Anatomy of Vision & Ocular Exam

What is special about dog and cat orbit?

Do not have a bony orbit like horses and humans fo

Optical canal

CN II

Orbital fissure

CN III

CN IV

CN V (ophthalmic)

CN VI

Rostral alar foramen

CNV (maxillary)

What opens the eyes

-skeletal muscle: levator palpebrae

-smooth muscle with sympathetic control: mullers muscle

What closes the eyes

Skeletal muscle: orbicularis oculi

What produces liquid tears?

Lacrimal gland 65-70%

Third eyelid 30-35%

What are the tunics of the globe?

Fibrous, vascular, nervous

Fibrous tunic

Cornea and sclera

Vascular tunic

Iris, ciliary body, choroid

Nervous tunic

Retina and optic nerve

Why is the cornea clear?

-regularity of collagen fiber

-low cellularity

-lack of blood and lymph vessels

-relatively dehydrated

What does the cornea do?

bends light as it enters the eye

What is the anterior chamber?

Aqueous humor filled space between the cornia and the lens. Aqueous humor maintains its shape and carries nutrients to the space

What does the iris do?

Sphincer and dilator muscles for the pupils

What does ciliary body do?

Creates aqueous humor and its muscles help to focus the lens by pulling down on it

What is the vitreous?

Gel structure between lens and retina that is attached via tiny ligaments = can have tears or detachment

What are the photoreceptors?

-rods: dim light, movement, contrast

-cones: color and detail, dogs have blue and yellow-green

What is the tapetum?

Reflective layer of chorid

Holangiotic tapetum

Whole retina has blood vessels. Dogs, cats, humans, ruminatns

Merangiotic tapetum

Rabbits have this. Only blood vessels in the meridian

Paurangiotic tapetum

Equine and elephants. Blood vessels come from behind and within = stars of winslow

CN II

-optic n

-vision

-afferent PLR

CN III

-oculomotor n

-dorsal, ventral, medial rectus, ventral oblique, levator palpebrae superiorus

-efferent PLR

CN IV

-trochlear n

-dorsal oblique

CN V

-trigeminal n

-sensory to skin, globe cronea

-mm of mastication

CN VI

-abducent n

-lateral rectus, retractor bulbi

CN VII

-facial n

-orbicularis oculi (blinking)

-parasympathetic fibers to lacrimal gland

CN VIII

-vestibulocochlear n

-nystagmus, head tilt

Sympathetic enervation to the eye

-pupil dilation

-control of mullers muscle

-rigidity to extraocular muscles

Equine eye exam

-sedation: alpha 2 and opioid

-nerve blocks: auriculopalpebral upper eyelid motor, frontal upper eyelid and cornea sensation

Cataracts vs NS

First need to dilate with tropicamide 1%.

NS: hazy appearance, can see the lens

Cataract: completely blocks light

When do you not dilate the eye?

Increase in IOP or questionable lens position

What do we evaluate in the anterior chamber?

Flare within thin bright light indicated inflammation and leakage of proteins

Persistent pupillary membrane

Birthmark from cornea and iris not splitting — rarely a concern

Neovascularization (rubeosis irides)

Iris inflammation from uveitis

Synechia

-iris sticking to other structures

-anterior = cornea

-posterior = lens

Schirmer teat test

Normal 15mm in 60 seconds

Fluorescein stain tests

-Jones tests: nasolacrimal duct patency

-Seidels test: leaking aqueous

Tonopen

-requires propaocaine

-angled to any surface of eye is okay

Tonovet

-does not require proparocaine

-must be straight against the eye because it uses a vector calculation

Normal IOP

8-25mmHg, no more than 3-5mmHg discrepancy between eyes

IOP elevated

Glaucoma

IOP decreased

Uveitis and old age