After years of pre-professional undergraduate education and four or more years at a college of optometry, an eye doctor earns a doctorate in optometry. Some also complete a medical residency.
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Refraction is the part of an eye exam determining your prescription for glasses or contacts. It involves the doctor shining a light into your eyes and looking at how it bounces off the retina (the light-sensitive tissue at the back of your eye). This gives the doctor a clue as to what kind of refractive error you might have. They can then use a machine called a phoropter or a handheld device to narrow down your vision problems, such as nearsightedness, farsightedness, and astigmatism.
Refractive errors occur because the cornea and lens of your eye can’t create a real image on the light-sensing retina in a way that is correct for all distances. Instead, the lens and cornea curve differently in different meridians of your eye, causing blurry up-close and distant vision. The refraction test tells the doctor what kind of lenses will help you see clearly at all distances, so the doctor can give you a complete prescription for your contact or glasses.
During the refraction portion of an eye exam, you’ll be seated in front of a piece of equipment called a phoropter or a hand-held device that looks like a large mask with holes for your eyes. On a wall about 20 feet away, you’ll see a row of letters that get smaller and smaller until you can no longer read them. The last row that you can no longer read is your visual acuity or 20/20 vision value.
The doctor uses the phoropter or handheld device to change the lenses to find the pair that will give you clearest vision at all distances. They can also adjust the lens for any astigmatism you have, which is caused by the irregular curvature of your cornea or the shape of your lens.
The final prescription is written on a slip of paper that you’ll take home with you. You can then go to a pharmacy or optometrist for your new lenses. Some vision insurance plans cover refraction tests, and Medicare Part C may also include some of the cost of your exam.
Visual Acuity Test
Most people have had a visual acuity test at some point without even knowing it. A wall chart with rows of letters that get smaller and smaller is a common sight in eye care specialist offices or vision screenings.
A visual acuity test measures how well you see things at a distance. Your eye care specialist will ask you to stand about 20 feet away from a chart with rows of capital letters that get smaller and smaller until you can no longer read them clearly. The eye doctor will then move to the next row, and you’ll repeat the process. The eye doctor may also ask you to do the test with just one eye and then with both eyes.
The results of the visual acuity test will be given as a fraction, such as 20/20. Having 20/20 vision means that you can see as well as the average person can from about 20 feet away. The higher your visual acuity score, the better your vision is.
To measure visual acuity, the eye doctor will use an eye chart that has 11 rows of letters that get progressively smaller. Each row has a number at the top that indicates how large the letter is. The eye doctor will then ask the patient to read each row, starting with the largest letter and moving down the chart until he or she can no longer see the letters clearly.
Some doctors use a less complicated chart that has just 10 rows of letters, all in lowercase. This type of chart is easier to read for children. Some doctors also test a patient’s visual acuity using a random E test, in which they project images of the capital letter E facing different directions and ask the patient to identify which direction each letter is facing. This test is often used in vision screenings in schools or other public places. The random E test is not used as often as the Snellen chart, but it can help diagnose certain vision problems. Patients who can see at least 6/6 on the Snellen chart typically do not need further evaluation.
Slit Lamp Examination
Slit lamp examination is a powerful tool that gives your eye care professional a look at the different structures in your eye, both inside and out. It is a mainstay of any comprehensive eye exam and can detect many diseases that cannot be seen with the naked eye. It can also help determine whether you need a prescription or not.
The slit lamp is a microscope with a bright light, allowing your eye doctor to see a variety of details about the front of your eyes. This test can diagnose many conditions, including glaucoma, macular degeneration, corneal ulcers, and other infections. The slit lamp can also identify other problems that might not be apparent to the naked eye, such as iris heterochromia and periorbital neoplasms.
Before the slit lamp exam, your eye doctor will place drops in your eyes to enlarge your pupils. He will then sit you down in a chair and ask you to rest your chin and forehead on supports that keep your head steady. The slit lamp will be placed in front of your eyes, and the doctor will then use various lenses to examine the different parts of your eye.
During the exam, your eye doctor will use the slit lamp to look at the surface of your cornea and the conjunctiva. He will then look for signs of inflammation, such as flares and cells, and he will evaluate the structure of your lens and the anterior chamber. He may also conduct a Seidel’s test to assess the integrity of your cornea.
If you are having a slit lamp exam done, be sure to wear your glasses. Your vision will be blurry, and your eyes will be very sensitive to light after the test is over. It is important to bring sunglasses with you to your appointment and plan on having someone drive you home afterward. Also, if you have any symptoms of nausea, vomiting, or eye pain while your eyes are dilated, please make an appointment to see your eye care provider right away, as these might be signs of increased fluid pressure in your eyes, which is an emergency.
Tonometry
Tonometry measures the pressure inside your eye, which is called intraocular pressure (IOP). It’s important to have this test performed because elevated IOP is a risk factor for glaucoma. Glaucoma is a disease that can cause blindness if left untreated. Your eye doctor can determine whether your IOP is within the normal range during a complete exam.
To perform the tonometry test, your doctor will use eyedrops to numb your eyes. You will then rest your chin on a padded surface and look directly into the machine’s light. A puff of air will be blown at your cornea, and the tonometer will measure the resistance to the indentation. Then it will provide a reading of your IOP, which is written in millimeters of mercury.
Your eye doctor will likely use a Goldmann applanation tonometer, which is the industry standard. It is a small instrument that attaches to the slit lamp biomicroscope used in all eye doctors’ offices. You may also see a tonometer shaped like a pen or the handheld Icare rebound tonometer, which can be used at home to monitor daily eye pressure. The PASCAL dynamic contour tonometer is a more advanced device that uses a technique similar to pachymetry, instead of applanation. It is especially useful when the patient can’t place his chin at the slit lamp instrument, as in children and elderly patients in wheelchairs.
The Goldmann tonometer requires a small amount of anesthesia, usually proparicaine or tetracaine. It is important to follow your doctor’s instructions for this test, as it can be dangerous if not performed properly.
Another way of measuring IOP is the Schiotz tonometer, which involves a curved footplate that touches the supine subject’s eye. A weighted plunger attached to the footplate sinks into the cornea, and a scale at the top of the plunger registers the kickback from the corneal surface that corresponds to a specific eye pressure measurement. The tonometer has several safety features that prevent complications such as corneal abrasion, corneal ulcer, ocular inflammation, and glaucoma aggravation. It is important that the IOP measured with an alternative tonometer be confirmed by GAT, because of the potential for elevated IOP to be due to factors other than ocular hypertension.