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What are the Tests for Glaucoma?

Author: Premier Eye Associates

January is Glaucoma month. That being said, we thought we’d talk about the specialized testing that goes along with glaucoma. In other words, how do we detect glaucoma?

Research is ever changing this field, becoming more accurate and precise every year. Today, there are three major tests used to detect and monitor glaucoma—eye pressure measurement, visual fields, and OCT imaging.

 

What is Glaucoma?

Before we get too far along with testing for glaucoma, we’ll briefly review what glaucoma is.

Glaucoma is a disease of the optic nerve.

The optic nerve is the nerve that connects the brain to the eye. All visual information is detected by photoreceptors—cells in the backmost layer of the retina.

The photoreceptors then send signals along axons, similar to cable TV being transferred via an optic fiber cable.

In glaucoma, damage to the optic nerve results in the inability for signals to be sent to the brain properly. This is seen by the affected individual as a permanent loss of vision, or visual field defects.

In glaucoma, peripheral vision is affected first—often described as “tunnel vision”. If the disease continues to progress it can extend to affect central vision, or even cause complete blindness.

You can think about this similar to a mouse chewing through a fiber optic cable. At first, only little bits and pieces are affected until the entire cable no longer transmit information.

 

Testing for Glaucoma

The best way to evaluate glaucoma is through annual eye exams that allow the doctor to closely evaluate the optic nerve and watch for subtle changes over time.

There are also several tests that help doctors further closely evaluate glaucoma progression.

 

Air Pressure Tests

The first of the lot is eye pressure measurement—or intraocular pressure (IOP).

While the exact mechanism for glaucoma is unknown, there are several hypotheses. One such is elevated eye pressure.

When eye pressure is too high, it adds extra pressure, and therefore stress, on ocular structures.

You can think about this as an over-inflated balloon. Too much pressure on the optic nerve puts stress on it, resulting in damage and death of the axons making up the nerve.

Thus, we want to measure eye pressure to ensure it is not too high. In fact, one of the main treatment options for glaucoma is eye pressure lowering eye drops or surgeries.

Doctors measure eye pressure through several different methods including Goldmann applanation tonometry, iCare rebound tonometry, and noncontact tonometry (the beloved air puff test) also known as NCT.

The normal range for eye pressure is around 10-21 mmHg (millimeters of mercury).

Semi-elevated pressures (ocular hypertension) do not always indicate glaucoma, however consistently elevated IOP has been linked to a greater risk of glaucoma development. Therefore, regular exams that watch pressures is an essential part of diagnosing and managing glaucoma.

If you are interested in learning more about the exact mechanisms of measuring eye pressure, be sure to check out our article from May of 2021, “Why is Eye Pressure Important?”.

 

Visual Field Analysis

Visual field testing involves sitting in a machine that flashes blinking lights of various sizes and intensities across your vision. The patient is asked to push a clicker button each time he/she sees a light stimulus.

The strength and extent of the visual field is then mapped out on a visual field analysis plot that allows your doctor to visually “see” what you are seeing. If there are any areas of visual field loss (aka visual field defects) your doctor will be able to see it.

This is important for glaucoma analysis as it allows your doctor to classify how significant your stage of glaucoma is—borderline, mild, moderate, or severe.

Visual field testing also allows your doctor to watch for changes to see if the glaucoma is progressing.

Ultimately, the goal of treatment is to slow the progression of glaucoma to preserve vision. If the nerve is continuing to die and vision is continuing to digress, treatment is not adequate.

We want to catch glaucoma as early as physically possible to prevent major vision loss. Visual fields are one of the best ways to monitor this.

 

Optical Coherence Tomography (OCT)

The last of the tests we will discuss here today is OCT (optical coherence tomography).

An OCT is a scanned image of the optic nerve that allows your doctor to better view its integrity and watch for minute changes.

OCT utilizes special cross-sectional imaging to create a map of the various layers of the retina and optic nerve.

In glaucoma, one of its most useful advantages is that it can measure the thickness of an individual layer of the retina called the nerve fiber layer.

The average thickness of a healthy nerve fiber layer is roughly 105 micrometers (um). This is a very thin layer. Changes to the nerve fiber layer are not typically detectable by the naked human eye unless they are extremely significant, which is what doctors are trying to prevent in glaucoma.

Thus, OCT imaging becomes the only way to truly measure and watch for small changes in thickness of this layer.

Not only that, but changes to the nerve fiber layer are not uniform, meaning they do not occur across the entire optic nerve at once.

In normal glaucomatous progression, the inferior aspect of the optic nerve is affected first, followed by the superior, nasal, and lastly temporal region.

Of course this may not be the case for everyone, however it is the more stereotypical pattern of progression of the disease.

OCT allows doctors to see and evaluate different areas of the nerve to watch for nerve fiber layer thinning over time.

OCT imaging also allows the doctor to see the nerve in a more zoomed in, up close fashion than typical examination allows for.

Through combination of OCT imaging and visual field analysis, the doctor will be able to have a better idea of exactly what is going on, and how quickly changes are occurring.

In fact, in those with glaucoma, follow up visits are typically every 3-4 months, and will alternate between a visual field and OCT imaging so that your doctor can watch for subtle changes and intervene when necessary.

While glaucoma can be a potentially devasting ocular disease, there is some hope with proper examination and treatment. Technology is ever-growing and doctors are able to better detect glaucoma sooner than ever, resulting in earlier treatment and thus better prevention of disease progression.

If you or someone you know is at high risk for glaucoma, or thinks they may be suffering from glaucoma, be sure to make an appointment with your eye doctor today.

 

Dr. Anthony Spina and the staff of Premier Eye Associates specialize in glasses, soft contact lenses, hard contact lenses, and medical eye exams. Call our eye doctor in Auburn, AL today at (334) 539-5391 or schedule an appointment online  if you are interested in learning more about glaucoma testing or if you want to be tested.  Our optometrist provides only the highest quality eye care services amongst eye doctors in the Auburn Alabama area.

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