Progressive
retinal
degeneration (PRD) is also known as progressive retinal atrophy (PRA)
and
refers to retinal diseases that cause blindness. Some breeds have
blindness by
abnormal development of the retina and this is called dysplasia. Other
breeds
have a slowly progressive degeneration or death of the retinal tissue
and this
is degeneration. These two types of diseases affect many breeds. In
general
these diseases are thought to be inherited but inherited differently in
each
breed.
In
all animals with
PRD the outcome, age of the patient and what the veterinary
ophthalmologist
sees are the basis for the classification of exactly what type of
condition the
patient has. Different breeds of dogs have variations in the age the
problem
starts and speed with which the blindness develops. The condition of PRD has
been seen in almost every registered breed and in mixed breed dogs as
well. This same condition occurs in humans and is known as retinitis pigmentosa.
As
the name PRD
implies, a slow death of retinal tissue occurs. It is a slowly
progressive
disease and the earliest signs may be overlooked. As stated above,
these
diseases are known to be passed from parents to offspring even though
the
parents may have normal eyes. Therefore, identification of breeding
animals
with PRD is essential to prevent spread of this condition.
To
better understand
PRD, a basic understanding of the function of the retina is needed. The
retina
is a highly complicated tissue located in the back of the eye. Light
strikes
the retina and starts a series of chemical reactions that causes a
nerve
impulse. The impulse passes through the layers of the retina to the
optic nerve
and from there to the brain where vision takes place. In the retina,
cells
called rods are involved with black and white or night vision and cells
called
cones are involved with color or day vision. Progressive retinal
degeneration
may effect either the rods alone, the cones alone or both the rods and
cones
together.
Progressive
retinal
degeneration is not a painful condition so your pet will not have a
reddened
eye or have increased blinking or squinting. For this reason most
clients will
not notice the early stages of the condition. Some clients will notice
an
abnormal shine coming from their pet's eyes. This abnormal shine is
because the
pupils are dilated and don't respond as quickly to light as pupils of
normal
dogs. The earliest signs of PRD include night vision difficulties that
in most
cases will progress to day blindness. Clients will often remember that
their
pets seemed disoriented when going out to the yard at night and they
had to
leave a light on for them. Night blindness may be manifested by a pet
that is
afraid to go into a dark room. Occasionally these pets will get lost in
their
own home after the lights have been turned off.
The
veterinary
ophthalmologist examines the retina with an instrument called an
indirect
ophthalmoscope. Changes in the retinal blood vessel pattern, the optic
nerve
head, and the reflective substance within the dog's eye called the tapetum can
be seen which are classic for PRD. However in some breeds PRD
characteristics
have little or no early changes. The eyes of these dogs may appear
normal until
they are in the later stages of the disease. Progressive retinal
degeneration
will progress at different rates in different breeds. This variation
causes
difficulty in determining just how long any particular dog will
continue
seeing.
There
is no possible
treatment for PRD although a number of vitamin therapies have been
suggested by
various people. One such vitamin "Ocuvite" manufactured by Stortz has
been recommended for people with retinitis pigmentosa and some patients
claim
that their vision is improved somewhat. At this time, none of the
vitamin
treatments have been proven to be effective scientifically, so use of Ocuvite
must be deemed a naturopathic remedy rather than a medical treatment. Use of
any other megavitamin treatment is discouraged.
Cataracts
may occur
in some patients with PRD and generally occur later in the disease. Formation
of cataracts may interfere with the ophthalmologist's direct
examination of the
retina and make other tests such as an electroretinogram (ERG)
essential for
diagnosis.
Diagnosis
is made
and confirmed by the ERG. This test involves sophisticated
instrumentation used
to measure the response of the retina to flashes of light. Your pet
would be
anesthetized for this test. The pet is then placed into a darkened
area, a
special contact lens with a gold ribbon is placed on the cornea and two
tiny
needles are placed under the skin around the eye. A light flash that
has been
dimmed with filters stimulates the retina and this procedure is
repeated
intermittently for 20 minutes. Finally, a bright red, blue and white
flash are
used for final analysis. A healthy retina will produce a characteristic
wave
form that builds from the time the lights are turned out. The ERG is
sensitive
enough to diagnose dogs with PRD before they begin to demonstrate signs
of the
disease.