|Zeiss Meditec’s Humphrey Matrix 800 can detect early glaucomatous visual field defects.|
|This Glaucoma Continuum illustrates the glaucoma disease state from no disease, through when a patient has the disease and is symptomatic. DIAGRAM SOURCE: WILSON MR, SINGH K. INTRAOCULAR PRESSURE: DOES IT MEASURE UP? THE OPEN OPHTHALMOLOGY JOURNAL. 2009; 3:32-37.|
|Haag-Streit Diagnostics’ Octopus 600 uses a water-drop”“like pattern.|
Visual field testing remains an important part in the diagnosis and treatment of glaucoma.
The utilization and importance of visual-field test findings has changed in recent years in the amalgamation of testing and observation, patient history, and demographic information that goes into the assessment of glaucoma patients.
Automated threshold visual field testing has been a mainstay of care for the diagnosis and monitoring of glaucoma. In the past, before initiating a patient on lifelong glaucoma therapy, a repeatable threshold visual defect was the standard of care on which we hung our hats. With new advancements available to us today””namely optical coherence tomography (OCT), electrodiagnostic testing such as pattern electroretinography (ERG), and visual evoked potential (VEP) that allow us to appreciate when ganglion cells are becoming stressed””waiting for white-on-white threshold visual field defects or functional vision loss shouldn’t be the deciding factor for beginning to treat or modify treatment. No single test should ever be used as the sole factor for patient management.
INTERPRETING VISUAL FIELD TEST RESULTS
When I got out of school in the late ’90s, I operated under the impression that a mild visual field defect was fairly benign. I wanted multiple, repeatable visual field tests of progression before a change in therapy or initiation of therapy was considered. That way of thinking now, I feel, puts the patient on a slippery slope to blindness so the way I utilize information from my threshold visual field analyzer has changed.
Visual field results should be used to monitor visual field defects and not for early diagnosis. Lack of a white-on-white threshold visual field defect for a newly diagnosed glaucoma patient is a really good thing. A mild white-on-white visual field defect or minor presence of progression is actually far more dire. It is crucial to closely monitor slight changes of small-threshold visual field defects, which means disease progression and, eventually, functional vision loss.
A helpful illustration of the progression of the glaucoma disease state is the Glaucoma Continuum (see at right). The arc’s left side starts with the beginning of the disease state before it’s detectable and the patient is asymptomatic. Proceeding up the arc, ganglion cell stress, and ultimately ganglion cell death, begins. It then continues into when retinal nerve fiber layer (RNFL) or optic nerve head (ONH) changes become detectable by optical coherence tomographers or electrodiagnostic equipment. The patient is still unaware that a problem is brewing at this point.
At the top of the curve is where blue-yellow or short wavelength automated perimetry (SWAP) starts to detect changes in visual function. It’s not until a patient is headed down the right side of the arc that white-on-white threshold visual field defects are detected. Patients typically are asymptomatic until they reach the moderate-to-severe white-on-white visual field portion of the continuum. At that point, they’re ultimately at the end of this continuum and at the risk of being rendered blind. Therefore, waiting for a repeatable white-on-white threshold visual field defect before initiating therapy appears to be fairly late in the game and is not in anyone’s best interest.
THE ROLE OF SWAP PERIMETRY
SWAP or blue-yellow visual field perimetry builds on the fact that blue cones number the fewest in the retina, so damage to those will manifest earlier than the full-color spectrum white light used in white-on-white threshold visual field testing. SWAP can catch mild visual field defects years earlier than a traditional white-on-white, but SWAP perimetry has fallen out of favor.
The yellowing of the crystalline lens of nuclear sclerotic cataracts can skew results, and SWAP has a limited dynamic range so the visual field defects will appear larger. It is more difficult for the patient to perform and thus this perimetry has a higher variability. However, these so-called negatives are all excellent reasons why it’s still a better strategy for young glaucoma suspects. The larger defects are great indicators of where future white-on-white threshold defects are going to begin first. Additionally, you can utilize the cluster analysis to correlate the visual field defects to where on the RNFL and ONH you should be watching. If a SWAP comes back reliable and clear, there’s little chance of a more profound white-on-white defect.
Frequency doubling perimetry (like Carl Zeiss Meditec Inc.’s Humphrey Matrix‘ 800 and FDT‘, which utilize a grating-type pattern) and Pulsar perimetry (like Haag-Streit Diagnostics’ Octopus, which uses a water-drop”“like pattern) are not on the Glaucoma Continuum, but I’d probably insert them just left of SWAP. These instrument types engage the magnocellular ganglion cell pathways responsible for flicker detection, thus detecting the early ganglion cell damage associated with glaucoma. The instrument types””which are similar to or even better than SWAP according to some clinical studies””can detect early glaucomatous visual field defects. Medicolegally though, standard of care still dictates use of traditional threshold visual field testing for glaucoma monitoring.
For glaucoma, the change in analysis software for monitoring field defects has made it much easier to compare multiple visual fields that are performed over the course of several years. The ability to see all of the testing on a single screen with the progression prediction is very helpful in monitoring glaucoma patients who already have visual field defects.
Some visual field analyzers will identify locations of RNFL damage or where changes should be made on the optic nerve head to assist with structure-to-function correlations. The ability to educate the patient about where their visual field results indicate damage on their ONH and RNFL””and for that information to correlate with your OCT findings””is priceless.
Visual field test results will always be patient dependent, so be sure to pay attention to reliability statistics: false negatives, false positives, fixations losses, etc. Variability and fluctuation in a patient’s performance from one testing to the next can make interpretation daunting and even fool analysis software in your particular instrument.
Most likely, your patients will hate doing visual field tests. They are difficult and if your patient hesitates on a response, the next test point flashes and the patient feels like he’s “failed” the test, so maybe we should call them visual field “measurements” instead. When patients ask why they still have to do visual field tests, I always say that the OCT monitors structural changes whereas visual field tests monitor functional changes, as most patients understand the importance of monitoring both structure and function.
White-on-white threshold visual field defects are signs of advanced level of disease. For detection of early milder glaucoma, a lack of defect seen on white-on-white threshold visual field testing results should factor less into your decision making than a defect on SWAP perimetry, optical coherence tomography, and/or other electrodiagnostic tests. In more advanced glaucoma patients that have any level of visual field defect, even the most minor changes in white-on-white visual field testing, though, should weigh heavily on your decision making for therapy change.
Shannon L. Steinhäuser, in private practice at Phoenix Eye Care in Phoenix, AZ, is a member of the Optometric Glaucoma Society.
WHERE TO FIND IT:
Carl Zeiss Meditec, Inc. •800-342-9821 •meditec.zeiss.com/usa
Haag-Streit USA •800-787-5426 •haag-streit-usa.com