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Types of Visual Impairments

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Introduction

Visual impairment is a decreased ability to see to a degree that causes problems not fixable by usual means, such as glasses. Visual impairment may cause people difficulties with normal daily activities such as driving, reading, socializing, and walking.

The World Health Organization estimates that as of 2012 there were 285 million people who were visually impaired in the world, of which 246 million had low vision and 39 million were blind. The majority of people with poor vision are in the developing world and are over the age of 50 years. 80% of visual impairment is either preventable or curable with treatment.

Types of Visual Impairments

Visual Impairment can be divided into 2 categories:

1.      Blind & Partial Blindness

2.      Colour Blindness

1. Blind & Partial Blindness

Visual impairment, also known as vision impairment or vision loss, is a decreased ability to see to a degree that causes problems not fixable by usual means, such as glasses. Some also include those who have a decreased ability to see because they do not have access to glasses or contact lenses. The term blindness is used for complete or nearly complete vision loss. Visual impairment may cause people difficulties with normal daily activities such as driving, reading, socializing, and walking.

The World Health Organization estimates that as of 2012 there were 285 million people who were visually impaired in the world, of which 246 million had low vision and 39 million were blind. The majority of people with poor vision are in the developing world and are over the age of 50 years. 80% of visual impairment is either preventable or curable with treatment.

Visual impairments have considerable economic costs both directly due to the cost of treatment and indirectly due to decreased ability to work.

Classification

The World Health Organization uses the following classifications of visual impairment. When the vision in the better eye with best possible glasses correction is:

20/30 to 20/60 : is considered mild vision loss, or near-normal vision
20/70 to 20/160 : is considered moderate visual impairment, or moderate low vision
20/200 to 20/400 : is considered severe visual impairment, or severe low vision
20/500 to 20/1,000 : is considered profound visual impairment, or profound low vision
More than 20/1,000 : is considered near-total visual impairment, or near total blindness
No light perception : is considered total visual impairment, or total blindness

Degrees of Visual Impairments

Visual impairments may take many forms and be of varying degrees. Some people who fall into this category can use their considerable residual vision (their remaining sight) to complete daily tasks without relying on alternative methods.

People with significantly reduced acuity may benefit from training conducted by individuals trained in the provision of technical aids. Low vision rehabilitation professionals, can provide advice on lighting and contrast to maximize remaining vision. These professionals also have access to non-visual aids, and can instruct patients in their uses.

The subjects making the most use of rehabilitation instruments, who lived alone, and preserved their own mobility and occupation were the least depressed, with the lowest risk of suicide and the highest level of social integration.

Those with worsening sight and the prognosis of eventual blindness are at comparatively high risk of suicide and thus may be in need of supportive services. These observations advocate the establishment and extension of therapeutic and preventative programs to include patients with impending and current severe visual impairment improving the quality of life of these patients. Such intervention should occur in the early stages of diagnosis.

Causes of Blindness & Partial Blindness

 The most common causes of visual impairment globally are:

  • uncorrected refractive errors (43%)
  • cataracts (33%)
  • glaucoma (2%). 
  • age related macular degeneration
  • diabetic retinopathy
  • corneal clouding
  • childhood blindness
  • infections
  • stroke
  • prematurity
  • trauma.

Blindness can occur in combination with such conditions as:

  • intellectual disability
  • autism spectrum disorders
  • cerebral palsy
  • hearing impairments
  • epilepsy

Read More...

Diagnosis of Blindness & Partial Blindness

Scientists track eye movements in glaucoma patients to check vision impairment while driving

It is critical that all people be examined by someone specializing in low vision care prior to other rehabilitation training to rule out potential medical or surgical correction for the problem and to establish a careful baseline refraction and prescription of both normal and low vision glasses and optical aids. Only a doctor is qualified to evaluate visual functioning of a compromised visual system effectively.

The World Health Organization estimates that 80% of visual loss is either preventable or curable with treatment.

Communication with people & surroundings

Communication with the visually impaired can be more difficult than communicating with someone who doesn't have vision loss. It is important to treat the blind the same way they would treat anyone else, rather than treating them like they have a disability, and need special attention.

Individuals with a visual disability not only have to find ways to communicate effectively with the people around them, but their environment as well. The blind or visually impaired rely largely on their other senses in order to understand their surroundings. They include:

  • Sound
  • Touch
  • Smell

Read More...

2. Colour Blindness

Colour blindness (colour vision deficiency, or CVD) affects approximately 1 in 12 men (8%) and 1 in 200 women in the world.

There are different causes of colour blindness. For the vast majority of people with deficient colour vision the condition is genetic and has been inherited from their mother, although some people become colour blind as a result of other diseases such as diabetes and multiple sclerosis or they acquire the condition over time due to the aging process, medication etc.

Most colour blind people are able to see things as clearly as other people but they unable to fully ‘see’ red, green or blue light. There are different types of colour blindness and there are extremely rare cases where people are unable to see any colour at all.

Types

The most common form of colour blindness is known as red/green colour blindness and most colour blind people suffer from this. Although known as red/green colour blindness this does not mean sufferers mix up red and green, it means they mix up all colours which have some red or green as part of the whole colour. For example, a red/green colour blind person will confuse a blue and a purple because they can’t ‘see’ the red element of the colour purple.

Similar problems can arise across the whole colour spectrum affecting all reds, greens, oranges, browns, purples, pinks and greys. Even black can be confused as dark green or dark blue.

Degrees of colour blindness

The effects of colour vision deficiency can be mild, moderate or severe so, for example, approximately 40% of colour blind pupils currently leaving secondary school are unaware that they are colour blind, whilst 60% of sufferers experience many problems in everyday life.

Statistically speaking most people with a moderate form of red/green colour blindness will only be able to identify accurately 5 or so coloured pencils from a standard box of 24 pencil crayons. Depending upon which type of the condition a colour blind person is suffering from they could see the set of pencil crayons similarly to the following images.

Diagnosis

Colour blindness can be difficult to detect, particularly in children with inherited colour vision deficiency as they may be unaware that they have any problems with their colour vision. A child with a severe condition such as deuteranopia may seemingly be able to accurately identify colours which they can’t see (e.g. red) because they have been taught the colour of objects from an early age and will know for example that grass is green and strawberries are red even if they have no concept of their true colours.

If you have any eye test with an optometrist (a registered health professional who examines eyes, tests sight and dispenses glasses and contact lenses) they should test your colour vision as a matter of routine, but not all chains of opticians test routinely and with some you may have to request a colour vision test specifically and sometimes even pay for it as an extra.

You can also see your GP if you have any problems seeing colours. Your GP will ask about your symptoms and examine you. He or she may also ask about your medical history.

There are many tests available to measure colour vision defects but the most common is the Ishihara Plate test. This can test for red/green colour blindness but not blue colour blindness. This is the test most likely to be used for routine colour vision screening in schools or medicals.

This test is the most widely used for testing for red-green colour vision deficiency and contains 38 plates of circles created by irregular coloured dots in two or more colours. The plates will be put in front of you and you will be asked what number you can see on the plate. Some plates contain information which people with normal colour vision can see whilst others contain information that only people with colour blindness can see. If you make a certain amount of errors you will be diagnosed with colour blindness. Special Plate tests have been devised to diagnose young children who are not old enough to identify numbers.

More sophisticated tests are also widely used to ascertain whether someone with a colour vision deficiency would be suitable for certain occupations. A Lantern test is one such test which is used to identify people not suitable to work as train drivers or in marine and aviation jobs or other occupations where the work requires the ability to accurately reading the colours of lights for safety reasons.

At www.colorvisiontesting.com you can find an initial test for colour blindness on the screen.

www.toledo-bend.com/colorblind/Ishihara.asp also shows some Ishihara plates tests for online testing.

Please note that these tests are for initial testing only and will not give an accurate formal diagnosis. If you suspect that you or your child might be colour blind please seek professional help from an optician.

Causes of Colour Blindness

The exact physical causes of colour blindness are still being researched but it is believed that colour blindness is usually caused by faulty cones but sometimes by a fault in the pathway from the cone to the brain.

The retina of the eye has two types of light-sensitive cells called rods and cones. Both are found in the retina which is the layer at the back of your eye which processes images. Rods work in low light conditions to help night vision, but cones work in daylight and are responsible for colour discrimination.

There are three types of cone cells and each type has a different sensitivity to light wavelengths. One type of cone perceives blue light, another perceives green and the third perceives red. When you look at an object, light enters your eye and stimulates the cone cells. Your brain then interprets the signals from the cones cells so that you can see the colour of the object. The red, green and blue cones all work together allowing you to see the whole spectrum of colours. For example, when the red and blue cones are simulated in a certain way you will see the colour purple.

People with normal colour vision have all three types of cone/pathway working correctly but colour blindness occurs when one or more of the cone types are faulty. For example, if the red cone is faulty you won’t be able to see colours containing red clearly. Most people with colour blindness can’t distinguish certain shades of red and green.

Colour blindness is a usually a genetic (hereditary) condition (you are born with it). Red/green and blue colour blindness is usually passed down from your parents. The gene which is responsible for the condition is carried on the X chromosome and this is the reason why many more men are affected than women.

There are estimated to be over 250 million colour blind people worldwide. The vast majority of people with a colour vision deficiency have inherited their condition from their mother, who is normally a ‘carrier’ but not colour blind herself. Some people also acquire the condition as a result of long-standing diseases such as:

  • Diabetes
  • Multiple sclerosis
  • Some liver diseases
  • Almost all eye diseases

The effects of colour vision deficiency can be mild, moderate or severe depending upon the defect. If you have inherited colour blindness your condition will stay the same throughout your life – it won’t get any better or worse.

Genetics

Color blindness is most commonly inherited from mutations on the X chromosome but the mapping of the human genome has shown there are many causative mutations—mutations capable of causing color blindness originate from at least 19 different chromosomes and 56 different genes (as shown online at the Online Mendelian Inheritance in Man (OMIM). Two of the most common inherited forms of color blindness are protanopia and deuteranopia. One of the common color vision defects is red-green deficiency which is present in about 8 percent of males and 0.5 percent of females of Northern European ancestry.

Some of the inherited diseases known to cause color blindness are:

  • cone dystrophy
  • cone-rod dystrophy
  • achromatopsia (a.k.a. rod monochromatism, stationary cone dystrophy or cone dysfunction syndrome)
  • blue cone monochromatism (a.k.a. blue cone monochromacy or X-linked achromatopsia)
  • Leber's congenital amaurosis
  • retinitis pigmentosa (initially affects rods but can later progress to cones and therefore color blindness).
  • Inherited color blindness can be congenital (from birth), or it can commence in childhood or adulthood. Depending on the mutation, it can be stationary, that is, remain the same throughout a person's lifetime, or progressive. As progressive phenotypes involve deterioration of the retina and other parts of the eye, certain forms of color blindness can progress to legal blindness, i.e., an acuity of 6/60 (20/200) or worse, and often leave a person with complete blindness.

Color blindness always pertains to the cone photoreceptors in retinas, as the cones are capable of detecting the color frequencies of light.

About 8 percent of males, but only 0.5 percent of females, are color blind in some way or another, whether it is one color, a color combination, or another mutation. The reason males are at a greater risk of inheriting an X linked mutation is that males only have one X chromosome (XY, with the Y chromosome carrying altogether different genes than the X chromosome), and females have two (XX); if a woman inherits a normal X chromosome in addition to the one that carries the mutation, she will not display the mutation. Men do not have a second X chromosome to override the chromosome that carries the mutation. If 5% of variants of a given gene are defective, the probability of a single copy being defective is 5%, but the probability that two copies are both defective is 0.05 × 0.05 = 0.0025, or just 0.25%.

Treatment

There is currently no treatment for inherited colour blindness. Colour filters or contact lenses can be used in some situations to enhance the brightness between some colours and these are occasionally used in the workplace, but many colour blind people find these actually confuse them further rather than help.

There is hope on the horizon for a ‘cure’ for inherited colour vision deficiency using gene technology – for more information visit www.genevolve.com. This will involve injecting genetic material into the eye so is not for the faint-hearted! At the moment there have been no trials on humans but the process has been proved to work in monkeys.

For acquired colour vision deficiency, once the cause has been established and treated, your vision may return to normal.

Read More: ....

 

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