Transcript The Anatomy of Vision

Trevor Arnold, MS, DVM, DACVO
Animal Eye Center
 Ophthalmology specialty Hospital
 Two Board Certified Veterinary Ophthalmologists
 2-3 Technicians
 2 Front Office staff
 5000+ cases a year
What is Veterinary Ophthalmology
 We provide a comprehensive list of services to all species (other
than human of course)
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Adnexal disease and surgery
Corneal disease and surgery
Cataract management
Glaucoma
Uveitis
Neurology
Oncology
Genetic /Inherited disease screening
Research services
 Must also be able to diagnose and manage ocular manifestations
of systemic disease
 Eg: Diabetes, Hyperthyroid, Infectious diseases etc
The Basics
 Understanding anatomy and embryology is essential
for the effective practice of ophthalmology
 There are some fundamental anatomical differences
between animals and humans
 Both on the gross anatomical and the cell level
 These differences do alter how we practice medicine,
however, for the most part the general principals of
ophthalmology are the same
“Primitive” Vision
 “Vision” most likely started as photoreceptive proteins
(the original opsins) in single celled organisms which
allowed these cells to move towards or away from light
 In multicellular organisms these proteins concentrated
in cells (the original photoreceptors)
 Light sensing cells then aggregated to provide a
primitive eye
 Evolutions Witness: How Eyes Evolved
 Ivan R. Schwab, Richard R. Dubielzig, Charles Schobert
“Primitive” Vision
 A flat surface cannot determine the direction of light
very well and so the “visual” surface became curved
 Some organisms curve the light sensitive cells outward
(eg. Compound eye)
 Others curve it inward (Humans, dogs, cats etc)
“Primitive” Vision
 Compound eye is made up of multiple Ommatidium
Cornea
Pigmented cells
Photoreceptors
Nerve
“Primitive” Vision
 Nautilus (Cephalopod)
 Developed an inwardly curved retina
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Forms as an invagination of the body as apposed to an
outpouching of the brain
 Works like a pinhole camera
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Maximizes focus without the need for a cornea or lens
“Primitive” Vision
 Mantis Shrimp: Proof that you don’t need a complex
eye to have complex vision
 Have at least 16 photopigments to help see color
 Can polarize multiple wavelengths of light
 Each eye moves independently and can provide depth
perception individually
Embryology
 The embryologic development of the human and dog
eye is generally guided by the same genes, proteins and
tissue induction
 The canine eye is slightly less developed at birth
compared to a human. It takes 2 weeks for the eyelids to
open, however it typically takes 4-6 months for the eye
to finish developing
 Recent advancements in gene therapy for Leber
congenital amaurosis were discovered through work on a
similar condition in dogs
Orbit
 Open orbit
 Dogs, cats etc
 The lateral orbit is composed of a ligament between the
zygomatic and frontal bones
 The floor of the orbit is primarily the Pteragoid muscle
 Closed orbit
 Primates (Human)
 Horses, Cows, Birds, Fish
Muscles
 Most species have some form of: Dorsal, Ventral,
Lateral and Medial rectus and the Dorsal and Ventral
Oblique
 Dogs and Cats also have a retractor bulbi muscle (CNVI)
 Larger species regularly require an Auriculopalpebral
nerve block to paralyze the levator palpebral
superiorus in order to examine the eye
Adnexal Structures
 Eyelids (Entropion, Distichia, Meibomian tumors)
 Third eyelid(Semilunar fold in Humans)
 Dog s have two Lacrimal Glands
 Immune mediated dry eye is a common problem in dogs
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Cyclosporine’s effect on tear production was incidentally
discovered by a veterinary ophthalmologist
Globe
 Birds, have bone, reptiles and fish have cartilaginous
plates in their sclera
 The dog globe is roughly spherical
 Average 21 mm in diameter
 The size of the eye is remarkably similar in all breeds
Cornea
 Average corneal diameter in dogs 18mm
 Curvature 40D (Larger breeds have flatter corneas)
 Due to the size of the cornea very little sclera is
exposed beyond the eyelids
 Makes scleral incisions for cataract surgery difficult
 The eye must be proptosed for vitreal surgery
Interesting Corneal Anatomy
 Some fish species have two corneal layers
 A scleral cornea and a dermal cornea
 Snakes have a scale that covers their eye (spectacle)
 This is normally shed when the skin is shed
 Big horn sheep and Manatees are two species with
non-pathologic corneal vascularization
Corneal Pathology
Corneal Refraction
 The curvature of the cornea bends light into the eye
 Provides the most refractive power of the eye in
terrestrial animals
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Tear film actually bends the light
Corneal Refraction
 What if you don’t live in air…or you live in both air and
water?
 Most fish have very round lenses and depend less on
their cornea
 Sea lions have a portion of their cornea that is flat
 They look through this part of the cornea when in air
 Some species constrict their pupil to a pinpoint to
minimize the refraction that needs to occur to focus
light on the retina
Iris and Pupillary Light Reflexes
 In cats there are bundles of iris sphincter muscles on
either side of the pupil to control constriction
 One branch of CNIII controls the medial aspect and
another controls the lateral aspect of the iris
 Birds have skeletal muscle in their pupil so they can
actively control their pupil
 Can be difficult to dilate (risk of systemic paralyzation)
Pupils around the World
Methods of Accommodation
 Dog and Human
 Ciliary body muscles contract, this releases tension on the lens,
allowing it to round up (I.e increases the curvature of the lens)
 Dogs only have an accommodation range of 3-4D (Human 15-16)
 Bird
 When ciliary muscles contract they push on the lens causing it to
round up
 Some birds can also change the curvature of the cornea
 Some diving birds have an accommodation range of 40-50 diopters
 Fish
 Have a special muscle (Retractor lentis) which moves the lens
forward or backward to change focus
 Several fish can also change the corneal curvature
 Cats
 Combination of dog and fish accommodation techniques.
Cataracts
 The canine lens is considerably larger than the human
 7mm thick (Up to 9mm with diabetic cataracts)
 Artificial PMMA or Acrylic lenses
 11, 12, 13mm or 14mm
 41D lens
 Dogs develop considerable inflammation
Glaucoma in Dogs
 Dogs typically present with an inherited Closed Angle
Glaucoma
 It is extremely rare for dogs to present for vision loss
prior to the onset of elevated intraocular pressures
 Common for dogs to have pressures of 50 mmHg or
more at the initial presentation
 We can rarely save the first eye
Medical Management
 Dogs respond well to Prostaglandin Analogues
 Typically Latanoprost 0.005%
 Causes profound miosis in all dogs due to prostaglandin
receptors in the smooth muscle
 Dorzolamide 2%
 Timolol does not tend to be as effective in dogs as it is
in humans
Surgical Management
 Trans-scleral or Endolaser Cyclophotocoagulation
 Shunt placement
 Ahmed valved shunts are used most often
 Often fail due to fibrin occlusion or Bleb capsule
thickening
 Trabeculectomy, Iridotomy and Iridectomy surgeries
do not work in dogs
 A likely reason for low surgical success rates is the
delay in intervention
Glaucoma In Cats
 Most often Secondary to chronic lymphoplasmacytic
uveitis
 Middle to older age cats occasionally develop glaucoma
secondary to Aqueous Misdirection
 Leads to over hydration of the vitreous, anterior
displacement of the lens and collapse of the anterior
chamber
 Interestingly Cats do not have PGF2A receptors inside
the eye, and so Prostaglandin analogues do not help to
decrease IOP
Retina
 Retinas are classified based on the vascular pattern
 Holangiotic, Merangiotic, Paurangiotic, Anangiotic
 Most primates and some birds, fish and reptiles have a
Fovea
 Some species have more than one to help focus on two
things at once
 Animals do have a location of the retina with a high
density of cones (Area Centralis)
 Typically a horizontal band dorsolateral to the ONH
Retina
 Animals do have color vision
 Humans have three types of color photoreceptors
 Blue, Green and Red
 Dogs have two
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Blue and Yellow-Green (Similar to Red-Green color blindness)
 Some birds have 3-4 types of rods
 Fish have colored oil droplets that sit around the
photoreceptors and to help maximize color perception
Tapetum
 Reflective layer of the choroid. Located below the RPE
and above the choroidal vessels (In most species)
 Dogs and cats have a Cellular tapetum
 Horses and Ruminants (cows, sheep etc) have a fibrous
tapetum
Holangiotic
 Dog, Cat, Primate, Ruminants
Merangiotic
 Rabbits
Paurangiotic
 Equine, Elephant
Anangiotic
 Birds, Reptiles
 Some mammals
Questions
Photo Credits
 www.vetmed.ucdavis.edu
 www.ocularservices.com
 www.biology-forums.com
 www.vims.edu
 wanderlustlizard.wordpress.com
 www.solarlightaustralia.com.au
 www.norcalblogs.com
 www.vhah.com
 dc378.4shared.com
 www.underwaterjournal.com
 www.djibnet.com
 tnrtb.wordpress.com