Clean Run - November 2017 - 34

and objective assessments of dogs' performances. Dogs with demanding visual tasks or signs of visual deterioration
should be evaluated retinoscopically to
determine the refractive state because
they may have ametropia."
In an interesting study from 1995 called
"Vision in dogs," I read about how a dog's
vision and what he sees is affected by
the actual shape and construct of the
eye.2 Different breeds of dogs have different ranges of vision as well, which is
affected by the structure of their head.
And, for the first time, I read about
depth perception. Dr. Murphy teaches
at UC Davis School of Veterinary Medicine and I mention this because of a
study I participated in with him in 2014.
Another article describes how myopia
is measured in dogs using retinoscopy
and the relevance in studying early
childhood myopia.3 I began to think we
need to be able to measure this value in
all dogs. In this study Dr. Murphy and his
colleagues tested vision in 1440 dogs in
90 breeds. The results pointed toward
some breeds having more prevalence
than others for myopia and astigmatism,
as well as other vision deficits. The article
states "the refractive state of canine eyes
is affected by breed and age." This is yet
another hint that our dogs' eyes differ a
bit from breed to breed and also change
over time as they age. In some cases,
entire breeds tested had higher rates of
issues and, in other cases, entire families
of dogs were tested and found to have
similar problems-a reminder that the
hereditary factor is important.

A 1991 study on 240 different dogs discusses different breeds and their subsequent myopia tendencies.4 Some of the
test dogs were from Guide Dogs for the
Blind in Santa Rosa.
Another study found that the clinical
handheld portable autorefractor technique used in human pediatrics is applicable to dogs.5 Using that technique
to take measurements, they found that
myopia gets worse with age in both
humans and dogs. And, the normal
cloudiness of the lens of an aging dog
causes myopia. This may be why some
ETO dogs get worse with age, no matter
what the cause, and why some normal
dogs develop ETOs.
And possibly just as important from
the same abstract: "Younger dogs were
able to make larger accommodation
changes from indirect light to direct
light conditions, indicating a more flexible lens." Accommodation is a reflex
action of the eye in response to focusing on a near object and then looking
at distant object. Our eyes "accommodate" almost immediately by changes
in our lenses. It is possible that dogs in
general, and especially dogs with ETOs,
do not accommodate as quickly from
seeing something up close to locating
something at a distance. Additionally,
if our eyes are operating correctly, we
adapt to the change from light and dark
quickly. When we walk into a dark room
and turn on the light we are usually not
blinded and vice versa. We don't know
everything about how our dogs' eyes
adapt going from light to dark, such as

a dog going into and out of a dark tunnel, but I do know my dog Scoop needs
to be supported on the next obstacle
after a tunnel more than my other dogs,
and this is often a trait of dogs with
ETOs. Some of the vision testing procedures an ophthalmologist can do may
determine some of those weaknesses.
I was totally fascinated when I read Caroline Coile's article "Bringing Dog Vision
into Focus."6 It is written with the layman
in mind and is definitely worth taking
the time to read. Here are a couple mentions from the article:
* Existing evidence suggests dogs have
very little ability to accommodate.
* Theoretical calculations and behavioral estimates of dog visual acuity
suggest it is at least six times poorer
than human acuity.
A study on inherited myopia in Labs
was one of the most interesting articles
I read.7 The results indicated "...myopia
in Labrador Retrievers has a significant
genetic component. To our knowledge,
this is the first study to demonstrate a
genetic component responsible for naturally occurring myopia in any species
other than humans." They also found
that "...dogs from small litters were likely to be more myopic than dogs from
large litters. The origin of this effect remains unclear. It may suggest some link
between the genetic determinants of
litter size and those of refractive error.
Alternatively, it may result from an environmental effect."

1-3 D (diopter) defocus can result in blurry vision simulated
based on human subject experience. Similar blurry vision can
be expected for dogs that have
-1 to -3 D defocus.
Hernandez, Jerome; Moore, Cecil; Si,
Xuemei; Richer, Stuart; Jackson, Janet;
Wang, Wei (2016): Simulated vision
blurriness with different diopter defocus. figshare. https://doi.org/10.1371/
journal.pone.0148436.g005

Clean Run | November 17

https://www.doi.org/10.1371/journal.pone.0148436.g005 https://www.doi.org/10.1371/journal.pone.0148436.g005

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