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Diagnosing Glaucoma
Comprehensive Eye Exam
The first step in diagnosing glaucoma or any other disease is a complete thorough dilated eye examination. This includes a thorough eye history, past medical history, medications review, review of body systems, and social history. The eye exam will consist of vision testing, pupil evaluation, extraocular muscle function, confrontation visual fields, external examination of the eyelids and surrounding structures, slit exam of the eye, eye pressure check, and dilated examination of the retina and structures inside the eye. There are many other ancillary tests for determining the presence of glaucoma as well.
Testing the Eye Pressure, Intraocular Pressure, or IOP
There are several instruments designed to measure the eye pressure.
Non-Contact or Air Puff Instrument
Canon Air Puff Tonometer
Tonopen
iCare Tonometer
Diaton Transpalpebral Tonometer
Perkins Tonometer
Pascal Tonometer
Goldmann Applanation Tonometer
The Goldmann applanation tonometer is considered the best method of testing the eye pressure and is what most eye doctors use to diagnose and/or monitor glaucoma. A normal eye pressure is considered to be 10 to 21 mmHg (millimeters of mercury).
If the eye pressure is elevated or the optic nerves look suspicious for glaucoma damage, further testing will be needed to diagnose whether the eye has glaucoma.
Gonioscopy
The eye doctor uses a contact lens with a prism to look into the angle of the anterior chamber. The angle will be evaluated on the width of the angle and the amount of pigment or debris in the trabecular meshwork. OCT and ultrasound Gonioscopy can be used to evaluate the angle.
Visual Field Testing or Side Vision Testing
Even though you may not notice any loss of side vision, you may be losing your side vision without your knowledge. Visual field instruments are able to find small changes in the field of vision. The instruments can detect a decrease in the amount of light the eye can see in the side vision and not just loss of side vision. The visual field instruments are controlled by a computer which can test the intensity or brightness of light the eye can see in the side vision. The eyes are tested one eye at a time. You are positioned in front of the instrument that looks like a white bowl. Small lights are briefly shown at specific light intensities and you are to push a button to confirm that you saw the light while you are staring a target.
The instrument records the intensity of light in different portions of the side vision. The computer in the instrument runs a report of the findings which the eye doctor evaluates. The visual field instrument can compare the results of the current test to the previous tests and show any changes in the side vision. These new instruments are able to detect tiny changes in the side vision. The visual field test is usually performed on a yearly basis for people with glaucoma or are at risk of developing glaucoma.
Examination of the Optic Nerve
After the pupils are dilated, the eye doctor will examine the optic nerve with a variety of lenses. The eye doctor will describe the appearance of the optic nerve and put it into the medical record. The are nearly 1.5 million nerve fibers in the optic nerve. If the optic nerve is small, the nerve fibers will fill up the optic nerve and there will be no open space, cupping, or cup-to-disc ratio in the nerve. If the optic nerve is large, there may be a large opening in the optic nerve but the peripheral portion of the nerve still contains the 1.5 million nerve fibers. The opening the nerve is called cupping or cup-to-disc ratio by eye doctors to describe the appearance of the optic nerve. When a person has glaucoma damage, there is a loss of nerve tissue and the cupping or cup-to-disc ratio increases.
When someone has a large nerve and large cupping it may look like glaucoma damage but it is the normal way their optic nerve is shaped. Sometimes, it can be difficult to tell if the nerve has glaucoma damage. Just like there are tall or short people, the optic nerve will vary in the size and shape of the nerve. Fortunately, there are other tests to help with the diagnosis. A photograph of the optic nerve can be taken as a reference point for future evaluation of the optic nerve.
Other Methods of Optic Nerve Evaluation
Optic Nerve and Retinal Nerve Fiber Layer Imaging
Optical Coherence Tomography or OCT
Optical coherence tomography or OCT is an imaging system that is non-invasive used to image or visualize and quantify the layers of the nerve fibers in the retina and evaluate the optic nerve. The OCT provides information about the contour or topography of the optic nerve, measures the thickness of the retinal nerve fiber layer just before they enter the optic nerve, and images the macula. The OCT records all these measurements and provides analysis of the data. It provides a rating of the status of the health of the nerve fiber layer and optic nerve.
This instrument is one of the most sensitive tests for determining the presence of glaucoma damage to the optic nerve. This test is usually performed and an annual basis for people with glaucoma or at risk of developing glaucoma. There are several different companies that make OCTs which are Heidelberg Engineering Inc., Nidek, Optovue Inc., and Topcon.
Confocal Scanning Laser Ophthalmoscopy or CSLO
Confocal scanning laser ophthalmoscopy or CSLO is a technique for imaging of the optic nerve and nerve fiber layer. CSLO uses laser scanning which produces up to 64 transaxial laser scans through the optic nerve and retina adjacent to the optic nerve. This produces a 3D image of the optic nerve. The data from the tests is used to produce a report about the status of the optic nerve and retinal fiber layer.
Scanning Laser Polarimetry
Scanning laser polarimetry or SLP measures the retinal nerve fiber layer thickness adjacent to the optic nerve. The instrument records the data and produces a report of the status of the nerve fiber layer and also compares the results to normal data as a comparison.
These instruments provide valuable information about the health of the optic nerve.
Pachymetry
Pachymetry is a simple office test that measures the thickness of the cornea. A numbing drop is instilled into the eye and a small probe is placed on the cornea which measures the thickness of the cornea. Other machines do not touch the cornea and the measurement is performed without numbing eye drops. The measurement of the cornea is important because the thickness of the cornea has an effect on the measurement of the eye pressure. In 2002, the results of the Ocular Hypertension Study (OHTS) was reported. As a result of the study, it was found that corneal thickness as a significant role in determining eye pressure and the development of glaucoma.
The measurement of the eye pressure was influenced by the thickness of the cornea. A cornea that was thicker than normal resulted in the eye pressure being measured higher than the true eye pressure due to the increased resistance of the cornea. A cornea that is thinner than normal resulted in the eye pressure being measured lower that the true eye pressure as the thinner cornea has less resistance. A normal corneal thickness is 555 microns. Someone who has had LASIK, LASEK, or PRK have thinner corneas after their laser and these people will have their eye pressures measured lower that their true eye pressure. This can present a problem in diagnosing glaucoma in these people.
Ocular Response Analyzer
Ocular response analyzer or ORA is a tonometer that measures corneal hysteresis. Corneal hysteresis is the difference in the inward and outward pressure values obtained during dynamic bi-directional applanation process with the ORA. Corneal hysteresis is the characterization of the cornea’s ability to absorb and dissipate energy which is the function of the visco-elastic properties of the cornea. In other words, how pliable is the cornea. This can affect the measurement of the eye pressure.
VEP (Visual Evoked Potentials)
VEPs have been used in universities for many years as a way to objectively measure and evaluate the presence of any pathology or abnormality in the afferent vision system. Originally a light was flashed into the eyes of the test subject but now a black and white checkered board pattern is used. Electrodes are placed on the scalp over the occipital lobe on the back of the skull. The pattern of the black and white checker board is reversed back and forth. The size of the checker board blocks and the variation of the contrast of the grey to white to black can be adjusted. The patient is seated in front of a computer screen and is asked to watch the pattern change. Sensors are placed on the head in the area of the occipital lobe or vision center. The amplitude or strength of the response in the occipital is measured and the time it takes for the stimulus to reach the occipital lobe is measured. Many different diseases can diminish the strength of the response and the time or latency of the response. Multiple sclerosis classically increases the latency of the response and amplitude. There is a high contrast test and a low contrast test. The low contrast test is the most sensitive way of determining the presence of disease.
The VEP is able to detect a reduction in cell function before any damage to the nerve has occurred. It will also show damage as well. This is a very sensitive test and needs to be done in a well-controlled environment for consistent results.
The VEP can show early damage to the optic nerve from glaucoma or even show that the nerve is at risk of developing glaucoma damage.
ERG (Electroretinography)
ERG is a test that measures the size and speed of the electrical activity in the retinal cells in the retina with exposure to a visual stimulus. A specific type of ERG is called a pattern electroretinography or PERG. This measures the level of activity in the retinal ganglion cells. Like the VEP, the PERG is able to detect small abnormalities in cell function before cell loss or damage has occurred.
The patient is placed in front of the instrument computer screen and electrodes are placed just under the lower eyelid and forehead. There are two protocols used with the PERG.
The first is a concentric stimulus field. A 24-degree circle is projected onto the retina and a waveform is generated from the stimulus. This pattern is useful in detecting early glaucoma damage.
A 16-degree circle is projected onto the retina and another waveform is generated. This PERG is useful is diagnosing diabetic macular edema, age-related macular degeneration, and retinal toxicity from many causes such as damage from the medication called Plaquenil.
Another protocol is using contrast sensitivity which can further help in the diagnosis of glaucoma and diabetic retinopathy.
The PERG is useful in the early detection of glaucoma and diabetic retinopathy.
JAMES E. CROLEY III, M.D.
Dr. James E. Croley III is a board-certified ophthalmologist and the founder of the Cataract & Refractive Institute of Florida. He graduated magna cum laude from Cumberland College before attending medical school at the University of Miami and completing his residency at the University of Alabama Eye Foundation Hospital. Additionally, he studied at Stanford University where he gained further education in advanced ophthalmology techniques. Dr. Croley is a member of the American Academy of Ophthalmology and the Florida Society of Ophthalmology.