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LENScience

An Insight into Blindness

This page supports specialist and non-specialist teachers by providing background information about the concepts that underpin the LENScience resources on vision impairment.

  1. » How Do We See?
  2. » Diseases Causing Vision Impairment and Blindness
  3. » Genetic Causes of Vision Impairment and Blindness

How Do We See?

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To be able to ‘see’, light first needs to enter the eye. Light enters the eye through the cornea which is a transparent, protective outer layer. Light then passes through the aqueous humour (a thin, transparent fluid) and through the pupil, which is an opening in the centre of the iris of the eye. Light then passes through the lens.

 

Just like a camera, the eye can adjust the lens (using muscles)  to focus the light so that the 'picture' we see is as clear as possible. As light hits the cornea and then the lens, it refracts (or bends) to form an upside down image on the retina (which is a bit like a projector screen). Finally, light passes through the vitreous humour (a thick, see-through jelly that fills the eyeball and helps maintain shape) before it reaches the retina. The retina is a layer of light-sensitive cells that line the interior of the eye. The light that hits the retina stimulate nerves which carry an electrical signal to the brain. The brain translates the information it has received into an image.

The Retina

The retina is the light sensitive layer at the back of the eye. It contains the light sensitive cells called rods and cones as well as nerves. The rods and cones are connected to nerve fibres which leave the top layer of the retina to join up and form the optic nerve. Cones are sensitive to colour and work best in brighter lights, rods are not sensitive to colour and work best in dim light.
Retina
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Diseases Causing Vision Impairment and Blindness

Disease

Cause

Effect on Vision
Cataract Cataracts are a leading cause of vision impairment. The lens of the eye becomes cloudy Blurring - leading to blindness
Glaucoma Normal fluid pressure inside the eye slowly increases, damaging the optic nerve Reduced field of vision, leading to blindness
Diabetic Retinopathy Damage to blood vessels in the retina caused by complications of diabetes Vision loss potentially leading to blindness

Over one billion people worldwide live with some form of vision impairment. There are various diseases and conditions which can impair vision and cause blindness. Some conditions can be easily corrected but can cause significant vision impairment if left untreated. For example, refractive errors such as nearsightedness and farsightedness are very common. Refractive errors can be treated with glasses, contact lenses, or refractive surgery. However, many people worldwide live with visual impairment due to uncorrected refractive errors. Scientists have shown that heredity (passing on of characteristics) and genetics can play a part in common vision problems such as refraction errors.

 

Some conditions are linked to age-related degeneration, for example, cataracts and glaucoma.

 

Other types of vision impairments develop as a side-effect of disease, for example, diabetic retinopathy. Some conditions are age-related but the tendency to develop the condition is genetically inherited.

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Genetic Causes of Vision Impairment and Blindness

Condition

Cause

Effect on Vision
Albinism Recessive allele that results in little or no pigment in the eyes, skin and hair. Sight problems are caused because the retina does not develop properly Uncontrolled eye movement; squinting, bright light is uncomfortable, blurred vision, varying degrees of partial sightedness
Leber Congenital Amaurosis LCA is a family of inherited retinal degenerative diseases that cause the rod and cone cells to deteriorate and die in early childhood Severe visual impairment (blindness). Sight is limited to detecting bright lights or hand motions
X-Linked Congenital Stationary Night Blindness (CSNB) X-Linked CSNB is caused by a mutation on the X chromosome. There are two genes known to be mutated in the condition. This affects the ability of the retina to function properly.  Stationary means it does not get worse over a life time  Difficulty seeing in low light, reduced acuity (sharp vision), low vision - usually severe short-sightedness, involuntary eye movements, eyes that do not look in the same direction 

 

60% of conditions which cause childhood blindness are genetically inherited. These conditions include congenital cataracts and congenital glaucoma. The word ‘congenital’ refers to the fact that the condition is present from birth. Colourblindness is a congenital condition that affects more males than females. 

 

Vision loss affects many everyday activities such as reading, watching TV, cooking, walking, playing sport, and shopping. In New Zealand, around 30 000 people are affected by blindness or vision impairment.

 

What it means to be ‘legally blind’ varies across different countries. In New Zealand you are considered to be legally blind if you cannot see at six metres what someone with normal vision can see at 60 metres and if your field of vision is less than 20 degrees in diameter (a person with normal vision can see 180 degrees). 

 

People who experience vision problems and vision loss, depending upon severity, sometimes need help or support to continue to live independently. 

 

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Fig. 1: Effect of disease on vision. Images courtesy of National Eye Institute, National Institute of Health.
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