Figure 7.5 Relationship between subjects’ Motion Displacement Threshold and HSRP matrix map mean threshold. The solid line represents the least-squares linear fit through the data. Arrow indicates 1 outlier o ff the y-scale.
7.5 Discussion
This aim o f this study is to test the hypothesis that abnormal M otion Displacement Thresholds coexist with scotomas on a finer scale than is measurable by conventional Humphrey perimetry. We identified 10 glaucoma eyes with normal Humphrey 24-2 field nearest the MDT test site. O f these 7 had abnormally elevated M otion Displacement Thresholds and 6 had fine scale scotomas detected with High Spatial Resolution Perimetry. This result suggests that glaucomatous elevations o f M otion Displacement Threshold may be present in areas o f normal Humphrey 24-2 field, and this may coexist with measurable scotomas beyond the resolution o f conventional Humphrey perimetry in some, but not all patients.
We identified 2 glaucoma eyes with abnormal HSRP in the presence o f a normal motion 50% threshold test. Whilst it is possible that these defects may be non-glaucomatous, and secondary to the influence o f refractive error or media absorption, we were unable to identify such factors in our patients. Further studies will be required to confirm this and to examine for additional unrecognized factors which may be responsible for this. A further proportion o f the glaucoma patients (30%) had Motion Displacement Thresholds defects in areas o f field with no detectable scotomas, measured with conventional perimetry or High Spatial Resolution Perimetry. One possible explanation would be the existence o f scotomas beyond the resolution o f our technique. The maximum displacement o f the motion stimulus is less than 0.3 degrees, whilst the High Spatial Resolution Perimetry has been performed by measuring field thresholds across a grid at 1 degree intervals. Thus we cannot exclude the possibility o f abnormal Motion Displacement Thresholds coexisting with scotomas beyond the resolution o f our technique.
In order to avoid artifacts due to the trial frame, we elected to perform M otion Displacement testing and High Spatial Resolution Perimetry without near correction. M otion Displacement Thresholds have been shown to be relatively unaffected by refractive errors (Fitzke, et al., 1989; Whitaker and Buckingham, 1987), and the subjects had low refractive errors. Refractive error would be expected to globally suppress Humphrey field threshold sensitivity (Goldstick and Weinreb, 1987). The uniformity index (Ul) we used reflects focal threshold depression, which would not be expected to
be affected by refractive error. 13/14 o f the glaucoma eyes identified as abnormal with the HSRP had an abnormal Uniformity Index. Refractive changes would not be expected to account for these localized threshold abnormalities, as in general the effect o f refractive error is to smear out localized abnormalities.
One finding o f this study was that over half o f the glaucoma eyes with abnormal motion sensitivity had normal Humphrey 24-2 field at the motion test site. This finding has also been reported by Johnson et al, who identified elevated M otion Displacement Thresholds in glaucoma in areas appearing normal on conventional perimetry (Johnson, et al., 1995). I shall examine two major hypotheses to explain this finding.
One hypothesis which has been invoked to account for the occurrence o f early abnormalities o f motion perception before conventional field defects is the “selective loss hypothesis” .
Quigley has hypothesized that there is selective damage to the larger diameter optic nerve fibres in early glaucoma (Quigley, et al., 1988). Since magnocellular cells are associated with larger mean diameters, the “selective loss hypothesis” would imply a preferential loss o f magnocellular function. The evidence for this is described in
chapter 3.
Johnson has proposed an alternative hypothesis based on the concept o f reduced redundancy to explain the presence o f a variety o f psychophysical abnormalities o f visual function (including motion perception) in glaucoma before conventional perimetric field defects (Johnson, 1994). According to the reduced redundancy theory, the poor performance o f conventional perimetry may be a consequence o f the non-selective nature o f the stimulus, which stimulates a broad spectrum o f retinal ganglion cells. The large overlap in ganglion cell receptive fields results in considerable redundancy, that may mask early losses if all classes o f ganglion cells are stimulated (Johnson, 1994). Tests that stimulate only a subpopulation o f ganglion cells to isolate one aspect o f visual function may identify the earliest losses.
Psychophysical evidence for both the selective cell hypothesis and the reduced redundancy hypothesis depends upon comparisons o f the extent and the sequence o f occurrence o f selective losses o f visual function, compared with non selective losses identified with conventional perimetry (Johnson, 1994; Sample
etal.,
1994a).An important finding o f this study was that elevations o f M otion Displacement Threshold may coexist with scotomas beyond the resolution o f conventional Humphrey perimetry. This finding demonstrates the importance o f considering the spatial resolution o f the tests, which must be taken into account when making comparisons between tests. The differing spatial resolution o f the tests may be as significant as any intrinsic differences that may exist in the sensitivity o f a broad spectrum stimulus such as the Humphrey stimulus, compared with more selective stimuli such as motion stimulus. In summary, the results o f this study suggest that a principal limitation o f conventional Humphrey 24-2 perimetry in detecting early glaucoma is its inadequate spatial resolution.