Researchers recently found that functional change appears to precede and predict thinning of the RNFL in glaucoma. The time lag they observed of about six months implies that, although RNFL thickness changes are detectable with OCT, true change found with standard automated perimetry occurs sooner and is predictive of subsequent RNFL fluctuations.

Rates of change in function (mean linearized total deviation between visits) and structure (RNFL thickness) were calculated over 1,135 pairs of consecutive patient visits from 318 eyes of 164 participants. Participants had follow-ups approximately once every six months.

The researchers found that the rate of change in function was predicted by its rate in the previous time interval but not by rates of RNFL thickness change in either the concurrent or previous time interval. The rate of RNFL thickness change was not predicted by the concurrent mean change in function after adjusting for its own previous rate. However, the rate of change in function in the previous time interval did significantly improve the prediction of the current rate for RNFL thickness. Overall, the study reported a time lag of around six months between changes in function and RNFL thickness.

According to the researchers, the presence of a time lag “encourages the development of improved and less variable functional testing, as well as the use of alternative structural measures, which may allow earlier detection of damage and provide better prognostic information about disease progression.”