Vision testing involves making a person read standard sized letters at a specified distance. The doctors record the vision as a fraction eg. 6/6 or 20/20. The top number denotes the distance (in feet or meters) at which the patient has been able to read the particular sized letter while the bottom number indicates the distance at which a normal person is expected to read the same letter. Near vision is tested separately in good illumination using special test charts held at normal reading distance. The testing is done with each eye separately. The doctors often test the vision using a pinhole. This gives an estimate of improvement possible with glasses. The pinhole cannot be used by the patient in place of glasses.
This is an important test that is done by the ophthalmologist or more often by the optometrist. The eye is like a camera. The light rays are focused on to the light sensitive film in the back of the eye called the retina. This focusing is made possible by the cornea (a clear watch glass like structure in the front of the eye) and by the lens in the eye (similar to the lens of a camera). Refraction is done usually in the normal state. On occasion (especially in children) it may have to be done using special eye drops (cycloplegics). In this situation one may have to retest the power of the required glasses 2-3 days after the testing with the use of drops. Refraction involves two parts. The first part is objective where in the refractionist estimates the power needed by using a test called retinoscopy. This test can also be done with a machine called the automatic refractometer (so called computer testing). However one still needs to do the all important subjective testing (i.e testing the response of the patient with different powered glasses) before prescribing the glasses. Hence do not be misled by the so called computer testing.
AMSLER GRID TESTING
This test is done in selected group of patients depending upon their symptoms. The test involves looking at a chart that has a grid drawn with a central dot. The test is done using the near vision glasses (if one is using the same). The test permits the evaluation of function of the central 20 degrees of the retina. The patient is asked to look at the central dot and tell whether all the corners of the chart are seen, whether all the lines are seen straight and not crooked. Are there any areas of grey patches where the lines are not seen. Whether the central part of the chart or the peripheral part of the chart is clear. The Amsler’s chart is very useful as a home monitoring device. If any defect is noted, immediate ophthalmologic examination is called for.
One of the most common procedures that is done in an eye specialist’s office is dilatation. The pupils of the eye constrict or dilate depending upon the light that thrown at the eye. For examining the back part of the eye (fundus), the doctor uses an instrument called ophthalmoscope. To get a good view of the back of the eye, one needs to dilate the pupils. This permits more light to enter the eye and gives a better image of the fundus. To keep the pupils dilated despite the intense light, one needs to dilate the pupils. There are various types of dilating drops available. The faster acting ones may dilate the pupil in 15-20 minutes time. Other variety of drops may take up to 30-45 minutes for good dilatation. The effect of dilatation usually lasts up to 6 hours. Some of them may retain the effect for 24 hours. Usually the drops used for routine eye examination do not have long lasting effect. A patient is expected to have glare in the sun light while still under the effect of the drops. Hence driving may become difficult. If you had similar dilatation in the past and have been noted to be allergic to any one of them, please inform the same to your doctor
SLIT LAMP EXAMINATION
Slit lamp is an instrument that has an in built microscope and a bright illumination system. The special arrangement of the light and the microscope allows the doctor to view the eye in great detail under high magnification. The front part of the eye is examined without any other aids while the back part of the eye (fundus) is examined with help of special lenses held in front of the eye ball.
TONOMETRY (EYE PRESSURE CHECK)
Tonometry involves the check of the pressure of the eye. Normal pressure is a range and not a finite number. Raised pressure in the eye can be harmful to the nerve connecting the eye with the brain (optic nerve). The pressure is normally checked in one of the following ways.
Here a metallic device is placed on the eye ball and the deflections of the needle on the scale is used as a guide to measure the eye pressure. At the best, this modality of testing can be used only as screening device.
In this a small prism mounted on the slit lamp is used to contact t
Eyeball and measure the pressure. This modality of testing is more accurate and is the standard today.
A drop of anesthetic is placed on the eye before these tests are carried out. The test is totally painless.
Gonioscopy is the test in which the angle of the eye is examined. The eye pressure is maintained by a fine balance between the inflow of fluid and its outflow. The outflow is through the angle of the eye. Studying this angle gives a lot of insight into the cause of a condition called glaucoma. This test is done with the help of the slit lamp and a special lens called gonioscope. The eye is anesthetized by placing a few drops of anesthetic to facilitate placement of the lens. This test totally painless.
This is a very important step in the total examination of the eyes. The visible portion of the eye is easily examined by the slitlamp examination. The back portion of the eye can only be examined by using the ophthalmoscope. This step usually needs dilatation of the pupils. This test involves throwing bright light into the eye and examining the image of the back of the eye using special lenses. For Indirect opththalmoscopy, the patient has to be in the reclining position for proper examination. Sometimes the slit lamp may be used for detailed evaluation of the areas of the back of the eye such as macula, optic disc etc.