Abnormal, harmful, or undesirable effect on an organism that causes anatomical or functional damage, irreversible physical changes, or increases the susceptibility to other biological, chemical, or environmental stresses.
Optometrists can prescribe glasses, contact lenses, eye exercises, vision aids and therapy, and (in some states) oral and topical medications to treat certain eye diseases. Ophthalmologists are doctors of the eye. They have graduated from medical school and are licensed to practice medicine, so their credentials are either doctor of medicine (MD) or doctor of osteopathy (DO).
Opticians are individuals who are trained to design, verify, and fit eyeglass lenses and frames, contact lenses, and other devices to correct eyesight. Opticians are not trained to diagnose or treat eye diseases, but when they do suspect a medical eye problem they should refer the patient to an ophthalmologist.