Much ophthalmic disease has been described and classified using the microscope. In spite of this, many of the important eye diseases can be diagnosed using a hand magnifier and an ophthalmoscope. At this point, it is important to understand the principle of examining the eye with a focused beam of light. If a pencil of light is directed obliquely through the cornea and anterior chamber, it can be made to illuminate structures or abnormalities that are otherwise invisible. One might inspect the glass sides and water of a fish tank using a strong,focused torch in the same manner (Figure 3.7). Many ophthalmoscopes incorporate a focused beam of light
that can be used for this purpose. A magnified image of the anterior segment of the eye can be viewed with a direct ophthalmoscope held about 1/3m away from the eye through a +10 or +12 lens. The principle has been developed to a high degree in the slit-lamp (Figure 3.8). This instrument allows a focused slit of light to be shone through the eye, which can then be examined by a binocular microscope. By this means, an optical section of the eye can be created. The method can be compared with making a histological section, where the slice of tissue is made with a knife rather than a beam
of light. The slit-lamp is sometimes called the biomicroscope. By means of such optical aids, the cornea must be carefully inspected for scars or foreign bodies. The presence of vascular congestion around the corneal margin might be of significance. Closer inspection of the iris might show that it is atrophic or fixed by adhesions. Turbidity or cells in the aqueous might be seen in the beam of the inspection light. The lens and anterior parts of the vitreous can be examined by the same means.
Once the anterior segment of the eye has been examined, the intraocular pressure is measured. The "gold-standard" method of measurement is to use the Goldmann tonometer (Figure 3.9), which relies on the principle of "applanation". In essence, the application of this principle provides a derived measurement of intraocular pressure by flattening a small known area of cornea with a variable force. The amount of force required to flatten a specific area is proportional to the intraocular pressure reading, and this is
read from a dial. The readings provided by this measurement are highly reproducible and are given in millimetres of mercury (mmHg).
Some optometrists, however, employ "airpuff" tonometers, which are more portable and do not require attachment to a slit-lamp. These instruments are excellent for screening but are generally not as accurate as applanation tonometers. A convenient hand-held instrument (the Tonopen) is available (Figure 3.10) and is commonly used by ophthalmologists when a slit-lamp is not available.
At this stage, the pupil can be dilated for better examination of the fundi and optical media. A short-acting mydriatic is preferable, for example tropicamide 1% (Mydriacyl). These particular drops take effect after 10 min and take 2-4h to wear off. Patients should be warned that their vision will be blurred and that they will be more sensitive to light over this period. Most people find that their ability to drive a car is unimpaired, but there is a potential medicolegal risk if the patient subsequently has a car accident. Once the pupils have been dilated, the eye can then be examined with the ophthalmoscope.
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