Animal Clinical Applications of ERG

Animal Clinical Applications of ERG

Inherited RetinopathiesMiscellaneous
Hemeralopia (achromatopsia)-progressive and non-progressivePreoperative evaluation before cataract surgery
CSNBSARDS
PRADiagnosis of retinal disorders with no ophthalmoscopic abnormalities

Why Animal ERG Testing?

Routine preoperative evaluation of retinal function before cataract surgery

Animals may be affected with both retinal degeneration and cataract at the same time, so it is obvious that cataract surgery will not restore vision if the retina is not functioning. ERG must be performed on every dog and cat that is a candidate for cataract surgery to rule out the presence of inherited retinopathy.

Because the cataract prevents a readable ophthalmoscopic evaluation of the retina, an ERG is required to determine the prognosis of the surgery. It is important to note that even in the presence of cataracts (or a corneal opacity) that affect vision, sufficient light reaches the retina to generate an electrophysiologic response, provided that the retina is functional. 

Differentiating between retinal and post-retinal causes of blindness

For example, cases of SARDS and retrobulbar optic neuritis may present similarly with acute loss of vision; a normal-looking fundus; and fixed, dilated pupils. An ERG may be used to differentiate between the two because the response will be extinguished in SARDS (which is a retinal disease) but normal in optic neuritis (which is a postretinal disease).

Early diagnosis of inherited retinopathies

Both dysplasia’s and degenerations can be detected with the help of ERG. In many dog breeds and in some cat breeds, the ERG may detect changes in retinal function long before ophthalmoscopic or behavioral signs of disease are observed. This early detection is invaluable to breeders wishing to screen their animals for inherited retinal diseases prior to breeding.

In some inherited retinopathies, such as cd in the Alaskan malamute, Gordon setter, and German shorthaired pointer or CSNB in the collie dog and Appaloosa horse, the animal presents with a normal-looking fundus. A definitive diagnosis can only be made with the ERG.

Specific Forms of Inherited Retinopathy

There are several breeds affected by inherited retinopathies, but it’s rarely reported in cats. Abyssinians seem to be susceptible, and lately Persian and Bengal cat breeds were reported too.

Hemeralopia (achromatopsia)

Non-progressive:

The age at onset of clinical signs is at 8 to 10 weeks, night vision is not affected, behavioural signs are obvious at birth. 

Ophthalmoscopic examination reveals no funduscopic abnormalities, but an ERG will show lack of cone function, making it essential for diagnosis. 

Progressive:

Unlike the non-progressive cd, the disease progresses and eventually rod function is also affected, and funduscopic abnormalities may be seen. 

Progressive Retinal Atrophy is one of the diseases that affect photoreceptor cells. 

There are two main forms of PRA recognized in dogs, early-onset or inherited form, also called retinal dysplasia, which is typically diagnosed in puppies around 2-3 months of age, and a late-onset form that is detected in adult dogs, usually between the ages of 3-9 years, called PRA.

Congenital Stationary Night Blindness

CSNB is an inherited, congenital disease affecting night vision in horses and dogs (particularly collies).   The disease does not progress to affect day vision. Ophthalmoscopic examination shows no funduscopic abnormalities, but an ERG examination demonstrates impaired bipolar cell activity, making it essential for diagnosis.

Sudden Acquired Retinal Degeneration Syndrome

SARDS is most common in middle-aged adult dogs, especially obese, spayed females. The typical symptom is acute blindness.

As the name implies, it is a retinal degeneration of sudden onset with both cones and rods affected; the blindness is irreversible. 

Pupils are dilated and non-responsive, initially, the fundus looks normal, although ophthalmoscopic signs of progressive retinal degeneration may appear over the next few months.

Onset of Ophthalmoscopic, Behavioral, and Electroretinogram Signs of Inherited Retinal Dystrophies and Degenerations in Dogs

BREED DISEASE OPHTHALMOSCOPIC SIGNS BEHAVIORAL SIGNS ERG ABNORMALITIES
Akita PRA 1.5-2 yrs 1-3yrs 1.5-2 yrs
Alaskan malamute cd Normal looking fundus 8-10 wks 6 wks
American cocker spaniel prcd 3-5 yrs 3-5 yrs 9 mos
Bull Mastiff PRA 6 mos 6 mos 12 mos
Cardigan Welsh corgi rcd3 6-16 wks 6-8 wks 3-6 wks
Collie rcd2 6 wks 6 wks 2 wks
Dachshund (miniature longhaired) crd1 6-12 mos 6 mos 4-9 mos
Dachshund (shorthaired) crd 3 yrs 3 yrs 5 wks
English cocker spaniel prcd 4-8 yrs 3-5 yrs 12 mos
German shorthaired pointer cd Normal looking fundus 8-10 wks 6 wks
Irish setter rcd1 12-16 wks 6-8 wks 3-6 wks
Labrador retriever prcd 4-6 yrs 3-5 yrs 1.5 yrs
Mastiff (Old English) PRA 6 mos 6 mos 12 mos
Miniature schnauzer PRA-A 1-2 yrs 6-12 mos 6-8 wks
Norwegian elkhound erd 6-12 mos 6 wks 5-6 wks
rd 6-18 mos 6 mos 6 wks
Papillon PRA 1.2-5 yrs 7 yrs 9 mos-1.5 yrs
Pit bull terrier crd2 3-6 mos 8 wks 7 wks
Poodle (toy and miniature) prcd 3-5 yrs 3-5 yrs 6-9 mos
Portuguese water dog prcd 3-6 yrs 3-5 yrs 1.5 yrs
Samoyed XLPRA1 1.5-2 yrs 2-4 yrs 6 mos
Siberian husky XLPRA1 1.5-2 yrs 2-4 yrs 6 mos
Tibetan terrier PRA 10-18 mos 6-12 mos 10 mos
“This article was published in Slatter’s Veterinary Ophthalmology, 6th edition By David Maggs, BVSc(Hons), DAVCO, Paul Miller, DVM, DACVO and Ron Ofri, DVM, PhD, DECVO , Table 15-6, Copyright Elsevier (2018).”

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