Hereditary retinal dystrophy

They are a group of genetic diseases that affect the retina and/or the choroid, causing gradual changes both anatomical as well as physical.  

Its prevalence is around 3% of the general population. 

They present with different inheritance patterns that determine the degree of severity and the age of onset of the symptoms as well as the clinical signs at the back of the eye. 

The variety of symptomology of patients is very wide. The most common are:

• Visual field changes. There are dystrophies that gradually cause a contraction of the peripheral visual field, but the central vision remains intact until the very advanced phases of the disease. In other cases, they affect the central visual field in the very early stages, reducing visual acuity from the beginning.
• Night blindness. There is an inability to adapt to poorly lit places. It is a symptom that restricts normal life a lot during the night as it hinders the individual from locating objects, identifying their movements and characteristics. 
• Metamorphopsia. It is the presence of waves on straight lines, which characteristically reveals changes to the central area of the retina named the macula. 
• Altered perception of colours. It usually occurs in the advanced stages of the disease.

They are diseases with a genetic cause. Their onset is determined by genes inherited from the parents. They are due to the presence of genetic modification that induce the development of dystrophy. To present, mutations of more than 250 genes have been identified.

There are three main inheritance patterns: 

Dominant autosomal. Usually various generations are affected, men and women in the same propotion. Whoever transmits the disease is the person who suffers from it. The probability of passing on the disease is 50%.  

Recessive autosomal. The children of parents carrying the disease have a 25% probability of developing it. This probability is the same for each pregnancy and it affects men and women equally. Those who pass on the disease are the carriers but they don't necessarily suffer from it. For the disease to appear, it must have undergone a mutation in the same gene of the mother and the father. 

Only men develop the disease when it is transmitted by the mutated gene of the X chromosone. Carrier women may present a clinical sign of the disease.

There are many classifications but at a teaching level we can classify them into macular and peripheral.

• Macular dystrophies. They start in the central area of the retina, named the macula. This is the part responsible for visual acuity. Dystrophies of this type affect the level of vision at earlier ages than those of peripheral retinal origin. The most common are Stargadt's disease and Best disease.
• Peripheral dystrophies. They cause a gradual reduction of the peripheral visual field until it affects the central portion of the vision in the last phase. The most common are retinitis pigmentosa and choroideremia. 

There are many ways to diagnose retinal dystrophies. In many cases, a direct examination of the back of the eye will give us a very reliable diagnosis. In other cases, complementary tests will be required, be they imaging tests like autofluorescence tests or an optical coherence tomography; or functional tests like an electroretinogram, electro-oculargram or evoked potentials.    

Currently, in order to answer a diagnostic question or as a final confirmation test, a genetic test can be carried out to substantiate the presence of the mutated gene/genes responsible for the disease.

Currently, there is no treatment available for retinal dystrophies. The different clinical trials point to the fact that both gene therapy and cell therapy may play a therapeutic role in the future. 

In specific advanced cases of patients with retinitis pigmentoda, there is the option to have an implant on top of the macula (ARGUS II) which improves the visual acuity of patients from light perception to locating objects.