Eye examination

What happens during an eye exam?

A comprehensive eye exam includes a variety of tests and evaluations to determine the health of your eyes and the quality of your eyesight.

Some of the preliminary testing and data gathering will be performed by an assistant or technician. An optometrist will always perform the health evaluation of your eyes, the prescription of your eyeglasses and/or contact lenses, and the analysis of the findings of your exam.

Depending on your needs, your eye exam will last anywhere from 30 to 60 minutes. When you make an appointment for your exam, ask how long your exam will be so you can plan your day accordingly. Be aware that your optometrist may wish to dilate your pupils, which may affect your near and driving vision for an hour or so after your examination. If you have diabetes then you will be required to have your pupils dilated so we can better exam your retina.

Though each eye exam is unique, the following tests are performed at most complete eye exams. More tests or fewer tests may be performed at your optometrist's discretion.

Interview and Medical History

You will be asked a number of questions about your health, your vision, and your personal and family medical history. This information helps your optometrist determine if you are at risk for certain eye diseases and vision disorders. Some of the questions that may be asked include:

  • How old are you?
  • Are you satisfied with your current vision?
  • What specific vision problems (if any) are you having?
  • Do you currently wear glasses full-time, part-time, or not at all?
  • Do you wear contact lenses?
  • Do you have any health problems?
  • Do you take any medicines, including non-prescription products?
  • Do you have any allergies, including allergies to medicines?
  • Does anyone in your immediate family have Glaucoma?
  • Does anyone in your immediate family have diabetes, hypertension, or other health problems?

Visual Acuity Test

The clarity of your eyesight is called your visual acuity. Your optometrist or an assistant will test your visual acuity at distances of approximately 6 metres (distance vision) and 40 cm (near vision). Determining the smallest letters or numbers you can read on a chart typically assesses visual acuity. It is usually performed one eye at a time. The untested eye is covered with a small hand-held plastic paddle called an occluder.

Visual acuity is recorded as a comparative fraction called the Snellen fraction. If you have "6/6" visual acuity ("20/20" in USA), this means you can read the same size letters that a person with perfect vision can read when you are both 6 metres away. If your vision is "6/12," the smallest letters you can read at 6 metres a person with perfect vision can see at 12 metres. Thus, if the bottom number of the Snellen fraction is larger, your visual acuity is worse. (A person who has 6/24 vision has only half the visual acuity of a person who is 6/12.)

Eye Alignment Test

Using an occluder, your optometrist will cover and uncover each of your eyes several times as you focus on an object across the room. By observing how much your eyes must move when uncovered to refocus on the object, your optometrist can determine if your eyes are properly aligned and how well they work together as a team. The test (called the cover test) is repeated as you look at an object positioned at a normal reading distance. Proper eye synchronisation is necessary for clear, comfortable, binocular vision. Poor eye synchronisation can lead to eyestrain, blurred vision, double vision, strabismus, and amblyopia.

Pupil Response Test

The pupil is the normally circular opening in the eye that is formed by the inner border of the iris (the pigmented structure that gives our eyes their color). The pupil changes in size in response to different lighting conditions and controls the amount of light that reaches the light-sensitive retina at the back of the eye.

Your optometrist will look closely at the size of your pupils to see if they are the same size and shape. A significant difference in pupil size may indicate a problem with your eyes or parts of the brain. Next, your optometrist will direct a bright beam of light alternately at each of your eyes. By observing the change in size of your pupils in response to the light, your optometrist can determine if your pupils function normally.

Refraction

The refraction is the part of the exam when your optometrist determines the prescription of your lenses for eyeglasses. The optical instrument used by an optometrist to perform a refraction to determine the most appropriate prescription for corrective lenses is a Phoropter. The phoropter contains thousands of lens combinations that can be manually introduced in front of your eyes one lens at a time. Using the phoropter, your optometrist will show you a series of "either/or" lens choices and ask you which of the two lenses make things look clearer.

Based on your responses, your optometrist will fine-tune your eyeglass prescription to give you the clearest, most comfortable vision possible. To assist in this process, the optometrist may also shine a light across your eyes called a retinoscope. This allows the optometrist to get an idea of the prescription without your input. During the subjective refraction, don't be shy about asking your optometrist to show you the lens choices more than once.

Also, blink frequently to prevent your eyes from drying out or "tearing up" - both can cause blur that will make it more difficult for you to select the best lenses. In some cases (especially for children who may be farsighted), your optometrist may use eye drops that dilate the pupil and relax the focusing muscle inside the eye. This focusing muscle (called the ciliary muscle) changes the shape of the lens inside the eye to allow our eyes to focus on near objects. But over-action of this muscle can mask uncorrected farsightedness (especially in children). Certain dilating drops temporarily force this muscle to relax completely (an action called cycloplegia) and thus enable your optometrist to determine the most accurate prescription for farsightedness.

Slit Lamp Exam of the Anterior Eye

Your optometrist will use an instrument called a slit lamp (also called a biomicroscope) to take a close look at the surface of your eyes and the anterior portion of the interior of the eye. The instrument is called a slit lamp because the magnified beam of light it produces can be narrowed to a thin slit. This enables your optometrist to see subtle changes in thickness and contour of the cornea and structures inside the eye. With special stains and filters, the slit lamp can also be used to evaluate the tear film on the cornea to make sure your eyes have sufficient tears for successful contact lens wear. The slit lamp is also used to examine the crystalline lens behind the pupil to determine if cataracts are developing.

Evaluation of Eye Pressure (The Glaucoma Test)

Excessive pressure inside the eye can damage the optic nerve, leading to glaucoma. This serious eye disease can affect people of any age and is a leading cause of blindness. Eye pressure is typically measured with a tonometer. The tonometer may be attached to the slit lamp or hand-held and measures the pressure of the eye by resting a small device directly against the cornea. Your optometrist will first apply an eye drop that contains a yellow dye and a short-acting anaesthetic to decrease the sensitivity of your eye. The tip of the tonometer (about the size of a small pencil eraser) is then gently placed against the centre of the cornea and measures the pressure required to flatten a small circular area on the cornea. The measurement takes only a few seconds and you will have no awareness that the tonometer is touching your eye.

Examination of Posterior Eye (Retina and Optic Nerve)

The retina is the light-sensitive inner lining of the back of the eye. The optic nerve transmits the electrical impulses from the retina to the visual cortex of the brain. Evaluation of the retina and optic nerve is important to rule out glaucoma, macula degeneration (MD), diabetic retinopathy, and other potentially sight-threatening eye conditions. To get a view of the entire retina, your optometrist may apply eye drops that will dilate your pupils. With your pupils dilated, your optometrist may use several techniques to examine your retina and optic nerve. One method requires a hand-held illuminated microscope called a direct ophthalmoscope. Your optometrist holds the direct ophthalmoscope up to his/her eye and moves very close to your face to look inside your eye. The direct ophthalmoscope provides a highly magnified view of the optic nerve and specific areas of the retina, including the macula. A second method requires an instrument called a binocular indirect ophthalmoscope. This is a combination of a light emitting device worn on the optometrist's head and a large, hand-held lens. Your optometrist will hold the lens close to your eye, fully extending his/her arm while directing a beam of light through the lens. The binocular indirect ophthalmoscope provides a wide angle, three-dimensional view of larger areas of the retina. A third method requires the use of the slit lamp and a smaller hand-held lens. This method provides a more magnified three-dimensional view of the optic nerve and macula.

 

Computerised Perimetry or Visual Fields test

Testing your acuity only examines the function of your macula or central vision.  Occasionally it is necessary to test the function of your peripheral or side vision using a computerised perimeter. Taking 5-10 minutes per eye, you are sat in in front of a white bowl and your head positioned on a chin rest. You acknowledge any lights flashing inside the bowl by pressing a button. At the completion of the test we will have an indication of the sensitivity of your retina and whether your visual fields are normal or depressed.  Visual field defects can be characteristic of retinal and neurological disease such as glaucoma, stroke, trauma and tumour.  Computerised perimetry allows assessment of functional abnormalities. 

 

Imaging of Retinal Structures

The use of the Zeiss Cirrus HD Optical Coherence Tomographer (OCT) enables high resolution cross-sectional imaging of the ocular structures. This is essential to differentiate "wet" from "dry" macular degeneration. It is also useful in the diagnosis of retinal structural abnormalities related to glaucoma and retinal disease such as leaks at the macula caused from diabetes. The optometrist may perform this scan of your retina which takes 5-10 minutes and sometimes requires prior dilation of the pupils.  OCT allows assessment of structural abnormalities.

Summary

Though these are the most common tests performed during a routine eye exam, your optometrist will determine the scope of your exam and what tests need to be performed to provide an accurate diagnosis and treatment. The fitting of contact lenses requires additional testing and follow-up exams before a contact lens prescription can be written. In some cases, your optometrist will feel it's necessary for you to have additional specialised tests to rule out or treat eye disease or general health problems. These tests may be performed by your optometrist or may require that you see an ophthalmologist or specialist. Be sure to listen to your optometrist carefully at your eye exam so you fully understand your optometrist's recommendations and why additional testing may be required.