Figure 1. Left - Goldmann Bowl Perimeter; Right – illustration of Goldmann testing.
by Lorne Yudcovitch, O.D., M.S., F.A.A.O.
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Short Wavelength Automated Perimetry
Comparison of Frequency Doubling Perimetry versus White-On-White and SWAP Perimetry
Case Studies Demonstrating the Use of SWAP and FDT Perimetry
Most eye care providers agree that glaucoma is difficult to diagnose with a single test. Often, analysis of results from several tests including history, visual fields, intraocular pressure, gonioscopy, pachymetry, optic nerve head assessment, and nerve fiber layer analysis must all be used to make a correct diagnosis. In fact, sometimes a patient needs to be followed for an extended period of time before a decision to treat can be made. Conversely, a patient may occasionally present for the first time with extensive vision loss that could have been prevented if treatment had been initiated years ago.
The main goal of treatment for a glaucoma patient is to preserve as much of the visual field as possible for the rest of his or her life. One of the key tests that is used to achieve this goal is visual field testing, also known as perimetry. When combined with measurements of ocular pressure and corneal thickness, and anterior and posterior structural examinations, visual field measurement is an essential tool in glaucoma diagnosis and management.
For more than a century, visual field tests were based mainly on kinetic stimulus presentations – that is, responses to moving peripheral stimuli were made while the patient looked straight ahead at a stationary fixation point. Typically white spots of varying size, intensity, and contrast were used as stimuli. The classic Tangent Screen and Goldmann Bowl Perimeter are good examples of this type of perimeter, and both are still used today in certain practice situations.
Figure 1. Left - Goldmann Bowl Perimeter; Right – illustration of Goldmann testing.
Other variations on the kinetic perimeter principle include the arc and auto tangent screen perimeters.
Kinetic perimetry allows relatively simple patient responses to the moving stimuli, it provides flexibility as to what areas of the visual field to test, and it can be used with patients who may give questionable responses on automated tests (e.g., patients with low vision, neurological defect, or advanced eye diseases).
Disadvantages of kinetic perimetry include:
As computerized, automated visual field testing became more popular, various companies (e.g., Zeiss-Humphrey, Dicon, Synemed, Medmont, Oculus, Kowa Optimed, Hansen and InterZeag) utilized static achromatic "white-on-white" stimulus presentations in which small, static, white light dots of various intensities were presented at selected locations on a uniform white background. This has resulted in highly quantifiable, sensitive visual field results that provide the ability to separate ‘abnormals’ from the general patient population. Standard ‘white-on-white’ perimetry is currently the most common type of automated perimetry performed.
Figure 2. Examples of contemporary automated visual field analyzers. From left to right: Dicon Autoperimeter, Oculus Easyfield, Zeiss-Humphrey Field Analyzer II.
Although most automated perimeters stabilize the patient's gaze by requiring him or her to fixate a static central target, Kinetic Fixation Perimetry, in which the patient is required to fixate a moving fixation target during testing, is a feature available on Dicon perimeters. This testing feature makes visual field testing more dynamic and interesting for the patient.
Some perimeter manufacturers, such as Dicon, also have a Multiple Stimulus Presentation option in which one to four stimuli are presented at a time, requiring the patient to respond one to four times, respectively. Meant to speed the visual field testing, multiple stimulus presentations are not used for most patients because it requires quick, multiple (i.e., one to four) click responses by the patient, rather than the single click response typical of most visual field testing.
The Humphrey Field Analyzer II perimeter has been utilized as a standard test for visual field assessment by many vision care providers. Its long history of utilization in automated visual field testing, its standardized printout, and its large comparative statistical database has made the Humphrey II a popular tool for managing glaucoma patients. Along with the standard ‘white-on-white’ screening and threshold testing strategies common with most autoperimeters, this instrument has several additional test strategies including FASTPAC and SITA.
SITA (Swedish Interactive Testing Algorithm) Standard, and SITA Fast programs use "intelligent" analyses of the patient’s responses and age-normed statistical data to significantly shorten the testing time required to map the visual field. For example, SITA computes real-time estimates of threshold values and threshold error measurements as the patient is performing the test. This speeds the examination by omitting the presentation of stimuli with certain parameters (e.g., brightness) that would be unlikely to provide useful data. The SITA algorithm also makes use of response time information and can regulate the pace of stimulus presentations to match the patient's own pace of responding.
The SITA-Fast algorithm uses SITA strategies and additionally speeds up testing even more by requiring somewhat less certainty for the threshold values obtained at each point in the field. However, there may be some limitations in terms of analyzing field loss progression with the SITA-Fast strategy because it performs fewer re-checks of missed points in order to speed the test, but statistical correlation with traditional threshold testing is still generally good.
GPA (Glaucoma Progression Analysis) is a new normative database combined with progression analysis in a concise printout that has recently been developed for specifically identifying and monitoring glaucomatous visual field loss.
Kinetic Perimetry is available on specific models of the Humphrey Field Analyzer II. This test simulates the traditional Goldmann bowl kinetic stimulus visual field testing mentioned earlier.
Another test strategy option available on some Humphrey field analyzers is short wavelength automated perimetry (SWAP) in which blue stimulus dots are presented on a yellow background.
Current research suggests that visual information is sent from the retina via the optic nerve to the lateral geniculate nucleus (LGN) and then on to higher neural areas via three general pathways: the magnocellular (M-cell), parvocellular (P-cell), and koniocellular (K-cell) pathways. These pathways include retinal ganglion cell axons and their synaptic connections to neurons in the LGN, the axons of geniculate cells that carry information to the primary visual cortex, and the fibers from the visual cortex that connect to higher brain centers. Figure 3 shows a schematic drawing of the retinal to geniculate portion of the pathway.
Fig 3. Retinal M-, P-, and K-cell retino-geniculate pathways and ganglion cell types. M = Magnocellular, P = Parvocellular (Green color = mid-spectrum color responsive; Red color = low-spectrum color responsive), B/Y = Koniocellular. (Modified from www.krasnow.gmu.edu/.../ Psyc372_01/Cla6.html)
Each pathway conveys a basic component of visual information, as shown in Table 1.
Table 1. Characteristics of the M-, P-, and K-cell pathways.
| Type of cell pathway | M | P | K |
| Approximate percent of retinal ganglion cells: | 10% | 80% | 9% |
| Receive input from: | Parasol retinal ganglion cells | Midget retinal ganglion cells | Mainly bistratified (blue-ON) retinal ganglion cells |
| Location in LGN: | Most ventral layers 1 and 2 | Most dorsal layers 3 to 6 | Within and between principal layers (interlaminar) |
| Sensitive to: | Higher temporal frequencies (movement) | Higher spatial frequencies (detail), color | Shorter wavelengths (blue-yellow color) |
Early studies demonstrated tritan-like defects (i.e., blue-yellow confusions) on tests evaluating foveal function in glaucoma. This finding led to research on evaluating short-wavelength function in the retinal periphery as well. A high luminance uniform yellow background bowl with blue test stimuli was developed to isolate the short-wavelength sensitive portion of the K-cell pathway, which is responsible for encoding and transmission of blue-yellow information. Specifically, the yellow background helps saturate the green (medium wavelength) and red (long wavelength) pathways, isolating the blue (short wavelength) pathway. The blue stimuli that are presented as dots have a peak wavelength that approximates the peak response of the blue cones (sometimes called S-cones) that connect to the K-cell pathway. The stimuli presented are larger in diameter than the traditional white stimuli used in white-on-white testing due to the lower stimulus brightness and the larger stimulus dots required to test the K-type nerve fibers. The high luminance of the yellow background also suppresses rod activity, while leaving the S-cones largely unaffected.
This type of perimetry has been called Short Wavelength Automated Perimetry, or ‘blue-on-yellow’ perimetry. Table 2 compares the parameters of traditional white-on-white to SWAP perimetry.
Table 2. Comparison of parameters of white-on-white perimetry and SWAP
| Parameter | White-on-White Perimetry | SWAP |
| Background color: | White | Yellow |
| Stimulus color: | White | Blue |
| Background brightness (cd/m2): | 10 | 100 |
| Stimulus duration (msec): | 200 | 200 |
| Maximum stimulus brightness (cd/m2): | 65 | 10,000 |
| Stimulus size Goldmann (degrees): | III (0.47) | V (1.8) |
Although it was previously thought that glaucoma caused M ganglion cells to die before P- and K-cells, more recent primate studies have shown that glaucoma is associated with the death of all types of ganglion cell neurons in the affected retina and in corresponding areas of the lateral geniculate nucleus and visual cortex.
Because glaucomatous ganglion-caused cell death occurs for all types of ganglion cells, and because there is empirical evidence for loss of blue-yellow perception in glaucoma, it is reasonable to assume that K-cell neurons die as a result of glaucoma. However, because there are relatively few K-cells (i.e., they provide less redundant coverage of areas in the retina where they are located), their loss is relatively easy to detect with SWAP perimetry.
Because SWAP perimetry allows the relatively small population (9 percent) of retinal neurons (specifically, the K-cells) to be isolated and tested, redundancy and overlap from a variety of retinal pathways that can potentially ‘mask’ visual field defects from neural damage due to glaucoma is reduced. This makes SWAP testing highly sensitive to glaucoma-caused damage (and other retinal and neurological disorders that affect the K-cells).
Commercially Available SWAP Perimeters
Blue-yellow perimetry has been incorporated into the Humphrey Field Analyzer II (Carl Zeiss Meditec, Dublin, CA), the Oculus Twin Field and Centerfield perimeters (Oculus, Dutenhofen, Germany), and the Octopus 1-2-3, 101, 301 (as an option) and 311 models (Interzeag AG, Schlieren, Switzerland). Figure 4 shows several of these perimeters.
Figure 4. Several commercial perimeters that incorporate blue-yellow testing. From left to right: Octopus 101, Octopus 311, Oculus Twin Field, Humphrey Field Analyzer II.
SITA SWAP is a combination of Short Wavelength Automated Perimetry and the SITA testing algorithm that significantly decreases testing time.
To obtain an intuitive understanding of the SITA testing algorithm (i.e., strategy), assume that a totally non-intelligent algorithm was used to select stimuli for presentation during field testing. Using this non-intelligent algorithm, the brightness of a point in the field to be tested is initially set as the highest possible level and then decreased in increments until the patient's threshold is found. Obviously this process would need to be repeated for each point in the field and would be very time consuming.
Using SITA and similar intelligent algorithms, best "guesses" are made for the threshold of a point in the field and stimulus brightness is varied around this best guess value until the actual threshold is found. Best guess values are typically based on the probable "hill of vision" height derived from the foveal threshold values measured before testing is started, the patient's age, results of testing adjacent points in the field, and other factors.
Depending on the patient being tested, typical 24-2 SITA SWAP threshold fields can often be measured in under four minutes. This is significantly faster than the ten to fifteen minutes per eye for traditional SWAP testing. Correlation of results to traditional SWAP has been good, but further refinement is anticipated.
Ability of SWAP Testing to Diagnose Glaucoma
But how effective is blue-yellow perimetry in identifying early glaucomatous visual field loss as compared to traditional white-on-white perimetry? Several studies have shown that blue-yellow field loss may precede white-on-white field loss by up to 3 to 5 years. This has important implications for identifying glaucoma because early diagnosis means that treatment can be initiated at an earlier stage in the disease.
Figure 5 shows field test results from a patient with early glaucoma who was tested with white-on-white and blue-on-yellow perimetry on the same day. The left eye from each test is shown for comparison. It can be seen that the white-on-white field shows mild areas of field loss; however, the blue-on-yellow field of the same eye shows much more extensive visual field loss from glaucoma.
Figure 5. White-on-white field OS (left side); SWAP (blue-yellow) field OS (right side) tested on same day. Darker squares indicate reduced areas of sensitivity.
In addition to SWAP field defects generally preceding white-on-white field defects, it has been shown that SWAP field defects may correspond more closely to structural changes of the optic nerve head than white-on-white field defects. For example, inferior optic nerve head rim thinning (i.e., inferior notching) of the optic nerve may correspond more closely with accompanying superior field loss measured with SWAP as compared to white-on-white perimetry. This higher structure-function correlation adds to the clinical value of utilizing SWAP in glaucoma management.
In research correlating visual fields to structural nerve appearance and other factors for glaucoma, SWAP has demonstrated, on average, a greater ability to separate glaucoma patients from normal patients as compared to white-on-white perimetry. In addition, more defects were noted for high-risk ocular hypertensives with SWAP than with white-on-white, and this demonstrates the predictive power of SWAP for identifying early glaucoma. To summarize various studies, functional vision loss noted by SWAP seems to precede white-on-white perimetry losses in 20 to 33 percent of glaucoma/ocular hypertensive cases.
Use of SWAP Testing in Clinical Situations
With the weight of evidence leaning towards SWAP as a preferred visual field test for glaucoma, the question arises as to why it is not utilized more often in a clinical setting. Several limitations currently exist with regard to this testing strategy. One factor is the longer (10-15 minutes per eye) test time. However, this should be alleviated with faster strategies such as SITA-SWAP.
Other drawbacks include a higher inter-test variability than with white-on-white perimetry, i.e., field results are more variable with repeated testing. SWAP results are also significantly more affected by cataracts and other media opacification because of a reduction in short wavelength transmission through cataractous lenses. Posterior subcapsular cataracts may have more of an adverse effect on SWAP fields as compared to white-on-white fields in undilated patients because the higher background illumination with SWAP causes pupillary constriction around the cataract.
Advanced patient age can also be a problem with SWAP testing. The absorption of short wavelength light due to yellowing of the crystalline lens increases with age, being most significant after 60 years of age.
The slope of the hill of vision is also steeper with SWAP than with white-on-white perimetry, and this steepness increases with age.
Because of the combination of variability and age effects, the grayscale appearance for a SWAP field is usually darker than for a white-on-white grayscale, and this may mimic an arcuate field loss that isn’t seen on a white-on-white perimeter grayscale. Therefore, the SWAP grayscale should be interpreted with great care. In addition, monitoring progressive field loss from glaucoma may be more difficult with SWAP than white-on-white perimetry due to variability and age-related factors – in fact, possibly 30 percent of patients with progressive optic nerve structural changes may not show any correlated progression of field loss using SWAP. Table 3 summarizes the benefits and limitations of SWAP.
Table 3. SWAP benefits and limitations
| SWAP Benefits | SWAP Limitations |
| Sooner identification of field loss | Greater inter-test variability |
| Higher correlation to structural nerve changes | Cataracts and media opacities may affect test results |
| May identify field loss in ocular hypertensives | Gray scale artifact may mimic arcuate defect |
| Greater amount of field loss shown than with white on white testing | Field loss progression difficult to monitor |
Understanding the limitations of SWAP, this test can still be used to advantage with selected glaucoma patients. Preferred candidates for SWAP testing are patients without significant media opacities, who are good responders in the testing condition, and who are high-risk ocular hypertensives or patients with early glaucoma. Traditional white-on-white fields are preferred when significant field loss has been established and/or when monitoring of progression over time is required.
As a side-note, SWAP has been found to be useful in identifying field defects due to certain non-glaucomatous conditions such as diabetic retinopathy, macular edema, HIV-related field loss, and neuro-ophthalmic disorders such as pseudotumor cerebri, optic neuritis, and/or multiple sclerosis.
Another relatively recent innovation in perimetry involves the presentation of phase-reversing gratings to assess field loss. Gratings are simply patterns of light and dark stripes (sometimes the stripes are called bands), and phase-reversing means that the light and dark stripes repeatedly exchange positions. Gratings can be defined by their contrast (the difference in brightness between adjacent light and dark stripes), their spatial frequency (the number of bright and dark stripe pairs per degree of visual angle, which is referred to as spatial frequency and is measured in cycles per degree), and the rate of phase reversal rate, which is measured in cycles per second - the unit for cycles per second is Hertz (Hz).
Theory of Frequency Doubling Perimetry
The human retina has approximately a million retinal ganglion cells with nerve fiber axons that bundle together to form the optic nerve. The irreversible loss of these nerve fibers occurs in glaucoma, which results in the classic increase in optic nerve "cup" size.
Figure 6 shows histological cross-section examples of a normal optic nerve and a glaucomatous optic nerve. Figure 7 shows direct images of a normal and glaucomatous optic nerve head, as would be viewed by standard ophthalmoscopy.
Figure 6. Cross-section of normal optic nerve (left) and glaucomatous optic nerve (right). Note the excavation of the glaucomatous nerve cup due to neuronal death. (From http://www.glaucoma-association.com/nqcontent.cfm?a_id=341&=fromcfc&tt=article&lang=en&site_id=176)
Figure 7. Direct ophthalmoscope view of normal optic nerve head (left) and glaucomatous optic nerve head (right). Note the enlarged cupping and excavation of the glaucomatous optic nerve along with bowing of the retinal vasculature. (From www.kseyes.com/ Glaucoma.htm)
Some studies have suggested that up to 40 percent of retinal nerve fibers die before any notable visual field loss can be detected, and both the patient and the doctor may be unaware that there is a visual problem. It can take an average of 4 to 6 years of gradual nerve fiber loss before glaucomatous visual field loss becomes apparent. Other measurements, including intraocular pressure, pachymetry, and gonioscopy may not be diagnostically conclusive on their own.
The development of several scanning laser ophthalmoscopes over the last few years has helped in detailing subtle structural changes in nerve head shape an/or retinal nerve fiber thickness. However, cases arise in which the patient has glaucoma, yet the nerve appearance is normal. This might occur if the patient has a smaller than normal optic nerve in which the nerve fibers are bundled more closely together as they exit the eye. Another example might be a glaucomatous optic nerve with buried disc drusen in which the cup is obscured by calcific material. In these and other cases, visual field testing could be the most valuable tool for identifying and managing glaucoma, as well as differentially diagnosing various other optic neuropathies, e.g., retrobulbar optic neuritis.
When a low spatial frequency sinusoidal grating with wide light and dark stripes undergoes a rapid phase reversal (i.e., the light and dark stripes exchange positions), the grating appears to the viewer to have approximately twice as many light/dark bars, i.e., its spatial frequency appears doubled (see example below). This phenomenon, which was discovered many years ago, is called the frequency doubling effect or illusion.
Figure 8. When the grating on the left is alternated in counterphase (black and white bars alternate with each other) at a relatively rapid rate, the grating on the right appears with twice the spatial frequency of the original grating, e.g., if the grating on the left has one light and dark band per degree of visual angle (one cycle per degree), the grating that appears will have 2 light and dark bands per degree (2 cycles per degree).
Neurological Basis for Frequency Doubling Perimetry
As discussed previously, retinal ganglion cell fibers can be classified into three main types of neurons: magnocellular (or M) cells, parvocellular (or P) cells, and koniocellular (or K) cells.
Figure 9. Schematic diagram of visual pathway, including lateral geniculate nucleus (LGN), occipital lobe, and striate cortex (V1).
Figure 10. Histological cross section of LGN. The koniocellular (K) layers are located between individual LGN layers.
The M-cell pathway is responsible for encoding and transmitting information about low-contrast, high temporal frequency (i.e., motion) stimuli. For example, a black car rapidly passing by a driver’s side window at night would selectively stimulate M-pathway neurons. The P-pathway is responsible for encoding and transmitting information about colored, high-contrast, low temporal frequency (i.e., static) stimuli. The smallest letters on a standard projected Snellen eye chart would selectively stimulate the patient’s P-cell neurons.
Figure 11. Magnocellular (M) cell pathways are responsible for low contrast, high motion stimuli such as would be seen during night driving.
Figure 12. Parvocellular (P) cell pathways are responsible for detecting high contrast, static stimuli such as during visual acuity testing with small letters.
It has been speculated that a portion of the M-cell population serves as the primary basis for the frequency doubling phenomenon. These respond to spatial grating stimuli in a non-linear manner (i.e., they change the apparent spatial frequency of the stimulus by approximately doubling it – the actual range is from 1.5 to 2.5 times the original spatial frequency). This non-linearity may account for the perception of doubling elicited during FDT testing.
(To help understand the concept of non-linearity, assume that a black box system has input and output connections. If the box behaved in a linear manner, an input of one would produce an output of one, two would produce two, etc. A non-linear box might take an input of one and produce an output of two, an input of two would produce an output of five, etc. The inputs and the outputs would not be related in a linear manner.)
New information suggests that the frequency doubling visual perception might also involve complex interactions at higher levels of the visual pathway. For example, metabolic changes have been found to occur in the visual cortex proportional to the degree of glaucomatous retinal ganglion cell loss. It is not clear whether these metabolic changes in the visual cortex are due to loss, atrophy, or reduced activity of neurons. Further functional neuroimaging and/or post-mortem histopathological neural analysis is needed to resolve this question. Regardless, large reductions in ganglion cell fibers have far-reaching neurological implications.
It is the M-cell neurons that are thought to be responsible for the doubling phenomenon. Because these ganglion cell neurons die as a result of glaucoma, and because there are only a relatively small number of M-cells, selective testing by presenting phase reversing grating stimuli was developed to identify retinal neuron loss (specifically M-cell loss) earlier than could be done by using traditional automated white-on-white perimetry (which may actually be a test of P-cell function).
SWAP testing was useful because it detected the loss of K-cells at an early stage in glaucoma. This was because there was relatively little redundancy in the retinal coverage by K-cells, and the loss of a small number of K-cells in any retinal area was detectable by using blue-yellow perimetry. In a similar manner, there are relatively few M-cells so there is only limited redundancy in coverage of retinal areas, i.e., if a few M-cells die, there are no M-cells with overlapping coverage and the effects of the loss can be assessed by using frequency doubling perimetry. Standard white-on-white perimetry, which probably measures primarily P-cell loss, is thought to be less sensitive to glaucoma damage because the high number of redundant P-cells allows many of them to die before there is a perceptual effect that can be measured.
Discovery of the frequency doubling phenomenon resulted in development of the Frequency Doubling Technology (FDT) Perimeter by Humphrey/Welch-Allyn/Zeiss.
Figure 13. Frequency Doubling Perimeter
The Humphrey Frequency Doubling Technology (FDT) Perimeter (Welch Allyn, Skaneateles, N.Y. and Carl Zeiss Meditec, Dublin, Calif.) is a portable device that specifically tests for visual field loss due to M-cell neuron death, typically from glaucoma. It includes a screening mode as well as two full-threshold modes. The screening mode is accurate and typically takes less than a minute to perform per eye. Contrast of the targets is reduced at various levels for the threshold testing mode.
Patients who undergo this form of testing do not have to determine whether the stimulus grating stripes are doubled in frequency, they only have to press a response button each time they see a grating within their field of view.
Figure 14. Performing the FDT Perimetry Test
In the screening mode, the FDT uses patient age to select a single grating contrast for each of 17 different locations on the display screen. (Contrasts for the different locations can be different, but each location has only one contrast presented.) When the grating is presented, the patient either detects it or he or she does not. There is no searching for a threshold.
In the threshold mode, the contrast is varied for each of the test locations until the threshold contrast for that location is found. Threshold contrast is the minimum difference in brightness between the stripes that just allows the grating to be seen.
The FDT is relatively easy to use with a series of menu screens that allow selection of the test to be conducted (e.g., screening versus threshold), the age of the patient, report printing, etc. Instructions to the patient are also quite simple: "look at a black dot in the center of the screen and press the button any time the black and white stripes are seen." During the test, a square grating stimulus is presented for 720 millisecond durations at multiple locations within the central visual field. A built-in photometer monitors and calibrates display brightness, and can detect any output problems.
Figure 15. FDT Stimulus presentation example
A special threshold visual field protocol allows two extra 10-degree points in the nasal visual field to be tested, which results in a 50 degree total horizontal field. Because this instrument targets a specific sub-set of nerve fibers that transmit information about larger, low-contrast, motion-based stimuli, FDT perimetry results are not affected by up to 6 diopters of blur. Results are also not affected external room illumination or variations in pupil size so long as the pupil diameter is greater than 2.0 mm.
Table 4. Humphrey FDT Perimeter Tests
| Test Pattern | C-20 Screening | C-20 Threshold | N-30 Threshold |
| Stimulus Size and Location | 20 degree radius (comprised of seventeen 10 degree squares including one 10 degree central target) | 20 degree radius (comprised of seventeen 10 degree squares including one 10 degree central target) | 20 to 25 degree radius (comprised of nineteen 10 degree squares including one 10 degree central target) |
| Stimulus Presentation | 0.25 cycles/degree; 25 Hz | 0.25 cycles/degree; 25 Hz | 0.25 cycles/degree; 25 Hz |
| Test Time* | 45 seconds | 4 minutes | 5 minutes |
* Times may be longer depending on patient response time and visual field loss
Two FDT screening test protocols are available. The C-20-1 test uses age-corrected normal responses selected so that 1% of the normal population is likely to miss one or more stimuli, and the C-20-5 test uses age-corrected normal responses selected so that 5% of the normal population is likely to miss one or more stimuli.
The FDT perimeter uses central static fixation with classic Heijl-Krakau (blind spot) fixation checks. A smaller size (5 degree) stimulus presentation at approximately 15 degrees temporal to central fixation serves this purpose. Any patient response given at this location, which is over the blind spot, indicates poor fixation.
FDT screening mode perimetry is considered abnormal when the following are present:
In the threshold mode, field defects are shown on printouts as varied gray scale densities called probability symbols. The darker the gray, the less probable (based on age-related norms) that the defect is a normal occurrence. As seen on the FDT threshold printout example in Figure 16, probability varies from 5% (somewhat unlikely that the defect is normal) to less than 0.5% (very unlikely that the defect is normal).
The FDT perimeter test strategy uses a MOBS (Modified Binary Search) threshold determination procedure, which is more advantageous over the standard staircase threshold determination algorithm.
Extended testing time can suggest a slow responding patient, uncertainty regarding instructions for the test, slow perception of the stimuli, or problems finding thresholds. Any of these could indicate an invalid test.
Figure 16. Normal FDT Threshold N-30 printout.
Reliability indices (fixation, false positive, and false negative errors) are shown on the printout along with Mean Deviation (average deviation from a normal visual field based on age-related norms) and the Pattern Standard Deviation (a measure of how different the patient's thresholds are from a normal profile in the overall field). Clinically, deviations from normal for any of these statistical measures might imply either difficulty in testing (based on reliability indices) and/or a disease process affecting the visual field (based on deviation values).
Even when just using the screening test paradigm, several studies have demonstrated the FDT sensitivities above 77 percent and specificities above 84 percent for the detection of glaucoma when the diagnosis of glaucoma was based on criteria including abnormal optic nerve photographs, white-on-white visual fields, and/or other clinical criteria. With high sensitivity and specificity for detecting glaucoma-related visual field loss, the FDT perimeter shows great potential for the early and rapid detection of glaucoma.
Along with testing for glaucoma, current software upgrades allow presentation of stimuli displaced on either side of the vertical midline to facilitate identification of post-chiasmal neurological visual field defects.
Figure 17. Humphrey Matrix Perimeter
The Humphrey Matrix Perimeter (Welch-Allyn, Skaneateles, NY and Carl Zeiss Meditec, Dublin, CA) was developed to use the FDT paradigm but also to address limitations of the original Humphrey FDT perimeter. Several of the new features of the Matrix as compared to FDT include:
Like the FDT, the Matrix can perform screening and full threshold visual field tests. Screening tests present stimuli that are well above the threshold contrast level determined on the basis of patient age. Screening tests are useful when full threshold tests are found to be too time-consuming or difficult for certain patients (those who are elderly, slow responders, etc.), when the visual field defect is severe enough to be evident even with suprathreshold contrast stimuli, or when a basic field test is required without the need for detailed assessment.
Along with providing the standard screening tests available with the original FDT perimeter, the Matrix utilizes 2 degree and 5 degree angular subtense stimulus targets in new threshold and screening tests. The smaller stimulus sizes increase the ability of the Matrix to identify small scotomas and correspond with the stimulus presentation pattern of standard 24-2 and 30-2 white-on-white visual fields presented with the Humphrey II Visual Field Analyzer.
A summary table showing the various types of Matrix threshold and screening tests are shown Tables 5 and 6.
Table 5. Humphrey Matrix Threshold Test parameters
| Test Pattern | Macula | 10-2 | 24-2 | 30-2 | N-30-F |
| Stimulus Size and Location | Central 4 degree radius (comprised of sixteen 2 degree by 2 degree squares) | Central 10 degree radius (comprised of forty-four 2 degree by 2 degree squares) | 24 degree radius (comprised of fifty-five 5 degree by 5 degree squares including one circular 5 degree central target) | 30 degree radius (comprised of sixty-nine 5 degree by 5 degree squares including one circular 5 degree central target) | 20-25 degree radius (comprised of nineteen 8-9 degree x 9-10 degree squares including one 10 degree central target) |
| Stimulus | 0.5 cycles per degree; 12 Hz | 0.5 cycles per degree; 12 Hz | 0.5 cycles per degree; 18 Hz | 0.5 cycles per degree; 18 Hz | 0.25 cycles per degree; 25 Hz |
| Test Time* | 1.5 minutes | 4.5 minutes | 5.5 minutes | 6.5 minutes | 2.5 minutes |
*Times may differ depending on patient response and degree of visual field loss.
Table 6. Humphrey Matrix Screening Tests
| Test Pattern | N-30 Screening | 24-2 Screening |
| Test Stimuli | Nineteen 8-9 degree by 9 to 10 degree squares including one circular 10 degree central target | Fifty-five 5 degree by 5 degree squares including one circular 5 degree central target |
| Stimulus Presentation | 0.25 cycles per degree; 25 Hz | 0.5 cycles per degree; 18 Hz |
| Test Protocols Available | 1% (99% of population should see stimulus) and 5% (95% of population should see stimulus) | 1% (99% population should see stimulus) and 5% (95% of population should see stimulus) |
| Test Time* | Under 1 minute | 1.5 minutes |
* Times may differ depending on patient response and degree of visual field loss.
Zippy Estimation of Sequential Thresholds
Full threshold testing using the Matrix incorporates a new algorithm called ZEST (Zippy Estimation of Sequential Thresholds). This algorithm shortens the process of determining thresholds in a similar manner to that used by SITA (Swedish Interactive Threshold Algorithm). Like SITA, it makes intelligent predictions of the patient's most likely threshold values and uses these predictions to avoid presentation of contrast values that are too high or too low.
ZEST uses Bayesian statistics to compare response probabilities and behaviors of normal patients to those of patients with visual field loss. Specifically, to determine contrast values to be presented, ZEST considers:
The 24-2 and 30-2 threshold test parameters used by the Matrix compare well with stimulus presentation area and display patterns used in the 24-2 and 30-2 threshold tests on the Humphrey II visual field analyzer. The stimulus point positions for each instrument correspond closely in terms of field location. However, the Matrix has a 3 degree sparing along the vertical meridian and a 1 degree sparing along the horizontal meridian. The 3 degree vertical sparing allows better definition of post-chiasmal neurological-based field loss (which tends to obey the vertical midline of the visual field) and the 1 degree horizontal sparing allows better definition of glaucoma-based field loss (which tends to obey the horizontal midline of a visual field).
An age-related statistical analysis program comparable to the program available for the Humphrey II comes with the Matrix. The Matrix program was derived from measurements from both eyes of more than 270 normal control subjects between the ages of 18 and 85.
The traditional visual field indices (grey scale, mean deviation, total and pattern deviations, pattern standard deviation, and glaucoma hemifield test) are displayed on the Matrix output using a format comparable to the Humphrey II visual field analyzer. Each component has a specific purpose. The Grey Scale allows a qualitative view of the visual field, which may be mainly useful for patient education purposes. The Mean Deviation (MD) shows how much, on average, the whole field departs from normal and is a weighted average of the decibel deviations from normal. The Pattern Standard Deviation (PSD) reflects irregularities in the field, such as those caused by localized field defects. Total Deviation (TD) Numeric Plot shows the point-by-point differences in decibel (sensitivity) levels compared to corresponding age-normed decibel levels. The significance of these deviations from normal are indicated in the associated Total Deviation Probability Plot in which sensitivities that are worse than those found in 5% of normal subjects of the same age are indicated with appropriate symbols. Pattern Deviation Numeric Plot reduces the effects of media opacities (i.e., cataracts) and age effects to provide point-by-point decibel differences compared to corresponding normal decibel levels. The significance of these deviations from normal are indicated in the associated Pattern Deviation Probability Plot with the same symbols used in the Total Deviation Probability Plot.
As with the Humphrey II, a Glaucoma Hemifield Test (GHT) analysis is performed by the Matrix for the 24-2 and 30-2 fields. This analysis compares a cluster of stimulus points above the horizontal meridian to a mirror-image cluster of stimulus points below the horizontal meridian. Results of this analysis are categorized as "normal" (upper and lower clusters show little or no sensitivity difference between corresponding points), "borderline" (upper and lower clusters show mild differences in sensitivities between corresponding points), "outside normal limits" (upper and lower clusters show significant differences in sensitivities between corresponding points), or "abnormally high sensitivity" (decibel levels are abnormal due to media or foveal threshold factors).
In glaucoma diagnosis and management, the Pattern Deviation Probability Plot and Glaucoma Hemifield Tests are two of the most useful analyses that can help to determine whether the patient has glaucoma and whether it is progressing because they highlight only significant localized visual field loss. Matrix threshold tests can be evaluated similarly to the traditional white-on-white visual field tests in this regard.
The Matrix macular tests (10-2 and Macula) do not utilize the frequency doubling phenomenon because of the relatively small number of M-cells in the macular area due to the spatial and temporal properties in the macular area. Therefore, a 12 Hertz flickering stimulus is used for these tests which is more suitable for detecting problems involving the macular area, e.g., macular degeneration and diabetic maculopathy.
Although newer objective instruments allow a detailed, quantifiable means of determining retinal nerve fiber thickness, optic nerve cupping, ocular pressure, pachymetry, and retinal blood flow, there are still many variances in what is considered "normal" with these tests. In addition, a "normal" retina does not necessarily mean normal visual functioning. As such, the ultimate criteria for glaucoma is still functional retinal sensitivity loss. This functional sensitivity loss can only be determined by subjective measurement through visual field testing. Unfortunately, even automated perimetry can be a burden for both the patient and the tester, because threshold visual field testing can take an exceedingly long time.
Frequency doubling perimetry can provide a more rapid means of testing, thus reducing patient fatigue. However, even with more rapid testing, frequency doubling perimetry has been found to correspond closely with glaucomatous field defects found with traditional white-on-white Humphrey 30-2 threshold visual fields. This is especially the case for the FDT Matrix. One study showed that when compared to white-on-white perimetry results as obtained from the Humphrey II Visual Field Analyzer, the traditional FDT perimeter results are approximately:
Another study found that both inter- and intra-test variability was lower for FDT 24-2 threshold fields as compared to traditional white-on-white automated Humphrey perimetry fields.
When the results of FDT and SWAP visual field testing were compared, it was found that both stimulus testing modalities identified field loss in ocular hypertensives who had normal white-on-white threshold visual fields. This suggests that defects detectable with FDT precede traditional white-on-white defects, and this strengthens the diagnostic value of the FDT.
However, some research has shown that FDT perimetry may not be more sensitive than conventional perimetry in certain situations, such as in early peripheral glaucomatous visual field loss outside of 30 degrees (which is beyond the testing range of some FDT fields), and with neurological defects that spare the central visual field. However, there does appear to be a strong correlation with conventional threshold perimetry for central glaucomatous field changes, and may be superior in identifying moderate to advanced glaucomatous visual field loss. The newer FDT Matrix may address these field limitation issues due to its larger visual field radius as compared to the original FDT perimeter.
Table 7. Frequency Doubling Perimetry Benefits
| Ease of Use – very simple menu, easy for both operator and patient |
| Speed – screening tests approximately 1 minute, threshold tests 3 to 8 minutes/eye |
| Environment-Resistant – refractive error blur up to +/-6 diopters and room lighting are not significant factors in screening |
| Relatively Small Size and Portability |
| Few Internal Moving Parts |
| Threshold tests potentially may identify glaucomatous defects sooner than traditional white-on-white visual fields |
At this time, the original FDT perimeter may serve as a good diagnostic tool, whereas the Matrix is a good instrument for both diagnosis and detailed monitoring of glaucomatous progression. Frequency Doubling Technology perimetry can be incorporated as either an adjunct test for those practitioners who are managing glaucoma patients and/or as a routine diagnostic test for lower-risk patients.
Several years ago, glaucoma was considered by some to be a relatively straightforward condition in which elevated IOP caused mechanical compression of axons or blood vessels in the optic nerve as it exited the eye. This constriction resulted in ganglion cell death, with the larger M-type ganglion cells dying first.
More recently, this simple theory of glaucoma and selective M-cell death has been questioned and some now believe that elevated IOP is only a symptom of an underlying disease process that kills ganglion cells. Additionally, it is no longer universally accepted that M-cells are the first to die as a result of glaucoma. Instead, P-, K-, and M-cells all may die at about the same rate, but the effects of K- and M-cell loss are more readily detectable because there are fewer of these cells.
A key concept is redundancy. There are a relatively large number of P-cells and they overlap each other in terms of retinal location and function. Hence, many P-cells can be lost before there is a noticeable field loss - as might be assessed by using white-on-white perimetry.
There are significantly fewer K- and M-cells so losing a proportion of these cells is more readily detected with perimetry. The two perimetry techniques described in this course assess K- and M-cells losses, respectively. SWAP makes use of the blue-yellow chromatic opponent characteristics of the K-cells and is sensitive to loss of these cells. FDT makes use of the non-linearity property of some M-cells and is designed to assess the death of these cells. Taken together or used separately, SWAP and FDT can be very useful in the early detection of ganglion cells death associated with glaucoma and other visual system diseases.
In conclusion, both SWAP and FDT perimetry provide unique, efficient means of evaluating visual field loss, particularly with glaucoma patients. Further hardware and software upgrades are likely to refine these forms of visual field testing. In addition, continued longitudinal studies evaluating the efficacy of these instruments and their correlation to structural nerve damage will likely help refine the utility of their use. The simplicity and effectiveness of SWAP and FDT perimetry will likely make them powerful tools in the management of glaucoma patients for years to come.
A 57-year-old Caucasian male presented with a prior diagnosis of early glaucoma. Dicon Threshold Grid 76/30 visual field testing (Figures 18 and 19) showed a diffuse, superior temporal arcuate area of reduced sensitivity OD and non-significant changes OS. Frequency Doubling Technology N-30 Threshold Visual Field Testing (Figures 20 and 21) showed much greater visual field loss, with inferior arcuate pattern scotomas in each eye, with the right eye loss greater than left eye loss. Based on the FDT test results, the patient was subsequently placed on more aggressive treatment by adding a topical prostaglandin to his drug regimen.
Figure 18. OD Dicon 76/30 Threshold Visual Field – Note superior arcuate-like defects.
Figure 19. OS Dicon 76/30 Threshold Visual Field – No significant defects noted.
Figure 20. FDT N-30 Threshold Visual Field from right eye. Note the significant inferior arcuate scotoma.
Figure 21. FDT N-30 Threshold Visual Field from left eye. Note the significant inferior arcuate scotoma.
A 67-year-old Caucasian female with a history of primary open angle glaucoma presented for optic nerve photos (Figure 22) and visual field testing. She was currently taking Xalatan® (latanoprost). No other health problems were noted. Visual acuities were 20/15- in each eye, and hyperopia and astigmatism were noted. Intraocular pressures were 14 mmHg in each eye. Left eye optic nerve and visual field findings showed no abnormalities and are not presented here.
Figure 22. OD Optic Nerve Head. Image contrast is degraded due to the patient's cataract/media opacity.
24-2 SWAP (blue-on-yellow) and FDT Matrix threshold visual field tests were performed on the same day for the patient's right eye. SWAP test results (Figure 23) showed an inferior cluster of mild defects on the Pattern Deviation plot and a normal Glaucoma Hemifield Test result.
Results of FDT Matrix testing (Figure 24) showed an inferior arcuate-like pattern of sensitivity reduction and an abnormal GHT result. The overall sensitivity reduction (in dB) was also worse than for the SWAP visual field.
Based on the FDT Matrix test results, the diagnosis of early glaucomatous field loss was confirmed for this patient’s right eye.
Figure 23. SWAP 24-2 field from right eye – Note mild inferior clustered defects and ‘normal’ GHT.
Figure 24. FDT Matrix 24-2 field from right eye. Note inferior arcuate pattern loss and ‘abnormal’ GHT.
A 53 year-old Caucasian male presented for optic nerve photos (Figures 25 and 26) and visual field testing. His history was positive for glaucoma, and he was taking timolol and Xalatan® (latanoprost). Visual acuities were stable at 20/20 (6/6) in each eye, ocular pressures were in the mid-teens and pachymetry gave central corneal thickness of 486 microns each eye. The patient had low myopia and astigmatism, and no other health problems.
Figure 25. OD Optic Nerve Head
Figure 26. OS Optic Nerve Head
Threshold 24-2 SITA-Standard (white-on-white), SWAP (blue-on-yellow), and FDT Matrix visual field testing was performed on each eye. All of these tests were performed on the same day.
Results from the SITA-Standard White-On-White tests (Figures 27 and 28) show that the Glaucoma Hemifield Test (GHT) from the right eye is ‘Within Normal Limits" although a few isolated temporal defects show in the Pattern Deviation plot. The GHT results for the left eye are "Outside Normal Limits" and the field shows a deep superior arcuate scotoma.
SWAP test results (Figures 29 and 30) show that the GHT is "Within Normal Limits" for the right eye with no pattern defects noted. The GHT is "Outside Normal Limits" for the left eye and the field shows a deep superior arcuate scotoma consistent with the SITA-Standard visual field.
FDT Matrix results from the right eye (Figure 31) show that GHT analysis is "Within Normal Limits." However, several mild-moderate temporal field defects are noted and there is a greater overall sensitivity reduction than was found with white-on-white or SWAP testing. The GHT result from the left eye (Figure 31) is "Outside Normal Limits" and the field shows a deep superior arcuate scotoma. Both the defect depth and the overall sensitivity reduction are greater than are shown on either the SITA or the SWAP visual fields.
Figure 27. SITA 24-2 field from right eye. Note the mild temporal defects and ‘normal’ GHT.
Figure 28. SITA 24-2 field from left eye. Note the dense superior arcuate defect and ‘abnormal’ GHT.
Figure 29. SWAP 24-2 field from right eye. Note the absence of defects and ‘normal’ GHT.
Figure 30. SWAP 24-2 field from left eye. Note the superior arcuate defect and ‘abnormal’ GHT.
Figure 31. FDT Matrix 24-2 field from right eye. Note the clusters of temporal defects and ‘normal’ GHT.
Figure 32. FDT Matrix 24-2 from left eye. Note the deep superior arcuate scotoma, ‘abnormal’ GHT, and large overall sensitivity reduction (MD or Mean Deviation = -10.97 dB)
For each of the three case studies, FDT testing tended to show more pronounced field defects, which suggests that the FDT testing method might be more sensitive to field defects than traditional white-on white or SWAP testing. Further clinical and research studies will be needed to confirm this suggestion.
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Acknowledgements: The author of this course extends sincere thanks to Dr. Chris Johnson, Cindy Blachly, and Thie Smith at Discoveries in Sight/Devers Eye Institute, Portland Oregon for their gracious assistance and resources. These acknowledgements do not imply agreement with or acceptance of any material presented in this course.
Disclaimer: The author does not endorse or have proprietary financial interest in any of the products mentioned in this article.
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Lorne Yudcovitch, O.D., M.S., F.A.A.O. Pacific University College of Optometry 2043 College Way Forest Grove, OR 97116Note: Pacific University College of Optometry provides On-Line CE as a service to optometrists. The college does not endorse or recommend any products, equipment, or services that might be discussed in the courses. Courses are prepared by individuals believed to be experts in their areas of specialization who are compensated for their efforts. The College relies on their expertise to produce accurate and timely courses. Questions or concerns about courses should be directed to the individual authors and/or the Continuing Education Department at the College of Optometry at kundart@pacificu.edu.
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