Visual Disorders

Painting is a blind man’s profession. He never paints what he sees, but what he feels.

Pablo Picasso

Light entering the eye is focused by the lens onto a light-sensitive panel of cells known as the retina at the rear of the eye. It is here that the light is detected and converted into electrical signals by photoreceptor cells of which there are two types: cone cells are responsible for sharp vision and colour vision while rod cells (named for their cylindrical shape) are highly sensitive to light and so are responsible for night and peripheral vision. These then transmit the signals to the brain via the optic nerve.

Myopia (short-sighted vision) is the most common eye problem in the world resulting from the lens being unable to focus light properly. There are many different diseases of the retina; disrupting both types of photoreceptor cells leads to complete blindness (Leber's congenital amaurosis), while disruption of only the rod cells cause diseases characterised by nightblindess (retinitis pigmentosa) and cone cells dayblindness (Stargardt’s disease). An uncontrolled growth of the retina, leading to a cancer of this tissue is known as retinoblastoma.

Humans have three types of cone cells each containing a different photopigment which respond to variation in colour in different ways so the perception of colour results from a mixture of the outputs of all three. An absence of only one of the particular photopigments will lead to different variations of colour deficiencies. The most common deficiency is red-green colour blindness, followed by blue-yellow colour blindess. A total lack of colour detection is known as achromatopsia.

There are two types of inherited diseases resulting in optic nerve damage leading to an irreversible blindness in young adulthood, both of which effect the functioning of mitochondria, and so possibly disrupting energy utilization in the nerve tissues, in particular the optic nerve (Leber’s hereditary optic neuropathy). Another group of diseases of the optic nerve is glaucoma when the fluid pressure inside the eye is too high.

Eye colour, is determined by the levels of melanin pigment in the iris (Gr. iris; rainbow) - the circular coloured curtain of the eye that opens to form the pupil, regulating the amount of light entering the eye. The melanin is produced here to absorb excess light too strong or bright for vision, with the differing amounts determining the colour of a person’s eyes; blue resulting from low levels and brown from high. The genetic basis for eye colour is quite complicated, with one relatively common phenomenon being the presence of two differently coloured eyes in an individual, known as heterochromia. A lack of pigment in the eye, such as occurring in albinism often associates visual defects.