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Retinitis pigmentosa is a degenerative disease that leads to severely decreased sight, even causing blindness. It comprises a wide group of hereditary diseases characterized by a progressive loss of photoreceptors and, subsequently, of other retinal cells.
Photoreceptors are the most abundant cells in the retina and are subdivided into rods and cones. The main role of these nervous cells is phototransduction, i.e. converting the captured light into electric signals that are then transmitted to the brain.
Rods enable night and black-and-white vision, while cones are responsible for daylight and color vision and are the ones that determine visual acuity.
Retinitis pigmentosa symptoms often appear during childhood, but severe sight problems usually do not occur until the first portion of adulthood. One of the earliest and most common symptoms is night blindness. This symptom can have two sides: on one hand, it refers to sight difficulties in low-light environments, and on the other hand, it can also refer to sight difficulties when moving from a well-lit environment to a low-light one, in which only after a significantly longer period than what is considered normal can the individual distinguish objects. Another typical symptom of retinitis pigmentosa that usually occurs after the above described one is a visual field restriction; in this case, the individual gradually loses sight from the peripheral area towards the center of the eye. Decreased visual acuity is another important symptom that can appear after the two aforementioned ones. When the disease is in an advanced stage, color perception is usually altered. The so-called photopsias (small lights or flashes) and glare are also characteristic symptoms.
Once symptoms begin, the disorder keeps on progressing at a speed that depends on the affected genes, the mutations, and the type of inheritance. Total blindness is uncommon. Certain clinical signs such as central retinal lesions, myopia, cataracts, and vitreous degeneration can be observed in these patients. Less commonly, other disorders, such as glaucoma, can be seen.
So far, no effective treatments for retinitis pigmentosa are available, but the use of sunglasses to protect the retina may help preserve vision. Some studies suggest that treatment with antioxidants, such as vitamin A palmitate at high doses, may delay disease progression. However, this treatment can cause important liver problems. Ongoing clinical studies are researching new treatments, such as omega-3 fatty acid therapy. A therapy to treat blindness associated with this and other eye diseases that is currently in its first stages of development consists of microchip implants inside the retina which would work similarly to a camera.
The genetic origin of this disease is highly complex and heterogeneous; so far, a large number of genes have been identified as responsible for the disease. The inheritance pattern of retinitis pigmentosa is also variable and can be autosomal dominant, autosomal recessive or X-linked recessive. welve genes have been identified as causative of dominant retinitis pigmentosa, three of which (RHO, RP1, and RD5) are considered to account for more than half of the cases of this type of retinitis. The locations of 16 recessive genes and 6 sex-linked genes that cause the disease are also known; in the case of sex-linked genes, a large proportion of cases are caused by mutations in the RPGR gene. Mutations in a relevant protein known as beta-phosphodiesterase are related to some cases of recessive retinitis pigmentosa.
On the other hand, different alterations in the same gene may cause the same or a different type of retinitis; this phenomenon is known as allelic heterogeneity. Moreover, individuals with the same mutation can differ in their symptoms or their severity even within the same family, which may be due to the interaction among several genetic and environmental factors.
The incidence of this disease in Spain is approximately one in every 2,500-3,000 live newborns. A family history is present in half of the cases.
The mutations associated with this disease can be transmitted from parents to children following three different inheritance patterns: autosomal dominant, autosomal recessive or X-linked recessive inheritance.
In the case of autosomal dominant, inheritance, at least one of the parents suffers from retinitis pigmentosa, and the probability of transmitting it to the offspring is 50%. This is one of the least severe forms of retinitis, since symptoms appear later and progress slowly. This type of inheritance is usually considered when the disease is observed in three generations, although, according to some authors, sometimes two generations are enough.
When the inheritance is autosomal recessive, parents are carriers of the disease but do not present with the symptoms, and they have a 25% probability of transmitting the altered gene o each of their children. In this case, symptoms are more severe.
When the disease follows a sex-linked (or X-linked) recessive inheritance pattern, men are affected and women are carriers of the affected gene. Therefore, fathers cannot transmit the disease to male children. Women do not experience severe symptoms. This is the most severe form of retinitis pigmentosa, since it is characterized by an early onset and rapid progression of symptoms. This type of inheritance is found in approximately 10% of affected families.
In some retinitis pigmentosa cases, the disease can occur without any previous family history, i.e. the affected individual is the first member of the family that suffers from this disease due to a new mutation.