Progressive
Retinal Atrophy (PRA)
Cells in the retina, lining the back of the eye, receive
light from the external environment and transmit this
information to the brain, where it is interpreted as
vision. In progressive retinal atrophy (PRA), deterioration
of the retinal cells causes blindness.
The
inner layer at the back of the eye is the retina. The
retina has 9 layers, the outermost of which consists
of the light-receiving cells - the rods and cones. The
outer layer of the retina is called the retinal pigmented
epithelium (RPE). In most breeds of dogs the retina
is not mature until 6 or 7 weeks of age.
The
retina contains specialized cells called photoreceptors.
These absorb light focused on them by the eye’s
lens, and convert that light - through a series of chemical
reactions - into electrical nerve signals. The nerve
signals from the retina are passed by the optic nerve
to the brain, where they are perceived as vision. The
retinal photoreceptors have specialised forms. Rods
for vision in dim light (night vision), and cones for
vision in bright light (day and colour vision). PRA
usually affects the rods first, and then the cones later
on. PRA in dogs is equivalent to retinitis pigmentosa
in Humans.
Where
PRA has existed for some time, the pupils of a dog’s
eyes become increasingly dilated, in an attempt to gather
more light, causing a noticeable "shine" to
the eyes. Also, the lens of the eyes may become cloudy,
or opaque, resulting in a cataract. Cataracts are often
a symptom associated with advanced PRA.
Secondary
cataracts are often associated with other eye disorders,
such as PRA. It has been noted that some dogs affected
by Posterior Polar Cataracts (PPC) have also been affected
by Progressive Retinal Atrophy (PRA). Thus, it could
be concluded that the PPC could be an indicator that
PRA has the potential to develop in certain lines of
dogs. PPC seems to be inheritable.
Several
different types of inherited deterioration of the retina
are lumped together under the heading of progressive
retinal atrophy. Sub-classifications of PRA are based
upon the age at which dogs show clinical signs of the
disease, and upon the type of retinal cell that is affected.
PRA
is an inherited condition with an autosomal recessive
method of gene expression. Rod-cone dysplasia (rcd1
form) can be detected in Irish Setters by DNA testing.
The advantage of such testing is that it can identify
dogs whose sight is unaffected, but who are carriers
of the recessive gene. This is particularly important
for responsible Breeders, as the onset of blindness
occurs long after the puppy has been taken to his new
home.
No
effective treatment for PRA exists. The degree of visual
impairment varies within breeds. Most affected dogs
will eventually become completely blind. With their
acute senses of smell and hearing, dogs can learn to
cope very well, particularly in familiar surroundings,
to the point where owners may be unaware of the extent
of vision loss. That the loss of vision occurs slowly
is also a factor in the dog’s ability to adjust
to becoming blind.
You
can help your dog by developing regular routes for exercise,
maintaining familiar surroundings, introducing any changes
gradually, and being patient.
There are no obvious changes to the eyes that you can
initially see. You may notice that your dog has increasing
difficulty getting around at night or when the lights
are turned off.
Your
Vet will look for abnormalities in the eye with an indirect
ophthalmoscope, that requires the dog’s pupils
to be dilated by using special eyedrops. PRA may also
be detected by an electroretinogram (ERG), before your
dog has any noticeable visual problems. These problems
include:
~ increased reflectivity (shininess) of the fundus (the
inside of the back of the eye, overlain by the retina)
~ reduction in the diameter and branching pattern of
the retina’s blood vessels
~
shrinking of the optic nerve head (the nerve connecting
the retina to the brain)
These
changes occur in all forms of PRA, but at different
times in the various breed-specific forms. Usually by
the time these changes are noticed in the affected dog,
there is already significant evidence of loss of vision.
Breeding
is not advised for any dog found to have PRA, or for
that dog’s parents (assumed to be carriers). Siblings
should be carefully screened by electroretinogram, where
DNA tests do not exist, if they are to be considered
for breeding. This type of testing is best done annually.
Problems may arise where symptoms only appear after
the dog has already been bred from.
Where
DNA tests are available and have been done, the gene
expression is known and carriers as well as affected
dogs are positively identified. DNA testing need only
be done once in a dog’s lifetime, making it definitive,
accurate and affordable.
Generalized
PRA
These
diseases affect mostly the photoreceptor cells. Both
eyes are similarly affected. Dogs eventually become
completely blind. There is no effective cure.
Breeds
affected by Progressive Retinal Atrophy Many breeds
are affected by one, and sometimes several, types of
PRA.
Generalized
PRA - early onset:
Usually, the first symptom is increasing night-blindness,
as early as 6 weeks of age. This progresses to a complete
loss of vision by around 1 to 2 years of age.
