abnormal visual function in glaucoma
3.4 Other psychophysical tests
3.4.1 Short Wavelength Automated perimetry (SWAP)
SWAP (otherwise known as blue-on-yellow perimetry) uses a blue stimulus on a yellow background to isolate and measure the sensitivity o f the short-wavelength pathway. This pathway is mediated by a subpopulation o f P-pathway ganglion cells believed to be the bistratified ganglion cells. These cells have slightly larger mean diameters and it is postulated that they may be susceptible to selective loss. They are also relatively sparsely distributed, comprising only 1-2% o f total ganglion cell pool. Thus it is postulated that functional losses may occur early in the blue pathway because o f the reduced redundancy o f this system (Johnson, 1994). SWAP is usually performed on the Humphrey Field Analyzer using a size V stimulus with a yellow background.
The use o f SWAP in glaucoma has now been under investigation for 10 years, and a full review o f the extensive literature is beyond the scope o f this thesis. In POAG patients, SWAP defects have been shown to correspond with, and often overlap conventional white on white defects (Johnson
et al.,
1993a; Sample and Weinreb, 1992). SWAP defects have also been shown to correspond to the retinal nerve fibre layer pattern. The greater extent o f SWAP deficits compared with conventional perimetric deficits appears to be most marked in POAG patients who undergo progression o f their conventional field loss (Johnsonet a l,
1993b).In ocular hypertensive patients, a longitudinal study o f 76 subjects showed that o f nine patients with initial SWAP defects, 5 developed conventional field defects within 5 years (a sensitivity o f 100 %, specificity o f 94 %) (Johnson, et al., 1993a). Sample et al reported significantly larger SWAP defects in suspect eyes that subsequently converted over at least 1 year o f follow up (Sample
et a l,
1993).Unfortunately SWAP has a number o f major disadvantages which may seriously limit its clinical usefiilness. These include a shorter dynamic range, and a greater short-term fluctuation (Wild
et a l,
1996; Wildet a l,
1995). Wild et al have also shown that SWAP has a greater long-term fluctuation compared with white-on-white perimetry (Wildet al,
1997). This would be expected to make the identification o f progressive field loss more difficult using SWAP perimetry compared with conventional perimetry.
Another problem with SWAP is the individual variation in the degree o f absorption o f short wavelengths by the lens. The accepted method o f correcting for this requires the
measurement of dark adapted thresholds which are time consuming and clinically impractical. Sample et al have attempted to address this problem using an approach similar to the Glaucoma Hemifield Test to avoid the need for such correction, although this analysis has yet to be replicated (Sample
et a l,
1994b).3.4.2 Flicker perimetry
Flicker perimetry has been shown to identify early abnormalities o f visual function in glaucoma. High frequency flicker is believed to mediated by the magnocellular cell pathway, whereas low frequency flicker is believed to be mediated by the parvocellular pathway (Schiller, et al., 1990a).
One threshold measure that has been used is the “critical flicker fiision frequency”, which is the highest frequency o f flicker that can be detected for a target at 100% contrast. A cross-sectional study by Lachenmeyer showed increasing depth o f flicker fusion defects relative to conventional perimetric defects, with increasing level o f lOP in glaucoma patients (Lachenmayr and Drance, 1992).
An alternative threshold measure is to determine the minimum contrast needed to detect a flickering stimulus maintained at a constant frequency (temporal modulation perimetry). Casson et al reported that temporal modulation defects were more extensive than conventional field defects in ocular hypertension and early glaucoma. In a prospective follow-up, temporal modulation defects at the 8 and 16 Hz frequencies performed best in predicting the locality o f subsequent conventional field deterioration (Casson a/., 1993).
Finally, flicker perimetry has the clinically useful properties o f being very resistant to the effects o f blur and cataract (Lachenmayr and Gleissner, 1992).
3.4.3 High-pass resolution perimetry
Another test that has been used to investigate early abnormalities o f visual function is high-pass resolution perimetry. This is a test o f peripheral spatial resolution, and was first pioneered by Frisen as a test o f parvocellular pathway function (Frisen, 1995). Frisen has suggested theoretical reasons for a close relationship between the high pass resolution threshold and the number o f functioning retinocortical neural channels. The test uses a ring-shaped target with a bright core and dark borders, and is quick to perform. Various sizes o f rings are presented in the field, and the threshold is the minimum size o f the ring
when it is first seen. The agreement between high pass resolution defects and conventional fieild defects has been reported as moderately good, ranging from 67% to 77% in glaucom a patients (Chauhan
et al,
1993a; Sampleet a l,
1992). However Sampleet a l
found no differences in high pass resolution thresholds in ocular hypertensive patients and age matched normal controls (Sample, et al., 1992). At present there are no results on the long-term usefulness o f high pass resolution perimetry in predicting conversion o f ocular hypertensives to glaucoma.3.4.4 Frequency doubling perimetry
There has recently been considerable interest in the use o f the frequency doubling illusion as a screening method for glaucoma. The frequency doubling illusion, as originally described by Kelly, occurs when a low spatial frequency grating (<1 cyc/deg) undergoes rapid counterphase flicker (i.e. the light bars become dark and vice versa). Under these conditions a normal physiological illusion occurs in that the grating appears to be twice its actual frequency (Kelly, 1981). The stimulus characteristics required for this illusion suggest that the frequency doubling response is mediated by the magnocellular pathway, via a subset o f M -cells with non-linear response properties (Johnson and Samuels, 1997; Maddess and Henry, 1992). Johnson & Samuels have investigated the screening ability o f a frequency doubling contrast threshold test o f discrete locations o f the visual field, and reported a sensitivity o f 93% at 100% specificity against the Humphrey perimeter (Johnson and Samiuels, 1997). However this degree o f separation could only be achieved using a frequency doubling test that sampled a relatively large number (16) o f test locations in the visual field. Nevertheless this test appears promising as a screening test and further studies; are awaited.