THE EXAMINATION AT THE OPHTHALMOLOGIST

    When visiting an ophthalmologist, the physician will want to do a number of tests in order to get a proper picture of the condition of the eyes. Always take a companion and a pair of sunglasses with you to the ophthalmologist, for your eyes can be irritated and extra sensitive to light for some time after some tests. 

    MEASURING THE VISUAL ACUITY

    Determination of the visual acuity is mostly done using a letter card. This contains letters of various sizes. As this card was developed by Dr Snellen, this test is also called the Snellen method. You have to read the letters at a distance of 6 metres. With a decreasing visual acuity, you will only be able to read the larger letters. A visual acuity of 1/10 means that you can only read letters that are 10 times bigger than the letters that can normally be read at a distance of 6 metres. At the moment that even the biggest letters can no longer be read, the fingers can be counted. A person with a normal visual acuity will be able to do this at a 60 metres distance. If someone can do this at a distance of 4 metres, the visual acuity will be 4/60. By the time that counting the fingers is no longer possible, hand movements can be used, which are normally recognisable at a distance of 300 metres. If someone can see these at a distance of 2 metres, the visual acuity will be 2/300. Of course, the ophthalmologist will make sure that the decreased visual acuity is not caused by faults in the lens system of the eye. These can be corrected by glasses. 

    ELECTRICAL CHANGES CAUSED BY LIGHT (ERG)

     ERG is an abbreviation of Electro-Retino-Gram. This is an image written on paper of electrical currents that are created in the retina and go through the retina under the influence of a light stimulus. Like an EEG, an Electro-Encephalo-Gram, can measure the electrical current running through the brain at the scalp and it is also possible to measure the electrical current running through the eye on the outside of the eye. This is done with the help electrodes. Such an electrode usually is a kind of large contact lens. 

    This contact lens will be put onto the eye after the cornea has been anaesthetised by means of eye drops. The contact lens is transparent and by means of a wire connected with the ERG device. Now light stimuli can be offered. This can be done by making a patient look at a lamp or a television screen. The light stimuli vary with respect to shape, light intensity and speed of offering successive light stimuli. Sometimes the colour of the light stimulus varies. 

    Before the start of the actual measurement, the patient first has to sit in a dark room for some time. The retina completely calms down in the dark. In a calm retina a light stimulus will cause a stronger current and this is necessary. This involves very small electrical signals sent by the eye that are really difficult to record. By offering the same light stimulus a few times in succession and adding up the results using a computer, the ERG device can accurately show the electrical currents. 

    Dropping out of mainly cones and dropping out of mainly rods each results in its own typically changing ERG registration. In this way the ERG makes an important contribution to the research into Usher Syndrome of retinitis pigmentosa. Because of its great sensitivity, the study makes it possible to detect changes at a stage of the course of the disease in which no deviations are noticeable in the eyes. Even at a stage that there are not even any complaints! 

    EXAMINATION WITH THE SLIT LAMP

    A slit lamp helps the ophthalmologist to very accurately examine the eyes. The eyes should not move too much during this examination. Therefore you will be asked to put your head in a bracket with the chin resting on a support and the forehead resting against a band. This places the eyes exactly in the right position to be examined. 

    The ophthalmologist will look through a microscope to the eye that is lit by a very narrow beam of light. This narrow beam of light comes from a slit-shaped light source, which explains the name of this instrument. The enlargement of the image by the microscope and the special way of lighting enable the ophthalmologist to take a good look at the cornea, the iris and the lens. Deviations, such as clouding of the lens, will be clearly visible. Clouding of the lens (cataract) often occurs with people suffering from Usher Syndrome or retinitis pigmentosa. Regular checks by an ophthalmologist are useful, as cataract can be remedied by surgery. 

    ADDITIONAL EXAMINATION

    Usually the additional examination cannot take place in the examination room of an ophthalmologist right away. This requires really expensive special equipment that is often available in academic hospitals only. The additional examination has been divided into two categories: one part of the examination tests the functions of the visual system, whereas another part measures the electrical changes caused by various light stimuli in the visual system. 

    THE FIELD OF VISION TEST

    The result of the field of vision test of the left eye. The black squares indicate which part of the field of vision is not observed by the person examined. The central field of vision is limited to about 10 degrees. 

    The field of vision is the part of our surroundings that we can see without moving the eyes or head. A special instrument has been developed for determining this function of the retina. In front of the head support there is a large half sphere with the concave inner side facing the head support. You have to look at a point in the centre of the sphere. The eyes are tested separately. The test light will appear somewhere on the inside of the sphere. As soon as you see the light, you say ‘yes’ (or press a button). In this way the entire retina is tested with the various test lights spread across the inside of the sphere and any drop-out or decrease of the functioning of the retina can be mapped out. 

    O.C.T.-scan

    O.C.T. stands for Optical Coherence Tomographu. It is a way to better image and capture the retina or optic nerve. The image is obtained using an infrared light beam. The patient receives eye drops that make the pupils wider. During the examination you sit behind a camera with your chin on a support. The patient looks at a flashing light with one eye. The ophthalmologist then takes pictures of your eyes with infrared light.

    THE EXAMINATION WITH THE EYE MIRROR

    Thanks to the eye mirror the ophthalmologist can look at the retina in the eyes through the pupil. In order to be able to see as much as possible, it is necessary to make and keep the pupil as large as possible. Special drops have such an effect on the iris, that the pupil becomes large and prevent the pupil from getting smaller again, despite the bright light of the ophthalmologist. This bright light comes from a lamp in the grip of the eye mirror and shines into the eye by means of a mirror. The drops do not only have an effect on the size of the pupil, but also on the muscle that varies the refraction power of the lens. By disabling this, reading will be difficult or impossible for some time after the application of the drops. It is advised to wear sunglasses after this examination. 

    Usually the colour of the papilla is yellow-red. Drop-out of the rods and cones goes hand in hand with a change of the nerve fibres of the dropped out cells and this is noticeable as a change of colour of the papilla. Initially, the papilla becomes paler and in extreme cases eventually as pale as wax. From the papilla the blood vessels are spread across the retina. Sometimes the blood vessels are narrower than they usually are. This is, just like the disappearance of nerve fibres, another consequence and not a cause of the disease. Then the ophthalmologist will pay attention to the retina. By making the eye of the patient look in all directions, an image can be obtained from the entire retina through the widened pupil. The typical pigment accumulations, which have given retinitis pigmentosa its name, will become visible. The pigmentations form black-brown fanciful figures, often compared with crow’s feet or trabeculations. 

    ADAPTATION TO THE DARK

    The adaptation to the dark is a measure for the speed with which the retina can adjust itself to the various light intensities in the surroundings. Everyone knows that when stepping from a brightly lit room into a dark room, you initially do not see anything. However, the objects in the dark room will slowly but surely become visible. The time that the eye needs to adjust itself as well as the smallest amount of light that one can still see in the dark are measured by the test of adaptation to the dark.
    The examination is done with the help of a similar sphere as with the field of vision examination. Again, one eye at a time, the head in the support and continuously looking at a small light in the centre of the sphere. Initially, the inside of the sphere will be brightly illuminated. Then the light in the sphere is turned off (but not the central light that you are to look at). You step as it were into a dark room. Now a test light is offered, just next to the central light. You have to indicate again whether or not you see the light. Just after the light was turned off a bright light is required to make you see it. The longer it has been dark, the faster you will be able to see a weaker light. The eye adjusts itself to the dark. Although the test light will be weaker and weaker, it will still be seen until the limit is reached of the part of the retina that is being examined. 

    Seeing in the dark depends on the functioning of the rods. With people suffering from Usher Syndrome and retinitis pigmentosa, whose rods have been affected, the result of this test of adaptation to the dark will probably be upset. Both the speed with which the adaptation to the dark takes place and the eventually reached limit value will be deviating. The adaptation to light/dark will take place much slower and the light intensity of the test light that will eventually be just observed is higher, which is to say that the tested part of the retina is less sensitive. People with an upset test for adaptation to the dark will usually suffer from night-blindness 

    Retina with pigment accumulation