A defect of the visual structures that prevents light images from being perceived by the visual center of the brain may be caused by diseases of the eye, e.g., conjunctivitis neonatorum, trachoma, cataract, or from complications of other diseases, e.g., diabetes mellitus, or an injury to the optic nerve, retina, or brain; or from genetic anomalies.
The fact of not being able to see.
Total or partial loss of sight caused by a disease or disorder of the eye, optic nerve, or brain. In most cases, blindness refers to a loss of vision that cannot be corrected with glasses or contact lenses. Blindness does not always refer to a total loss of sight. Some people who are legally blind can perceive slow-moving objects or colors. The term “low vision” is used for people who have moderately impaired vision but are not classified as blind. Color blindness is not actually a form of blindness, since it refers only to a lack of perception of certain colors and not to a loss of vision.
The inability to see. Lack of all light perception constitutes total blindness but there are degrees of visual impairment far less severe than this that may be classed as blindness for administrative or statutory purposes. For example, marked reduction in the visual field is classified as blindness, even if objects are still seen sharply. The commonest causes of blindness are trachoma, onchocerciasis, and vitamin A deficiency but there is wide geographic variation. Among the commonest causes in the US are diabetes mellitus, myopic degeneration, and glaucoma.
Total blindness is a simple inability to see light or form. Legal blindness is defined differently in different countries. In most of Europe and in North America, it is defined as having less than 6/60 (20/200) vision in the better eye despite correction with lenses. 6/60 means that the person concerned would have to be 6 feet away from something an ordinary person could see from 60 feet. In the UK, a very restricted field of vision (20° rather than 180°) would represent legal blindness even if acuity is better than 6/60.
Inability to see. The leading causes of blindness in the U.S. are age-related macular degeneration, diabetic retinopathy, and glaucoma.
The definitions of blindness and partial sight can vary. In the United Kingdom, blindness is defined as having a corrected visual acuity of 3/60 or lower, or a visual field no wider than 20 degrees, in the better eye.
Vision loss can occur due to various factors such as injury, disease, or degeneration affecting the eyeball, the optic nerve, the nerve pathways connecting the eye to the brain, or even the brain itself.
Normal vision relies on the unimpeded flow of light from the front of the eye to the light-sensitive retina located at the back. Any obstruction that hinders the passage of light to the retina can result in blindness.
Several conditions can lead to the clouding of the cornea located at the front of the eye. These conditions encompass disorders such as Sjögren’s syndrome, characterized by excessive dryness of the eyes, vitamin A deficiency, chemical damage, infections, and injury. Corneal ulcers, which commonly occur following severe infections, have the potential to cause blindness due to corneal scarring. Loss of vision can also result from uveitis, which refers to the inflammation of the iris, ciliary body, or choroid.
Cataract, a condition characterized by the cloudiness of the lens, stands as a prevalent factor contributing to vision impairment. Typically, this ailment arises from the gradual reduction in the lens’ transparency during the process of aging. However, there are instances where cataract manifests either at birth or during childhood, accentuating its diverse onset.
Loss of vision can ensue from various factors such as diabetes mellitus, hypertension (high blood pressure), or injury, which can lead to bleeding within the eye’s cavity. Additionally, vision impairment may result from bleeding either in the fluid positioned in front of or behind the lens.
Blindness can be attributed to various retinal disorders, including age-related macular degeneration, characterized by the degeneration of the central region of the retina, typically occurring in old age. Other causes encompass retinopathy caused by diabetes or hypertension, retinal artery or vein occlusion leading to impaired blood flow to and from the retina, retinal detachment, certain types of eye tumors such as retinoblastoma and malignant melanoma, and retinal hemorrhage resulting from diabetes, hypertension, vascular disease, or injury.
Glaucoma, a condition characterized by elevated fluid pressure within the eye, leads to the deterioration of nerve fibers located at the anterior portion of the optic nerve.
The retina receives incoming light energy, which undergoes a transformation into nerve impulses. These impulses then journey through the optic nerve and nerve pathways, eventually reaching the brain. However, when there is a hindrance in the conduction of these nerve impulses, it can lead to a loss of vision.
Various factors can contribute to damage in the nerve pathways, including the pressure exerted by an orbital tumor, which occupies the bony cavity housing the eyeball. Additionally, a diminished blood supply to the optic nerve can arise due to conditions such as diabetes mellitus, hypertension, tumors, injuries, or temporal arteritis, characterized by inflammation of the scalp’s arteries. Optic neuritis, inflammation of the optic nerve associated with multiple sclerosis, toxic effects of certain chemicals, and specific nutritional deficiencies also present potential causes for such damage.
Nerve impulses originating from the retina ultimately reach a specific area within the cerebrum known as the visual cortex, which constitutes the main mass of the brain. In the event of pressure exerted on the visual cortex due to a brain tumor or brain hemorrhage, or if there is a reduction in blood supply to the visual cortex following a stroke, it can lead to blindness.
The cause of blindness is often identifiable through direct examination of the eye using techniques such as ophthalmoscopy, slit-lamp examination, and tonometry. These diagnostic methods allow for the detection of various eye conditions. Additionally, the conduction of nerve impulses can be assessed by measuring evoked responses, providing valuable insights into the functionality of the visual system.
The treatment approach for blindness varies depending on the underlying cause. In cases where vision loss cannot be corrected, individuals may be officially registered as legally blind or partially sighted. This registration grants them access to specific benefits and services aimed at supporting their unique needs and circumstances.