Why Do People Become Blind?

Figure showing a cross-section of the eye and the
location of a cataract lens. The actual cloudy
appearance of the cataract can be seen through
the pupil. (Figures from www.eyemdlink.com)

Blindness is commonly caused by an eye disease or condition such as glaucoma, diabetic retinopathy, cataract, age-related macular degeneration and retinitis pigmentosa.

A cataract is an opacity (cloudiness) within the lens. In the treatment of a cataract, the lens inside the eye is removed and replaced with a clear artificial lens.

In diabetic retinopathy, retinal blood vessels are adversely affected and begin to leak. Treatment involves photocoagulation with a laser to destroy leaky blood vessels and to prevent the growth of abnormal blood vessels (angiogenesis).

In glaucoma, the retinal ganglion cells (which send visual information from the retina to the brain) slowly die over time. The exact cause of this loss is unknown but frequently high eye (intraocular) pressure is often associated with this condition. Current treatment is with medications and surgical techniques aimed at trying to lower intraocular pressure.

Figure showing the back of the eye (retinal fundus)
in a patient with diabetic retinopathy.
Hemorrhages (bleeding) and fluid leakage
(exudates) can be seen and are indicative
of advanced complications due to diabetes.
(Figures from www.eyemdlink.com)

It is unlikely that the retinal prosthesis we envision would be useful for individuals with either glaucoma or diabetic retinopathy. This is due to the fact that in these two diseases, retinal ganglion cells are also damaged and therefore can no longer be stimulated. Healthy retinal ganglion cells are necessary in order for our retinal prosthesis to work. In retinitis pigmentosa and age-related macular degeneration, a significant proportion of ganglion cells remain healthy and thus can be potentially activated.

The most common cause of blindness in the aging population is age-related macular degeneration (ARMD) affecting over 10 million Americans. ARMD causes loss of central vision due to the loss of photoreceptors (the light-sensing cells).This condition is particularly debilitating for the elderly who may have difficulty walking and remain confined to their homes.

Retinitis pigmentosa (RP) affects 1.6 million people worldwide and is the leading cause of inherited blindness. RP causes a slow and progressive blindness across the entire visual field. As with ARMD, this disease is associated with photoreceptor loss. To date, there are no treatments for either ARMD or RP. Molecular genetic therapy may represent a viable long-range hope although there are formidable challenges facing this therapeutic approach. It is also important to keep in mind that even successful repair of genetic function would only slow or prevent further deterioration rather than restore lost visual function (as would be possible with a prosthesis).

Figure showing a close up view of the optic nerve head in the back of the eye.
The optic nerve relays visual information from the eyeto the rest of the brain
to be processed as visual information. Progressive enlargement of the optic nerve
head “cup” (green outline) over time is indicative of damage to the nerve.
(Figures from www.eyemdlink.com)

Age-related Macular Degeneration

Age-related macular degeneration (ARMD) is a disease that affects the macula - the area responsible for central and fine detail vision. There are two forms of the disease: a less severe non-exudative ("dry") form and the more devastating exudative ("wet") form. Clinically, the "wet" form is characterized by the presence of newly formed blood vessels (subretinal choriodal neovascularization), exudation and hemorrhages. For some individuals, the "dry" form can convert and progress into the "wet" form. It is therefore important that individuals with ARMD have frequent eye examinations.

Figure showing the back of the eye (retinal fundus)in the “dry” form of ARMD.
Typically, there is thinning, scarring and atrophy of
the central macular region. (Figures from www.eyemdlink.com)

Figure showing the back of the eye (retinal fundus)in the “wet” form of ARMD.
In this condition, there is growth of abnormal blood vessels and hemorrhaging beneath
the central macula region.(Figures from www.eyemdlink.com)

Retinitis Pigmentosa

Figure showing the back of the eye
(retinal fundus) in a patient with RP.
The presence of black pigment flecks
in the retina (bone spicules)
is characteristic of this condition.
This condition is also commonly associated
with a narrowing of blood vessels
and abnormalities of the optic nerve head.

Retinitis Pigmentosa (RP) is an inherited eye condition affecting initially the mid peripheral portion of the retina. Central vision is generally not affected until the very late stages of the disease. Clinically, the first observable sign of RP is a narrowing of the retinal arterioles (small arteries in the retina). Later, retinal pigment formations known as "bone spicules" and changes in the appearance of the optic nerve head are apparent.

page updated: 9/16/05