Reti-whatty pigmen-who? Retinitis pigmentosa (RP), almost as complicated to spell as Gnadenhutten, Ohio and as awkward to say as Coxsackie, New York, is an inherited disease of the retina that often results in blindness. The prevalence of this disease sits around 1 in 4000 and though much research is being devoted to a cure or at least a treatment, currently nothing has been found to fully restore sight in RP sufferers.
The vision loss experienced in Retinitis Pigmentosa develops due to a mutation in any one of over 50 genes, which causes the photoreceptors of the retina to slowly die. The retina is the sensory layer of tissue inside the eyeball and is comprised of a number of layers in itself, one of which is a layer of light-sensitive cells known as photoreceptors that detect light entering the eye before sending the signal down a pathway which eventually ends in the brain for interpretation into what we know as “vision.” Two types of photoreceptors exist – rods and cones, named as such because they resemble – wait for it – rods and cones! In simplified terms, rods predominantly populate the periphery of the retina and thus deal with peripheral vision, while cones are found in their highest density at the central retina in the area known as the macula and are responsible for fine detailed central vision. Other differences between rod and cone photoreceptors include the fact that rods function better in dim lighting while vision in bright light relies on cones; cones are able to detect color while rods are more or less color blind.
RP is known as a rod-cone dystrophy, meaning that rods are affected first and more so than cone photoreceptors. Eventually, due to the loss of rod photoreceptors in RP, cones also become affected.
The disease typically onsets during early childhood within the first decade of life though some cases may be detected only after the age of 50. Its progression can vary from one individual to the next as so many different combinations of genes may be involved. The inheritance of RP can be in one of three ways:
- Autosomal dominant: the inheritance is not linked to a sex chromosome from the parents but to one of the other 22 non-gender-specific chromosomes, known as autosomes. In a dominant mode of inheritance, only one copy of the mutated gene from one parent is required to manifest the disease.
- Autosomal recessive: two copies of the mutated gene from each parent is required to exhibit RP. If one of the paired genes from one parent is normal then the child will not have the disorder.
- X-linked recessive: the X chromosome is known as a sex chromosome, along with the Y chromosome. Females have two X chromosomes, one from each parent, while males have both X and Y chromosomes. In this mode of inheritance, the mutated gene causing RP is found on the X chromosome. In females this means that to exhibit RP, both X chromosomes must contain the mutant genes, while in males, if his mother passes down an X chromosome with a mutated RP gene, he will manifest the RP disease.
Symptoms in RP
The first and most common symptom is often reported as night-blindness. This makes sense as rods are responsible for night vision but are the first to go in RP. However, frequently the increasing difficulties with night vision can be dismissed as simply “it’s just dark” (which is a fair point as night-time is often known to be dark). This symptom may be present as early as infancy and in the early stages of RP it can be difficult to make a diagnosis as symptoms are mild and even clinical tests may not yet be able to detect the disease.
As the disease progresses, night-blindness becomes more obvious. A noticeable constriction of the peripheral vision also develops, even in bright daylight conditions, making it difficult to navigate unfamiliar places or to avoid bumping into objects. Central vision often becomes affected, whether through loss of cone photoreceptors or due to other associated issues such as cataract or swelling of the macula. At this stage, color vision may become compromised and glare sensitivity can become an issue. An eyecare specialist will also begin to notice changes to the retina, such as altered pigmentation.
Testing for RP
Genetic testing can help to determine the presence of genetic mutations associated with RP. This information may be used to predict the course and severity of the individual’s disease and may also be of benefit when it comes to future family planning.
A visual field test involves sitting in a dark room in front of a machine and clicking a button in response to a flicker or a point of light presented in the peripheral vision. This gives a map of the patient’s extent of their remaining field of vision and is useful for monitoring progression of the disease. A visual field test is basically the most boring video game known to man.
Electrodiagnostic testing such as electroretinography (ERG) sounds high-tech and cool and it kind of is until someone puts an electrode in your eye. The purpose of an ERG is to objectively measure the response of the retina to light without requiring patients to subjectively report what they can or cannot see.
All the routine examinations such as measuring visual acuity and the prescription, as well as checking the health of the eye under the slit lamp microscope is also important.
Finding a treatment for RP
Research has found that high doses of vitamin A can slow the deterioration of retinal function in adults with RP; however it may not be appropriate for all patients considering the variable presentation and progression of the disease.
A retinal implant known as the Argus II has had promising results in allowing previously blind RP patients to see well enough to make out the outlines of objects and even navigate their way without the use of a cane or guide dog. This implant, or “bionic eye”, is not currently conveniently available and is priced at over $100k, though Medicare may contribute in part to the cost of the device and the surgery.
For all RP patients, the use of low vision aids can be very beneficial. This may involve a cane or guide dog for mobility, specific magnifying lenses, or even the high-tech and cool closed-circuit TVs, which can capture text and enlarge it to an appropriate size.
Though some loss of vision in retinitis pigmentosa is as certain as Kissimmee, Florida has three pairs of double letters, it is important to be proactive in exploring all options to maintain as much quality of life as possible. An eyecare specialist is best to provide advice and referral to genetic counselling or low vision services.
Facts About Retinitis Pigmentosa. https://nei.nih.gov/health/pigmentosa/pigmentosa_facts
Retinitis Pigmentosa. http://www.allaboutvision.com/conditions/retinapigment.htm
Retinitis Pigmentosa. https://www.aao.org/eye-health/diseases/what-is-retinitis-pigmentosa
Retinitis Pigmentosa. https://ojrd.biomedcentral.com/articles/10.1186/1750-1172-1-40
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