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Cause: The optic nerve is damaged at the point where it leaves the eye. This may be caused by increased intracocular pressure (pressure of the fluid inside the eye) or by weakness in the optic nerve. In the normal eye, the ciliary body (a layer of cells behind the iris) secretes a watery fluid called aqueous. This fluid then flows through the pupil before leaving the eye via the trabecular meshwork (drainage channels between the back of the cornea and the iris).
Primary open angle or chronic glaucoma occurs when these drainage channels become slowly blocked and the eye pressure rises, causing damage to the optic nerve. The field of vision gradually becomes impaired. Acute angle closure glaucoma occurs
when there is a sudden and more complete blockage to the flow of aqueous caused by the
apposition of the lens to the back of the iris.
Symptoms: Reduction in the peripheral field of vision (tunnel vision).
Retinitis Pigmentosa |
Cause: retinitis pigmentosa is the name given to a group of hereditary eye conditions that affect the retina's ability to respond to light.
Symptoms: Reduction in the peripheral field of vision (tunnel vision) and difficulty in seeing in poor light.
Macular Degeneration |
There are two types of macular degeneration:
Disciform or 'wet' macular degeneration accounts for about 10% of all macular degeneration and occurs rapidly.
Age-related or 'dry' macular degeneration may occur more gradually.
Cause: Wet macular degeneration results in a build-up of fluid under the retina. This causes bleeding and scarring which leads to sight
loss. Dry macular degeneration is caused by pigmentary changes at the macula.
Symptoms: Macular degeneration usually occurs in both eyes, but it may be
asymmetrical. In the early stages central vision may be disturbed or blurred. Straight lines may seem wavy and objects may become distorted or appear an unusual size or shape. Peripheral vision is
unaffected. Eventually, central vision may be completely lost resulting in a
blank patch or dark
spot. This central area of visual distortion or loss moves as the patient tries to look around
it.
Diabetic Retinopathy |
Cause: diabetic retinopathy affects the fine network of blood vessels within the retina. It is usually graded in three main stages according to how severe it is.
Background diabetic retinopathy: Blood vessels may bulge slightly and may leak blood (haemorrhages) or fluid. The macula is unaffected.
Diabetic Maculopathy: This is more severe than Background diabetic retinopathy and affects the macula.
Proliferative diabetic retinopathy: Blood vessels in the retina may become blocked. New blood vessels form on the retina or in the vitreous cavity. These vessels can bleed very easily causing a sudden decrease in vision. Scar tissue may form, pulling at the retina and resulting in retinal detachment.
Symptoms: Peripheral vision will be initially preserved. Some central vision loss may occur. As the condition progresses to Proliferative diabetic retinopathy, pigment or blood in the vitreous may cause blurred or patchy vision with floaters or blobs.
Hemianopia |
Hemianopia is also known as Hemianopsia.
Cause: Damage to the brain as a result of stroke, tumour or trauma.
Symptoms: Loss of vision in one half of the visual field.
Cataracts |
Cause: A cataract is the opacification (clouding) of all or part of the lens which prevents light passing through. Cataracts often form slowly with age. The lens turns from being clear to yellow and then eventually to brown.
Symptoms: Sensitivity to glaring light. Blurred or misty vision, multiple images, double vision or ghost images. Print appears hazy. Colours may appear faded, and contrast is limited.
Colour Blindness |
Colour blindness is the reduced ability to discriminate between different wavelengths of
light.
Cause: The retina contains three types of cone cells. The short-wave (blue), medium-wave (green) and long-wave (red) cone cells each contain a different photosensitive pigment. They have peak sensitivities in the blue, green and red wavelengths of the visible spectrum. colour blindness is the result of deficiencies in the cone photopigments, which can be classified into three main types:
Anomalous Trichromacy is due to an alteration in the sensitivity of one of the three photopigments. Three-dimensional colour vision is not fully impaired. Protanomaly is a red-deficiency. Deuteranomaly is a green-deficiency.
Dichromacy is the loss of function of one of the photopigments. Colour vision is reduced to two-dimensions. Protanopia is a red-deficiency. Deuteranopia is a green-deficiency.
Monochromacy is the loss of function of two, or all three of the photopigments, and results in total colour blindness.
Symptoms:
Protanomaly: Red, orange and yellow and green are difficult to discriminate, as they are all shifted in hue towards green. The brightness of these colours is reduced. Violet and purple appear blue because of the dimness of the red component.
Deuteranomaly: Red, orange and yellow and green are difficult to discriminate, as they are all shifted in hue towards red. The brightness is not reduced.
Protanopia: Red, orange, yellow and green are perceived as the same colour and the brightness is reduced. Violet, purple and blue are indistinguishable.
Deuteranopia: Red, orange, yellow and green are perceived as the same colour. The brightness is not reduced.
Myopia |
Myopia is also known as short-sightedness.
Cause: Refractive error. Parallel light rays are focussed in front of the retina when the eye is in its relaxed state. This is because the eyeball is too long for the natural converging power of the cornea and lens.
Symptoms: The eye can see close objects clearly whist distant objects appear blurred.
Hyperopia |
Hyperopia is also known as Hypermetropia or long-sightedness.
Cause: Refractive error. Parallel light rays are focussed behind the retina when the eye is in its relaxed state. This is because the eyeball is too short for the natural converging power of the cornea and lens.
Symptoms: The eye can see distant objects clearly whist close objects appear blurred.
© Stephen Ratcliffe. 2005