular pressure for horses?
15-30 mmHg
which intraocular pressure measurement tool requires the use of topical anesthetic?
applanation tonometry (Tonopen)
What does the Jones test evaluate?
nasolacrimal duct patency
What are the indications to sedate a horse and perform irrigation of the nasolacrimal duct?
- epiphora
- mucoid ocular discharge
- nasal puncta discharge
If you were performing retrograde irrigation of the NLD in a horse, what size catheter would you use and hose much sterile eyewash would you infuse?
5 or 6 polyethylene urinary catheter
12-20 mL sterile eyewash
T/F: a grey line at the edge of a horses iris is considered an abnormality and can indicate iris atresia
false – this is a normal finding in horses and is indicative of the iridocorneal angle.
What does the Tyndall Effect indicate?
Aqueous flare – proteins in the anterior chamber which can be resulting from equine recurrent uveitis or acute uveitis.
What could be a cause of corpora nigra atrophy?
chronic inflammation (such as in ERU cases)
What causes posterior synechia?
uveitis
What drug should you use to dilate the eyes for complete examination of the intraocular structures such as the lens, etc.?
1% tropicamide (20 min prior to exam)
this will last for 4-6 hours
What are the 3 differences of the equine fundus?
optic disc is oval, salmon-pink, and located in the non-tapetal fundus
the retina is paurangiotic (partially vascularized)
they have a fibrous tapetum which has color variation and stars of winslow (black dots that are choroid vessels)
T/F: Direct fundoscopy provides you with an inverted and reversed image
false – this is true of indirect fundoscopy (the one with the handheld lightsource and lens)
indirect provides you with a wider field of view, but less magnification
T/F: direct fundoscopy provides you with a smaller field of view and more magnification than indirect fundoscopy
true
T/F: horses have an incomplete orbit which makes fractures more likely
false – they have a complete bony orbit comprised of frontal, lacrimal, zygomatic, and temporal bones.
Which nerve innervates the dorsal oblique ocular muscle?
trochlear nerve
which nerve innervates the retractor oculi (bulbi) muscle?
abducens
which nerve innervates the lateral rectus ocular muscle?
abducens
T/F: most of the extraocular muscles are innervated by the oculomotor nerve
true
the muscles are: levator palpebrae superioris (opens eye), the superior, medial and inferior rectus muscles, and the inferior oblique muscle.
what are the 3 possible sources of a “sunken” appearance to the eye?
- enophthalmos
- microphthalmos
- phthisis bulbi
_________ is recession of the globe within the orbit usually secondary to loss of the orbital contents.
enophthalmos
what are 4 potential causes of enophthalmos in horses?
- orbital fractures
- resorption of orbital fat (weight loss or aging)
- dehydration in foals (+entropion)
- sympathetic denervation (horners)
__________ is a congenital anomaly resulting in a small globe
microphthalmia
_________ is the gradual shrinkage of the globe due to chronic inflammation and low intraocular pressure. This is a non-visual eye.
Phthisis bulbi
damage to the CB results in decreased aqueous humor production leading to chronically low intraocular pressure.
what are the 2 potential sources of a bulging ocular appearance and whats the difference between them?
- buphthalmos – enlarged globe assoc. with chronically increased IOP secondary to glaucoma
- exophthalmos – anterior displacement of a normal-sized globe
what causes buphthalmos?
chronically increased IOP (secondary to glaucoma)
look for other signs of glaucoma such as corneal edema or haab’s striae
T/F: glaucoma carries a poor prognosis in horses
true horses dont usually respond well to glaucoma therapy.
what are 3 potential causes of exophthalmos?
- retrobulbar mass
- orbital cellulitis / abscess
- trauma (reduced orbital space)
to differentiate, do digital retropulsion to see if pain is elicited. if so, more likely inflammatory/infectious. if non-painful, more likely neoplastic or cystic mass.
which areas of the orbit are at highest risk for fracture?
dorsal orbital rim
zygomatic arch
what are clinical features of orbital fractures in horses?
- facial asymmetry
- blepharaedema
- periocular laceration
- conjunctival hyperemia
- chemosis
- SQ emphysema
What is the typical approach to treating orbital fractures in horses?
- ensure corneal protection and lubrication
- check visual status
- if eyelid/skin laceration, clean and appose
- if minor non-displaced orbital fractures, may be able to leave it alone
- if displaced, reposition (surgery)
- if comminuted fracture, may require repositioning and wiring or bone grafts (surgery)
What causes orbital fat prolapse?
weak episcleral fascia or trauma causing the fat to herniate
how do you treat orbital fat prolapse?
resect it and suture closed the conjunctival surface over the exposed area.
what is the clinical appearance of orbital cellulitis?
- exophthalmos
- blepharaedema/ blepharitis
- severe conjunctival swelling
- elevated 3rd eyelid
- mucoid ocular discharge
- IOP normal or elevated
what causes orbital cellulitis?
- direct trauma
- seeding septic emboli
- foreign body
- uncontrolled septic endophthalmitis
what is the most important diagnostic to diagnose orbital cellulitis?
imaging or FNA
how do you treat orbital cellulitis?
systemic antimicrobials
aggressive NSAIDs
topical lubricants
drain abscess or remove FB (if present)
enucleation if necessary
what are the 4 most common orbital neoplasms in horses?
- neuroendocrine tumors
- extra-adrenal paranglioma
- nasal and orbital adenocarcinoma
- sarcoma, lymphoma, SCC
what is the clinical appearance of a horse with an orbital neoplasm?
exophthalmos
3rd eyelid elevation
strabismus
anisocoria, blindness
chemosis
epistaxis
how do you diagnose orbital neoplasia in horses?
advanced imaging! (CT/MRI)
+/- orbital ultrasound + guided FNA
how do you treat orbital neoplasia in horses?
exenteration
radiation
chemo
or euthanasia
the prognosis is grave if advanced stage when presenting
