Progressive retinal atrophy (PRA) is a common inherited eye condition known by the name “inherited retinal degeneration”.
Recognized in over 90 dog breeds, PRA is an insidious and, sadly, irreversible condition that ultimately causes blindness.
PRA – Definition And Meaning
Let us analyze the name of the condition. The term progressive describes the condition’s nature; the term retinal shows the changes’ anatomical location, and the term atrophy translates to deteriorating or merely wasting.
So, PRA is a condition in which the retinal layer of the eye slowly but continuously deteriorates.
First, the dog loses its scotopic (night) vision, then its photopic (day) vision, and finally, the condition culminates with permanent bilateral blindness. Bilateral indicates that both eyes are affected.
PRA – Causes
PRA is an inherited condition that occurs due to a specific gene mutation known as CNGB1. The gene is inherited recessively, which means a puppy only develops PRA if both parents carry the CNGB1 gene.
The only two exceptions to the CNGB1 gene inheritance pattern are the Old English Mastiff and the Bullmastiff. In these breeds, CNGB1 is dominant. This means a pup can develop PRA even if only one of the parents carries the responsible gene.
PRA – Types
The CNGB1 gene mutation pre-programs the retinal cells to start deteriorating at a certain point in the future. That point is different among different breeds.
Based on the time when the retinal cells are programmed to die, there are two types of PRA:
- Early-onset PRA, also known as retinal dysplasia, is inherited and develops in puppies (between two and three months old) when their retinal cells fail to develop correctly.
- Late-onset PRA, also known as adult PRA is inherited and develops in adult dogs (between three and nine years of age) when the retinal cells die.
PRA – Pathophysiology And Development
PRA affects the retina and the pigmented layer below the retina cells. The retina consists of two types of cells:
- Rods – responsible for seeing movements and seeing in low light conditions
- Cones – responsible for seeing in color and seeing in bright conditions
The pigmented layer is responsible for protecting the rods and cones. Basically, the retina acts like a film inside a camera – it enables vision by adding light.
The eye is a light-accumulating organ – it collects light and focuses the light rays on the retina. Once light touches the retina, a cascade of chemical reactions occurs and creates an electrical impulse.
The impulse travels through the retinal layers and via the optic nerve to the brain’s visual cortex, where it is processed. When the brain interprets the electric impulse’s light signal, the dog gets a visible picture.
In dogs with PRA, the rods and cones are genetically pre-programmed to die. The rods die first, and then the cones follow. That is why PRA starts with night vision loss.
As the retina loses its ability to register light, the pupil dilates to allow as much light as possible into the eye.
Then, the condition progresses to day vision loss, and ultimately blindness occurs. Due to oxidative stress on the lens, the affected eyes develop toxic cataracts and become greyish with a slight sheen.
PRA can be compared to a dimming switch adjusted to reduce a room’s brightness. If the light is reduced slowly and over a more extended period, the eyes adapt to the gradually darkening environment.
The change becomes apparent when the room turns completely dark. The same concept applies for dogs with PRA – their eyes adjust, and by the time the first signs and symptoms become apparent, the condition is already quite advanced.
PRA – Incidence And Predisposition
Both purebred and mixed breed dogs can develop PRA. However, according to reports and statistics, the condition is more common among the following breeds:
- Akita: onset – 3-6 years, affected cells – rods and cones
- Cairn Terrier: onset – <1 year, affected cells – rods and cones
- Cocker Spaniel: onset – 2-7 years, affected cells – rods and cones
- Collie: onset – <1 year, affected cells – rods and cones
- Golden Retriever: affected cells – rods and cones
- Irish Setter: onset – <1 year, affected cells – rods and cones
- Labrador Retriever: affected cells – rods and cones
- Miniature Long-Haired Dachshund: onset – <1 year, affected cells – rods and cones
- Miniature Poodle: onset – 3-6 years, affected cells – rods and cones
- Miniature Schnauzer: onset – 3-6 years, affected cells – rods and cones
- Norwegian Elkhound: onset – 2-3 years, affected cells – rods
- Samoyed: onset – 3 years, affected cells – rods and cones
- Schnauzer: onset – >3 years, affected cells – rods and cones
Other breeds frequently diagnosed with PRA include:
- Alaskan Malamute
- Australian Cattle Dog
- Australian Shepherd
- Belgian Shepherd
- Briard
- Cardigan Welsh Corgi
- Glen of Imaal Terrier
- Mastiff
- Papillon
- Portuguese Water dog
- Siberian Husky
- Tibetan Terrier
When it comes to predispositions and incidence, there are two main peculiarities. First, the above-listed dog breeds’ European lines are more likely to carry the CNGB1 gene.
Secondly, in certain dog breeds, such as the Samoyed and the Siberia Husky, PRA is a sex-linked condition prevalent in males.
PRA – Signs And Symptoms
Sadly, PRA is an insidious condition, which means by the time the affected dog starts manifesting signs and symptoms, the disease is already significantly advanced.
Plus, the condition is slowly progressing. It takes around a year or two for the blindness to develop in most cases.
The fact that the dog has plenty of time to adjust contributes to the late clinical manifestation.
Usually, the first things the owner notices are:
- Increased eye reflectiveness (eye shine)
- Unusually dilated pupils
- Clumsiness, tripping, and bumping into walls, doors, furniture
- Reluctance to go out when it is dark
- Reluctance to enter a dark room
- Reluctance to move around the yard at night
- Inability to follow hand signals
PRA – Differential Diagnosis
Several eye conditions may cause slow-developing blindness in dogs:
- Glaucoma
- Slowly progressive cataracts
- Lens luxation
- Uveitis
- Optic nerve disease
While on the subject of similarly manifesting issues, two diagnoses are worth mentioning.
Central Progressive Atrophy
This condition is similar to PRA but affects only the central part of both eyes’ retina. It usually occurs in older dogs of the following breeds: Golden Retriever, Labrador Retriever, Border Collie, and Rough Collie.
The affected dog retains its peripheral vision, which means that the animal will lose the ability to see stationary but will still be able to see moving objects.
An ophthalmologic examination is enough to set up the diagnosis, but sadly, there is no known available treatment.
Sudden Acquired Retinal Degeneration Syndrome (SARDS)
As the names suggest, SARDS occurs unexpectedly, and the affected dogs lose their eyesight fast – in a matter of days to few weeks. The condition is non-inherited, but the exact trigger is not determined yet.
Clinically, SARDS manifests with dilated pupils, but the eye exam reveals no significant changes. The examination is enough to differentiate this condition from PRA.
PRA – Diagnosis
As in any other case, the vet will start by performing a full physical examination. If there are no other pre-existing conditions, the physical examination results will be clear since PRA does not cause systemic changes.
Then, the vet will perform a general ophthalmic examination. The general ophthalmic examination usually shows:
- Dilated pupils at rest (a condition scientifically known as resting mydriasis)
- Delayed and sluggish pupillary light response
The vet will proceed by examining the deeper eye structures through an ophthalmoscope. When seen under an ophthalmoscope, PRA’s retina is much different from a normal retina.
Namely, in a retina affected by advanced PRA, the blood vessels are shrunk and almost disappearing. The retina’s color is also significantly altered – it is paler and flatter.
Theoretically, these changes in the deeper eye structures are enough for setting the diagnosis. However, there are two problems. First, during the initial phases of PRA, the described changes are subtle and hard to spot.
Second, if there is already a developed cataract, the retina and other deeper structures are not visible under an ophthalmoscope.
Simply put, if the condition is in its early or advanced stages, the regular ophthalmic examination has no diagnostic value. Your regular vet will refer you to a veterinary ophthalmologist in such cases.
The eye specialist will conclusively confirm or rule out PRA based on one test – electroretinogram (ERG). ERG is an extra sensitive diagnostic test that measures the retina’s light response and can diagnose PRA before any clinical manifestations.
PRA – Treatment Options
Sadly, there are no widely and routinely available treatment options for PRA. Lately, there have been advances in the field of sub-retinal gene therapy.
This novel treatment includes inserting a normal (non-mutated) copy of the CNGB1 gene. However, this option is still in its experimental stages and is not widely available.
Nevertheless, based on results, it is safe to say that sub-retinal gene therapy is a promising option for the future treatment of PRA.
Oral antioxidant therapies and lutein (carotenoid) supplementations are also showing some promising results. Namely, if started during the early stages, they can have three benefits:
- slow down the PRA’s progression
- delay the toxic cataract’s onset
- promote overall eye health
Finally, it is a good idea to say a word or two about cataracts when discussing treatment options. Many dog parents are concerned with its presence simply because it is more easily visible than the PRA.
Consequently, they often ask whether it is advisable to have it removed. The simple answer is no. In a dog with PRA, the cataract is a purely cosmetic issue.
Its removal will not improve the dog’s already deteriorated or completely lost vision, nor does it slow down the PRA’s progression.
PRA – Prognosis And Complications
The faith of a dog with PRA is sealed – it will go bind, it is just a matter of time. However, a dog with PRA needs regular check-ups. As mentioned, a dog with PRA will eventually develop a toxic cataract.
The cataract causes the eye proteins to leak from where they are stored in the eye. The leaked proteins can sometimes cause an inflammation, which, if left untreated, becomes chronic and, more often than not, culminates in glaucoma.
Glaucoma is a blinding and excruciating condition that manifests with increased intraocular pressure.
Glaucoma’s blinding part is not an issue since the dog’s vision is already compromised or lost due to the PRA. However, the painful part is concerning.
Dogs with PRA do not have pain problems, but the pain will be inevitable if they develop glaucoma. That is why dogs with PRA benefit from frequent check-ups and early management of potential complications.
PRA – Prevention
As with any untreatable condition, all efforts should be put into prevention. The best way of preventing PRA is the early detection of affected and carrier dogs and their exclusion of breeding programs.
If a dog is either affected or a carrier removing it from breeding is not enough. The dog’s parents, siblings, and offspring (if existing) must also be excluded.
You can check whether your dog is a carrier or not by testing its DNA. Luckily, many laboratories offer such tests, and what is best, the price tag of these analyses is not as hefty as it used to be.
PRA – Coping With Blindness
From our perspective, losing the ability to see is a devastating and life-changing experience. However, dogs are different than us. They are excellent at accepting life as it is.
Therefore, when we think about how dogs feel about going blind, chances are we are extrapolating. With lots of love and some small changes in the dog’s environment and routine, a blind dog can thrive and live a happy and high-quality life.
Here are some tips on how to successfully navigate your dog through the going blind experience:
- Avoid rearranging the furniture to prevent your dog from bumping
- Always keep your blind dog on a leash when not in an enclosed area
- Use a short and rigid leash so you can direct your dog
- Replace the collar with a harness – it gives you better control and spares the eyes the unnecessary pressure (significant for dogs with glaucoma as complication)
- When walking, allow your blind dog to sniff as much as it wants – now it perceives the world through its nose
- Install a baby gate to block the stairs or any area you consider unsafe
- Pad all sharp edges in the house with a bubble wrap
- Buy toys with sound, vibration, or smell features
- Buy and install scent markers around the home to help your dog make a mental map of its environment
- Put plastic mats under your dog’s food and water bowl so it can be aware of their proximity by the different textures of the surface
Conclusion
PRA is characterized by retinal cell death and the choroid layer’s blood vessels’ wilting. In laymen’s terms – progressive and destructive changes cause gradual vision deterioration.
In most cases, the first sign of PRA onset is night blindness. As the condition progresses, the dog becomes less confident due to the decreasing eyesight. The situation culminates when the whole retina dies, and the eye goes completely blind.
The condition is diagnosed through ophthalmological examination, and sadly, there is no routinely available treatment. As with any other hereditary condition, examination programs have been set up to certify that breeding individuals are free from PRA signs.
It is also essential to know that several previous generations of a dog’s lineage are free of late-onset PRA.
Finally, with small adjustments and a healthy support system, even a blind dog can have a long and happy life. Just keep on loving your dog the way you did.
