Combining diagnostic tools improves glaucoma diagnosis

Investigators tested a new index they developed that estimates glaucoma severity based on functional measurements obtained by standard automated perimetry (SAP) and structural assessment by spectral domain optical coherence tomography (SD-OCT).

Subjects included glaucoma patients (333 eyes) and healthy controls (330 eyes). All eyes were tested with SAP and SD-OCT with no more than six months between measurements.

Estimates of the number of retinal ganglion cells (RGCs) were obtained from SAP and SD-OCT and a weighted averaging scheme was used to obtain a final estimate of the number of RGCs for each eye. The combined structure-function index was calculated as the percent loss of RGCs obtained by subtracting estimated from expected RGC numbers. The performance of the index for discriminating glaucoma from normal eyes and the different stages of disease was evaluated by receiver operating characteristic curves.

The index performed significantly better than isolated measures of structure and function for diagnosing preperimetric and perimetric glaucoma, as well as for discriminating disease stages.

The authors write that this structure-function index has several desirable properties for use as a staging index. It discriminates well among the different stages of the disease and has an intuitive interpretation as the overall percent loss of neuronal tissue. In addition, it is provided on a continuous scale, avoiding the artificial categorization of the disease continuum.

However, they add, an ideal staging system for glaucoma would be highly predictive of the degree of disability from the disease. Although SAP measurements have been related to measures of quality of vision in patients with glaucoma, such a relationship is usually weak. Future studies should be performed to attempt to develop staging systems based on results of tests directly measuring functional impairment in glaucoma. The methods described in this study to estimate RGC counts from a combination of structure and function could also be used to provide a useful parameter for longitudinal monitoring of glaucomatous changes.