Eye Anatomy
The human eye functions like a camera. It consists of a series of structures that act as lenses to focus light on the back of the eye.
- The cornea, the clear surface on the front of the eye, provides the primary source of focus as light enters the eye.
- The iris controls the amount of light reaching the back of the eye by changing aperture or pupil. The pupil, is a lack of tissue, alters its size depending on lighting and distance of stimuli.
- The eye's crystalline lens is located directly behind the pupil and further focuses light transmitted through the cornea. Through a process called accommodation, this lens helps the eye automatically focus on near and approaching objects, like an autofocus camera lens.
- Light moves through these structures and eventually reaches the retina, the structure that interprets light information into optical images and electric signals.
- The optic nerve then transmits these signals to the visual cortex — the part of the brain that controls our sense of sight.
Refractive Errors
- Refractive error is the primary reason why people wear spectacles, contact lenses, or undergo refractive surgery.
- If the eye is too long to focus light properly, myopia is present and images are blurred at a distance. If the eye is too short, hyperopia/hypermetropia is present and images are blurred at near. At higher levels of hyperopia, images are also blurred at a distance. Astigmatism refers to a condition where light enters the eye and does not have a single point of focus. Astigmatism occurs due to a football-shaped cornea or lens.
- Presbyopia is a condition that occurs along with aging that leads to difficulty when attempting to focus on near targets. This is corrected with multifocal lenses.
- Myopia is corrected by minus powered lenses. Hyperopia is corrected by plus powered lenses. Astigmatism is corrected by lenses that realign distorted images.
Glaucoma
Glaucoma refers to any condition which causes damage to the optic nerve that is responsible for the communication between the eye and the brain. It is an extremely concerning condition because it usually has no symptoms in its early stages. Screening for this condition is conducted at every comprehensive eye examination. Screening includes a measurement of eye pressure, careful examination of the optic nerve, and an assessment of peripheral vision.
Types of glaucoma
There are two major types of glaucoma: open-angle glaucoma and narrow angle glaucoma. The angle refers to the internal drainage structure within in the eye that controls the outflow of the fluid that is created in the eye.
Risk Factors
- Age
- History of poor cardiovascular health
- Ethnicity (African, Asian, Native, and Hispanic)
- History of tobacco usage
- High degrees of myopia
Cataracts
A cataract is the clouding of the eye's natural lens (also known as the crystalline lens) which lies behind the iris and the pupil. Cataracts are the most common cause of vision loss in people over age 40 and are the leading cause of blindness in the world. Cataracts predominantly occur in the aging population. There are also forms of cataracts that occur that are congenital in nature, occur due to medication side effects, or complications of systemic diseases.
Signs and symptoms
Cataracts usually progress slowly. Major symptoms of cataracts include experiencing hazy vision, requiring more light to see clearly, increasingly difficulty with glare especially during night driving, and changes in color perception.
Prevention
There is a great deal of research that is sometimes in conflict regarding preventing or slowing cataract progression. One large study suggested that higher dietary intakes of vitamin E, lutein, and zeaxanthin significantly decreased cataract development/progression. The most important set that can be taken is to protect against harmful UV radiation by wearing protective sunglasses when outdoors.
Treatment
When symptoms begin to appear, the best course of action is to ensure that you are wearing your most current spectacle prescription with anti-reflective coating. Others find increasing lighting for difficult tasks to be helpful. Once the cataract has progressed enough to seriously impair your vision, it is important to consider cataract extraction.
Cataract surgery has become the best way to improve the quality of your vision. A surgeon removes the cloudy lens and replaces it with a plastic intraocular lens (IOL) that improves your vision at distance. There are more advanced IOLs available that correct astigmatism, near vision, or provide an extended range of focus.
Macular Degeneration
Age-related macular degeneration (also known as AMD or ARMD) is a condition in which the central area of the vision or the macula deteriorates. This affects the ability for a person to see faces clearly, drive, read, or other visual tasks that require us to see clear details. On examination, the degeneration can be evidenced by the appearance of yellow spots called drusen as pictured to the left. This can progress to larger, coalesced drusen (dry AMD)or possible ruptures in the retina itself which can lead to the growth of new, leaky blood vessels (wet AMD).
Causes
Although macular degeneration is associated with aging, newer studies have isolated several genes that make it more likely for you to develop this condition. Excessive UV radiation, tobacco usage, and fair complexions are also important risk factors.
Treatment
Despite a better understanding of this condition than ever before, there is as of yet no cure for macular degeneration. An important piece of management that is thought to control the condition is oral supplements that contain key elements of good ocular nutrition such as lutein and zeaxanthin. Eating a healthy diet rich in Vitamin A and antioxidants can be helpful as well.
Should you progress to the wet form, injections directly into the eye can cause the leaky blood vessels to recede and inhibit their growth.