Glaucoma monitoring: merits of mean deviation and visual field testing

[caption id="attachment_6887" align="alignnone" width="350"] Francisco Javier Goni MD[/caption] An experienced clinician and one of best pioneers in the modern statistical analysis of visual field testing debated the relative merits of mean deviation (MD) and visual field index (VFI) in the detection of glaucomatous progression, at the 12th European Glaucoma Society Congress in Prague, Czech Republic. Francisco Javier Goñi MD, Barcelona, Spain, said that he prefers MD, a value derived from the total deviation which represents the overall mean departure from the age-corrected norm. That is because MD is better at detecting diffuse loss in early glaucoma, he said. He noted that 15% of eyes with early glaucoma will have a VFI value of 99% or more, meaning almost perfectly normal vision for a patient’s age. “With VFI we are missing some glaucoma, at least for 15% of our patients with early damage,” Dr Goñi said. Moreover, diffuse retinal nerve fibre loss corresponds to diffuse sensitivity loss on preserved hemifields of glaucoma eyes. In addition, pattern deviation analyses, like VFI, classify 15% fewer glaucomatous eyes more progressive than would be classified as such with total deviation analyses, like MD. “So that means that mean defect is better at detecting this diffuse loss,” he said. He added that although only 4.4% of patients present with only diffuse vision loss, focal visual field loss is usually associated with a diffuse component. For example, most of the eyes that developed glaucomatous endpoints in the Ocular Hypertension Treatment Study (OHTS) showed both diffuse and focal visual field changes. One often cited potential disadvantage of MD is that it is less sensitive than VFI to central vision loss-threatening fixation. However, Dr Goñi questioned whether that is relevant when assessing progression and predicting future vision loss. He noted that a study conducted in Sweden showed that eyes with glaucomatous fixation-threatening visual field loss within the innermost points had significantly worse MD values than those without any macular threat. EVENTUAL BLINDNESS RATES Moreover, with regard to survival 
curves, there was no significance difference between the eventual blindness rates of eyes with and without a threat to fixation, once adjusted for MD values. On that basis, the study’s authors concluded that the risk of blindness can be based solely on MD, 
Dr Goñi pointed out. MD also has the advantage of providing a more precisely graded assessment of visual function that is the same across the entire standard automated perimetry range. In contrast, VFI is calculated from the pattern deviation (PD) probability map when MD is better than 20dB, and from the total deviation (TD) map when MD is worse than 20dB. That means if MD is going across the 20dB threshold, the VFI value can vary up to 15% with the change of just one decibel in MD. [caption id="attachment_6886" align="alignnone" width="350"] Boel Bengtsson MD[/caption] Boel Bengtsson MD, Lund University, Malmo, Sweden, who developed the VFI with Anders Heijl MD, countered Dr Goñi’s argument by pointing out that the VFI was designed to measure the rate of progression of glaucoma visual field loss in eyes with manifest glaucoma. It was not designed for detection of visual field defects nor was it designed to detect progression. Event analysis is designed for those purposes. Trend analysis is not sensitive at all early scatter in proceeding repeatable visual field defects. She added that VFI values are usually similar to MD, but not always. One exception is in glaucomatous eyes with paracentral visual field defects. “VFI is much more heavily weighted towards the centre of the visual field, because, even where there is not such a great risk for blindness, we think that the most central part of the visual field is more important than the periphery,” Dr Bengtsson said. Another exception is in glaucomatous eyes with concomitant cataract. VFI is considerably less affected by cataracts than MD. As an illustration, she described a study she and her associates conducted which compared visual fields of glaucoma patients with and without (pseudophakic eyes) increasing cataract. It showed that the mean annual rate of progression was 3.6% using MD values versus only 2.1% using VFI in eyes with increasing cataract, and 2.7% and 2.6% respectively in eyes without increasing cataract. Similarly, a glaucomatous cataract patient’s MD values will typically have a pronounced recovery towards normal values, but there will only be a slight change in VFI values. Dr Bengtsson said that she also prefers VFI’s graphical display of the rate of progression to that of MD. She noted that the purpose of trend analysis is to identify patients who are at risk of developing field defects that will reduce their quality of life during their expected lifetime. Plotting MD over several visits can provide an estimate of the rate of progression, but since (unlike VFI) it does not plot change against time or age, it provides a less clear picture of a patient’s lifetime risk of severe visual impairment or blindness. Francisco Javier Goñi: 
francisgoni@yahoo.com Boel Bengtsson: boel.bengtsson@med.lu.se