Equine Retina and Optic Nerve

Question 1

What type of retinal vascular pattern do horses have surrounding their optic disc?

Answer 1

Paurangiotic

Question 2

What cells are responsible for transduction (converting light energy into electrical energy)?

Answer 2

Photoreceptors

Question 3

What does an ERG measure?

Answer 3

Electrical response of retina (retinal function NOT vision)

Question 4

On ophthalmic exam, your patient has an absent menace response (blind), but has normal PLRs, what are some possibilities?

Answer 4

Something obstructing vision (cataract) or cortical disease (somewhere behind the reflex arc)

Question 5

On ophthalmic exam, your patient has an intact menace response (visual), but has absent PLRs, what are some possibilities?

Answer 5

Efferent problem (CN 3 or iris sphincter) OR mechanical or pharmacologic problem

Question 6

On ophthalmic exam, your patient has an absent menace response (blind) and absent PLRs, what are some possibilities?

Answer 6

The issue is localized to the retina or optic nerve

Question 7

How do you diagnose congenital stationary night blindness?

Answer 7

History and ERG (a-wave dominant)

Question 8

A 2 year old Appaloosa mare presents to your clinic because the owner reports that she “acts weird as the sunsets and it gets darker.” You do an ophthalmic exam and note no obvious abnormalities (your fundic exam was normal), what is the BEST differential diagnosis in this case?

Answer 8

Congenital Stationary Night Blindness

Question 9

A 2 year old Appaloosa mare presents to your clinic because the owner reports that she “acts weird as the sunsets and it gets darker.” You do an ophthalmic exam and note no obvious abnormalities (your fundic exam was normal), what diagnostic tool would you use next to confirm your suspicion?

Answer 9

ERG

Question 10

A 2 year old Appaloosa mare presents to your clinic because the owner reports that she “acts weird as the sunsets and it gets darker.” You do an ophthalmic exam and note no obvious abnormalities (your fundic exam was normal). You decide to do an ERG on this patient, what would you expect to see if your differential is correct?

Answer 10

An a-wave dominant graph (very large dip in the a wave, and the b wave is absent) - this is pathognomonic for this!

Question 11

What are “Stars of Winslow” that you may see on a fundic exam?

Answer 11

End-on choroidal vessels in the non-pigmented retina.

Question 12

What is a common retinal anomaly seen in Rocky Mountain horses?

Answer 12

Retinal dysplasia

Question 13

A 5 year old QH mare presents for muscle wasting and other neurologic signs. On ophthalmic examination you note a honeycomb appearance of the fundus. What is your top differential diagnosis in this case?

Answer 13

Equine Motor Neuron Disease

Question 14

A 5 year old QH mare presents for muscle wasting and other neurologic signs. On ophthalmic examination you note a honeycomb appearance of the fundus. What is the cause of this disease?

Answer 14

Vitamin E Deficiency

Question 15

What is the most common cause of retinal detachments?

Answer 15

ERU!

Question 16

On ophthalmic exam a horse is blind, mydriatic, and has absent PLRs. You look at the fundus and see hemorrhage around the optic disc and edema of the nerve head (fuzzy indistinct margins to the optic disc). This horse has no history of trauma, what is a possible differential?

Answer 16

Optic neuritis