As the number of patients who have glaucoma grows, having an efficient testing system becomes crucial to appropriately care for them. An efficient testing system is one in which technicians are able to quickly and accurately acquire the data of both functional and structural testing to relate to the doctor.
Here, we review the common glaucoma tests as well as tips for each and links for more resources.
Tonometry
Measuring intraocular pressure (IOP), or tonometry, is essential in assisting in the detection of undiagnosed glaucoma and in identifying disease progression. The Goldmann tonometer is the most commonly used. Other tonometers include the Icare rebound tonometer, the digital dynamic contour tonometer, and several lightweight handheld tonometers, such as the Tono-Pen (Reichert) and AccuPen (Keeler).
A new prism, the CATS Tonometer Reusable Prism, was designed to improve IOP accuracy. The CATS prism integrates easily with current tonometer equipment.
Visual field testing
In terms of functional measurements, visual field testing is the major player. Defects, or blind spots, are the major sign of the disease and a method by which to monitor progression of the disease. This is done via perimetry, and, most commonly, the Humphrey field analyzer. For tips on acquiring results, see “Determine the visual field,” page 21.
Optical coherence tomography (OCT)
OCT aids in diagnosing glaucoma in evaluating both the optic disk and retinal nerve fiber layer (RNFL). Specifically, images captured can provide an objective evaluation of the optic nerve. Identifying areas of thinning in the RNFL can aid in early detection of the disease as it may be apparent before visual field defects become evident.
According to The Ophthalmic Assistant, if the OCT is new to the practice, “proper training should come from the distributor.” If the device has been in the practice, then a knowledgeable in-house technician can provide training.
Anterior segment OCT (AS-OCT) provides a visual of both the cornea and the angle to identify glaucoma as either open, narrow, or closed.
OCT angiography (OCT-A), used to evaluate microvasculature of the optic nerve, retina, and choroid, can aid in detecting blood perfusion ahead of damage related to glaucoma, Davinder S. Grover, MD, MPH, says in “What’s new in glaucoma diagnostics” for the February 2019 issue of Ophthalmology Management (bit.ly/2TgNTCB ).
Pachymetry
Optical pachymeters aid the technician in providing corneal thickness measurement and assessment.
Thin corneas, common in those who have undergone laser refractive surgery, can affect tonometry readings. Specifically, it can cause the pressure reading to be lower than the eye’s actual pressure. Correction tables can be helpful to ascertain true IOPs in these patients. A thin cornea itself may also be a risk factor for glaucoma.
Ophthalmic technician Luis Campos says that a standard deviation is important to getting an accurate measurement of corneal thickness. “This requires the tech keep a steady hand, keeping the device located on the central cornea, while instructing the patient to look straight ahead,” says Mr. Campos in “Battling the silent thief” in the January 2017 issue of OP (bit.ly/2RYGc2w ).
“Many EMR systems auto-adjust after pachymetry info is added, or a common table is used,” says Amanda Waggoner, COMT, in “Overcome challenges with IOP readings” in the January 2017 issue of OP (bit.ly/2M2FBsX ).
Corneal hysteresis (CH)
Acquired via Ocular Response Analyzer (Reichert), CH measures the cornea’s ability to absorb and dissipate energy. Studies have shown that lower CH values correlate to faster rates of visual field loss.
By calculating both the IOP and CH, the ORA can analyze what the ocular pressure would be without the influence of corneal thickness, according to The Ophthalmic Assistant.
Dr. Grover, in “What’s new in glaucoma diagnostics” says, “CH helps me improve the quality of glaucoma care I provide by better risk stratifying my patients and tailoring my treatment of the patients and their eye’s shock-absorbing ability.”
CH values can be particularly useful if glaucoma is asymmetrical or pressure varies among the eyes of a patient, via “Uncovering early glaucoma,” in the June 2019 issue of Ophthalmology Management (bit.ly/34s6fSu ).
Determine the visual field
To obtain optimum visual field test results the technician should remember the following:
- Make sure the patient’s name is entered correctly into database. Things like incorrect spelling will interfere with the ability to compare the data against future results.Along the same lines, enter the correct date of birth. This is important to be able to compare patients with a normative database.
- In terms of setting up the machine, put the correct refractive error in to allow it to calculate the proper lens required for the distance of center of the bowl to eye (30 cm). In addition, make sure the machine is selected for the correct eye. When placing multiple lenses in the lens holder, the spherical lens is placed closest to the patient’s eye. Confirm that the axis of the cylinder lens is lined up properly. Also, make sure the lens is close to the eye but not touching. If the lens is too far from the eye, it will induce a rim artifact.
- The biggest deal breaker in these tests is patient positioning and comfort. For positioning, this requires proper eye alignment with the center and the height of the machine. Also make sure the chin and forehead stay in contact with the machine. For comfort, if the patient is not comfortable, he will focus on his discomfort instead of the test. As such, once you line up the eye in monitor, ask the patient if he wants the height adjusted for comfort.
- An important factor in acquiring good results is the instruction of how to do the test. This is critical since miscommunication can lead to poor test results that the provider cannot interpret. Patients must understand that not all lights are expected to be perceived, that some are very dim (intentionally). Patients must know to fixate at the center target and not move the eye to search for lights, as the test is measuring peripheral vision. Also, patients should be encouraged to blink so the eye does not dry out.
- During the test, monitor the video screen for proper fixation throughout the entire test. Be aware of fixation losses. Once you have measured the foveal threshold and gaze-tracking — immediately after starting the test, relocate the blind spot. A lot of times there is movement at the beginning of the test, and, if the blind spot measured incorrectly, it will lead to high fixation losses. Finally, encourage the patient throughout the test as positive reinforcement to contribute to good test results.
- Be aware of false positive and negative errors. False positive errors occurs when the patient is just pressing the buzzer — sometimes referred to as “trigger happy” — although no lights are being presented. In this case, patients may be responding to the noise inside the machine. If this should happen, re-establish understanding and instruct them to only click when lights present and that there are times when no lights are presented.
False negatives occur when the patient fails to see a significantly brighter light at a location where the patient was already tested and had responded. If there are a high number of false negatives, either the patient is not alert/paying attention or there is pathology in this area.
— CAROLYN SHEA, COMT
Utilize resources
The American Academy of Ophthalmology Preferred Practice Patterns, The Ophthalmic Assistant and Ophthalmic Medical Assisting, in addition to other ophthalmology publications, such as Ophthalmology Management, OP’s sister publication, are all valuable resources for information on glaucoma testing. OP
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