By Eyecare NGHistory taking | Visual acuity test | Refraction | Binocular vision tests | External examination | Ophthalmoscopy | Tonometry | Slit lamp examination | Supplemental tests
A comprehensive eye examination is an assessment of your eye health and visual system through several standard and specialized tests or procedures performed by an eye doctor. It is important for the prevention and early detection of eye diseases and conditions. The common tests performed during a comprehensive eye examination are explained below.
An eye doctor starts every eye examination by taking a detailed history of a patient’s eye health and general medical status. This is done to obtain all the information needed to understand an eye condition better, so that appropriate decisions and actions with respect to eye examination procedures, diagnosis and care can be taken.
While taking your history, your doctor will ask detailed questions about your symptoms, past eye examination, other eye health history, medical history, family health history, known allergies, occupation, hobbies and other relevant information. To ensure you make the most of your eye care examination, note down all these information and take them along with you to your eye doctor’s office.
Visual Acuity Test
Visual acuity refers to how well you can see. The test is done to measure how well you can see at distance and at near, with each eye alone and both eyes together.
To determine your distance visual acuity, you are asked to read the letters on a standard chart (Snellen chart) placed at a distance of 6 metres or 20 feet away from you. The letters on the chart are arranged in rows and the size of the letters decrease as you read from the top, down. Your visual acuity equals to the lowest row you are able to read.
Your near visual acuity (how well you can see while performing near tasks) is also measured. This is done by asking you to read the paragraphs on a standard card placed at the distance of 40 cm from your eyes.
This test is done to determine the lens prescription that will compensate for any refractive error you may have. It has two parts; objective refraction and subjective refraction.
Objective refraction determines the refractive error in your eyes without relying on your feedback to arrive at a result. It can be done using an instrument called retinoscope or a computerized equipment known as autorefractor.
In subjective refraction, your eye doctor requires your feedback to arrive at the best prescription for you. Your doctor does this by first placing lenses equivalent to the result of your objective refraction in a trial frame or phoropter positioned in front of your eyes. The lens power is then adjusted until your preferred prescription is determined.
Binocular vision tests
This group of tests assess how well your two eyes work together synchronously to produce a single visual image. There are so many binocular vision tests. The most common ones are ocular motility test, cover and uncover test and near point of convergence test.
Ocular motility test (broad H-test)
This test is used to determine how well your ocular muscles work. It is done with one eye covered at a time and then with both eyes open. To determine how well your ocular muscles move your eyes individually and synchronously, a penlight is pointed at you and then you are asked to look at the light and follow it as it is moved to your different positions of gaze. Basically, a giant “H” is traced with the penlight in front of you while you follow it. During the test, your eyes are observed for poor tracking or jerky movements. You may also be asked to report double vision and other observations you may make while the test is being done.
Cover and uncover test
This test is used to determine whether you have squint (tropia) or phoria (latent deviation) and also measures the degree of deviation using a prism. It is usually done at far (6metres) and at near (40cm). The test requires that you focus on an object in front of you while the eye doctor covers each of your eyes alternately and observes the uncovered eye for movements. Abnormal movements usually indicate the presence of a binocular vision problem.
Near point of convergence test
This tests the ability of your eyes to converge at near. It involves moving a penlight towards your nose until you see two penlights and then moving it away until you can see only one penlight. To perform the test, a penlight is pointed at you; then you are asked to focus on the light as it is moved towards you and to report when the light becomes two. Your eyes are also observed for deviations and jerky movements. Once you report that the light has become two, the professional will move the light away until you report that the light has become one again. The distance from your nose to the point where the light became two is measured using a meter rule and recorded as the breakpoint (that is, the near point of convergence) and the distance from the nose to the point where the light became one again is the convergence recovery point. Abnormal break and recovery values usually indicate that a convergence problem is present.
This is an inspection of the external part of the eye and its structures for abnormalities. It is done using the slit lamp bimicroscope or a penlight and a magnifying glass. The areas inspected include the eyeballs (for size and positioning within the eye sockets), eyelids, eyelashes, the conjunctiva, the cornea, the pupil, iris and lens. Abnormalities in size, shape, colour or general appearance are noted and may be further investigated with other tests if the diagnosis is not conclusive.
Ophthalmoscopy is the examination of the retina and other structures at the back of the eye using an instrument called an ophthalmoscope. To perform the test, the eye doctor asks you to focus on an object in front of you. Keeping a distance of about 50cm from you, he/she shines the light of the ophthalmoscope into your eyes and moves towards you while adjusting the lens of the ophthalmoscope until a clear view of your retina is obtained. He/she then examines the retina, the blood vessels, optic nerve head and macular for abnormalities.
This is a test done to check the pressure inside the eye. The result of the test is important for evaluating your risk for glaucoma.
Tonometry is done with an instrument known as tonometer. There are different types of tonometers and their methods of measuring eye pressure differ. The common types are the indentation tonometer, applanation tonometers and the non-contact tonometers.
These are also known as impression tonometers. They determine eye pressure by measuring the depth produced on the surface of the cornea by a plunger of known weight. An example of the indentation tonometer is the Schiotz tonometer. To measure your eye pressure using this tonometer,
- The surface of your eyes are numbed with an anaesthetic eye drop.
- Thereafter, you are asked to lie on a flat surface and look up towards the ceiling and focus at a particular point or object.
- The tonometer is then placed on the surface of your cornea.
- The reading on the scale is noted by your eye doctor.
Applanation tonometers determine eye pressure by measuring the force that is used to flatten the cornea. This is currently the commonest method of measuring the pressure in the eye. An example of an applanation tonometer is the Goldmann applanation tonometer (which is a Perkins tonometer mounted on a slit lamp). The use of a handheld version of this applanation tonometer is also popular. To measure your eye pressure using the Goldmann tonometer,
- The surface of your eyes are numbed with an anaesthetic eye drop and fluorescein dye is applied to stain the corneal surface.
- The slit lamp is placed in front of you and you are asked to place your chin and forehead on the chin rest and forehead rest of the slit lamp respectively.
- The tip of the tonometer is moved until it touches your cornea.
- The eye doctor looks through the eyepiece of the tonometer and adjusts the knob of the tonometer until your eye pressure reading is obtained.
This measures the pressure in the eye, by releasing a puff of air into the eye, which flattens an area of the cornea. Non-contact tonometers are easy to use and are particularly useful for measuring eye pressure in children and other people who may not comply with the use of other types of tonometers. The American Academy of Ophthalmology, however, notes that in comparison with the Goldmann applanation tonometer, non-contact tonometers may underestimate pressure in the eye at high ranges and overestimate it at low ranges.
Slit lamp examination
Slit lamp examination is done to obtain a detailed view of the eyes both externally and internally, from the front to the back using a bimicroscope that magnifies the structures of the eye.
To examine your eyes using the slit lamp, you are asked to place your chin and forehead on the chin rest and forehead rest of the slit lamp respectively. The doctor then looks through the eyepiece of the instrument and with a combination of bright light and different magnifying lenses views the different structures of your eyes and notes abnormalities if any is observed.
The slit lamp examination is an important part of the comprehensive eye examination. It provides so much information about your eye health and can make it easy to identify signs of vision-threatening conditions and diseases such as glaucoma, age-related macular degeneration, and some cancers.
Sometimes, more specialized tests may be recommended to aid a definite diagnosis of abnormalities observed during the standard tests. Supplemental tests either clarify uncertainties in the result of the standard tests, provide a more in-depth assessment of the eye or confirms or rule outs the doctor’s tentative diagnosis. Some of these tests include:
- Perimetry or visual field test
- Optical coherence tomography