New index combines best of structure and function testing
Combining the results of structural and functional tests into an estimate of retinal ganglion cell (RGC) loss in eyes with glaucoma can provide a clearer quantification of disease progression than either type of test on its own, according to Felipe Medeiros MD, Duke University, Durham, North Carolina, USA.
“There is a strong need for using a combined approach for evaluating structure and function in glaucoma,” he told the 7th World Glaucoma Congress in Helsinki, Finland.
INVERSE DETECTABILITY EFFECT
He noted that abundant research has shown that in some eyes glaucomatous change is detectable by changes in the optic nerve before it is detectable by visual field changes, but in other eyes the reverse is true. For example, in a recent study where he and his associates followed 462 eyes over 3.6 years with 24-2 standard automated perimetry (SAP) and optical coherence tomography (OCT), glaucoma progressed with both tests in only 4.1% of cases, while SAP detected progression in around 10% and OCT detected it in 19%.
Closer examination of the data revealed that those most likely to progress based on SAP testing were those with more advanced disease, whereas those with less advanced disease were more likely to progress on OCT. Other studies have shown the same tendency of better detection of progression with structural tests in the early stages of glaucoma and with functional testing in the later stages, he noted.
One explanation for the finding is that in the early stages of the disease a high proportion of RGC loss will cause only minor changes in the visual field, he said. In fact, research suggests that patients can lose around half of their RGCs before glaucoma is detectable at all by perimetry. In contrast, small amounts of structural change occurring in the later stages of glaucoma can be below the limit of detectability using current OCT technology.
INTEGRATED STRUCTURE/FUNCTION INDEX
Dr Medeiros and his associates have therefore devised an index of glaucoma that derives estimates of RGC count from both types of measurement, the combined index of structure and function (CSFI).
The new index derives functional-based estimates of RGC loss from research data from an empirical monkey model of glaucoma comparing behavioural perimetry with post-mortem optic nerve histology. The index couples the functional estimates with structural estimates of RGC loss derived from spectral-domain OCT.
The CSFI is the average of structural and functional estimates weighted according to the normal effect of ageing and the effects of disease severity.
In an observational study, the CFSI performed significantly better than isolated measures of structure and function for diagnosing glaucoma and discriminating different stages of the disease. The index is now available in a form that is optimised for integrating data from the Humphrey visual field analyser and Cirrus OCT (Zeiss).