Common glaucoma tests

Intraocular pressure test

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 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).  Your doctor will review the pressure measurements in conjunction with other testing results to assess whether to treat suspected glaucoma.

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.

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.

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
A nerve fiber layer analyzer is a computerized camera that provides a graphic and statistical view of a patient’s optic nerve.  As noted above, 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 analyzers used at Midwest Eye Care are called OCTs.

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 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 glaucomatous changes.