Understanding IRD’s

Inherited Retinal Diseases

Inherited retinal diseases (also called inherited retinal dystrophies, or IRDs) are a group of rare eye disorders caused by an inherited gene mutation and can result in vision loss or blindness.

Some people with inherited retinal diseases, for instance those with retinitis pigmentosa (RP) or choroideremia (CHM), experience a gradual loss of vision, eventually leading to complete blindness.Others, with conditions like Leber congenital amaurosis (LCA), may be born with or experience vision loss in infancy or early childhood.

Stargardt Disease

Stargardt disease is the most common form of inherited macular degeneration, affecting about 30,000 people in the U.S. The progressive vision loss associated with Stargardt disease is caused by the degeneration of photoreceptor cells in the central portion of the retina called the macula.

The retina is the delicate light-sensing tissue lining the inside wall of the back of the eye. Photoreceptor cells in the retina convert light into electrical signals, which are sent to the brain where they are processed to create the images we see. The macula, which is rich in cone photoreceptors, is responsible for sharp central vision — for tasks like reading, watching television, and looking at faces. Cones also provide vision in lighted settings and color perception.

Decreased central vision due to loss of photoreceptors in the macula is the hallmark of Stargardt disease. Some peripheral vision is usually preserved. Stargardt disease typically develops during childhood or adolescence, but the age of onset and rate of progression can vary. The retinal pigment epithelium (RPE), a layer of cells supporting photoreceptors, is also affected in people with Stargardt disease.

Retinitis Pigmentosa

Retinitis pigmentosa, also known as RP, refers to a group of inherited diseases causing retinal degeneration. The retina is a thin piece of tissue lining the back of the eye. Rod and cone photoreceptors in the retina convert light into electrical signals that the brain interprets as vision. People with RP experience a gradual decline in their vision, because photoreceptors degenerate.

Forms of RP and related diseases include Usher syndrome, Leber congenital amaurosis, and Bardet-Biedl syndrome, among others.

Choroideremia

Choroideremia is an inherited disease that causes progressive loss of vision due to degeneration of cell layers in the retina, the tissue that lines the back wall of the eye and makes vision possible. The affected layers include the choroid, the retinal pigment epithelium or RPE, and the photoreceptors. The choroid consists of blood vessel layers located between the retina and the sclera. Choroidal vessels provide the RPE and photoreceptors with oxygen and nutrients necessary for normal function. The RPE provides essential support functions for photoreceptors. The photoreceptors are responsible for converting light into the electrical impulses that are transferred to the brain where the images we see are created.

Cone-Rod Dystrophy

Cone-rod dystrophy is a group of related eye disorders that causes vision loss, which becomes more severe over time. These disorders affect the retina, which is the layer of light-sensitive tissue at the back of the eye. In people with cone-rod dystrophy, vision loss occurs as the light-sensing cells of the retina gradually deteriorate.

The first signs and symptoms of cone-rod dystrophy, which often occur in childhood, are usually decreased sharpness of vision (visual acuity) and increased sensitivity to light (photophobia). These features are typically followed by impaired color vision (dyschromatopsia), blind spots (scotomas) in the center of the visual field, and partial side (peripheral) vision loss. Over time, affected individuals develop night blindness and a worsening of their peripheral vision, which can limit independent mobility. Decreasing visual acuity makes reading increasingly difficult and most affected individuals are legally blind by mid-adulthood. As the condition progresses, individuals may develop involuntary eye movements (nystagmus).

There are more than 30 types of cone-rod dystrophy, which are distinguished by their genetic cause and their pattern of inheritance: autosomal recessive, autosomal dominant, and X-linked. Additionally, cone-rod dystrophy can occur alone without any other signs and symptoms or it can occur as part of a syndrome that affects multiple parts of the body.

Achromatopsia

Achromatopsia is an inherited retinal condition causing extreme light sensitivity (i.e., day blindness), as well as reduced visual acuity and color discrimination. Achromatopsia is caused by mutations in any of several genes. The most common genes associated with the condition are the CNGB3 and CNGA3 genes — mutations in these cause about 75 percent of cases.

Leber Congenital Amarosis

Leber congenital amaurosis (LCA) is a group of inherited retinal diseases characterized by severe impairment vision or blindness at birth. Some retinal experts consider LCA to be a severe form of retinitis pigmentosa (RP). The condition is caused by degeneration and/or dysfunction of photoreceptors, the cells in the retina that make vision possible.  Photoreceptors capture light, converting it to electrical signals which are sent to the back of the brain to create the images we see. Mutations in one of more than two dozen genes can cause LCA.

Leber hereditary Optic Neuropathy

Leber hereditary optic neuropathy (LHON) is an inherited optic nerve disease that leads to sudden, painless vision loss during young adult life, most commonly affecting men. It is caused by mutations in the genetic code of the mitochondria, which are small subunits that reside within the cell.

Mitochondria are also known as the “powerhouses of the cell” as they constantly convert energy locked in our food into energy that the cell can use. Our eyes are our most energy hungry organs and a lack of energy production can lead to degeneration and death of retinal ganglion cells (RGCs), which are the nerve cells that communicate visual information to the brain. Loss of these cells leads to subsequent degeneration of the optic nerve and visual loss. However, it is worth noting that a significant percentage of people who possess a mutation that causes LHON do not develop any features of the disorder.

Loss of vision due to LHON can be quite an alarming experience as the loss of central vision presents suddenly and can progress quite quickly, leaving only peripheral vision. This means that the majority of people with LHON retain independent mobility but cannot focus on anything straight ahead or see fine detail. The prevalence of this condition is thought to be around 1 in 50,000 people worldwide.

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