The following tests are commonly used to diagnose and monitor eye conditions:
The pressure within the eye is measured with a tool called a tonometer (tuh-nom-uh-ter). A tonometer may either be non-contact (aka the ‘puff test’) or come into contact with the eye; the contact tonometer provides a more reliable reading and is used at all Midwest Eye Care offices.
To perform this test, the doctor or technician will instill a mild anesthetic drop and then ask the patient to rest his or her chin on a chin rest and look straight ahead. The doctor or technician will slowly move the dimly-lit tonometer towards each eye until it makes gentle contact with the eye. This procedure is not painful but the sensation of something touching the eye may cause a patient to blink; if so, a technician or doctor will simply hold the patient’s eyelids in place for the test. This pressure reading typically takes less than 30 seconds per eye.
A normal intraocular pressure is considered to be between 10 and 21 mm HG (millimeters of mercury), but ‘normal’ may vary based on your corneal thickness. Your doctor will review the pressure measurements in conjunction with other testing results to assess whether to treat suspected glaucoma.
A refraction is the test that is performed to determine your eyeglass prescription. A refraction may be performed by either the doctor or a technician, and typically involves questioning along the lines of, “Is 1 better than 2?” Medicare and many other insurance plans consider a refraction to be routine medical care not covered under their medical coverage, so this amount is charged separately and is paid directly by the patient. Medicare secondary insurance plans will also not pay the charge since it is not a Medicare-covered service, so the $35.00 fee is to be paid by the patient.
A pachymeter (puh-kim-u-ter) is used to measure the thickness of the cornea. The end of a pencil-like wand is placed on the surface of the eye at several different points to record an average corneal thickness. The procedure is performed on each eye and typically takes less than thirty seconds per eye.
Similar to the B-scan machine, a diagnostic A-scan is an ultrasound machine. While the B-scan provides a view of the cross-section of the eye, the A-scan provides a view of the top section. The diagnostic A-scan allows the technician to calculate the size of a lesion within the eye.
A B-scan is an ultrasound machine used to view the internal structures of the eye. With the B-scan, the operator can see a cross-section of the eye. It is used when, during a dilated exam, the physician cannot obtain a clear view of the patient’s retina or other internal structures due to bleeding, a dense cataract, corneal cloudiness or lesions. It can also be used to locate and follow cancerous tumors and other abnormalities.
In order to perform a B-scan, an anesthetic drop is applied to the surface of the eye. While the patient is reclined in an exam chair, a technician places a probe on the surface of the eye and moves it to view the rear of the eye. Photographs may be taken using the probe.
Fluorescein angiography is a common test used to diagnose and monitor the impact of diabetic retinopathy and macular degeneration. Fluorescein angiography is a diagnostic procedure that uses a special camera to take a series of photographs of the retina, the light sensitive tissue in the back of the eye. The photographs are transmitted to a computer, allowing your doctor to view a series of digital retinal images.
To prepare for a fluorescein angiography test, you will receive dilating drops to expand the size of your pupils, allowing for an easier view of the back of your eye. A nurse will then inject a vegetable-based dye (fluorescein) in your arm. The dye travels through the veins and into the arteries that circulate throughout the body. As the dye passes through the blood vessels of the retina, a technician uses a special camera to take approximately two dozen photographs of the retina.
The main purpose of this test is to reveal the condition of the blood vessels within the retina. If the blood vessels in the retina are abnormal, the photographs will show dye leaking into the retina or staining the blood vessels. Damage to the lining underneath the retina or the appearance of abnormal new blood vessels growing beneath the retina may also be revealed. The precise location of these abnormalities can be determined by a careful interpretation of the fluorescein angiogram by your ophthalmologist. The ophthalmologist uses this information to determine whether additional monitoring, laser procedures or injections are warranted.
There are some risks associated with fluorescein angiography, but the diagnostic benefits of the tests overwhelmingly outweigh these risks. After the dye is injected, your skin may turn yellowish for several hours. This color disappears as the dye is filtered out of the body by the kidneys. Because the dye is removed by the kidneys, your urine will turn bright yellow for up to 48 hours following the test.
Some individuals may experience slight nausea during the procedure, but this usually passes within a few seconds. If the dye leaks out of a fragile vein during the injection, some localized burning and yellow staining of the skin may occur. This burning usually lasts only a few minutes and the staining will go away in a few days.
Allergic reactions to fluorescein dye are rare. If they occur, they may cause a skin rash and itching. This is usually treated with oral or injectable antihistamines, depending on the severity of the symptoms. Even more rarely, severe allergic reactions (anaphylaxis) can occur and be life threatening.
Goldman visual field test
The Goldman visual field machine is operated manually by a test technician to measure a patient’s central and peripheral (side) vision. The machine is more time intensive than a Humphrey visual field test from both a patient and technician standpoint since a computer is not utilized. The test takes approximately 30 minutes to complete, and is primarily used for ptosis (droopy lids) evaluations and driver’s license visual field exams.
Humphrey visual field test
The visual field is the entire area that a person can see when the eye is focused on a central point. It includes central and peripheral (side) vision. Changes in the visual field may be difficult to detect since one eye may compensate for visual field loss in the other eye. This compensation is one reason why visual field tests are performed separately on each eye.
The most common reason to have a visual field test is because of glaucoma or suspected glaucoma. Other indications include droopy lids and other less common but serious eye conditions.
Most visual field tests incorporate computerized machines that require the patient to stare at a fixed point in the middle of a large domed area. A computer program will flash small lights in different locations on the surface of the domed area. While staring straight ahead, the patient is asked to press a button when he sees the small lights in his peripheral vision. The computer summarizes the patient’s responses and prints a graphic interpretation of the patient’s visual field. While just one visual field test can be extremely helpful in identifying visual field loss, a series of tests over a period of years allows your doctor to assess whether visual field loss is stable or progressing. Visual field tests will take between 20 and 45 minutes depending on the level of test ordered by the doctor.
The picture in this section shows the results of a visual field test; the black shading in the upper section of the results indicates that this patient has very little vision remaining in the top half of their visual field. Visual field loss cannot be reversed, which is why patients with glaucoma need to be monitored by an ophthalmologist or optometrist for the rest of their lives.
Nerve fiber layer analyzer (OCT)
A nerve fiber layer analyzer is a computerized camera that provides a graphic and statistical view of a patient’s optic nerve. Glaucoma progression can permanently damage the optic nerve, so the ability to photograph the nerve and analyze it is quite useful to eye doctors. In particular, the camera measures the depth of the nerve fiber layer; a thinning of the layer over time will indicate to the doctor that the glaucoma is progressing. The nerve fiber layer analyzer used at Midwest Eye Care is called an OCT (optical coherence tomographer).
The OCT machine is also a useful diagnostic tool for visualizing macular holes and macular edema (swelling).
The OCT test procedure is quite brief and painless. The patient sits in front of the camera, and the camera scans the eye for approximately five minutes each. The patient will focus on a fixation light within the camera and the technician will provide any additional directives.
Optic disc (fundus) photos
Optic disc photos are performed with a specialized camera. After instilling dilating drops, the technician waits several minutes before taking pictures of the optic disc (optic nerve). Optic disc photos are a helpful to the doctor when he or she has a series of photos taken over a period of several years; in this way, the doctor can determine if damaging changes are taking place in the optic disc.
The nerve fiber layer analyzer provides similar photographic capabilities, but some doctors continue to order optic disc photos because they place greater trust in this ‘tried and true’ technology or because they do not have access to a nerve fiber layer analyzer. At Midwest Eye Care, your ophthalmologist or optometrist will determine which approach is best suited to track changes due to glaucoma, macular degeneration or diabetes.
Pre-operative tests for cataract surgery
Depending on the location a patient is seen, the patient will have either an axial length or IOLMaster test prior to cataract surgery. Both tests measure (in millimeters) the length of the ye from the front surface to the retina. With this measurement, the surgeon can calculate the exact power and type of the artificial lens to insert when a cataract is removed.
During an axial length test, ointment is applied to the eye and the tip of an instrument wand comes into contact wit the eye. Several measurements are made from different points of the eye. The patient either reclines in a chair or may lay down on a bed for the test.
The IOLMaster machine is similar to the OCT machine. The patient sits in front of the machine’s camera and rests his or her chin on a chin rest. The camera measures the eye using several different scanning approaches, and the measurement for each eye is completed in approximately five minutes.