Basic ocular anatomy and diagnostics (Proceedings)

Basic Anatomy of the Eye

• Orbit

• Eyelids

• Nasolacrimal System

• Cornea/Sclera

• Lens

• Uvea

• Aqueous/Vitreous

• Iridocorneal Angle

• Retina

• Optic Nerve & Brain

The Orbit

• Orbit= the "camera case" enclosing the eye

• Made of bone, ligament, fat, muscle, salivary gland, roof of mouth

• A mass behind the eye can push the eye out and/or protrude through soft tissue into the mouth*

The Eyelids

• Eyelids= "lens cover" or "windshield wipers" of the eye

• Act to protect and lubricate the surface of the eye

o Mechanical protection

o Meibomian glands of eyelids produce the oily component of the tears

o Spread tears (and foreign material) across surface & toward drainage system

• Outer layer is thin skin

• Muscles to close (orbicularis oculi) and open (levator palpebrae superioris) lids

• Poorly developed supportive tarsal plate in dogs/cats

• Meibomian glands in tarsus

• Inner surface lined with conjunctiva

• Cilia (eyelashes) arise from the lid margins*

• Eyelids= skin, so can get all the skin diseases

o Bacterial

o Parasitic

o Allergic

o Auto-Immune

o Neoplastic

• Eyelid glands can also become infected and/or inflamed, develop tumors

The Third Eyelid

• Most domestic species have some variation of this structure

• Acts to protect the eye when it is drawn over its surface passively*

• T-shaped cartilage core coated by conjunctiva

• Important tear-producing gland located behind and at its base

• Von Graefe or other atraumatic forceps are used to look behind the third eyelid for foreign bodies, follicles

The Nasolacrimal System

• Tear film coats cornea

o Meibomian glands produce thin fatty outer portion of tear film

o The lacrimal & third eyelid glands produce the thick middle watery tear layer

o Goblet cells within conjunctiva produce inner mucus layer

• After tear production, tears are flushed by lids toward upper & lower punctae

• Tears drain from lacrimal punctae→ canaliculi→ sac→ NL ducts→ nose/mouth

• Obstruction at any point causes failure of drainage (tearing)*

Cornea & Sclera

• Together make up the outer, fibrous coat of the eye

• CORNEA= clear "windshield" and most important focusing structure of eye

• SCLERA= the support mechanism

• CORNEA & SCLERA

• Cornea is a multilayered structure

• Pumps in the innermost endothelium (esp) and outermost epithelium act to dehydrate it

• The middle stroma is hydrophilic (water-loving)*

• CORNEA & SCLERA

• Corneal clarity achieved due to:

o Relative dehydration

o Lack of blood vessels

o Strict organization of collagen in stroma

• The sclera is opaque because it lacks these qualities

• Scarred cornea is also opaque to varying degrees b/c of fibrosis

The Lens

• LENS= "fine- focus" mechanism of eye

• Spherical structure responsible for focusing images on retina

• Similar to cornea, clarity achieved through:

o Relative dehydration

o Acellularity

o Lack of blood vessels

• Production of new lens fibers occurs only from anterior lens epithelium

• Individual lens fibers do not reach all the way around; where they meet is staggered, forming lens sutures

o Y-shaped in dog/cat

• Cells lose organelles with maturity

• Layers added consecutively throughout life ("onionskin")

o Volume stays same, so compression of inner layers occurs with age

o Oldest layers are innermost

• Normal lens held in place by numerous peripheral lens zonules

• When all zonules rupture, lens luxation occurs

• Without a lens, near images appear

o Out of focus

o Larger than life

The Uvea

• Pigmented & highly vascular middle coat of eye

• Three segments

o Iris

o Ciliary body

o Choroid

• Iris and CB= anterior uvea

• Because of its intense vascularity, uvea sensitive to systemic inflammation and infections

• A blood-aqueous barrier (BAB) is intended to dampen this response, but can be broken down and takes months or longer to re-establish

The Uvea- Iris

• Iris= pigmented "sunshade"

• Pupil is "aperture"

o Controls amount of light entering eye

• Pupil shape differs with species

The Uvea- Ciliary Body

• Ciliary body= Aqueous-producing factory

• Target of IOP-lowering drugs and procedures which reduce aqueous production

o Methazolamide

o Trusopt

o Laser CPC

o Cryoablation

o Gentocin injection

The Uvea- Aqueous Humor

• Aqueous is normally clear liquid

• Fills anterior & posterior chambers

• Normally low in protein

o Increase in protein content with BAB breakdown (seen as aqueous flare)

• Drains through iridocorneal angle

The Uvea- Choroid

• Blood and oxygen supply to the retina

o Exclusive supply to some species' retinas

• Located between visual retina and supportive sclera

• Contains reflective tapetum

Vitreous

• Vitreous is normally gelled and fills vitreous chamber between lens and retina

• Even degenerated vitreous held in back of eye by intact lens zonules*

• Places pressure on neuroretina to help keep it in place

o Vitreal degeneration may result from or predispose to retinal detachment

The Iridocorneal Angle

• IRIDOCORNEAL ANGLE= the eye's "drain"

• If this is abnormally formed or damaged, drainage occurs less freely and intraocular pressure builds up→ glaucoma

The Retina

• RETINA= the "film" in the "camera"

• Responsible for converting light energy into chemical energy and passing it as a neuro signal to the brain

• Ten layered structure

o 9 layers of neuroretina

o Outer retinal pigment epithelium (RPE)

o Reflective tapetum actually within choroid

o Retinal detachment not entirely accurate term, since separation occurs b/w neuroretina and RPE

The Retina-the Cells

• Photoreceptors

o "rods" and "cones"

o Absorb incoming light

o Narrow range of specific wavelengths absorb by each cone- varies by species

• Bipolar cells connect PRs to...

• Retinal ganglion cells

• RGCs converge to ON

Optic Nerve & Brain

• OPTIC NERVE= "cable" connecting the eye to the "computer" (BRAIN)

• The optic nerve is the only nerve in the body that can be directly examined (by ophthalmoscopy)

• If the ophthalmic exam and ERG are normal, blindness must be secondary to a ON or brain problem

• The nerve is a collection of axons of the retinal photoreceptors

• Takes visual message to the brain

o Travels through weak point in sclera called lamina cribrosa

o Padded in orbit by fat

• Passes thru the optic foramen

• Considered cranial nerve II

• Brain is responsible for interpreting the signal from the eye so that it forms a meaningful image

• Damage to the occipital or "visual cortex" can result in blindness even if the eye is completely normal