More sensitive tests aim to detect glaucoma-related function and structure changes

September 19, 2005

San Francisco - Developers of specialized perimetric tests and quantitative optic nerve and retinal nerve fiber layer (RNFL) analyzers have introduced hardware and software modifications to improve those new tools for assessing function and structure in patients with glaucoma. However, their roles in clinical practice still await definition pending further study, said Christopher A. Girkin, MD, MPH, associate professor of ophthalmology and director, glaucoma service, Callahan Eye Foundation Hospital, University of Alabama at Birmingham.

San Francisco - Developers of specialized perimetric tests and quantitative optic nerve and retinal nerve fiber layer (RNFL) analyzers have introduced hardware and software modifications to improve those new tools for assessing function and structure in patients with glaucoma. However, their roles in clinical practice still await definition pending further study, said Christopher A. Girkin, MD, MPH, associate professor of ophthalmology and director, glaucoma service, Callahan Eye Foundation Hospital, University of Alabama at Birmingham.

Short-wavelength automated perimetry (SWAP) and frequency-doubling technology perimetry (FDTP) were developed to emphasize response characteristics of subpopulations of ganglion cells and thereby to improve the sensitivity of visual field testing to ganglion cell loss relative to standard automated perimetry (SAP). SWAP has been shown to detect visual function loss due to glaucoma at least 5 years earlier than SAP. While the older version of SWAP suffered from being a difficult and prolonged test with high threshold variability, SWAP using the Swedish Interactive Thresholding Algorithm (SITA) seems to overcome those drawbacks and it is hoped it will be available soon.

FDTP has also been shown to correlate well with SAP for glaucoma detection. While its original version checked only a limited number of locations, the now available Humphrey Matrix (Carl Zeiss Meditec) offers a 24-2 test pattern similar to SAP.

Newer technology for evaluating structural changes due to glaucoma include scanning laser polarimetry with variable corneal compensation (SLP; GDx-VCC, Carl Zeiss Meditec), which evaluates the RNFL, confocal scanning laser ophthalmoscopy (CSLO; HRT II, Heidelberg Engineering) that creates a topographic map of the optic nerve head, and optical coherence tomography (OCT; Stratus OCT, Carl Zeiss Meditec), which also analyzes the optic disc and RNFL.

Studies with those instruments demonstrate each seems to perform similarly for detecting glaucoma when compared with expert photograder evaluation of the optic disc. Interestingly, agreement between the various devices is not high, indicating each is providing different information.

“The ideal method for detecting structural changes due to glaucoma would provide a multimodal assessment of the optic disc topography and RNFL. Currently, the only instrument we have that functions in a multimodal fashion is our brains,” Dr. Girkin said.