What Happens at an Eye Examination
This will vary depending on age, range of symptoms and reason for attending but a general pattern would be as below.
Core elements:
- Take your ocular, medical and family history and symptoms by asking questions, listening and then possibly asking more questions.
- Examine the outside and inside of your eyes- using an ophthalmoscope and possibly a slit lamp.
- Assess objectively the power of your eyes using a retinoscope.
- Fine tune your prescription subjectively using a test chart and check how your eyes co-ordinate.
- Normally for over 30 year olds we would measure the pressure in the eye using a tonometer.
- Fields of vision may be checked using field tests. Occasionally this shows an abnormality that requires further investigation with a more in-depth test, a further appointment may then be needed.
- Further tests as necessary depending on age / symptoms etc, sometimes you may be referred to your GP to arrange some of these.
Retinal Camera in use
We have a retinal camera, which takes a picture of the back of the eye and shows us a different view of the retina. This image would be saved on a computer to give us a very good record to refer back to in future. Experience of using this instrument over the last 2 years has shown that at times this will highlight changes that had not been noticed with careful ophthalmoscopy.
We offer a special glaucoma package that we consider to be of particular benefit to people at higher risk of developing glaucoma, please click on the word glaucoma to find out more.
Dilated 3D viewing of the back of the eye when necessary, as in diabetes.
We can perform a wide range of supplementary field tests to rule out, help diagnose or monitor certain conditions and check for suitability for driving.
The Ophthalmoscope in use
A hand held instrument that looks a bit like a torch. The optometrist will get close to you and shine a light into your eye. By looking along the same direction as the light we are able to view the structures inside the eye that have been illuminated. It has the advantage of being very portable / manoeuvrable but gives an illuminated area on the back of the eye that is only small although with good magnification. To get an idea of the whole of the retina you need to scan lots of times and then mentally piece the scans together.
The Slit Lamp in use
This is a binocular microscope tilted on its side. We illuminate the eye with a variable slit of light and are then able to see a 3D image of the eye. This instrument is used mainly to look at the outside of the eye (particularly in contact lens work) but can also be used to look inside the eye for cataracts and other problems where an enhanced view is required.
The Retinoscope in use
A hand held instrument that looks a bit like a torch. We shine a light at the eye from approximately half a metre and view the reflex that the eye returns. This reflex is very similar to the light you see shining from a cat’s eye when you catch one in your car headlights. As we move our light so the reflex changes and by putting the correct combination of lenses in front of the eye we get the reflex to behave in a particular fashion. From this combination of lenses we can work out the power of the eye. We then use this power as the starting point for our fine-tuning of the prescription when we ask you questions. When we are dealing with someone where this fine-tuning may be difficult (very young children, deaf and dumb patients etc.) we rely on the retinoscopic findings more fully.
A combination of figures and symbols that are normally illuminated
and help in our fine-tuning of your prescription, not all sections are used on all people. The main letter chart, known as the Snellen chart, enables us to quantify how small an image your eye can appreciate and record it in a commonly used form. It is from this chart that the often quoted expression of “six six” or “twenty twenty” vision comes from; it means the eye is able to read a line of letters of a particular size, usually near the bottom of the chart.
The circles and / or the fan at the top are used to tie down the astigmatism. The OXO with red lines tells us about the way the eyes co-ordinate.
There are various ways to measure the pressure inside the eye
(IOP) and the instrument used is called a tonometer. To measure the pressure you have to see how easy it is to indent the eye (it’s fairly easy to indent a tennis ball but more difficult to indent a golf ball because it is harder). The most commonly used form of tonometer in opticians these days is a ‘non contact tonometer’ where a puff of air is blown at the eye and the tonometer works out the IOP by monitoring the reflections off the front of the eye, it is this type of tonometer that we use mainly.
There is another type called an ‘applanation tonometer’
where a small prism is placed against the eye and an increased load is placed on it until a certain area of eye is flattened. This method is the norm in hospitals and is arguably a more medically accepted value. To use this method a drop of local anaesthetic and a yellow dye is placed in the eye and the tonometer can be hand held or is more commonly attached to a slit lamp.
The Non Contact Tonometer
The Perkins (Applanation) Tonometer and in use
The eyes see to the side of straight ahead and our visual field (the area we can see) can be measured. This measurement is done in various ways but the more modern ways tend to be computer led. The patient is asked to look at a central target and respond when they see an off centre target. These tests are very subjective (relies on the subject giving responses) and as the field tests become more detailed some patients find them more difficult and there is sometimes thought to be a lack of accuracy developing as the tests become more involved. To enable a quick evaluation of the visual fields most instrument have some screening programmes (a field screener) that will show up most defects. A fuller field test is then required to quantify the possible problem.
We mainly use two different types of field-testing equipment.
The Humphrey Frequency Doubling Technique (FDT) instrument is our main screening tool, it tests visual fields in a very different way from most other machines using what is called frequency doubling technology. This method of testing preferentially stimulates certain cells in the optic nerve that are thought to be the first to be damaged in glaucoma and has a particularly patient friendly screening test mode, quick yet easy to follow whilst still giving very useful information. The screening test takes less than one minute per eye and involves resting your forehead against the instrument and viewing a white screen on which a pattern of lines appear to flicker, when this is seen a button is pressed. The instrument calibrates itself for different ages and varies the test depending on the results recorded. If an abnormal result is found a fuller test is done on another instrument.
The Humphrey FDT machine
The main field-testing tool we use is the Humphrey Visual Field Analyser,
which is the instrument that is used in most hospital eye departments and is requested by the Driving Vehicle Licencing Authority when we undertake tests for them. This is a large self-illuminated bowl that is looked into at a central fixation spot. A spot of light is projected onto the bowl surface and a button is pressed when it is seen. The intensity of the stimulus light can be varied and allows a huge range of types of test of different complexities to be performed.
The Humphrey Visual Field Analyser
Our camera is described as a Digital Non-Mydriatic Camera. It gives an instantaneous picture of the back of the eye on a computer screen. We can manipulate the image to help use it as a diagnostic tool and can save it on the computer to give an absolute record of the state of the eye for future reference or for referral purposes via a print out or by e-mail. The non-mydriatic element of the title means that normally we are able to take an image without the need to dilate (enlarge) the pupil. As a patient the experience is totally painless and is just like having an ordinary photograph taken but with a much less bright flash.
Often changes at the back of the eye are very difficult to record
with words or a hand drawn sketch and to tell whether there have been subtle changes since the last time is notoriously difficult, particularly if someone else recorded their interpretation of what they saw at the time. This instrument is best at recording the central area of the retina and gives an overall view of the central 45 degrees of the retina rather than the smaller isolated areas that are seen with an ophthalmoscope. The image on the computer is ideal for showing you what we see in your eye and explaining its significance. Unfortunately if there is any appreciable cataract present it is unlikely to allow a good image to be captured
