Diagnostic Evaluation
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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