Combined index of optic nerve structure, function, and anatomy promising for detection of glaucoma

Fort Lauderdale, FL-Researchers at the Wilmer Eye Institute, Johns Hopkins University School of Medicine, Baltimore, have developed a combined index of optic nerve structure, function, and anatomy to improve the detection of glaucoma. The structure function index (SFI) might perform better than either structure or function alone, said Michael V. Boland, MD, PhD, assistant professor of ophthalmology at the institute.

“We think the SFI represents a new approach to the integration of optic nerve structure and function,” Dr. Boland said. “It has some promise for distinguishing glaucoma from suspects.”

The model is based on combined optic nerve imaging and visual field testing using what is known about retinal nerve fiber layer anatomy.

“I propose that the probability of an abnormal structure-function relationship is a function of the probability that the function is abnormal-the visual field; the probability that the structure is abnormal-imaging, and the probability that those two are tied together,” Dr Boland said.

The reference population used for this model consisted of 1,662 eyes from the Wilmer Eye Institute clinical database of visual fields and tomography images (Heidelberg Retina Tomography, Heidelberg Engineering), linked by billing data. These patients had a billing diagnosis of suspected glaucoma, reliable visual field test, acceptable tomography imaging, and no subsequent diagnosis of glaucoma in the records. Investigators used 1,000 of the glaucoma suspect eyes to define normative distributions for the total deviation of each point in the visual field and for the numeric value of the Moorfields regression function for each optic disc sector.

The next step was to calculate the new measure, SFI, for the remaining subjects. A modified version of the glaucoma hemifield test was calculated using the SFI for each patient, and the SFI hemifield test (SFI-HT) then was used to perform receiver operating characteristic (ROC) analysis.

Applying the SFI-HT, the investigators found that the area under the ROC curve was 0.77. The optimal sensitivity was 80%, and the optimal specificity was 65%. Dr. Boland said that the specificity lags, possibly because some of the glaucoma suspects may have glaucoma. He also noted the need for longitudinal data.