What kinds of eye examinations are there?

Eye examinations tend to follow a certain pattern: After talking with your doctor about the type and severity of your symptoms, both eyes will usually be tested to measure the visual acuity (sharpness of sight). Then the doctor will look at your eyes from the outside, take a closer look at them using a slit lamp, and measure the intraocular pressure (the pressure inside the eyeball). Eye drops are given before some tests to dilate (enlarge) the pupil of the eye for a certain amount of time. This makes it easier for the doctor to see the back of the eye. Depending on the symptoms and any other medical conditions you may have, the eye examination might be carried out differently and additional tests might be done

Illustration: Structure of the eyeStructure of the eye

What do general eye examinations involve? 

Eye examinations may be necessary for various reasons, for instance due to external eye injuries or sudden vision problems. Vision also tends to get worse with age, so many people end up going to see an eye doctor at some point.

General eye examinations typically involve testing your vision and your pupillary response (how your pupils react). Pupils usually automatically contract and get smaller when light enters the eye.

The doctor first looks at the eye from the outside to see whether, for instance, the conjunctiva or eyelids are red and inflamed. The conjunctiva are the membranes lining the exposed part of the eyeball and inner surface of the eyelids. If your eye hurts, it is important to describe exactly where it hurts – e.g. on the surface of the eye, inside the eye or only when you move your eye? Eye pain can also radiate to your forehead or temples.

How is visual acuity measured?

If you aren't sure whether your vision has got worse, you can have your visual acuity (sharpness of sight) tested by an eye specialist. To do this eye test, one of your eyes is covered and you are asked to look at an eye chart with your other eye. The chart may have letters, numbers or C-shapes on it. These get smaller as you go down the chart.

Your visual acuity is measured by determining which of the figures you can still recognize from five meters away. The eye doctor will also check your near vision by holding a card with a similar chart on it at reading distance (about 30 centimeters). If necessary, this eye test can then be repeated using corrective lenses. Technically speaking, visual acuity is a measure of how well you can see two points as being separate. The closer together the points are, the more difficult this becomes.

How else can your vision be tested?

Vision is often assessed using other tests too. These include color vision tests, where you are asked to identify certain patterns in multicolored pictures. Eye movement and the position of your eyes relative to one another are important too. In children who have a squint, for example, the relative position of their eyes is abnormal due to different abilities of their left and right eye to see clearly. Tests to check your field of vision are also common.

What are visual field tests?

Your field of vision (or visual field) is what you can see without moving your eye. The test is done using a bowl-shaped instrument known as a perimeter. With your head supported by a chin rest, you focus on a fixed spot at the center of the bowl. Lights flash in the bowl, and may move around or become increasingly brighter. If you see a flash or notice a change, you press a button. The more areas in your field of vision are tested, the more accurate the diagnosis – but the test will then take longer as a result. 

Illustration: Perimetry test: Press a button everytime you see a flashPerimetry test: Press a button everytime you see a flash

Your field of vision is very important as it helps you know what is around you. Because various medical conditions affect the field of vision, perimetry tests are often done if damage to the retina or optic nerve is suspected, or if someone has headaches or dizziness for no known reason. Eye diseases like glaucoma and age-related macular degeneration also lead to changes in the field of vision. Although perimetry tests aren't painful, they can be quite tiring because you have to concentrate for a long time.

What are slit-lamp examinations?

A slit lamp is a microscope that projects a narrow, slit-shaped light. It is one of the main instruments used by eye specialists. You are asked to place your head on a chin rest. The doctor sits across from you and shines the light into your eyes, one at a time.

The intense line of light and various lighting options (direct, indirect, diffuse) enable the doctor to see most of the different areas of the eye magnified up to 60 times: the front, middle and back of the eye, and even as far back as the retina in the fundus of the eye. Modern slit lamps also include a device that measures intraocular pressure (the pressure inside the eyeball).

What is ophthalmoscopy?

A special device known as an ophthalmoscope can be used to look at the fundus (back) of the eye through the pupil and examine structures like the retina, optic nerve, blood vessels and the choroid coat. This examination is known as ophthalmoscopy or funduscopy ("fundus" is the Latin word for "bottom" or "base"). About 20 minutes before the examination, the pupils of the eyes are dilated (enlarged) using eye drops.

In the direct exam, the doctor positions the ophthalmoscope close to the eye and shines a beam of light directly into it to see a small magnified section of the back of the eye. This is particularly useful for examining the structures at the center of the eye's fundus, including the optic nerve, the blood vessels and the macula.

In the indirect exam, the doctor uses a condensing lens with a bright light to examine the eye from a distance of about 60 centimeters. This allows them to see a larger area of the fundus, providing a better overview as well as a three-dimensional image. A slit lamp is sometimes used at the same time. Doing so greatly magnifies the retina and provides better lighting.

Conditions such as retinal detachment and optic nerve damage can be diagnosed using ophthalmoscopy. This examination can also be a good idea in people who have diabetes or high blood pressure because both of these conditions can damage blood vessels in the eye.

If your pupils are dilated using eye drops, your eyes become very sensitive to light and your vision becomes blurred. For this reason, you are only allowed to drive a car or operate machinery again once the effect of the eye drops has worn off.

How is intraocular pressure measured?

The measurement of intraocular pressure (tonometry) is one of the most common eye examinations.  The pressure inside the eyeball becomes too high if a fluid produced in the eye (called aqueous humor) cannot drain properly and starts to build up. An intraocular pressure of between 10 and 21 mmHg (millimeters of mercury) is considered to be normal in adults.

Various instruments can be used to measure the pressure in the eye. In what is known as applanation tonometry, the cornea is first numbed using eye drops. Then a small measuring device is placed against the surface of the cornea, putting a little pressure on the eye.

The device measures how much pressure is needed to flatten the cornea by 3 millimeters. This is a measure of the pressure inside the eye.

Intraocular pressure can also be measured using an approach known as non-contact tonometry. Here the pressure is measured by applying a short puff of air to the cornea. Neither of these examinations are painful.

Further eye examinations

Other eye examinations can be done to confirm certain diagnoses and help plan treatment. For instance, the retina can be examined using fluorescein angiography. This involves injecting a fluorescent dye into a vein in the arm. The dye spreads through all the blood vessels in the body, including those in the retina. The doctor can then use a special camera to detect changes in the blood vessels of the eye.

Optical coherence tomography (OCT) is another type of eye examination. OCT uses a laser to measure things like the thickness of the retina. This technique can be applied to detect the build-up of fluid in the retina.