Blindness & Low Vision Guide

Visual impairment is classically defined as a visual disability that cannot be corrected medically, surgically, or with the use of eyeglasses. The threshold for defining low vision or vision impairment falls within the visual acuity range of 20/60 vision in both eyes, this is the minimum acuity required to read a standard newspaper print.  Blindness on the other hand is a person with a visual acuity of 20/200 or less or a visual field of 20 degrees or less. This is the minimum requirement for a person to be classified as legally blind in the UK. Once a person is classified legally blind they are entitled to claim the appropriate benefits.     

Effects of Blindness & Low Vision

Visually impaired individuals often have difficulty performing day-to-day tasks and activities and the resulting long-term effects can be a reduced quality of life. The person’s health and mental well-being may suffer but this is certainly not always the case. People with low vision or no vision are more prone to accidents and falls, especially the elderly who are at high risk of hip fractures and injury. There is also a higher mortality rate amongst the visually impaired.

Causes of Blindness & Low Vision

Our eyes suffer from many forms of visual impairment both naturally through age, poor diet and even computer use and other lifestyle factors. Several disorders occur ophthalmologically or neurologically. Disorders such as age-related macular degeneration (AMD), and diabetic retinopathy are the main causes of low vision in the elderly. Whilst many elderly people suffer from cataracts, another common cause of low vision, cataracts can now be successfully corrected through surgical removal of the effected lens. People suffering from cataracts who receive surgical correction do not meet the current definition of low vision or blindness.

Other disorders including glaucoma, retinitis pigmentosa, central retinal vein occlusion (CRVO), corneal damage, stroke, atherosclerosis, temporal arteritis, trauma, tumors and accidental eye damage, can all lead to low vision or blindness.

Millions of pounds are being spent each year, to find treatment for vision impairment and blindness. Much research is focused on developing cures and treatments for macular degeneration. Retina transplants and stem cell implants are amongst the few miracle cures tried and tested to date with tests still ongoing.  

The Aim of this Guide

This guide is designed to be a comprehensive, though summarised, look at the eye, how it works, how it malfunctions, and how to help someone cope with loss of vision. In order to understand blindness, the eye itself must first be understood as should the definitions of ‘blind’ and how it is categorised.

How the Eye Works

Knowledge on the human eye and how it functions has come a long way since Isaac Newton forced a bodkin into his eye-socket to discover how light in the eyeball affected colour, but much of his work on light is still relevant to modern optical science.

Blindness is defined as the inability of the eye to perceive light, the source of our vision. On a basic level, the eye takes incoming light, turns it into nerve impulses which are directed to the brain where the impulses are then interpreted into an image.  This is done through a variety of anatomical pieces of the eye which must be explored to understand how exactly the visual acuity and the visual field can become corrupted and can lead to impaired vision.

  • Front of the Eye

Light enters through the outermost part of the eye, the cornea, which is protected by a layer of tears.  The amount of light which is allowed to enter into the eye is gauged by the coloured part of the eye, or iris, and the dark centre of the eye, or pupil, both of which are covered by the cornea.  When the light entering the eye is bright, the iris contracts forcing the pupil to get smaller in order to restrict the amount of light taken in; when in a dark environment, the iris expands making the pupil larger to receive a greater amount of light.

  • Inside the Eye

After the initial entrance process, the eye then sets about converting light into neural signals to be sent to the brain. Light rays are first directed to the lens, the part of the eye which gauges the distance of various objects by changing the depth of its convex shape. If the lens is too narrow, then the viewer suffers from near-sightedness, if it’s too broad then far-sightedness results. The lens then sends the light to the back of the eye to an area called the retina where photoreceptor cells, or rod and cone cells, turn the light into nerve impulses.

  • Back of the Eye

Although the cone and rod cells are both involved in interpreting the light, they also have separate visual functions. Cone cells are found in the area of the eye called the macula which is at the back of the eye and contains the centre of the eye, or fovea, which is made up of a dense area of cone cells. Cone cells give our vision colour, the ability to see in bright light, as well as detailed vision, or acuity.  It also allows us to perform specific recognition tasks like reading or recognising people’s faces. Rod cells do not cluster inside the macula like cones, instead, they brim the retina and provide us with the more basic visual functions as well as our visual field, or peripheral vision. These cells allow us to view movement, see in low light and, like a cat’s whiskers, they orient our sense of space, depth, and general peripheral awareness of the world around us which also prevents us from running into things.

  • Optic Nerve and the Brain

Once the light passes through the retinal cells, it finally makes it to the nerves at the back of the eye-socket which send it through the optic disc. The nerves then transmit the electrical signals made from the light to the brain through nerve fibres stemming from the optic nerve. Although the right and left sides of our vision are controlled by the opposite eye like our motor function, each eye sends signals along the nerves to both sides of the brain. The split optic nerves come together in the optic chiasm which then redirects the signals to the back of the brain where sight is controlled in the visual cortex. Here the brain can interpret the image which began at the front of the eye as light, the entire process taking place in an instant.

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