Blue-on-yellow perimetry, also called short wavelength automated perimetry (SWAP), is a non-conventional visual field test. It uses a high-luminance yellow background light to suppress the responses of red and green cones (selective chromatic adaptation) and measures only blue cone sensitivity with a blue test stimulus.
Standard automated perimetry (SAP) uses a white stimulus on a white background and tests all retinal ganglion cell (RGC) populations. In glaucoma, visual field defects on SAP are thought to appear only after about 40% of RGCs have been lost. SWAP aims to detect earlier functional damage by selectively evaluating the koniocellular cells (K cells) that mediate the blue cone system.
The main targets are early open-angle glaucoma and ocular hypertension. The SWAP program built into the Humphrey automated perimeter is widely used clinically.
Non-conventional perimetry tests including SWAP (such as FDT and flicker perimetry) were developed to detect glaucomatous visual field damage earlier than SAP, but all major glaucoma clinical trials used SAP, and evidence clearly demonstrating the superiority of SWAP is insufficient 1)2)3).
SAP uses a white stimulus on a white background to test all retinal ganglion cell populations. SWAP suppresses red and green cones with a yellow background and selectively evaluates only the blue cone system (K cells) using a blue stimulus. This may allow detection of early glaucomatous damage, but there are also disadvantages such as large variability and the influence of cataracts see disadvantages.
The main parameters of SWAP are shown below.
| Parameter | Specification |
|---|---|
| Background light | 100 cd/m², 530 nm yellow |
| Test stimulus | 440 nm blue, Goldmann V |
Similar to standard SAP (W-on-W), central programs 30-2, 24-2, 10-2, and macular program can be used. SITA (Swedish Interactive Threshold Algorithm) is also supported, and SITA SWAP is available in addition to the conventional Full Threshold program.
It is available on the Humphrey Field Analyzer II (model 700 and above) and Octopus 311, and includes a normal database and statistical analysis package. The Octopus has a dynamic range of 18 dB, which is wider than that of the Humphrey under the same conditions.
Cataracts (especially nuclear sclerosis) significantly affect SWAP results. Yellowing of the lens blocks short-wavelength light transmission, which can cause false-positive visual field defects or false progression. In cases with advanced cataracts, the reliability of SWAP decreases, so careful interpretation of results is necessary.
Retinal ganglion cells (RGCs) are functionally classified into three main populations.
| Cell type | Proportion | Function |
|---|---|---|
| P cells (parvocellular) | Approximately 80% | Color and contrast sensitivity |
| M cells (magnocellular) | Approximately 15% | Motion and temporal modulation |
| K cells (koniocellular cells) | Approximately 5% | Blue-yellow opponency |
SWAP targets K cells (small bistratified ganglion cells). K cells connect to the koniocellular pathway in the lateral geniculate nucleus and transmit signals from blue cones.
The rationale for detecting early damage with SWAP via K cells is as follows.
The theoretical basis of SWAP dates back to Stiles’ two-color increment threshold method from the 1950s. This technique uses chromatic adapting backgrounds to reduce sensitivity of certain color mechanisms (π mechanisms) and measures thresholds of specific mechanisms. SWAP is based on isolating the principal short-wavelength-sensitive mechanism (π1).
Multiple long-term studies have reported that SWAP can predict the location and timing of glaucomatous visual field defects 3 to 5 years (and in some cases up to 10 years) earlier than SAP. SWAP abnormalities are found in 20–25% of ocular hypertensive patients with normal SAP.
Ocular hypertensive patients who show normal results on W-on-W but abnormal results on SWAP may develop scotomas on W-on-W several years later and progress to open-angle glaucoma, suggesting that SWAP has the ability to predict glaucoma progression. Combining optic disc evaluation with SWAP results may improve the accuracy of assessing the risk of developing glaucoma.
Reports indicate that SWAP has a sensitivity of 88% and a specificity of 92%. However, the EGS guidelines and AAO PPP state that there is insufficient evidence to show a clear advantage of SWAP over SAP, and it is not widely used in current glaucoma management1)2)3).
Advantages
Early detection capability: May predict visual field defects several years earlier than SAP.
Pattern correspondence: SWAP defect patterns correspond to glaucomatous nerve fiber bundle damage.
Clarity of defects: SWAP abnormalities are larger in size and more prominently detected than corresponding defects on SAP.
Disadvantages
Effect of cataracts: False positives and false progression due to nuclear sclerosis are problematic.
High variability: Short-term fluctuation is 25–30% greater than SAP. False positives and false negatives are also more frequent.
Test duration: The Full Threshold method takes 2–3 minutes longer than SAP, requiring 15–20 minutes per eye. Adaptation time of 2–3 minutes is also needed.
The introduction of the SITA SWAP strategy has reduced test time and improved detection accuracy. Sensitivity at each test point is improved by 4–5 dB, and the dynamic range is expanded. Detection sensitivity is reported to be equal to or better than the Full Threshold method, and variability is less than or equal to the Full Threshold method.
The FDT (frequency doubling technology) perimeter is a test that detects damage to the M-cell system (magnocellular cells, 10-15% of all RGCs) and targets a different population of ganglion cells than SWAP. FDT is greatly affected by cataracts, but has the advantages of short test time and low susceptibility to refractive error (within ±7D). The Tajimi Study in Japan reported that FDT has high specificity but insufficient sensitivity for early glaucoma.
Both SWAP and FDT aim to detect early glaucoma as non-conventional perimetry, but evidence from major clinical trials is lacking, and they are not the standard for glaucoma management1)2)3).
SWAP may be superior for early detection, but has limitations such as the influence of cataracts, large variability, and long test time. All major glaucoma clinical trials have used SAP (W-on-W), and guidelines have not shown a clear advantage for SWAP1)2)3). Currently, SAP is the standard for glaucoma management, and SWAP is positioned as a supplementary test.
