Vision Overview and Basic Eye Exam

The vision pathway starts when...

light enters the eye

Different wavelengths of light activate different ___________ in the retina

photoreceptors

When light hits photoreceptors, it is transformed into...

an electrical signal

Two types of photoreceptors:

1.) Rods

2.) Cones

Rods

low light and motion

Cones

color and acuity/clarity

Humans have ______ types of cones while dogs have _____ types

three; two

Most animals have a higher ________ to ________ ratio

rod to cone

How does a higher rod to cone ratio affect vision?

can see better in dim light, but have worse visual acuity

Animals who have two types of cone have _____________ vision

dichromatic

Animals who have three types of cone have _____________ vision

trichromatic

In terms of color, what do dogs see?

similar to red/green color blindness; can detect differences between blues, yellows, and greens, but red and green look the same

Visual acuity/clarity

the ability to resolve fine spatial details; discrimination between two points

What photoreceptor is important for visual acuity?

cones

Dogs have _________ visual acuity than humans

lower

Three factors determining visual acuity

1.) amount of cones

2.) ability to form well focused image

3.) accomidation

What factors effect the ability to form a well focused image

Clarity of the aqueous and vitreous humor and shape of cornea and lens

Accommodation

the ability to adjust focus; very limited in dogs and cats

Are dogs more near sighted or far sighted than humans?

near sighted

near sighted

unable to see distant objects clearly

Tapetum lucidum

part of the choroid underneath the retina and above the sclera; reflects light to help with night vision

What animals don't have a tapetum?

pigs, camelids, squirrels, birds, primates

The tapetum can cause light scatter, which results in ________ acuity

decreased

binocular vision

vision using two eyes with overlapping fields of view, allowing good perception of depth

monocular vision

the eyesight in which each eye sees a very different portion of the world around them

Carnivores have more ___________ vision

binocular

Why do carnivores have more binocular vision?

they have forward oriented eyes with better depth perception, but smaller visual field

Prey species have more ___________ vision

monocular

Why do prey species have more monocular vision?

their eyes are more laterally oriented, giving them a wider visual field

Dogs have a _________ field of view compared to humans, but ________ binocular vision

wider; narrower

A good _______ source is key to a thorough exam of the eye

light

What tool is used for diffuse light? Direct light?

finoff tip

opthalmoscope

Four steps of eye exam

1.) distance exam

2.) neuro-opthalmic exam

3.) preliminary testing

4.) dilate the pupil

distance exam

look for eye symmetry, dropping, swelling, size and behavior in the exam room

Neuro opthalmic exam

tests reflexes and reposes of the eye; assessing vision and cranial nerves

Menace response tests what nerves?

afferent: retina, optic nerve (CN II)

efferent: cerebral cortex, facial nerve (CN VII)

Palpebral reflex tests what nerves?

afferent: trigeminal nerve (CN V)

efferent: facial nerve (CN VII)

Pupillary light reflex tests what nerves?

afferent: retina (CN II) and optic chasm and tracts, pretectal nucleus (CN III)

efferent: ciliary ganglion, iris sphincter muscle (CN III)

Dazzle reflex tests what nerves?

afferent: retina, optic nerve (CN II)

efferent: sub-cortical, facial nerve (CN VII)

Tests for assessing vision

cotton ball test and maze test

Preliminary testing

testing done before dilating the pupil

Three preliminary tests

1.) schirmer tear test

2.) tonometry

3.) fluorescein stain

schirmer tear test

measures tear production

tonometry

measures eye pressure

What does dilating the pupil allow you to see?

the entire lens and fundus

What medication is used to dilate the pupil?

tropicamide

When should you proceed to dilation?

if PLRs and topometry are normal

Examination of the anterior segment is done in a ___________ room with a _________ beam light source

dark; diffuse

Examination of the anterior segment includes:

1.) eyelids

2.) conjunctiva

3.) nasal lacrimal system

4.) cornea

5.) sclera

6.) anterior chamber

7.) iris and pupil

8.) lens

eyelids examination

-position

-function (blinking)

-skin/hair

conjunctiva examination

should be pink and smooth with normal tear film (no discharge) and normal location of nicitating membrane

lacrimal system examination

lacrimal punch should be clear with not swelling or discharge; clear tear film

cornea examination

should be clear and transparent with a shiny tear film

sclera examination

should be clear white with no increased vessels or masses

anterior chamber examination

aqueous humor should be clear with no cloudiness/haziness

corpora nigra

the black pigmentation at the edge of the iris in equine and ruminants; normal

lens examination

normal lens is clear and behind the iris; can visualize using slit beam tool (left line is cornea, right line is iris face and anterior lens capsule)

Exam of the posterior segment includes:

1.) vitreous humor

2.) retina

3.) choroid

4.) tapetum

5.) optic nerve

vitreous humor examination

should be clear

retina examination

should see plenty of blood vessels

indirect opthalmocopy

low magnification with wide field of view, examiner far away (2-3 inches) from patient

The image for indirect opthalmocopy will be...

upside down and backwards

direct opthalmocopy

high magnification with low field of view, examiner is close to patient

The image for direct opthalmocopy will be...

upright