About Dr Brian Ang

Causes of Blindness and Visual Impairment Worldwide

What is blindness?

Blindness is essentially the inability to see well enough to be able to have a reasonable quality of life. It does not necessarily have to mean not being able to see absolutely anything at all. It is the worst outcome of any eye disease.

You are considered legally blind when you are not able to see more than the top letter on the Snellen chart, even when wearing correctly prescribed spectacles or contact lenses.

An important consideration is your peripheral field of vision. Even though you may have excellent visual acuity, if your peripheral visual field is constricted to no more than 20 degrees, you are also considered legally blind.

What are the causes of blindness?

How does blindness, visual impairment and low vision come about? What causes our eyes to go blind? According to The World Health Organization (WHO), the 5 commonest causes are:

     –   Cataract
     –   Glaucoma
     –   Age-related macular degeneration / AMD / ARMD
     –   Corneal opacities
     –   Diabetic retinopathy

This list does not include refractive error such as myopiahyperopia and astigmatism. Refractive errors are extremely common and are generally easily corrected with spectacles or contact lenses.

The unfortunate thing about this list is that all these eye disorders are either preventable or treatable. Yet, so many people suffer from blindness and visual impairment from these conditions. Taking the simple measures of looking after your vision and eye health will reduce your risk of suffering from one of these major causes of blindness.

1. Cataract

Cataract is the clouding of the lens. The lens is the middle window which separates the front and back segments of the eye. In a normal eye, any light entering the eye is focused by the lens onto the retina. The retina is the innermost layer of the eye which converts the light into biochemical signals which are subsequently transmitted to the brain.

When cataract develops, the lens becomes gradually more opaque. This affects the amount of light that is focused onto the retina. The eyesight gradually becomes more hazy and blurry. The more severe the cataract, the less light enters the eye and the less the eye sees.

Unfortunately, age is the most common cause of cataract. This means that you and I will develop them as we grow older. At present, the only proven effective treatment is with cataract surgery and intraocular lens implantation. Advances in surgical technologies mean that cataract can now be treated earlier and with increasingly better outcomes.

2. Glaucoma

Glaucoma is the progressive damage to the optic nerve due to the pressure in the eye (intraocular pressure) being too high for the optic nerve’s liking. The optic nerve transmits the biochemical signals from the retina to the brain for processing.

Glaucoma is generally classed as either open angle glaucoma or angle closure glaucoma because the treatment is slightly different. In both types, the main target is to lower intraocular pressures as much as possible either by eye dropslaser or glaucoma surgery. In angle closure glaucoma, an additional separate laser or surgery is needed to re-open the angle.

Glaucoma causes progressive loss of field of vision. As the contraction of the visual field is gradual, most do not even know that they suffer from it. If untreated, tunnel vision occurs. Eventually even the central vision can become lost, causing blindness and low vision. Unlike cataract, visual impairment from glaucoma is not reversible and cannot be recovered. Early detection and treatment is therefore vital.

3. Age-related Macular Degeneration (Amd / Armd)

Age-related macular degeneration (AMD/ARMD) refers to degeneration or damage to the macula. It is the commonest cause of blindness, visual impairment and low vision in the Western world. The macula is the central part of the retina. You use the macula to look at fine detail, for reading, for focusing at near and distant objects, and for recognizing faces.

As the name implies, AMD occurs more commonly as you grow older. The effect on the macula causes central visual impairment and blindness. Patients with AMD will notice distortion or reduction in central vision. There are 2 broad types: dry AMD and wet AMD.

Dry age-related macular degeneration causes gradual visual distortion and low vision, usually over many months. It is due to the degeneration or atrophy of the macular photoreceptor cells. This form of AMD is considered not treatable by ophthalmologists.

Wet age-related macular degeneration causes a more sudden visual impairment. It is due to leaking and bleeding of abnormal blood vessels at the macula. Wet AMD is treatable with intravitreal injections (injections of medication into the vitreous cavity in the eyeball) as long as scarring has not occurred at the macula. The injected anti-vascular endothelial growth factor (anti-VEGF) acts to stop these abnormal blood vessels from growing and leaking further.

As with glaucoma, early detection and treatment is essential to prevent macular degeneration from becoming a blinding condition. Once scarring develops at the macula, any vision loss is permanent and cannot be recovered.

4. Corneal opacities

Corneal opacities refer to the clouding of the cornea. The cornea is the front window of the eye. It is the clear part of the eye that we can see. Anything that affects this clarity can impair the vision. The cornea is also important in focusing the light as it enters the eye. Laser refractive surgery (including LASIK), corrects refractive errors by altering the shape of the cornea.

Corneal opacities cause visual impairment by affecting light as it passes the cornea into the eye. Common causes include: eye infections (viral and bacterial), injury, allergic eye disease, eyelid disease, dry eyes and keratoconus.

It is important to treat the underlying cause of the corneal opacity first. Treatment of the corneal opacity itself may require corneal transplant surgery if severe enough.

5. Diabetic retinopathy

Diabetic retinopathy occurs when there are changes in the retina as a result of diabetes. The retina is the innermost layer of the eye which converts the light into biochemical signals which are subsequently transmitted to the brain. In diabetes mellitus, the body is unable to metabolize sugar in the body. The high blood sugar levels cause alterations in the blood vessels throughout the body, including in the retina. The result in the retina is leakage, bleeding and retinal detachment if not managed properly.

Diabetic retinopathy causes blindness and visual impairment from proliferative diabetic retinopathy and diabetic maculopathy.

In proliferative diabetic retinopathy, the changes in the retinal blood vessels trigger a process that encourages the growth of abnormal new blood vessels. These new blood vessels develop on the optic nerve head and retina. Low vision and blindness occurs from bleeding of these blood vessels and retinal detachment.

In diabetic maculopathy, there is damage to the macula due to fluid leakage or ischemia. The vision is impaired because the function of the macular photoreceptor cells becomes disrupted. Treatment may be possible with laser and intravitreal injections into the eyeball. As with AMD, prevention is possible and efforts must include excellent blood sugar and blood pressure control.

Learn more about how to maintain your vision and eye health here


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