Diagnosis
Diagnosis of eye disorders is initially based on the appearance of the eyes and on the symptoms that the person is experiencing. A variety of tests can be carried out to confirm a problem or to determine the extent or severity of the disorder.
Refraction
Refraction is the procedure by which focusing error is assessed. Problems with visual acuity (sharpness of vision) that result from refractive errors, such as nearsightedness, farsightedness, astigmatism, and presbyopia, are diagnosed by refraction. Acuity is usually measured on a scale that compares a person's vision at 20 feet with that of someone who has perfect vision. Thus, a person who has 20/20 vision sees objects that are 20 feet away with the same clarity as a person with normal vision, but a person who has 20/200 vision sees at 20 feet what a person with perfect vision sees at 200 feet. One important visual acuity test uses the Snellen chart (eye chart), which is a large card or lighted box that displays rows of letters in smaller and smaller sizes. The card is read from a standard distance. The degree of visual acuity is determined by the size of the row of letters that the person can read. For those who are unable to read, a modified chart can be used in which the letters are represented by an upper case "E," which is rotated randomly. The person is asked to describe the way the letter is facing.
Automated refraction is performed with machines that determine the refractive error of the eye by measuring how light is changed when it enters a person's eye. A person sits in front of the autorefractor, a beam of light is emitted from the device, and the eye's response is measured. The machine uses this information to calculate the lens prescription needed to correct the person's refractive error. This measurement takes only a few seconds.
A phoropter is the device commonly used, in conjunction with a Snellen chart, to allow determination of the best corrective lenses for a person being assessed for eyeglasses or contact lenses. The phoropter contains a complete range of corrective lenses, allowing the person to compare different levels of correction while viewing the chart. Typically, the eye doctor will use the phoropter to refine the information obtained from the autorefractor before prescribing lenses.
Visual Field Testing
The visual field is the entire area of vision that one sees out of each eye, including the corners (peripheral vision). The visual field is often tested as a regular part of an eye examination. It may also be tested if a person notices specific changes in vision, for example, if he keeps bumping into objects on one side. The simplest way to test peripheral vision is for a doctor to face the person and gradually move a finger from the left and the right at face level in toward the center of vision. The person tells the doctor when the moving finger is first detected. The person must fix his vision on the doctor's face (and not look for the finger) in order for the result of the test to be valid. Each eye is tested separately.
The visual field may be measured more precisely with a "tangent screen" or a Goldmann perimeter. With these tests, the person stares at the center of a black screen or a hollow, white, spherical device (which resembles a small satellite dish). An object or a light is moved slowly from the periphery toward the center of vision from many different directions. The person indicates when he first sees the light out of the corner of his eye. The doctor places a mark on the screen or perimeter indicating where the person can see, thus allowing recognition of blind spots. Visual fields can be measured using computerized automated perimetry. Here, the person stares at the center of a large shallow bowl and presses a button whenever he sees a flash of light.
The Amsler grid is used to test the central area of vision. The grid consists of a black card covered with a white grid and with a white dot in its center. Looking through one eye, the person notes any distortion in the lines of the grid, while staring at the white dot. Each eye is tested separately at a normal reading distance and while using reading glasses if the person normally uses them. If a person cannot see an area of the grid, an abnormal blind spot may exist. (There is a normal but very small blind spot in the area where the optic nerve leaves the eye; however, people are not aware of it.) Wavy lines suggest a possible problem with the macula. The test is simple enough to be used by people at home and is useful for monitoring macular degeneration.
Color Vision Testing
A variety of tests can be used to detect a reduced ability to perceive certain colors (color blindness). The Ishihara plates, which are most commonly used, are patterns of small, colored circles crowded together on a white background to form a large circle. The small circles are usually arranged so that people with normal color vision see a particular number. Those who have color blindness see another number or no number, depending on the type of color blindness.
Ophthalmoscopy
A direct ophthalmoscope is a handheld device like a small flashlight with magnifying lenses that shines a light into the eye to enable a doctor to examine the cornea, lens, and retina. The person looks straight ahead as the beam of light is shone into the eye. Often, eye drops are given to dilate the pupil, which allows the doctor to have a better view. Ophthalmoscopy is painless, but if eye drops are used to dilate the pupils, vision may be blurred and the person will be more sensitive to light for a few hours afterward.
 See the figure What Is an Ophthalmoscope?
Ophthalmoscopy is a standard part of every regular eye examination. Ophthalmoscopy is useful to detect not only changes in the retina due to eye disease but also changes due to certain diseases affecting other parts of the body. For instance, it is useful to detect the changes that occur in the retinal blood vessels in people with high blood pressure, arteriosclerosis, and diabetes mellitus. Ophthalmoscopy can also be used to diagnose elevated pressure within the brain, which results in a swelling (pushing-out) of the normally cupped optic disk (papilledema). Tumors on the retina can be seen with ophthalmoscopy. Macular degeneration can be diagnosed with ophthalmoscopy as well.
Sometimes the doctor uses an instrument called an indirect ophthalmoscope, in which a binocular device is placed on the doctor's head and a handheld lens is used in front of the person's eye to focus the image inside the eye. This method gives a three-dimensional view, allowing a better view of objects that have depth, including a detached retina or a swollen optic disk. It also allows a brighter light source to be used, which is important if the interior of the eye is cloudy, for instance due to infection or cataract. The indirect ophthalmoscope also allows a much wider field of view than a regular ophthalmoscope, so that the doctor can examine more of the retina at once.
Slit Lamp Examination
 See the figure What Is a Slit Lamp?
The slit lamp is a table-mounted binocular microscope that shines a light into the eye to allow the doctor to examine the entire eye under high magnification. The slit lamp has better optics than the ophthalmoscope, providing magnification and a three-dimensional view, which allows measurement of depth. Often, eye drops are used to dilate the pupils so that the doctor can view even more of the eye, including the lens, vitreous humor, retina, and optic nerve. Sometimes, in people with suspected or known glaucoma (see Section 20, Chapter 233), an additional lens is placed on or held in front of the eye to allow examination of the "angle" between the iris and the front part of the eye (inside surface of the cornea). This examination is called gonioscopy.
Tonometry
With tonometry, the pressure of the aqueous humor within the eye can be measured. The aqueous humor is the fluid in the front part of the eye. Normal pressure within the eye is 8 to 21 millimeters of mercury (mm Hg). Pressure in the eye is measured to detect certain types of glaucoma and monitor its treatment.
The noncontact ("air-puff") tonometer is used to screen for elevated pressure in the eye. This device is not highly accurate, but it is useful in screening. A small puff of air is blown against the cornea, which causes the person to blink but is not uncomfortable. The puff of air flattens the cornea, and the device measures the time (in thousandths of a second) it takes to flatten the cornea. It takes less time for the puff of air to flatten the cornea in an eye with normal pressure than it does an eye in which pressure is elevated.
Portable, handheld instruments are also used for tonometry. Eye drops that contain a drug to numb the eye are given, then the instrument is gently placed on the cornea, and a reading is obtained. Portable tonometers can be used in the emergency department or a doctor's office to quickly detect increased pressure in the eye.
Applanation tonometry is a more accurate method. The applanation tonometer is usually attached to a slit lamp. After numbing the eye with drops, the instrument is gently moved until it rests upon the cornea, while the doctor observes the cornea through a slit lamp. The amount of pressure it takes to indent the cornea is related to the pressure within the eye.
Fluorescein Angiography
Fluorescein angiography allows a doctor to clearly see the blood vessels at the back of the eye. A fluorescent dye, which is visible in blue light, is injected into a vein in the person's arm. The dye circulates throughout the person's bloodstream, including the blood vessels in the retina. Shortly after the dye is injected, a rapid sequence of photographs is taken of the retina. The dye inside the blood vessels fluoresces, making the vessels stand out. Fluorescein angiography is particularly useful in the diagnosis of macular degeneration, blocked retinal blood vessels, and diabetic retinopathy.
Electroretinography
Electroretinography allows a doctor to examine the function of the photoreceptors in the retina by measuring the response of the retina to flashes of light. Eye drops numb the eye and dilate the pupil. A recording electrode in the form of a contact lens is then placed on the cornea and another electrode is placed on the skin of the face nearby. The eyes are then propped open. The room is darkened and the person stares at a light source that emits flashes of light. The electrical activity generated by the retina in response to the flashes of light is recorded by the electrodes. Electroretinography is particularly useful for evaluating diseases, such as retinitis pigmentosa, in which the retina or photoreceptors are affected.
Ultrasound
The eye can be examined by ultrasound. A probe is placed gently against the closed eyelid and painlessly bounces sound waves off the eyeball. The reflected sound waves produce a two-dimensional image of the inside of the eye. Ultrasound is useful when an ophthalmoscope or slit lamp cannot show the retina because the inside of the eye is cloudy or something is blocking the line of sight. Ultrasound can also be used to determine the nature of abnormal structures, such as a tumor, inside the eye. Ultrasound also can be used to examine blood vessels supplying the eye (Doppler ultrasound) and to determine the thickness of the cornea in pachymetry.
Pachymetry
Pachymetry (measuring the thickness of the cornea) is usually carried out using ultrasound. Accurate measurement of the thickness of the cornea is very important in refractive eye surgery, such as LASIK (see Section 20, Chapter 226).
For ultrasound pachymetry, a drop of anesthetic is placed in the eye, and an ultrasound probe is placed gently onto the surface of the cornea. Optical pachymetry methods do not require anesthetic eye drops because the instruments do not touch the eye.
Computed Tomography and Magnetic Resonance Imaging
These imaging techniques can be used to provide detailed information about the structures inside the eye and the bony structure that surrounds the eye (the orbit). Computed tomography (CT) is particularly useful to locate foreign bodies inside the eye.
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