OPTICAL COHERENCE TOMOGRAPHY RESULTS

Ophthalmologist Carolina Valdivia, MD discusses optical coherence tomography (OCT), a test used in the diagnosis of glaucoma and other eye diseases.

Dear Dr. Valdivia,

For the past three months, I have been experiencing a problem of high eye strain. I was examined by two different eye doctors and both told me that everything was fine. My situation did not improve, so I visited a third eye doctor. He tested my eye pressures on two separate occasions and found them to be average: 20 (right) and 21 (left) for the first day, and 18 (right) and 19 (left) for the other day.

The doctor sent me for an OCT test. Results indicated that my cup is not within normal limits.

• Cup/Disk Area ratio OD is 0.70 and OS is 0.77
• Rim area (mm2) OD is 0.41 and OS is 0.39
• Cup Area (mm2) OD is 0.94 and OS is 1.27
• GLV % OD is 10.002 and OS is 14.265

The remaining parts of the OCT test were normal.

I have been myopic for last 20 years. My corrective lens prescription is -3.5 and -2.75/0.5.

Can you help me to make sense of all of this?



Thank you for your question and for your interest in glaucoma-eye-info.com.

You are experiencing eye strain and have asked me to help you to understand the results of your eye examination. I will begin with the test results.

Glaucoma is characterized by damage to the cells and fibers of the optic nerve. These nerve fibers are responsible for bringing information received by the retina to the visual center of the brain.

Optical Coherence Tomography (OCT) is a technique for obtaining high-resolution cross-sectional images through the thickness of the retina. A cross section is a cut that is perpendicular to the longitudinal axis of a solid object. Put simply, potato chips are cross-sectional slices of potato, whereas french fries are longitudinal cuts to the vegetable.

In ophthalmology, the purpose of OCT is to visualize and study a structure known as the retinal nerve fiber layer. It is a very important tool in the diagnosis of glaucoma and other eye diseases.

An optical coherence tomography test enables me to obtain a highly sensitive measurement of the quantity of nerve fibers present in the retinal nerve fiber layer. A decreased number is evidenced by the appearance of an optic disc excavation – the deeper the excavation, the presumed greater the loss of nerve fibers. But this is not the most important information generated by an OCT study.

Optical Coherence Tomography works in conjunction with computer software to compare the results obtained for the patient to available OCT studies of other persons in the same age group that do not have any ocular pathology. Optical coherence tomography accomplishes this by dividing the retinal nerve fiber layer into sectors and analyzing them. In the normal eye, each sector has its own characteristic concentration of nerve fibers, with some showing higher concentrations than others. The OCT software then locates a patient's results within a range of nerve fibers established for each sector from a healthy population of the same age.

As you can see, the analysis of optical coherence tomography results depends on many factors, not just an increased excavation.

Now, let us address the issue of your intraocular pressure. It is important to understand that a normal eye pressure is one that does not cause damage to YOUR optic nerve. For many people, this is a range between 10-20 mmHg, and one that ideally does not fluctuate more than 6 mmHg within this range on any given day. However, for other individuals, in particular those with normal tension glaucoma, pressures ranging from 16-20 mmHg can cause optic nerve damage.

In your particular case, intraocular pressures have been measured from 18-20 mmHg in one eye and 19-21 mmHg in the other eye. These are located at the upper end of the typical intraocular pressure range. It is now important to determine the degree to which your intraocular pressure fluctuates throughout the day. To accomplish this, your ophthalmologist will take pressure measurements at different times of the day and on different days. She or he will then use these data to develop an intraocular pressure curve that not only identifies the range of fluctuation throughout the day, but also establishes the length of time during the day that your eye pressure is at a level where it might be causing damage to the optic nerve.

The gold-standard method for measuring intraocular pressure is using an instrument called the Goldmann Applanation Tonometer. Although this is the most accurate method, it does have a drawback. Applanation tonometry works by measuring the force required to temporarily flatten the cornea. In eyes with thin corneas, the force required is less because the cornea itself does not provide as much resistance. This means that the instrument potentially can provide us with an intraocular pressure that is less than the real value. For example, a patient with thin corneas actually may have an intraocular pressure of 22 mmHg, but the applanation tonometer may indicate a pressure of only 19 mmHg.

The opposite occurs in corneas that are thicker than average. In this case, a thick cornea is more difficult to flatten, which means that the applanation tonometer must exert greater force. Consequently, the instrument indicates that the patient has an intraocular pressure greater than it actually is. Haag-Streit International, manufacturer of the Goldmann Applanation Tonometer, provides conversion values that allow ophthalmologists to obtain highly accurate readings, while taking into consideration corneal thickness.

Corneal thickness is determined using a simple test called pachymetry.

In most patients, the degree of excavation (visualized as cup-to-disc) is an indication of nerve fiber loss. However, remember that excavation is not the only measurement in an OCT test. It also examines the number of nerve fibers present by sector and compares these results to a population in the same age range with no ocular pathologies. This part of the exam revealed a presence of nerve fibers within a normal range. In a minority of patients, the cup-to-disc ratio is high but there is no apparent loss of nerve fibers. This could be true in your case.

On the other hand, your intraocular pressure is in the upper limits of the typical range. As I noted earlier, normal tension glaucoma is a type of glaucoma in which eye pressures between 16-20 mmHg can cause damage to optic nerve fibers. Thus, it is possible that some damage slowly is occurring to your optic nerve, even though your intraocular pressure has not yet reached a point where it falls outside typical ranges.

Fortunately, the same course of action is required to distinguish between these two possibilities.

• Return to your ophthalmologist and arrange to have multiple readings of your intraocular pressure taken at different times during the day and on different days, so that she or he can develop and intraocular pressure curve.
• Arrange to have additional optical coherence tomography examinations over the next 12-24 months. This will enable your ophthalmologist to determine if the values are changing in such a manner as to indicate nerve fiber loss.

If this extended observation demonstrates progressive optic nerve damage and/or elevations in intraocular pressure, then steps will need to be taken to control your eye pressure. Obviously, if the extended observation does not show problems, then no treatment will be required. However, it is important that for future visits to eye doctors, you disclose that you had an optical coherence tomography examination with atypical results.

Finally, your initial complaint was eye strain. This can be very uncomfortable, especially if it is experienced chronically. The eye-care professionals that you visited apparently ruled out the possibility that you need corrective lenses or, if you already have them, that you do not require a change in prescription.

This leaves the possibility that other factors may be the cause of your eye strain. I would like to direct you to my page about eye strain, where you can learn about its causes and things that you can do to adjust your behavior and environment to prevent it. I hope that these strategies will prove beneficial to you.

Carolina Valdivia, MD

REFERENCE:

McNaught A. New Imaging Techniques. In: James B and Benjamin L. Ophthalmology: Investigation and Examination Techniques. Philadelphia, PA: Butterworth Heinemann Elsevier, 2007:232-237.

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