FDT and SWAP: Advancing Glaucoma Diagnostics
Part 3 of our glaucoma management series focuses on the growing role of FDT and SWAP in early detection and diagnosis, how they can be used, and documenting for reimbursement.
BY ANDREW RABINOWITZ, M.D.

A great deal of research has been aimed at creating technologies that reveal functional losses earlier in the disease process. Additional emphasis has been placed on finding test modalities that reveal functional loss at a rate commensurate with axonal cell damage. This would allow us to adjust the IOP target range in a given patient over a period of weeks to months, as opposed to years.

Frequency doubling technology (FDT) perimetry and short-wavelength automated perimetry (SWAP) offer unique opportunities toward the goal of earlier detection and diagnosis. Both of these technologies are still in their infancy and continue to evolve. In spite of their relative immaturity, both are currently commercially available and have engendered huge interest among ophthalmologists, especially for their possible potential in providing more accurate diagnoses of younger ocular hypertensives who have "normal" SAP. Additionally, they offer cost-effective, user-friendly adjuncts to threshold SAP.

This article will discuss the theory behind the FDT and SWAP technologies, their applications in the management of glaucoma, and how to code and document their use.

Frequency Doubling Technology (FDT)

When a low spatial frequency sinusoidal grating (<1cycle/degree) undergoes high temporal frequency counter-phase flicker (>15Hz), the stimulus appears to have twice as many light and dark bars as are physically present.^1,2 This phenomenon is known as the frequency doubling illusion. Maddess and associates^3,4 postulated that the frequency doubling effect might be useful in detecting visual field damage in glaucoma. They proposed that targets are detected by a subpopulation of magnocellular ganglion cells, (My cells). The My cells project to the magnocellular layers of the lateral geniculate nucleus (LGN). My cells comprise roughly 25% of the entire magnocellular axon population. My cells have larger cell bodies than other M cells, little receptive field overlap, and a nonlinear response.

My cells have the largest diameter axons among retinal ganglion cells. Several studies have suggested that axons that project to the magnocellular layers of the LGN are more susceptible to early glaucomatous cell death.^5,6,7 Magnocellular axons are thought to be most vulnerable to ischemia, and therefore may be the first axons lost in early glaucoma.⁸

Testing magnocellular function by way of contrast sensitivity may reveal the earliest signs of glaucomatous visual field loss. Studies have demonstrated that contrast sensitivity testing provides relatively high sensitivity and specificity in diagnosing early glaucoma. Subsequent work has likewise shown good sensitivity and specificity for the frequency doubling test procedures for both threshold^9-12 and supra-threshold screening^13,14 test strategies.

Cello et al. evaluated the reliability of FDT in detecting glaucoma. For early glaucomatous visual field loss, sensitivity and specificity was 85% and 90% respectively. For moderate and advanced glaucomatous visual field loss, both the sensitivity and specificity exceeded 97%. These findings indicate that FDT perimetry can be used to detect glaucomatous visual field loss with performance levels comparable to those achieved by full-threshold test strategies in standard achromatic perimetry.¹⁵

Comparing FDT to SAP

A study by Quigley et al. found that FDT perimetry compared more favorably with standard threshold achromatic perimetry data when individual quadrant data were compared. The mean cup/disc ratio in eyes with normal FDT perimetry (fewer than two points missed) was 0.47 +/- 0.16 (n-38), compared to 0.76 +/- 0.19 in patients with abnormal FDT perimetry results. The sensitivity and specificity was greater than 90%, which is comparable to standard threshold achromatic perimetry (level II evidence). This evidence suggests that FDT perimetry can detect glaucoma damage in its earliest stages.¹³

A case series by Lester et al. compared FDT perimetry with standard threshold achromatic perimetry in POAG patients, glaucoma suspects, and ocular hypertensive patients. FDT perimetry showed a high sensitivity and specificity for screening and discriminating healthy individuals from those with glaucoma, and for quantifying glaucomatous damage. The data revealed a high correlation between FDT and pattern standard deviation on standard threshold perimetry.¹⁶ Results from Sponsel et al. support the existence of a linear correlation between FDT perimetry mean deviation and standard threshold achromatic perimetry 30-2 mean deviation.¹¹

FDT perimetry can be used to detect glaucomatous visual field loss as well as to characterize its severity. The ability of FDT perimetry to detect glaucomatous progression is currently being investigated. FDT perimetry may have less intertest and intratest variability when compared to standard threshold achromatic perimetry.¹⁷ Most importantly, FDT perimetry may detect deficits at an earlier stage than standard achromatic perimetry.¹⁶

Short-Wavelength Automated Perimetry (SWAP)

Short-wavelength automated perimetry is a visual field test designed to assess the short-wavelength-sensitive color system. A relationship between color vision deficits and glaucoma has been well documented.^18-21 SWAP was developed to capitalize on the involvement of the short-wavelength sensitive system in glaucomatous optic neuropathy, and to provide a perimetric measure of peripheral function.

SWAP isolates the short-wavelength-sensitive pathways throughout the retino-geniculo-cortical route. It originates at the receptor level in the retina with the short-wavelength (blue) cones. These cones send projections to the blue-cone bipolar cells, which in turn project to the small bistratified retinal ganglion cells. The small bistratified retinal ganglion cells are distinct from the parvocellular axons.²²

Small bistratified retinal ganglion cells make up approximately 9% of the entire population of retinal ganglion cells. They are sparsely distributed across the retina. Initially, it was thought that SWAP was able to detect early loss because the bistratified cells are affected first by glaucoma.²³ An alternative theory has suggested SWAP is able to detect early loss in glaucoma because it's specifically testing only one type of ganglion cell.²⁴ When the short-wavelength-sensitive pathway is isolated, a deficit may be manifest even when a small proportion of cells is affected. This occurs because even if other cell types are still functioning in a given retinal area, they are unable to detect the stimulus until it becomes much brighter than normal. The redundancy in the system is therefore reduced.²⁴

To isolate the short-wavelength-sensitive pathway, SWAP uses different stimulus and background parameters than SAP. A 440-nm narrow-band target that subtends 1.8" (Goldmann size V) of visual angle is presented for 200 ms on a bright broadband 100 cd/m2 yellow background (530 nm and higher) The background serves to adapt the middle- and long-wavelength-sensitive pathways while simultaneously saturating the activity of the rods so only the short-wavelength-sensitive pathway mediates target detection.²⁵

There are two commercially available perimeters with SWAP capability -- the Humphrey Field Analyzer II (HFAII) from Carl Zeiss Meditec, Dublin, Calif., and the Octopus 1-2-3, 101 and 311 (optional on model 301) from Haag-Streit USA, Mason, Ohio.

Studies have established that SWAP deficits occur prior to those for standard achromatic perimetry and are predictive of future glaucomatous visual field deficits for standard achromatic perimetry.^26,27

In glaucoma patients, SWAP defects are larger than those obtained with standard achromatic perimetry, and the rate of progression is greater for SWAP than for standard achromatic perimetry.²⁸ SWAP deficits are more prevalent in glaucoma suspects who have a higher risk probability of developing glaucomatous visual field loss.²⁹ SWAP deficits are highly correlated with optic disc abnormalities.^29,30 Although the prevalence of SWAP deficits is greater than for SAP, the incidence of new deficits is equivalent for SWAP and standard achromatic perimetry.³¹ This suggests that both standard achromatic perimetry and SWAP are detecting the same underlying pathophysiologic processes, but that SWAP is detecting it at an earlier stage.³²

SWAP results are affected by cataract. Moss has reported that posterior subcapsular cataracts have a greater effect on SWAP than on SAP, whereas anterior cortical cataracts have a larger effect on SAP than on SWAP.³³ SWAP takes slightly longer to perform than standard achromatic perimetry and has moderately greater intertest and intratest variability.²⁵

SWAP in Early Detection

The emphasis on early detection of glaucomatous optic neuropathy continues to grow. Test modalities that detect glaucoma earlier than evidenced by SAP are of paramount importance. Studies by Johnson and Sample have demonstrated that SWAP deficits can appear 3 to 5 years before SAP. Additionally, SWAP deficits are predictive of the onset of future visual field loss.^26,34 Johnson has shown that SWAP deficits are present in 15 to 30% of patients with ocular hypertension who have normal results on SAP.²⁴ Moreover, the prevalence of SWAP deficits is greatest in high-risk ocular hypertension patients, and decreases progressively in moderate- and low-risk ocular hypertension patients²⁴. Furthermore, SWAP deficits are more prevalent in glaucoma suspects who have a higher risk for glaucomatous visual field loss.²⁹

Monitoring Progression

SWAP has also been shown to be of value in monitoring progression. A study by Spry has suggested that with SAP, at least seven visual field tests are required to assess progression with certainty using trend analysis.³⁵ Among glaucoma patients, the rate of progression was faster for SWAP than for SAP.^24,28 SWAP deficits are predictive of future progression of glaucomatous visual field loss on SAP²⁴.

Structure Relates to Function

Numerous studies have demonstrated a significant relation between optic nerve head abnormalities and visual field defects observed with SWAP³⁰. SWAP deficits have been shown to correlate well with glaucomatous neuroretinal rim abnormalities, retinal nerve fiber layer defects, and optic disc damage.^36,27,38

The pattern of localized SWAP deficits conforms to the traditional nerve fiber bundle deficits that are typical of SAP.²⁴ Teesalu et al. have reported that the correlation between structural changes observed with scanning laser ophthalmoscopy in early glaucoma and the mean deviation of SWAP was equal to or better than that observed for the mean deviation of standard achromatic perimetry, depending on which parameter was assessed.³⁹

Historically, the testing time needed to complete SWAP has been greater than for standard achromatic perimetry. The introduction of SITA-SWAP is expected to greatly reduce test time and improve test reliability and reproducibility. It's evident from the data provided above that the value of SWAP in the early diagnosis, as well as the monitoring of progression among existing glaucoma patients can't be overemphasized. The utility of SWAP is clearly in its infancy, but will likely grow over the coming decades.^40,41

New Tools for Your Diagnostic Arsenal

FDT has many unique advantages as an adjunct to standard threshold achromatic perimetry. These include reduced testing time and relative immunity from the effects of pupil size and blur. It's currently accepted that FDT perimetry is most useful as a screening tool in a limited examination. But it's important to note that advances in the technology will likely expand its utility. Future applications of FDT are likely to include its use as tool for monitoring progression in moderate-to-advanced glaucomas.

SWAP has demonstrated value as a diagnostic tool for detecting early glaucomatous loss. Historically, one of the greatest challenges for SWAP has been long and tedious test algorithms. Fortunately, the impending introduction of SITA-SWAP will likely reintroduce short-wavelength testing for all stages of glaucomatous optic neuropathy.

Applying both FDT and SWAP can bolster the arsenal of functional glaucoma testing. The current gold standard of functional testing is standard threshold achromatic perimetry, and neither of these technologies is intended to replace the current gold standard. When one or both of these technologies are applied as adjuncts to standard threshold achromatic perimetry, we'll be able to detect damage earlier than previously possible. Additionally, we may be able to assess progression at a rate more commensurate with its structural occurrence. Currently, axonal cell loss may precede perimetric evidence of such cell loss by years. This delay in perimetric detection will hopefully be reduced from years to months with proper use of SWAP and FDT.

When to Use FDT

At the time of initial consultation, glaucoma suspects may be given the limited FDT test. FDT has a high sensitivity but a modest specificity. As a consequence, it suffers from a high number of false-positive test results.

If the results of the initial FDT test are suspicious for a pathologic result, the patient's subsequent visit should include threshold FDT testing to increase the specificity of the testing and weed out false positives.

Threshold SAP testing serves to help grade the degree of disease. A normal threshold SAP following an abnormal FDT test may suggest early glaucoma. An abnormal threshold SAP in conjunction with an abnormal FDT likely suggests at least moderate glaucomatous damage. If either threshold SAP or threshold FDT are abnormal, then each of these tests should be performed annually. The utilization of annual threshold SAP testing is widely viewed as standard of care if glaucoma is suspected or confirmed. This holds true whether the initial threshold SAP revealed normal or abnormal results. Utilization of annual threshold FDT has not yet been accepted as standard of care. Historically, performing annual threshold FDT was thought to be of marginal benefit once threshold SAP tests revealed abnormalities. Current thought, however, suggests that serial threshold FDT may be of great benefit even in the presence of abnormal threshold SAP. In this setting, serial threshold FDT may be helpful in screening for novel scotomas in quadrants that appear to be uninvolved on threshold SAP testing.

SWAP: a Versatile Tool

If both the initial FDT test and subsequent threshold FDT testing suggest glaucomatous scotomas, SWAP testing may then be employed to further discern which subpopulation of My cells may be involved. SWAP testing may also be used independently of FDT.

Many practices currently have threshold SAP and SWAP testing capabilities, but don't yet have FDT perimeters. Much like threshold FDT, SWAP can be used to detect the presence of early glaucomas in patients with normal threshold SAP test results.

If glaucoma is suspected, a patient may undergo baseline threshold SAP testing. If this test is interpreted as normal, then SWAP perimetry may be employed on the patient's subsequent visit. Both SWAP and threshold SAP may each be performed annually in suspect and confirmed glaucoma cases. The rationale for using each test annually is analogous to the logic in alternating threshold FDT and SAP. Thus, SWAP isn't just a tool for screening early glaucomas, but may also be used to monitor progression in patients with acknowledged SAP abnormalities. Serial SWAP perimetry can be used to follow uninvolved quadrants in patients with confirmed threshold SAP scotomas.

Code These Tests Properly

Coding for these ancillary functional tests varies with their application. When FDT perimetry is used in a limited examination, the appropriate code is 92081 for visual field limited. When the full threshold program is used, the 92083 for visual field extended may be utilized. The complexity of SWAP warrants the 92083 code as well. This code should also include the soon-to-be-introduced SITA-SWAP.

The national average Medicare reimbursement for 92081 is $45.92; for 92083 it's $69.07 (both are bilateral). FDT and SWAP can each be billed once a year.

Proper documentation should accompany each testing session. When documenting the plans for future visits, the chart should reflect the rationale for ordering subsequent perimetry. Providing that the documentation is thorough and well conceived, third-party payers will usually reimburse the ordering physician. Abuse of perimetric testing exposes the physician to scrutiny by government and private third-party payers. When used appropriately, combining the information provided by these technologies greatly increases the sensitivity and specificity in detecting glaucoma. A general rule of thumb is that only one perimetric test should be performed on any given visit.

Chart documentation for perimetric tests should include some basic core comments. These comments can be written directly onto the visual field test results along with the physician signature. Alternatively, a unique form may be generated to comment on the visual field results, which can serve as accompanying documentation.

Make Patients into Partners

It's my experience that patients often struggle to understand the reasons their physicians work so diligently to lower their IOP. I've found that sharing the results of their tests can increase their compliance. I routinely see new patients in consultation who've had glaucoma for nearly a decade. More often than not, these patients have never been shown their visual field results. Similarly, a majority of these patients have no concept of what their optic nerve looks like. I find it quite useful to review the visual field results with the patient present. I can show them what their results are relative to nonglaucomatous normal eyes. Every time a patient has an ancillary test such as scanning laser ophthalmoscopy, I make sure that they see the results.

Patients who are shown concrete evidence of their disease, or of disease progression, tend to be more active participants in their care. Glaucoma is a complex, chronic disease. In the early- to mid-stages of the disease, patients rarely have any subjective awareness of their functional losses. To the contrary, the only subjective awareness most glaucoma patients have is of the side-effects from their treatments.

Making patients visually and cognitively aware of their test results and disease status can dramatically improve their compliance. Additionally, the more that patients are made aware of their objective and subjective test results, the more likely and willing they will be to repeat these important studies in the future. Physicians who educate patients on the status of their disease and treatment options generally have greater success in achieving target IOP levels for patients.

In the next installment of this multipart series, I'll cover fundus photography, gonioscopy, pachymetry and serial tonometry including codes, indications for use, preferred practice pattern/frequency of use, mandatory documentation, coding tips and recommendations.

Dr. Andrew Rabinowitz is a board-certified ophthalmologist specializing in glaucoma management. He's currently in private practice at the Barnet Dulaney Perkins Eye Center in Phoenix, Ariz., and can be reached at Barnet Dulaney Perkins Eye Center, 4800 North 22nd St., Phoenix, AZ 85016.

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