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Your ability to “see” starts when light reflects off an object at which we are looking and enters the eye. As it enters the eye, the light is unfocused. The first step in seeing is to focus the light rays onto the retina, which is the light sensitive layer found inside the eye. Once the light is focused, it stimulates cells to send millions of electrochemical impulses along the optic nerve to the brain. The portion of the brain at the back of the head interprets the impulses, enabling us to see the object.
Light, refraction and its importance.
Light entering the eye is first bent, or refracted, by the cornea — the clear window on the outer front surface of the eyeball. The cornea provides most of the eye’s optical power or lightbending ability.
After the light passes through the cornea, it is bent again — to a more finely adjusted focus — by the crystalline lens inside the eye. The lens focuses the light on the retina. This is achieved by the ciliary muscles in the eye changing the shape of the lens, bending or flattening it to focus the light rays on the retina.
This adjustment in the lens, known as accommodation, is necessary for bringing near and far objects into focus. The process of bending light to produce a focused image on the retina is called “refraction”. Ideally, the light is “refracted,” or redirected, in such a manner that the rays are focused into a precise image on the retina. Most vision problems occur because of an error in how our eyes refract light. In nearsightedness (myopia), the light rays form an image in front of the retina. In farsightedness (hypermetropia), the rays focus behind the retina. In astigmatism, the curvature of the cornea is irregular, causing light rays to focus to more than one place so that a single clear image cannot be formed on the retina, resulting in blurred vision. As we age, we find reading or performing closeup activities more difficult. This condition is called presbyopia, and results from the crystalline lens being less flexible, and therefore less able to bend light.
Since changing the apparent refraction of the eye is relatively easy through the use of corrective spectacle or contact lenses, many of the conditions that contribute to unclear vision can be readily corrected.
Even with the light focused on the retina, the process of seeing is not complete. For one thing, the image is inverted, or upside down. Light from the various “pieces” of the object being observed stimulate nerve endings — photoreceptors or cells sensitive to light — in the retina.
Two types of receptors — rods and cones — are present. Rods are mainly found in the peripheral retina and enable us to see in dim light and to detect peripheral motion. They are primarily responsible for night vision and visual orientation. Cones are principally found in the central retina and provide detailed vision for such tasks as reading or distinguishing distant objects. They also are necessary for colour detection. These photoreceptors convert light to electrochemical impulses that are transmitted via the nerves to the brain.
Millions of impulses travel along the nerve fibers of the optic nerve at the back of the eye, eventually arriving at the visual cortex of the brain, located at the back of the head. Here, the electrochemical impulses are unscrambled and interpreted. The image is reinverted so that we see the object the right way up. This “sensory” part of seeing is much more complex than the refractive part — and therefore is much more difficult to influence accurately.
A comprehensive eye exam involves several different tests measuring peripheral vision, depth perception, visual acuity, color vision, and the ability to focus close up. Different kinds of instruments and lenses are employed to test vision and eye health. Every part of the eye is examined for any role it may play in various eye disorders.
Several tests exist to examine both the physical characteristics of the eye itself, as well as aspects of vision.
These tests include:
This is the eye test most people are familiar with. Using a Smellen chart containing various letters, numbers and/or symbols, a patient’s ability to read the chart indicates how clearly one can see from a distance. The results of a visual acuity test are expressed as a fraction, for example 20/20 vision. The top 20 indicates the distance from the chart at which the patient can read accurately (20 feet with normal vision), whereas the bottom number indicates the distance that one with normal eyesight can accurately read the chart.
Only light is used in this physical examination of the eye itself. The doctor assesses how the pupil responds to light, both the movement and position of the eyes, corneal health, and the clarity of the iris.
This exam allows the doctor to assess the cornea, iris, lens and anterior chamber of the eye under intense magnification. An intense light is beamed through a slit lamp so that the doctor can see a cross section of the front areas of the eye and check for any abnormalities. The doctor may choose to use a dye to check for corneal problems such as tears, scrapes, cuts, or infections. The doctor uses blue light, making the dye appear yellow as it spreads across the eyes.
Only light is used in this physical examination of the eye itself. The doctor assesses how the pupil responds to light, both the movement and position of the eyes, corneal health, and the clarity of the iris.
Visual field refers to the area in front of an individual that they are able to see without moving the eyes. The Perimetry test uses a computerized machine that flashes spots of light at varying brightness onto different locations of a screen. Each time the patient can see a spot, they press a button that records the results, providing a map of where vision is strong or weak. Eye disorders such as macular degeneration or glaucoma cause characteristic patterns of holes in a patient’s visual field, allowing the doctor to diagnose the disorder long before a patient even notices vision loss. The Amsler grid is another test used to assess a patient’s visual field. A grid with a black dot at the center is used to test each eye’s ability to focus clearly. Each eye is tested to assess whether the grid can be seen clearly as the eye focuses on the dot. Distortion of the graph can indicate retinal damage.
Crisp vision relies heavily on the eyes ability to correctly focus light waves that pass through the cornea and lens onto the center of the retina. The ability to assess a patient’s refraction helps a doctor determine the lens prescription best suited to a patient’s vision needs. Both a computerized refractor, or Retinoscopy can be used to test for refractive errors. Then, an instrument called a Phoroptor is used to fine tune the exact lens best able to correct the error.
Glaucoma is an eye disease that causes damage to the optic nerve, usually due to increased pressure within the eye. The deterioration of the optic nerve hampers peripheral (side) vision. When left untreated, glaucoma can lead to blindness; however, with treatment, it is a manageable eye disease. Tonometry measures internal eye pressure. Tonometry measures the amount of force required to briefly flatten the cornea. When internal eye pressure is high, glaucoma is typically diagnosed.
Regular eye exams are necessary in order to diagnose vision and ocular health conditions in a timely manner, and to aid in prevention of these conditions.
How often you will need to receive an eye exam will depend largely on your individual eye care needs. After your initial examination, your optometrist will help you determine when you should return. General guidelines, however, are in place to assist you in determining followup examination frequency.
The below list is a recommended minimum frequency for lowrisk patients in each age group:
* Infants and toddlers (birth to 24 months) – By age 6 months
* Preschool (2 to 5 years) – At age 3, and prior to entering elementary school
* School age (6 to 19 years) – Annually
* Adult (20 to 64 years) – Every one to two years
* Older adult (65 years and older) – Annually
* Contact lens patients – Annually Frequency for high risk patients will be determined by the examining optometrist based on the findings at your initial examination. Some factors which may indicate high risk are:
*Infants and toddlers and preschool: Premature birth; low birth weight; mother’s health during pregnancy; family medical history; strabismus; or congenital eye disorders.
*School age: children experiencing difficulty at school; children exhibiting reading and/or learning disabilities.
* Adult: diabetes; hypertension; family history of glaucoma; those whose work is visually demanding or who face eye hazards.
* Older adult: diabetes; hypertension; family history of glaucoma; those taking systemic medication with ocular side effects.
Your Optometrist will explain the purpose of your prescription and when they should be worn. In most cases, glasses will not cause any deterioration that would not otherwise occur. However, wearing glasses for activities different than recommended (i.e. distance glasses worn to read up close) may make the eyes work harder than they need to.
There are a number of things you can do to prevent your vision from getting worse. Here are a few suggestions from team! 1. You should always be taking frequent eye breaks. 2. Looking up and away from extended near or computer work 3. Spending time outdoors! 4. Don’t hold reading material too close to your face 5. Practice eye-focus skills as recommended by your doctor, and ensure you are wearing a current lens prescription.
Your child should have their first vision evaluation as early as 6 months old and annually thereafter unless a visual problem is suspected earlier.
The doctor will use symbols instead of letters. Much of the testing that is done by the doctor requires little input from the child. The child’s eye exam is focused on assessment of the strength of the eyes, eye muscle status, and eye health to ensure proper vision development. Critical information can be obtained about your child’s eyes without them needing to say a word!
If your child is frequently engaging in one or many of these actions it may be due to vision problem: rubbing their eyes, squinting, turning or tilting their head, losing their place or using a finger to follow along when reading, moving their head or mouthing words while reading, headaches, red eyes, wandering eyes, complaints of blurred vision. Many disorders have no symptoms. ALL children need eye examinations at age 3.
Replacement time varies depending on eye health and tear chemistry, lifestyle and environmental exposure, time worn per day and days worn per week. Disposable lens replacement options can vary from daily, weekly, biweekly, monthly, and even quarterly. Your doctor will assess your eye health and vision status with your contact lenses to ensure you are following an appropriate wearing schedule.
Cataracts are largely an agerelated condition, but many factors can enhance earlier onset. Ultraviolet Light exposure has a cumulative effect, as does smoking.
Laser eye surgery is used to correct many cases of myopia (blurred distance vision) and astigmatism safely and predictably. Laser correction of hyperopia (farsightedness) is in its advanced testing stages. Results are not as predictable, nor as successful. Presbyopia, which causes the need to wear reading glasses or bifocals, cannot be corrected by laser surgery. As a result, individuals over 45 years will require reading glasses in most cases following surgery. Amblyopia (lazy eye) or other existing conditions that have caused damage to the eye or loss of vision, cannot be repaired by laser surgery.
