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Editors Selection IGR 7-2

Clinical examination methods: Structure and function correlations

Comment by Nicholas Strouthidis & David Garway-Heath on:

13183 Evaluation of the structure-function relationship in glaucoma, Gardiner SK; Johnson CA; Cioffi GA, Investigative Ophthalmology and Visual Science, 2005; 46: 3712-3717

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The term 'glaucoma' is used to describe a spectrum of conditions where a progressive optic neuropathy is associated with characteristic visual field deficits. There is a widely held view that, using current techniques, structural changes are detected at an earlier stage than functional loss.¹ Despite the long-established availability of standard automated perimetry, and more recently the availability of optic nerve head imaging techniques, such as scanning laser tomography, the exact nature of the structural and functional relationship in glaucoma remains elusive. This is largely a consequence of the absence of an independent measure with which to define glaucoma.

In a study by Gardiner et al. (865), the relationship between neuroretinal rim area in ten-degree sectors measured with the Heidelberg retina tomograph (HRT) and Humphrey visual field test-point threshold sensitivity is examined in a cross section of 'healthy' subjects and 'glaucoma' subjects (established glaucoma and glaucoma suspects).² The 36 rim area sectors from each 'healthy' eye were used to construct a model of what would be expected to constitute a normal, healthy disc. The 36 rim area sectors from each glaucoma subject were divided by the matching healthy sector average rim area values. The sectors (glaucoma/healthy) were then normalised to account for absolute differences in rim area between eyes. The normalised rim area sector values were correlated against the threshold sensitivities of all 52 visual field test-points. A map, correlating structure and function, was constructed, allowing easy visualisation of the optic nerve head sectors with the highest correlation with each visual field test-point. In particular, locations in the superior hemifield were highly correlated to the inferior and temporal optic nerve head and locations in the inferior hemifield were highly correlated to the superior and temporal optic nerve head.

An interesting feature of the results was that, despite the fact that there was a normalisation for rim area, but not for field test point sensitivity, the use of normalised rim areas resulted in a 'less noisy' map. This perhaps suggests that variations in absolute rim area in normal subjects, as measured by HRT, are not necessarily associated with variations in visual field sensitivity.

The structure-function map identified in this study was compared with a previously published map in which points on the Humphrey visual field test grid were related to the position of prominent retinal nerve fibre layer (RNFL) bundles or defects in a series of red-free photographs.³ For test points in the central field, the most highly correlated sectors in the HRT map tended, however, to be more vertically located (near the poles of the disc) than in the RNFL map. There also appeared to be a tendency in the HRT map for identified sectors to be more frequently in the infero- or supero-temporal aspect of the disc than expected from the map derived from RNFL photographs. A possible explanation is the fact that the RNFL map contains no functional information, so may not identify true structure/function correlations. Alternatively, there may be factors that influence the outcome of the HRT correlation analysis, as recognised by the authors. These include a limited between-patient range of visual field sensitivity at some locations, the fact that the HRT rim analysis includes blood vessels, that some HRT sectors may have higher inter-individual variability than others, and that the rim areas of various sectors are not independent and are probably correlated.

There were some unexpected correlations: locations in the supero-temporal part of the visual field tended to relate to sectors in the superior (when inferior would be expected) part of the disc and locations in the field temporal to the blind spot were more often were associated with sectors in the temporal (when nasal would be expected) part of the disc. The inclusion of blood vessels in the nasal rim area calculation is a possible explanation for this latter finding.

There were few discs used in the construction of the map with advanced glaucomatous optic neuropathy and, whereas the inclusion of more advanced cases might strengthen the correlations, it is difficult to predict how it would affect the pattern of correlations.

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