Visual Impairment and Blindness
What is Visual Impairment?
When someone’s vision cannot be restored to a normal level they are said to have vision impairment.
Loss of visual acuity or the inability of the eye to perceive objects as clearly as usual can lead to vision impairment. A loss of visual field in which the eye cannot see as wide an area as usual without moving the eyes or turning the head could also be the cause.
The severity of a person’s vision loss can be expressed in a variety of ways. Low vision is defined by the World Health Organization as having a visual field of 20 degrees or less or visual acuity between 20/70 and 20/400 with the best possible correction. Blindness is defined as having a visual field of 10 degrees or less or a visual acuity of less than 20/400 with the best possible correction. At 20 feet a person with 20/70 visual acuity can see what a person with normal sight can see at 70 feet. A person with 20/400 visual acuity can see at 20 feet what a person with normal vision can see at 400 feet. A typical visual field has a horizontal angle of 160 to 170 degrees.
For some purposes the severity of a vision impairment may be classified differently. For instance, in the US the phrase ‘legal blindness’ refers to a person’s eligibility for specific federal programs or educational opportunities. Legal blindness is characterized by a visual field of 20 degrees or less or a visual acuity of 20/200 or worse with the best available correction.
How much a person’s life will be impacted by vision loss cannot be determined solely by visual acuity. Evaluating a persons ability to use their vision is also crucial. Even though two individuals have the same level of visual acuity one may be more adept at using their vision for daily tasks. The majority of blind people have at least some functional vision which enables them to navigate their surroundings and carry out daily tasks. It is possible to assess a person’s functional vision by seeing how they use their vision in various contexts. A functional vision evaluation can provide answers to the following queries. Is it possible for someone to search a room for someone or something? Which lighting is ideal for a person performing various tasks? How does a person move around in a room or outside using their vision?
Visual Impairment in Children
A child’s understanding and functioning in the world are altered when they have vision impairment. A child’s cognitive, emotional, neurological and physical development may be impacted by vision impairment because it may limit the variety of experiences and information a child is exposed to. Mental retardation, cerebral palsy, hearing loss, epilepsy and other developmental disabilities are present in nearly two-thirds of children with vision impairment. Compared to children with milder vision impairment, children with more severe vision impairment are more likely to have other disabilities.
Anatomy of the Human Eye
The human eye is a sophisticated organ that receives light and transforms it into electrical signals that the brain uses to create images. The cornea (the transparent outer layer) iris (the colored portion that regulates pupil size) pupil (the opening that lets in light) and lens (which focuses light onto the retina) are its principal components. Light-sensitive cells in the retina at the rear of the eye transform light into electrical signals that are transmitted to the brain through the optic nerve.
Structure and Operation:
Cornea: The transparent curved outer layer that aids in light focus and protects the eye.
Iris and Pupil: The iris is the colored portion of the eye that modifies the pupil’s size which is the black hole in the middle to regulate how much light enters. The pupil dilates in low light and constricts in high light.
Lens: Light is further focused onto the retina by this transparent structure behind the pupil. It can alter its form to concentrate on objects at various distances.
Retina: A light-sensitive layer at the back of the eye with millions of cells called rods (for black and white vision) and cones (for color vision). It converts light into electrical signals.
Optic Nerve: Transmits the electrical signals from the retina to the brain, which then interprets them as image
Vitreous Humor and Aqueous Humor: The eye contains two different kinds of fluid: vitreous humor and aqueous humor. The vitreous humor is a jelly-like material that fills the space between the lens and the retina whereas the aqueous humor is a watery liquid that lies between the cornea and iris. Both aid in keeping the eyes shape intact.
Sclera: The hard white outer layer that shields the eye from harm.
Protection: The eye is shielded from dust dryness and other dangerous substances by the eyelids lashes and tears.
