The eye

Week 9 NMSK

Label the parts of the eye in this image

1. Anterior chamber

2. ciliary body

3. ciliary muscles

4. cornea

5. eyelid

6. fornix

7. iris

8. lens

9. limbus

10. optic disk

11. optic nerve

12. pupil

13. retina

14. sclera

Label this diagram

1. cilia

2. lacrimal puncta

3. levator palpebrae superioris

4. meibomian glands

5. meibomian gland orifices

6. orbicularis oculi

7. outer skin

8. palpebral conjunctiva

9. tarsal plate

Label this diagram

1. cartilage skeleton

2. lymphoid nodules

3. nictitans gland

4. palpebral surface

Very bottom one is the T-shaped cartilage skeleton

label this diagram

1. bulbar conjunctiva

2. lateral canthus

3. limbus

4. lower eyelid, no lashes

5. third eyelid

6. upper eyelid with lashes

On the right with no number
Top = medial canthus

Bottom = iris

Label this diagram and what kind of segment of the eye is this?

• calotte/cadaver: posterior segment (view of fundus)

  1. choroid

  2. non-tapetal fundus

  3. optic nerve head/disc

  4. retina

  5. sclera

  6. tapetal fundus

what does ‘calotte’ mean

half of the eye after sectioning (cadaver specimen)

Define exophthalmos

abnormal position of the eye from the orbit

how can we recognise exophthalmos?

• globe is the normal size

• the globe position is pushed forward/protruding

• the globe is normal

define enophthalmos

• abnormal recession of the eye within the orbit

How can we recognise enophthalmos?

• globe size is reduced

• globe is sunken

• globe appears normal

define hydrophthalmos

Enlargement of the globe

How can we recognise hydrophthalmos?

• globe has an enlarged size

• globe is protruding

• globe appears abnormal

define microphthalmos

Congenitally abnormal (small) eye

How can we recognise microphthalmos?

• globe is reduced in size

• globe position is normal

• globe appearance is abnormal

what is the orbit

• a cavity within the skull that encloses the eye

what is the purpose of the orbit

• protection

• separation of the eye from the cranial cavity

why are there foramina in the walls of the orbit?

• pathway for blood vessels and nerves to reach the eye

what are the 2 kinds of orbit

1. open/incomplete

2. closed/complete

Compare an open orbit to a closed orbit

Open: - lateral orbital ligament, facilitates access for a biopsy, ultrasound and surgery

Closed: fusion of zygomatic and frontal bones

What are the 2 kinds of visual fields?

• monocular

• binocular

What kind of vision do a)carnivores e.g. cats and b)herbivores e.g. horses have?

• cats = binocular

• monocular

Compare:

a) orbit type

b) composition of later wall

c) degree of globe protection

d) visual field

e) degree of binocular vision

between herbivores and carnivores

a) Closed/complete Open/incomplete

b) fusion of frontal and zygomatic Lateral orbital ligament

c) greater protection less protection

d)wide field of vision narrower field of vision

e) more monocular more binocular

why do carnivores have an open/incomplete orbit

• enables greater jaw movement

Provide:

1. A definition

2. depth of orbit

3. degree of protection offered

4. breed examples

of brachycephalic breeds

1. Skull is short and broad

2. shallow

3. reduced

4. pug, boston, boxer, french bulldog

Provide:

1. A definition

2. depth of orbit

3. degree of protection offered

4. breed examples

of mesocephalic breeds (mesaticephalic)

1. medium skull length, ‘normal’

2. normal

3. moderate

4. lab, german shepard, doberman, dalmation, beagle

Provide:

1. A definition

2. depth of orbit

3. degree of protection offered

4. breed examples

of dolicocephalic breeds

1. long skull

2. deep

3. increased

4. great dane, greyhound, daschund, irish wolfhound, saluki and poodle

what problems might be associated with having a brachycephalic conformation

• these dogs have a greater risk of developmental disorders

some examples: shallow orbit, microphthalamia, anophthalamia and hydrocephalus with orbital malformation

Outline the anatomy of the orbit

• 5-7 bones (species dependent)

• orbital rim: frontal, lacrimal and zygomatic bones

• soft tissue structures

Label this image

1. frontal bone

2. maxilla

3. zygomatic bone

4. zygomatic process

Label this image

1. frontal bone

2. lacrimal bone

3. maxilla

4. orbital canal

5. orbital fissure

6. palatine bone

7. sphenoid bone

8. zygomatic arch

outline the limits of the bony orbit

1. medial limit - frontal bone (separates the orbit from the nasal cavity), very thin

2. Dorsal limit - projection of the frontal bone, contains frontal sinus

3. Rostral and lateral limits: zygomatic, lacrimal and maxillary bones

4. caudal limit: sphenoid bone (contains the optic canal and orbital fissure)

Outline the soft tissue orbit

1. Dorsolateral limit - temporal muscle and the orbital ligament

2. rostral and lateral limits: masseter muscle (medial and ventral to zygomatic arch)

3. ventral floor: pterygoid muscle and neurovascular structures that transverse the orbital floor

How many foramina do we have in the orbit

8 but species variation

what are the 2 most important foramina in the orbit?

• optical canal

• orbital fissure

what nerves/vessels pass through the optical canal?

• optic nerve

• internal opthalmic artery

what nerves pass through the orbital fissure/foramen?

• abducens nerve

• oculomotor nerve

• opthalmic nerve

• trochlear nerve

what muscles does CNIII innervate around the eye

• dorsal rectus

• medial rectus

• ventral rectus

• ventral oblique

what muscles does CNVI innervate around the eye?

• lateral rectus

• retractor bulbi

what muscles does CNIV innervate?

• dorsal oblique

where are the 2 places we can find soft tissue structures in the orbit?

• intraconal

• extraconal

how can we define the intraconal area?

• 4 rectus muscles and the periorbital fascial sheath

• shaped like an ice cream cone

what does the intraconal space include

• multiple nerves: optic nerve and nerves supplying extraocular muscles

• vessels

• smooth muscle

• fat

• orbital lacrimal gland

all other soft tissue orbital structures are within the extraconal space

what other important structures (not bone or soft tissue) are located within the orbit

1. tooth roots

2. zygomatic salivary glands (dogs)

3. paranasal sinuses

What other important structures (not bone or soft tissue) are found outside the orbit

1. nasolacrimal duct

2. base of the nictitating membrane

3. orbital fat cushion

where is the orbital fat cushion and what does it do

• surrounds the eye and the muscles

• protection

Outline the arterial blood supply to the eye

• eye has a high metabolic activity; needs a rich blood supply

• opthalmic artery - derived from the internal carotid artery

• ^ they supply the highly vascular uveal tract

• retina has a dual blood supply

Outline the venous drainage of the eye and orbit

• vortex veing and orbital venous plexus

• smaller route via opthalmic vein

• all goes to the external jugular vein

What is the clinical relevance of high ocular blood supply?

• systemic hypertension → ocular damage

• system diseases may cause uveitis (inflammation of the uveal tract)

what is the clinical relevance of venous damage?

• When enucleating in rabbits, they have a very well formed orbital venous plexus - BE CAREFUL

• careful when restraining around the neck

what is an example of a disease that affects the nasal cavity and paranasal sinuses

• disease within the nasal cavity or paranasal sinus

• neoplasia

what is an example of a disease that affects the caudal roots of the 4th premolar and 1st/2nd molar teeth

• risk trauma during tooth extraction

• tooth root abscess

what is an example of a disease that affects the brain

• neoplastic or inflammatory CNS disease

what is an example of a disease that affects the temporal and masseter muscles

• masticatory muscle myositis

what is an example of a disease that affects the zygomatic salivary glands

• sialadenitis

define anisocoria

• unequal size of the pupils

define miosis

• excessive constriction of the pupil

define mydriasis

dilation of the pupil

define strabismus

cross eyed, they have an inability to align both eyes simultaneously

define nystagmus

involuntary eye movement, may result in limited or reduced vision

what is the ramus of the mandible

• where the mandible moves between the zygomatic bone and cranium

• it enables the jaw to be opened and closedwhat is an example of a disease that affects the

how does the ramus move

• jaw opens - ramus moves towards the globe

• jaw closed - aways from the globe

what is the clinical relevance of the ramus

• orbital disease will often cause pain during:

  • eating

  • yawning

  • examination of the oral cavity

• animal may yelp when trying to open the mouth

what is a useful clinical method to identify exophthalmos

• looking at the animal’s head from above

How do we assess retropulsion

• gently pushing on the globe through the upper eyelid

• equal and non-painful protrusion of the globe

• brachycephalic breeds - it’s limited because the orbit is normally shallow anyway

What should we look for whilst conducting an oral exam for indicators of eye problems?

• pain on jaw movement

• assess for dental disease

• any history of reduced appetite, especially of dry food

What are the functions of the eye

• vision

• collects light from the environment

• focuses light onto photoreceptors in the retina

• converts light into a nerve impulse = PHOTOTRANSDUCTION

• sensation of vision

Outline where light is refracted and what this is

• refraction = bending of light rays

2 places:

1. cornea - major refraction 48/60 dioptres (man)

2. lens = aqueous-lens interface and the len-vitreous interface

why does major refraction take place at the cornea

• air-cornea interface

what is a dioptre

• unit of power of lens

• recriprocal of focal length in metres

how do our eye and brain orientate images?

Eye - image is upside-down on the retina

Processing in the visual cortex - converts the image to being the correct way up

what 2 kinds of lenses do we have and what do they do to light rays

1. convex lens - converges light rays to a focal point

2. concave lens - diverge light rays

what kind of lenses are the cornea and lens

• convex

What are 11 key points for vision in dogs

1. sensitive to motion

2. sensitive to flickering light

3. good day and night vision

4. visual perspective is closer to the ground

5. larger field of view

6. reduced depth perception

7. binocular vision

8. most are emmetropic (no visual error)

9. poor acuity

10. dichromatic

11. better at distinguishing shades of grey

what is the clinical relevance that dogs are emmetropic

• short and long-sighted vision is uncommon

What structure is this and what can we identify?

1. It’s the cornea

2. corneal epithelium on the left (multiple layers)

3. corneal stroma (90% corneal thickness)

4. Endothelium layer on the right

5. Descemet’s membrane (basement membrane for endothelium)

What structure is this and what can we identify?

1. Lens

2. Anterior lens capsule (thick) on the left

3. Posterior lens capsule (thin) on the right

What structure is this and what can we identify

• Lens equator

• Lens epithelium on the top

• lens fibres running through

what is the equator of the lens?

• the marginal circumference where the anterior and posterior surfaces meet, playing a crucial role in the lens’s structure and function

What are we looking at in this image?

1. Iris (top)

2. Iridocorneal drainage angle (middle)

3. Cornea (bottom left)

4. Limbus (bottom middle)

5. Sclera (bottom right)

What are we looking at in this section?

Iris - uveal tract

What are we looking at in this section and what can we identify

1. ciliary body (uveal tract)

2. lens zonule fragments (top)

3. ciliary processes

4. ciliary body stroma

What is the fundus?

• interior surface at the back of the eye, comprising of essential structures like the retina, optic disc and blood vessels

What are we looking at here and what structures can we identify?

1. vitreous (top)

2. Retina (middle)

3. Blood vessels in the choroid

4. sclera

What are we looking at in this image and what is the difference between the 2?

1. these are images of the retina

2. On the left = tapetal retina

3. On the right = non-tapetal retina

what are the layers that are visible in these structures

Top to bottom:

1. retina

2. tapetum (more on left, as it’s tapetal retina and less on the right)

3. choroid

4. sclera

What are we looking at and what structures can we identify

Optic nerve

What setting do we want for an ocular ultrasound

• B-mode US

what do we use ocular ultrasound for?

• imaging inside teh globe if we can’t see inside due to cloudy cornea, cataract etc.

• measure globe size

• assess orbital disease

How do we prepare our patient for ocular ultrasound

• conscious ± light sedation

• topical anaesthetic eye drops to numb cornea

• direct corneal contact, eyelids open

What do we see on the ocular ultrasound

What structures are identified here?

1. anterior lens capsule

2. posterior lens capsule

3. vitreous

4. retrobellar space

5. posterior limit of the globe (retina, choroid)

6. melanoma (tumour)

What problem can be identified in this ocular ultrasound

• retinal detachment

What problem can be identified on this ocular ultrasound

• cataract

What are some common indications we need a CT and MRI

• orbital disease: FB, neoplasia, abscess

• central blindness: brain tumour

• investigation of tear duct disorders

What are 2 indicators may we get on an ocular ultrasound that we need a CT/MRI?

• abnormal masses but can’t decipher what

• swollen muscles visible

what fungus can cause orbital fungal infection

blastomyces dermatitidis

What diagnostic test can we use to assess optic neuritis

• MRI scan

• will show an abnormal optic nerve, swollen and retinal detachment

What diagnostic test may we need to use for a presentation of a swelling near the tear duct

• skull radiograph with liquid contrast

• CT

Is this a normal canine orbit? What diagnostic tool is used?

Yes

CT scan

Are these normal canine orbits and what diagnostic imaging tool has been used?

• MRI
  yes