When we refer to blindness we usually think of a complete or near-complete lack of vision. The blind are often depicted as only those who are unable to see anything or can see very little and have very low visual abilities. However, the term 'blind' doesn't just refer to the different types of blindness and embodies a variety of vision impairments.
As vision is simply a combination of light (photons) hitting the back of the retina and the transmission of this information to the brain via the optic nerve, blindness simply means any condition where insufficient light is reaching the retina, or the electrical synapses either aren't firing correctly, or the information is being delivered correctly to the brain.
There are at least three completely different types of blindness that can occur within the human population.
Complete blindness is the form of blindness we usually think of when the term 'blind' is used. It is characterized by a complete loss of vision. Even in complete blindness, the severity can vary dramatically. The requirements of being declared legally blind (according to Merck Manuals) is simply to have a visual acuity of equal to or less than 20/200 in your better eye. The 20/200 refers to where a person with 'normal' vision can see an object from 200 feet away but a person with impaired vision can see an object at no greater than 20 feet away.
This essentially means that even those with complete blindness aren't always completely blind. We think of complete blindness as those with retinas, corneas or optic nerves so damaged that light that hits the retina is not picked up at all, or no information is sent to the brain - meaning that the person can see nothing but darkness. This simply isn't the only form of complete blindness. Most people who are legally blind can see light, shapes and even some objects from a close distance, however, many conditions that cause this kind of blindness can be progressive.
Continue reading for next types of blindness.
Complete blindness (where there is no sight at all) usually occurs due to trauma, injury, progressive disorders or is a congenital defect (from birth). Many conditions and diseases can cause complete blindness, with one of the leading causes in the US being diabetes (according to the National Eye Institute). Diabetes causes blindness by completely destroying the retina due to a condition called diabetic retinopathy.
Cataracts are a common cause of blindness which affects the lens of the eye. Opaque patches are present on the lens, causing light to be blocked out and vision to be impaired. Glaucoma is another disorder that can cause blindness and does so due to damage of the optic nerve, preventing information from being sent to the brain.
Dyschromatopsia (color blindness) is a well-known disorder that relates to a person not being able to distinguish between colors effectively. Color blindness doesn't necessarily mean that no color is seen, just that certain colors are difficult to distinguish from each other due to the lack of a third cone within the eye.
Most people are born with three cones, one for red, one for green and one for blue light - people who have color blindness are simply missing one of these cones, or the cones aren't functioning properly. The most common form is red-green color blindness, meaning that objects that are red or green can be difficult to tell apart as the brain cannot see one form of light properly, or at all. People suffering from the condition may not see as many colors as others, and the rarest cases involve no cones at all, with sufferers only seeing black, white and gray.
Color blindness in more common in men as the genetic defect occurs within the X chromosome. As women have two X chromosomes, both would need to contain the defect whereas men only have one X chromosome, meaning the defect only needs to be present in that one X chromosome for the condition to be present.
It is unlikely that color blindness will be acquired, but it is possible. Things such as age, glaucoma, injury and trauma have been known to induce color blindness by causing damage to the cones.
Night blindness refers to a visual impairment present in dim light (at night-time or in a dark room). People with the condition have difficulty seeing objects when the light is low. This is usually due to a defect in the natural way our eyes adjust to low-light (where rhodopsin is produced by the rods of the eye in reaction to darkness).
It can be caused by a vitamin A deficiency, birth defect, trauma, glaucoma and many other conditions.