Symptoms & Understanding

The Visual System

Millions of people across the world suffer varying degrees of irreversible vision loss because they have an untreatable, degenerative eye disorder like Retinitis Pigmentosa (RP), or Age-related Macular Degeneration (AMD). These conditions are retinal degenerative diseases and affect a specific layer of delicate tissue that lines the inside back of the eye. This part of the eye - the retina - is essential for vision.

The eye is like camera. The eyelid is the shutter; the iris and lens combine to control light entering the eye and focus it correctly on the retina at the back of the eye, the film of the camera. If this photo-reactive layer is damaged then the image will be distorted, blurred or impossible to see. Light entering the eye is detected by a layer of light-sensitive cells consisting of rods and cones.

The rods function in conditions of low illumination whereas cones are responsible for colour vision and all visual tasks that require high resolution, such as reading. The rods are located away from the centre of the eye and are most concentrated in the retinal periphery. The highest concentration of cones is found at the centre of the retina, the macula, which is necessary for visual acuity.

The most common forms of inherited retinal degenerative diseases are:

• Retinitis Pigmentosa
• Macular Degeneration
• Usher Syndrome

These diseases affect the light-sensitive cells in different ways and progress at varying rates. Breakdown and degeneration of the cells can be limited to the macula only or can slowly attack the whole retina.

The most common type of Macular Degeneration is called "Age-related Macular Degeneration", occurring in older adults. Hereditary forms with an early onset, such as Stargardts, Best's Disease or progressive Cone Dystrophy also exist. As a general rule these diseases cause severe visual impairment but rarely result in complete blindness.

Contrarily, hereditary retinal degenerations that attack the whole of the retina tend to be more severe. The most common types of these diseases are Retinitis Pigmentosa (RP) and Usher Syndrome. Early symptoms are night blindness and diminishing of the peripheral vision. Individuals with Ushers Syndrome suffer from congenital deafness or severe loss of hearing. Loss of the visual field usually begins on the periphery and may result in retention of just the central portion of the visual field. This is called "tunnel vision". In one form of Retinitis Pigmentosa the primary symptom is not loss of peripheral vision, but loss of central vision (inverse RP). RP is often diagnosed during childhood, early adulthood or only during middle age and progression is gradual.

When these conditions were first diagnosed they were grouped under the title Retinitis Pigmentosa. It is now known that there are a multitude of related conditions, all with their own very specific mechanisms and faults. It is a testament to the power of science that we now know the genetic origins and mechanisms of most of these conditions and are on the path to finding cures and treatments.