A direct ophthalmoscope is a hand-held instrument with various plus and minus lenses. The lenses can be rotated into place, en-abling the examiner to bring the cornea, lens, and retina into focus sequentially. The examiner holds the ophthalmoscope in the right hand and uses the right eye to examine the patient’s right eye. The examiner switches to the left hand and left eye when examining the patient’s left eye. During this examination, the room should be darkened, and the patient’s eye should be on the same level as the examiner’s eye. The patient and the exam-iner should be comfortable, and both should breathe normally. The patient is given a target to gaze on and is encouraged to keep both eyes open and steady.
When the fundus is examined, the vasculature comes into focus first. The veins are larger in diameter than the arteries. The examiner should focus on a large vessel and then follow it toward the midline of the body, which leads to the optic nerve. The cen-tral depression in the disc is known as the cup. The normal cup is about one third of the disc. The size of the physiologic optic cup should be estimated. Are the disc margins sharp, or are they blurred? Do the veins have a silvery or coppery appearance? The periphery of the retina can be examined by having the patient shift his or her gaze. The last area of the fundus to be examined should be the macula, because this area is the most light sensitive. The retina of a young person often has a glistening effect, which is sometimes referred to as a cellophane reflex.
The healthy fundus should be free of any lesions. The exam-iner should look for intraretinal hemorrhages, which may appear as red smudges or, if the patient has hypertension, may look some-what flame shaped. Lipid may be present in the retina of patients with hypercholesterolemia or diabetes. This lipid has a yellowish appearance. Soft exudates that have a fuzzy, white appearance (ie, cotton-wool spots) should be noted. The examiner looks for microaneurysms, which look like little red dots, and nevi. Drusen (ie, small, hyaline, globular growths), commonly found in mac-ular degeneration, appear to be yellowish areas with indistinct edges. Small drusen have a more distinct edge. The examiner should sketch the fundus and document any abnormalities.
The indirect ophthalmoscope is an instrument commonly used by the ophthalmologist. It produces a bright and intense light. The light source is affixed with a pair of binocular lenses, which are mounted on the examiner’s head. The ophthalmoscope is used with a hand-held, 20-diopter lens. This instrument enables the examiner to see larger areas of the retina, although in an un-magnified state.
The slit lamp is a binocular microscope mounted on a table. This instrument enables the user to examine the eye with magnifica-tion of 10 to 40 times the real image. The illumination can be varied from a broad to a narrow beam of light for different parts of the eye. For example, by varying the width and intensity of the light, the anterior chamber can be examined for signs of inflam-mation. Cataracts may be evaluated by changing the angle of the light. When a hand-held contact lens, such as a three-mirror lens, is used with the slit lamp, the angle of the anterior chamber may be examined, as may the ocular fundus
The ability to differentiate colors has a dramatic effect on the ac-tivities of daily living. For example, the inability to differentiate between red and green can compromise traffic safety. Some ca-reers (eg, commercial art, color photography, airline pilot, elec-trician) may be closed to people with significant color deficiencies. The photoreceptor cells responsible for color vision are the cones, and the greatest area of color sensitivity is in the macula, the area of densest cone concentration.
A screening test, such as the polychromatic plates discussed in the next paragraph, can be used to establish whether a person’s color vision is within normal range. Color vision deficits can be inherited. For example, red/green color deficiencies are inherited in an X-linked manner, affecting approximately 8% of men and 0.4% of women. Acquired color vision losses may be caused by medications (eg, digitalis toxicity) or pathology such as cataracts. A simple test, such as asking a patient if the red top on a bottle of eye drops appears redder to one eye than the other, can be an ef-fective tool. Changes in the appreciation of the gradations of the color red can indicate macular or optic nerve disease.
Because alteration in color vision is sometimes indicative of conditions of the optic nerve, color vision testing is often per-formed in a neuro-ophthalmologic workup. The most common color vision test is performed using Ishihara polychromatic plates. These plates are bound together in a booklet. On each plate of this booklet are dots of primary colors that are integrated into a background of secondary colors. The dots are arranged in sim-ple patterns, such as numbers or geometric shapes. Patients with diminished color vision may be unable to identify the hidden shapes. Patients with central vision conditions (eg, macular de-generation) have more difficulty identifying colors than those with peripheral vision conditions (eg, glaucoma) because central vision identifies color.
The Amsler grid is a test often used for patients with macular problems, such as macular degeneration. It consists of a geomet-ric grid of identical squares with a central fixation point. The grid should be viewed by the patient wearing normal reading glasses. Each eye is tested separately. The patient is instructed to stare at the central fixation spot on the grid and report any distortion in the squares of the grid itself. For patients with macular problems, some of the squares may look faded, or the lines may be wavy. Pa-tients with age-related macular degeneration are commonly given these Amsler grids to take home. The patient is encouraged to check them frequently, as often as daily, to detect any early signs of distortion that may indicate the development of a neovascular choroidal membrane, an advanced stage of macular degeneration characterized by the growth of abnormal choroidal vessels.
Lesions in the globe or the orbit may not be directly visible and are evaluated by ultrasonography. A probe placed against the eye aims the beam of sound. High-frequency sound waves emitted from a special transmitter are bounced back from the lesion and collected by a receiver that amplifies and displays the sound waves on a special screen. Ultrasonography can be used to iden-tify orbital tumors, retinal detachment, and changes in tissue composition.
Fundus photography is a technique used to detect and document retinal lesions. The patient’s pupils are widely dilated during the procedure, and visual acuity is diminished for about 30 minutes due to retinal “bleaching” by the intense flashing lights.
Fluorescein angiography evaluates clinically significant macular edema, documents macular capillary nonperfusion, and identi-fies retinal and choroidal neovascularization (ie, growth of ab-normal new blood vessels) in age-related macular degeneration. It is an invasive procedure in which fluorescein dye is injected, usually into an antecubital area vein. Within 10 to 15 seconds, this dye can be seen coursing through the retinal vessels. Over a 10-minute period, serial black-and-white photographs are taken of the retinal vasculature. The dye may impart a gold tone to the skin of some patients, and urine may turn deep yellow or orange. This discoloration usually disappears in 24 hours.
Tonometry measures IOP by determining the amount of force necessary to indent or flatten (applanate) a small anterior area of the globe of the eye. The principle involved is that a soft eye is dented more easily than a hard eye. Pressure is measured in milli-meters of mercury (mm Hg). High readings indicate high pres-sure; low readings, low pressure. The three most common types of tonometers are indentation, applanation, and noncontact. The procedure is noninvasive and is usually painless. A topical anes-thetic eye drop is instilled in the lower conjunctival sac, and the tonometer is then used to measure the IOP.
Gonioscopy visualizes the angle of the anterior chamber to iden-tify abnormalities in appearance and measurements. The gonio-scope uses a refracting lens that can be a direct or indirect lens. The indirect lens views the mirror image of the opposite anterior chamber angle and can be used only with a slit lamp. The direct gonioscopic lens gives a direct view of the angle and its structures.
Perimetry testing evaluates the field of vision. A visual field is the area or extent of physical space visible to an eye in a given posi-tion. Its average extent is 65 degrees upward, 75 degrees down-ward, 60 degrees inward, and 95 degrees outward when the eye is in the primary gaze (ie, looking directly forward). It is a three-dimensional contour representing areas of relative retinal sensi-tivity. Visual acuity is sharpest at the very top of the field and declines progressively toward the periphery. Visual field testing (ie, perimetry) helps to identify which parts of the patient’s cen-tral and peripheral visual fields have useful vision. It is most help-ful in detecting central scotomas (ie, blind areas in the visual field) in macular degeneration and the peripheral field defects in glaucoma and retinitis pigmentosa.
The two methods of perimetric testing are manual and auto-mated perimetry. Manual perimetry involves the use of moving (kinetic) or stationary (static) stimuli or targets. An example of kinetic manual perimetry is the tangent screen. A tangent screen is a black felt material mounted on a wall that has a series of concentric circles dissected by straight lines emanating from the cen-ter. It tests the central 30 degrees of the visual field. Automated perimetry uses stationary targets, which are harder to detect than moving targets. In this test, a computer projects light randomly in different areas of a hollow dome while the patient looks through a telescopic opening and depresses a button whenever he or she detects the light stimulus. Automated perimetry is more accurate than manual perimetry.
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