Visual impairment

# Introduction
Visual impairment or vision impairment is vision loss that constitutes a significant limitation of visual capability resulting from disease, trauma, or a congenital or degenerative condition that cannot be corrected by conventional means, including refractive correction, medication, or surgery. This functional loss of vision is typically defined to manifest with 1) best corrected visual acuity of less than 20/60, or significant central field defect, 2) significant peripheral field defect including homonymous or heteronymous bilateral visual field defect or generalized contraction or constriction of field, or 3) reduced peak contrast sensitivity either of the above conditions.
According to the U.S. , "the terms partially sighted, low vision, legally blind, and totally blind are used in the educational context to describe students with visual impairments. They are defined as follows:
- Partially sighted indicates some type of visual problem, with a need of person to receive special education in some cases;
- Low vision generally refers to a severe visual impairment, not necessarily limited to distance vision. Low vision applies to all individuals with sight who are unable to read the newspaper at a normal viewing distance, even with the aid of eyeglasses or contact lenses. They use a combination of vision and other senses to learn, although they may require adaptations in lighting or the size of print, and, sometimes, braille;
Myopic - unable to see distant objects clearly, commonly called near-sighted or short-sighted
Hyperopic - unable to see close objects clearly, commonly called far-sighted or long-sighted
- Myopic - unable to see distant objects clearly, commonly called near-sighted or short-sighted
- Hyperopic - unable to see close objects clearly, commonly called far-sighted or long-sighted
- Legally blind indicates that a person has less than 20/200 vision in the better eye or a very limited field of vision (20 degrees at its widest point); and
- Totally blind students learn via braille or other non-visual media.
Visual impairment is the consequence of a functional loss of vision, rather than the eye disorder itself. Eye disorders which can lead to visual impairments can include retinal degeneration, albinism, cataracts, glaucoma, muscular problems that result in visual disturbances, corneal disorders, diabetic retinopathy, congenital disorders, and infection." Visual impairment can also be caused by brain and nerve disorders, in which case it is usually termed cortical visual impairment (CVI).
The American Medical Association's Guides to the Evaluation of Permanent Impairment attempts to provide "a standardized, objective approach to evaluating medical impairments." The Visual System chapter "provides criteria for evaluating permanent impairment of the visual system as it affects an individual's ability to perform activities of daily living." The Guide has estimated that the loss of one eye equals 25% impairment of the visual system and 24% impairment of the whole person;  total loss of vision in both eyes is considered to be 100% visual impairment and 85% impairment of the whole person.
Visual impairments have considerable economic impact on even developed countries.
Drug Side Effect:
- Crizotinib, Denileukin diftitox