Both structure, function necessary in glaucomatous damage assessment

January 15, 2006

Chicago—While it is generally accepted that optic disc and retinal nerve fiber layer (RNFL) changes precede visual field changes, this is not always the case, according to Linda M. Zangwill, PhD, who spoke at the glaucoma subspecialty day program during the American Academy of Ophthalmology annual meeting.

"Structural or functional damage can be the first sign of glaucomatous change, so as clinicians, it is critical to assess the optic disc, and it is critical to monitor visual function," said Dr. Zangwill, professor, Department of Ophthalmology, Hamilton Glaucoma Center, University of California, San Diego.

An epidemiologist, Dr. Zangwill is the principal investigator of a National Eye Institute-sponsored study evaluating the ability of new diagnostic imaging instruments to detect glaucomatous damage to the optic nerve and to monitor its progression.

Dr. Zangwill reviewed the findings of the three studies and concluded that the first type of glaucomatous change detected can be either structural or functional. In the OHTS, structural change was noted first in 55% of cases and functional change first in 35%. In the EMGT, structural change was detected first in only 1% of patients, while functional change was noticed first in 86%. In the EGPS, the percentages for structural change and functional change were 40% and 60%, respectively.

Structural and functional changes were noted simultaneously in a much smaller proportion of patients: 10% in OHTS and 13% in EMGT. This fact was not included in the results of the EGPS.

Detecting damage

Numerous factors influence the detection of structural and functional changes and could explain this variability, Dr. Zangwill said. Patient factors are one category, encompassing components such as demographics, medical history, and clinical characteristics; for example, the size of the optic disc. The type and stage of glaucoma are also influential, because structural damage can be more difficult to observe in eyes with advanced glaucoma.

Definitions of glaucomatous change also affect the detection of such change, as do the choice of instrument used, the magnitude of the change, and the variability of the measurement. Other items to take into account include frequency of testing, requirements for repeatability of damage, and the measurement scale of the visual field (linear versus non-linear).

Study differences

In the three studies, variation in the proportion of eyes with structural or functional damage apparent first seems to be attributable primarily to differences in study design, end-point definition, and patient characteristics.