Retinitis pigmentosa is an inherited, degenerative eye disease that can cause severe vision loss, beginning with decreased night vision and loss of peripheral vision (tunnel vision).
The disease progresses at varying rates from person to person, and may eventually lead to blindness. Although there is currently no cure for retinitis pigmentosa, there are several treatments that can help increase the visual field and night vision, and stabilize or slow down the progression of vision loss. We will explore these treatments in coming blogs.
Symptoms of Retinitis Pigmentosa
A list of the potential symptoms are:
- Night blindness
- Slow visual adjustment moving from a dark to light environment, or vice versa
- Tunnel vision
- Blurry vision
- Poor color separation
- Aversion to glare
What Causes Retinitis Pigmentosa?
Retinitis pigmentosa is an inherited disease resulting from mutations in many different genes. Although most of the genes affected relate to specific structures and functions of the photoreceptors in the eye, there are still many mutations that are unknown as to why they cause this disease.
It is necessary to understand some basic anatomy of the eye, to understand what causes retinitis pigmentosa.
The back of the eye is lined with special cells that make up the retina. These cells are called photoreceptors, and are made up of two types of cells: rods and cones, which convert light rays into nerve impulses for the brain to interpret. Most of the photoreceptors are rods, which are concentrated on the periphery of the retina, and are more sensitive to light than cones. They provide peripheral and night vision. Cones are more centralized in the retina and are less sensitive to light, but provide color vision.
This little bit of anatomy will explain why mutations in rod cells will cause the symptoms listed above (tunnel vision, poor night vision). The mechanism how these mutations lead to photoreceptor cell death is not completely understood. However, research is showing that the breakdown of the photoreceptors in the periphery of the retina results in accumulation of metabolic by-products disrupting retinal function, and causing further photoreceptor damage, lipofuscin deposition, glial scarring, blood vessel occlusions, and sometimes macular edema.
Why do some people develop symptoms earlier in life than others? Why does the rate of degeneration vary so greatly between people with the same mutations? These questions are beginning to be answered by a new field of genetics called ‘epigentics’ (click HERE to learn more about epigentics) and show how there are many more factors involved to inherited diseases other than simply having a bad gene. The Foundation for Fighting Blindness provides a good booklet to explain how retinal diseases are inherited, and can be found HERE.
How is Retinitis Pigmentosa Diagnosed?
Ophthalmologists are able to diagnose retinitis pigmentosa through a series of tests and medical history. At first, the patient may complain of poor night vision, as well as losing some peripheral vision. Many times this will begin when the patient starts bumping into things more frequently at home and may at first think they are just being clumsy. As this happens more often and they realize that it is vision related, it will inspire a trip to the ophthalmologist.
The ophthalmologist will most likely first perform a fundus examination using an instrument to look through the pupil at the back of the eye. This often reveals what is called a bone spicule pigmentation in the periphery of the fundus.
The next step will be visual field testing, which will detect any defects in the peripheral vision. The amount of peripheral vision loss detected will depend on how far the disease has progressed.
An electroretinogram (ERG) is another test utilized to diagnose retinitis pigmentosa, and is performed by an ophthalmologist to measure how the different photoreceptors in the retina respond to stimuli. Electrodes are placed either on the cornea and/or the skin around the eye. When the eye is stimulated with flashes of light, the rods and cones make tiny amounts of electricity picked up by the electrodes. It is possible to find out how well the rods and cones are working by knowing exactly how much light is coming into the eye, and how much electricity is produced by the cells in response to this light.
A dark adaptation sensitivity test is performed to determine the ability of the rod photoreceptors to increase their sensitivity in the dark. The rate at which the photoreceptors recover sensitivity in the dark following a bright light stimulus is measured.
Finally, genetic testing can be utilized for some of the most common mutations found in those with retinitis pigmentosa. Since there can be over 100 mutations, and new mutations are currently being discovered, it does not diagnose all types of the disease. However, if the some of the known mutations are found, it can help predict how the disease is most likely to progress. Genetic testing is also good to identify any risk factors in children of those with retinitis pigmentosa.
This blog entry is primarily about what retinitis pigmentosa is and how it can be diagnosed. In the next few blog entries we will explore both conventional and alternative treatment strategies, as well as some of the latest research in regards to the disease.