Retina, eye fundus, vision impairment

Question 1

The most metabolically active tissue in the body

Answer 1

the retina.

• Outer retina is suplied by choroid.
• Inner- and mid-retina are supplied by inner retinal vessels which are visible ophthalmoscopically.
• Light strikes the photoreceptors: rods and cones after being focused by the lens.

Question 2

Rods and cones are found in what part of the retina?

Answer 2

In the outer retina.

Rods detect light intensity whilst cones detect color. Rods are for dim and night vision, cones for vision in bright light.

Question 3

Photosensitive ganglion cells are found in what part of the retina?

Answer 3

In the inner retina (see pic).

Are sensitive to bright and blue light.

Question 4

PLR is…?

A blind patient with normal bilat. PLR indicates?

One blind eye with normal bilat. PLR indicates?

Answer 4

pupillary light reflex.

• the constriction of the pupil in response to light
• dependent on functional retina, optic nerve, oculomotor nerves, midbrain and brainstem

A blind patient with normal bilat. PLR indicates bilateral cerebral lesions (cortical blindness).

One blind eye with normal bilat. PLR indicates unilateral cerebral lesion in the contralateral side.

Question 5

Describe color vision in dogs and cats.

Answer 5

Dogs and cats have dichromatic vision: red, green, yellow appear similar.

Question 6

Describe Electroretinography (ERG).

Answer 6

Electroretinography measures the electrical responses of various cell types in the retina, including the photoreceptors (rods and cones). non-invasive, objective test widely used in veterinary ophthalmology.

to assess retinal function by measuring electrical responses of the retina to light stimuli. It is commonly performed in dogs, cats, horses, and other species to diagnose retinal diseases, such as progressive retinal atrophy (PRA), sudden acquired retinal degeneration syndrome (SARDS), and retinal detachment.

May be used in e.g. sudden acquired retinal degeneration/disease in dogs cases.

Question 7

The fundus is typically divided into the

Answer 7

tapetal and nontapetal fundus areas.
* The tapetal area is in the superior half of the fundus and the non-tapetal area is in the inferior half of the fundus as well as the periphery of the superior fundus.

Question 8

The Optic disc (3) is typically located…?

Answer 8

centrally located but will vary in its location depending on the species:
- dog: at tapetal-nontapetal junction
- cat: tapetal area
- horse: non-tapetal area

The shape of the disc may also vary amongst species.

Question 9

Species?

Answer 9

cat, because cats’ optical disc is usually located centrally and within the tapetal area of the ocular fundus.

In dogs, the optical disc is at the tapetal-non-tapetal junction.
In horses, the optical disc is in the non-tapetal area.

Question 10

Species?

Answer 10

Horse, because their optical disc is located in the non-tapetal area of the ocular fundus.

In cats, the optical disc is in the tapetal area.
In dogs, the optical disc is at the tapetal-non-tapetal junction.

Question 11

Species?

Answer 11

Dog, because the optical disc is located at the tapetal-non-tapetal junction in dogs.

In cats, the optical disc is in the tapetal area.
In horses, the optical disc is in the non-tapetal area.

Question 12

Tapetum lucidum is…?

Answer 12

a reflective layer located in choroid. The tapetum like reflects light back toward the retina like a mirror, this
increases the stimulation of photoreceptors and visual sensation in dim light and increases the ability to see in the dark.

Question 13

Describe how the different layers of the globe mask each other as they are laid down and how this masking affects the final ophthalmoscopic appearance of the fundus.

Answer 13

Question 14

SARD

Answer 14

sudden acquired retinal degeneration/disease in dogs

May go to sleep visual, wake up blind. sudden, irreversible blindness in animals, primarily seen in middle-aged to older dogs.

The exact cause is unknown but is suspected to be related to autoimmune, endocrine (Cushing’s disease), or neurodegenerative factors.

Electroretinography may be used in these cases to confirm retinal cell death.

Question 15

Difference between direct and indirect ophthalmoscopy.

Answer 15

direct is handheld, indirect is a headset/loupes

In direct, the examiner looks directly through the device’s viewing aperture to visualize the fundus (the interior surface of the eye). The light from the instrument illuminates the retina, allowing the examiner to see its details.

Direct ophthalmoscopes provide a limited field of view of the retina, usually around 5-15 degrees. This narrow field of view can make it challenging to examine the entire retina at once.

Direct provides an upright, unreversed image of around 15 times magnification

Indirect consists of a light source mounted on a headband or handheld device, along with a condensing lens. They also require the use of a handheld lens, known as a condensing or viewing lens.

In indirect, the examiner sits at arm’s length from the patient and holds the condensing lens in front of the patient’s eye. The light from the ophthalmoscope is used to illuminate the retina, and the examiner views the magnified image of the fundus through the condensing lens.

Indirect ophthalmoscopes provide a wider field of view compared to direct ophthalmoscopes, typically around 20-30 degrees. This broader field of view allows for better visualization of the peripheral retina and a more comprehensive examination.

produces a reversed, inverted image magnified 2 to 5 times

Question 16

Describe Indirect Ophthalmoscopy.

Answer 16

Monocular or binocular.

• Wider field of view than direct ophthalmoscopy, typically around 20-30 degrees.
• Examiner’s face is a safe distance from patient.
• Binocular method allows for 3 dimensional image.
• Monocular method is great for quick clinical survey of fundus.
• The image is inverted!

tropicamide mydriatic used to dilate pupil to get a better view of the fundus

Question 17

Describe direct ophthalmoscopy.

Answer 17

• Small field of view but high magnification
• provides a limited field of view of the retina, usually around 5-15 degrees
• Examiners face is very close to patient’s face.
• Direct Head (1) or Panoptic Head (2): Panoptic Head gives larger field of view than Direct.

Use Mydriasis: artificial dilation of pupil necessary for full view of fundus. Use anticholenergic drops - use short acting only not atropine! E.g. tropicamide 1% (Mydriacyl®) = drug of choice (lasts 4 - 6 hours).

Question 18

Answer 18

“Tigroid (albinotic) Fundus” is a fairly common variation of normal in “color dilute” animals and especially with blue irises.

Question 19

Retinal dysplasia or detachment is…

Answer 19

a congenital, focal, geographic, or generalized maldevelopment of the retina that may arise from trauma, genetic defect, or intrauterine damage such as viral infections.

Most forms of retinal dysplasia in dogs are inherited, and many DNA mutations have been reported.

You can ultrasound an eye to visualize retinal detachment.

Signs of this disease as congenital can be screened in puppies that are old enough to have their eyes open.

In images: detached retina can be seen as round splotches or pale worm llooking things (not the blood vessels, the non-red ones).

So can be total or partial/focal.

Question 20

Answer 20

Total retinal dysplasia/ detachment /TRD

• Bedlington Terrier
• Sealyham Terrier
• TRD and MRD both in Retriever (Labrador)

Question 21

What is MRD?

Answer 21

Multifocal retinal dysplasia (MRD) is a type of inherited eye condition. Affected dogs can be diagnosed with MRD after an eye examination that finds specific changes to the light sensitive area at the back of the eye (the retina).

Question 22

Describe Generalised progressive retinal Atrophy (GPRA) / Progressive Retinal Atrophy (PRA)

Answer 22

hereditary, degenerative retinal disease in animals, primarily affecting dogs and, rarely, cats. “Non-congenital but inherited”

In the early stages of inherited retinal atrophy, there is loss of night vision due to early degeneration of
rods. Not painful.

As the disease progresses cones are also affected and day vision is also lost, making animal blind.

The pupils are more mydriatic than usual and their reaction to light is slower.

ERG = electroretinography can detect progressive retinal atrophy changes long before behavioral or fundoscopic changes occur.

Question 23

Describe fovea & macula.

Answer 23

dogs, cats, horses do NOT have a macula and fovea.

humans, reptiles and birds do!

Question 24

Answer 24

Retinal pigment epithelial dystrophy (RPED) /
Central progressive Retinal atrophy (CPRA)

is a retinal degenerative disease affecting the retinal pigment epithelium (RPE), leading to central vision loss in dogs.

Unlike generalized PRA, RPED primarily affects the central retina while peripheral vision remains intact for a longer period.

The disease is associated with lipofuscin accumulation in the RPE, often linked to vitamin E deficiency or metabolic disorders.

Clinical signs include reduced vision, particularly in bright light, and a distinctive multifocal retinal pigmentation pattern seen on fundoscopic examination.

Electroretinography (ERG) typically remains normal in early stages, differentiating it from PRA.

There is no cure, but dietary supplementation with antioxidants and vitamin E may help slow progression.

Question 25

Collie Eye Anomaly

Answer 25

Mutation of the gene that determines the development of the eye resulting in defects of many layers of the eye. * Retinal detachment may progress slightly * Vision depends on the extent of the changes * Choroidal hypoplasia (pics 2-4) - blood vessels do not develop under the retina and it cannot be viable or functional without a nutrient supply. +/- retinal detachment, retinal folds (pic 3) +/- optic disc coloboma (pic 4) Most dogs with collie eye anomaly can actually see, just not as well as a healthy animal. ## Footnote * Collies * Shetland sheepdogs * Australian shepherds * Border collies * Nova Scotia retrievers

Question 26

Answer 26

Question 27

Answer 27

Question 28

congenital optic nerve hypoplasia

Answer 28

**Congenital Optic Nerve Hypoplasia (ONH)** is a rare developmental abnormality in animals, primarily seen in **dogs and cats**, where the **optic nerve is underdeveloped** due to incomplete formation of retinal ganglion cells. It can be **unilateral or bilateral** and results in varying degrees of **vision impairment or blindness**. Affected animals may have **abnormal pupillary light reflexes** and small, pale optic discs on **ophthalmic examination**. Diagnosis is confirmed through **fundoscopy and electroretinography (ERG)**, which typically shows a normal retinal response despite vision loss. In optic nerve hypoplasia (ONH), the retina itself is structurally and functionally normal, so electroretinography (ERG) often shows normal retinal activity. However, because the optic nerve is underdeveloped, the visual signals from the retina cannot be properly transmitted to the brain, resulting in vision deficits or blindness. There is **no treatment**, and prognosis depends on the severity, but animals can adapt well to vision deficits with environmental support.

Question 29

Examination of blindness in an animal can include: (10)

Answer 29

* Menace response * Size and symmetry of the pupils * Pupillary light reflexes (PLRs) * Dazzle, palpebral and corneal reflexes * Facial nerve function/facial sensation * Motor function of the muscles of the eyes and face * Observe in a dark, familiar room in which the furniture has been rearranged. The same test can then be done with the lights on. The ophthalmic exam may include: fluorescein stain test, intraocular pressure. Evaluation of any opacity of eye structures. Ophthalmoscopy.