By Ann-Marie C. DePalma, RDH, MEd, FADIA, FAADH
Our eyes and our vision are precious resources. Regular eye exams can detect vision-affecting diseases such as nearsightedness, farsightedness, astigmatism, glaucoma, cataracts, or macular degeneration. Everyone should have regular eye exams even to confirm health. Whether health or disease is discovered, the eye-with its intricacies of the lens, cornea, retina, pupil, sclera, and conjunctiva-is a remarkable organ.
Similar to having regular dental checkups, it is important for all of us to have regular eye exams, especially as we age. In addition, comparable to first dental exams, the American Optometric Association (AOA) recommends infants have their first comprehensive eye exam at 6 months of age. Children are recommended to have additional exams at 3 years and before they enter kindergarten or first grade (age 5 or 6). These eye exams are important since basic visual skills are needed for learning. Exams involve:
- Near vision
- Distance vision
- Eye team (binocularity skills)
- Eye movement skills
- Focusing skills
- Peripheral awareness
- Hand/eye coordination
Because vision and eye health are so critical to learning, some states even require eye exams for all children entering school for the first time. We all remember the vision and hearing screenings done during our school days!
Brief review of anatomy
For a more in-depth review of the eye anatomy, the author recommends referring to an anatomy textbook. For purposes of this article, however, a brief review is warranted. The eye itself is an asymmetrical orb, about an inch in diameter nestled in the orbital bones. The orbit is the bony cavity that contains the orb (eyeball), muscles, nerves, and blood vessels, along with structures that produce and drain tears and nourish the eye. The outer covering of the eyeball consists of a tough, white layer called the sclera (white of the eye).
A thin, transparent membrane, the conjunctiva, runs to the edge of the cornea and covers the sclera. The cornea is the curved layer in front of the iris and the pupil where light enters. It serves as a protective covering for the front of the eye and focuses light onto the retina. Light travels through the cornea to the pupil and iris, both of which control the amount of light that enters the eye. The iris controls the size of the pupil. The pupil will constrict or dilate similar to the aperture of a camera lens as the amount of light in the immediate surroundings changes. Various pupillary muscles control the size of the pupil. Behind the iris is the lens. By changing shape, the lens focuses light onto the retina. Ciliary muscles in the lens allow it to become thicker to focus on nearby objects and thinner to focus on distant objects. The retina is the nerve layer that lines the back of the eye, senses light, and creates impulses that travel through the optic nerve to the brain. There is a small area, called the macula, in the retina that contains the special light-sensitive cells or photoreceptors, the rods, and cones. The retina also contains the optic nerve and blood vessels.
Anatomy of the Eye
Light enters the eye through the cornea. The cornea's refractive power bends the light rays so that they pass freely through the iris to the pupil. In some patients, however, the cornea becomes irregular and/or cone-shaped so that light enters the eye at different angles and does not focus on one point on the retina, but on many different points. This refraction causes a blurred, distorted image.
Astigmatism occurs when the curvature of the cornea is different in one direction than another. It is often described as football shaped (oblong) rather than basketball shaped (rounded). This change in curvature causes the eye not to focus light rays at a single point but multiple points on the retina, thus creating vision that is out of focus at any distance.
Astigmatism is a very common vision condition. Many people have some degree of astigmatism, where slight amounts don't affect vision and don't require treatment. Larger amounts of distorted images can cause eye discomfort and headaches.
Astigmatism often occurs with other conditions such as myopia (nearsightedness) or hyperopia (farsightedness). All of these conditions are referred to as refractive errors since they affect how eyes bend or "refract" light. Nearsightedness or myopia is a condition where close objects are seen clearly but objects farther away appear blurred. Myopia occurs if the eyeball is too long or the cornea has too much curvature. Farsightedness or hyperopia is the condition where distant objects are seen clearly but close objects do not appear in focus. Farsightedness occurs if the eyeball is too short or the cornea has too little curvature.
Anyone over the age of 40 may notice changes in their vision. Most notably, presbyopia, the normal age-related loss of near-focusing ability, usually develops around at that time. One may find that there is a tendency to hold reading material farther away to make the letters clearer, blurred vision may occur at the normal reading distance, and headaches or eyestrain may occur after reading or doing close work. This loss of focusing ability for near vision is the result of the lens becoming less flexible. This flexibility allows the eye to change focus from objects that are far away to objects that are close.
Another common age-related vision issue is cataract development. A cataract is a clouding of the lens of the eye, which results in decreased vision from the breakdown of lens proteins. In addition to age, other risk factors include diabetes, smoking/tobacco use, prolonged exposure to sunlight, and alcohol use. Cataracts are one of the leading causes of blindness. Many corrective lens and surgical options are available for patients who experience some form of presbyopia, myopia, hyperopia, or cataracts.
Sometimes, however, a condition known as keratoconus is diagnosed. Keratoconus is a corneal disease seen in approximately one to 430 of 2,000 people. Depending on the research study cited, the number of diagnosed cases can vary because of the inconsistencies within the diagnostic criteria. Some cases of extreme astigmatism are not classified as keratoconus. It is also estimated that there are approximately two new cases per 100,000 people each year, and inconsistencies exist in the male/female distribution with some studies stating a higher female to male incidence, while others state the opposite.
In keratoconus, the cornea becomes progressively thinner and cone shaped than in regular astigmatism. Since the corneal changes are more abrupt than regular astigmatism, it is often called irregular astigmatism. Keratoconus usually begins in the teenage years but can also be seen in early childhood or up to age 30. It can occur in people age 40 and older, but this is less common. There is no one known etiological factor that is responsible for the progression of keratoconus. A number of conditions can increase its likelihood of occurrence including genetic predispositions, hormonal changes, allergies/asthma/eczema (atopic disease), excessive eye rubbing, or oxidative stress. Studies state that in corneas diagnosed with keratoconus, the cornea lacks the ability to self-repair from routine damage.
Like many tissues in the body, the cornea creates harmful by-products of cell metabolism known as free radicals. Normal corneas can neutralize these free radicals; keratonic corneas lack this ability and, therefore, the collagen fibers that support the structure of the cornea are weakened by the increased activity of proteases. When the collagen fibers become weak, they cannot hold the normal shape of the cornea, so it will begin to bulge and become progressively thinner and cone shaped. This results in increased astigmatism that may or may not be correctable with normal glasses or contact lenses. Patients with predisposing genetics, allergies, hormonal changes, or other conditions aggravate this collagen breakdown.
Changes in the shape of the cornea can occur rapidly or may take years to develop with no prediction on how the disease will progress. Typical characteristics of keratoconus include blurred vision, glare, and halos during night driving, photophobia (sensitivity to bright light), eye strain, itching eye, and ghost images (monocular polyopia), but there is usually no pain. Symptoms appear worse in low light conditions since the dilated pupil exposes more of the irregular-shaped cornea. Both eyes can be affected, but not to the same extent. It usually develops in one eye first and then later in the other eye, but the first eye is usually the more severe. It does not lead to true blindness; however, general vision is compromised. If severe keratoconus exists, the stretched corneal collagen fibers can tear and lead to corneal scarring.
Patients with keratoconus will exhibit symptoms of:
- Increasing nearsightedness-objects near and far can appear distorted
- Sudden change in vision in one eye only
- Double vision when looking with just one eye
- Increasing halos around bright lights or light streaking
- Ghost images-double and triple images
- Increased difficulty driving due to blurred vision
Regular eye examinations, even for "healthy" individuals, are important. Ophthalmologists will examine the eye and if keratoconus is suspected will perform a corneal topography test along with other diagnostic evaluations. The corneal topography projects a series of illuminated rings onto the surface of the cornea that are reflected back onto the instrument. A computer then analyzes the rings to generate a topographical map of the cornea and thus the disease or health is determined. Other diagnostic images are also evaluated depending on the patients' condition.
Treatment for keratoconus varies from a simple change in eyeglass prescription to more invasive corneal transplantation. Many patients, however, do not seem to benefit from a change in eyeglass prescription, so rigid gas permeable contact lenses are often prescribed.
Rigid gas permeable lenses (GP or RGP) are made of fluorosilicone acrylate and were introduced to the contact lens world in 1987. Up until that time, only soft and hard contact lenses were options. Hard lenses are virtually obsolete now, while many patients benefit from soft contacts that are made of gel-like plastics for vision correction. Soft lenses are larger than GP lenses and fit under the eyelid, but do not allow enough oxygen to permeate through to the eyes. GP lenses allow for this oxygen exchange that is important in maintaining eye health. Due to their rigid nature, GP lenses retain their shape, and maintenance care is easier than with soft lenses.
For patients with keratoconus, this rigidity helps smooth out the irregularities of the cornea, thus improving vision. Some patients can use the lens only; others must supplement lenses with eyeglass use. Others may benefit from a "piggyback" lens of both GP and soft lenses, a hybrid lens that combines the rigidity of GP lenses with the comfort of soft lenses, or a scleral lens that fits over the cornea onto the sclera of the eye. Contact lens use with keratoconus is very individualized.
When eyeglasses and contact lenses fail to improve vision, other more invasive options can be considered. Intacs are microthin semicircular prescription inserts that are inserted into the middle layer of the cornea to change the shape of the keratonic cone. Corneal collagen crosslinking with riboflavin (CXL) is a procedure that has been used successfully in European countries for years. The Food and Drug Administration (FDA) considers CXL experimental at this time, and patients who are interested in this procedure must do so at clinical trial sites. CXL increases the collagen fibers that support the cornea by using custom-made riboflavin drops to saturate the cornea. The drops are activated via ultraviolet light, and studies have shown this to strengthen the cornea. Although not a cure for keratoconus, it can prevent further deterioration of vision.
Only 15%-20% of keratoconus patients require corneal transplantation. It is only considered when the cornea is extremely thin or when vision is so affected that no other treatment options are viable. Most cases can be done in outpatient surgery under local or general anesthesia while others require in-patient care. The diseased cornea is removed and a donor cornea received from a recently deceased individual is placed.
The patient receives an eye patch that remains in place for several days. Eye drops are prescribed to assist in healing and aid in rejection prevention. Since there is no blood supply to the corneal tissue (nutrients are received from the aqueous humor of the lens), it will heal slowly and care must be taken during the healing phase. Glasses or other protective eyewear are often recommended. Depending on the severity of the disease, contact lenses or glasses for vision correction may also be required postsurgically.
For many keratoconus patients, the costs of treatment are significant. From exam fees to contact lenses to surgery, there is a major cost factor for both patients and third-party payers. Combined with the usual early onset of the disease and the significantly impaired vision-related quality-of-life issues, the economic burden has been found to be a significant public health issue. As with any other medical or dental diagnosis, education plays a key role in the disease process. Understanding any vision disease process is important for dental patients and family. As hygienists, educating ourselves about the various types of eye diseases and their impact for dental patients can assist those experiencing eye disease to be more comfortable in our care. RDH
ANN-MARIE C. DEPALMA, RDH, MEd, FADIA, FAADH, is a Fellow of the American Academy of Dental Hygiene and the Association of Dental Implant Auxiliaries, as well as a continuous member of ADHA. She presents continuing education programs for dental team members on a variety of topics. Ann-Marie is collaborating with several authors on various books for dental hygiene and can be reached at [email protected].
1. American Optometric Association www.aoa.org
2. Contact Lens Manufacturers Association www.contactlenses.org
3. Global Keratoconus Foundation www.kcglobal.com
4. Merck Manuals www.merckmanuals.com
5. National Institutes of Health, National Eye Institute www.nei.nih.gov
6. National Keratoconus Foundation www.nkcf.org
7. National Library of Medicine, National Institutes of Health www.ncbi.nlm.nih.gov
8. WebMD www.webmd.com
9. Wikipedia www.wikipedia.org