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Editors Selection IGR 24-3

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Comment by Kaweh Mansouri on:

59604 Short-Term Enhancement of Visual Field Sensitivity in Glaucomatous Eyes Following Surgical Intraocular Pressure Reduction, Wright TM; Goharian I; Gardiner SK et al., American Journal of Ophthalmology, 2015; 159: 378-85.e1

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The hypothesis that significant IOP reduction can lead to visual field (VF) improvement in glaucoma has been discussed for decades. It is based on the putative reversal of retinal ganglion cell (RGC) dysfunction when IOP is lowered. In 1985, Dr George Spaeth laid the scientific foundation for this theory by suggesting it as an outcome measure for glaucoma treatment.1 Ever since, and disconcertingly, solid evidence for a phenomenon that many of us clinicians, albeit sporadically, observe has been lacking.

Skeptics point to the regression to the mean effect as an explanation for cases of observed VF "improvement" after successful surgery. Similar to IOP-based decisions, surgery is often recommended when VFs seem to have worsened. Given an additional stochastic element in VF series, it would then be expected that in some patients fields would improve, i.e. regress back to the mean.

Recently, the subject of VF restoration after IOP-lowering versus regression to the mean, or simply due to variation, was addressed in a re-analysis of the collaborative initial glaucoma treatment study (CIGTS).² (see Comment by Dr Kouros Nouri-Mahdavi, IGR 16-2). The authors found the percentage of participants showing substantial VF improvement (by 3dB or more) over time to be similar to that showing VF loss through 5 years after initial treatment. They demonstrated an association with lower mean and minimum IOP. If VF improvement had been entirely due to regression to the mean or random variation, there such a correlation would be highly unlikely.

In the present study, Wright et al. attempted to examine this hypothesis by designing a prospective study comparing surgical IOP reduction with medically controlled eyes with stable IOP. Eyes that underwent surgery had their baseline IOP reduced from a mean of 18.0 ± 6.7 mmHg to 9.9 ± 4.7 mmHg. They found that a greater number of these eyes had improved PSD at the three-month follow-up visit (30%, respectively) than the control group (7%). Similarly, surgical eyes had a greater number of test locations improving, while MD was not statistically significantly different. Contrary to the CIGTS data, and despite a 0.04 higher number of improving test locations per mmHg lower baseline IOP, no ‘dose-response relationship’ between IOP change and VF improvement was observed.

Interestingly, a 'risk factor' for VF improvement was a higher postoperative mean ocular perfusion pressure. Also, the fact that both groups retained a similar number of locations with deteriorating VF sensitivities (4.8 vs 6.4, p = 0.433) may give credence to the putative explanation for VF improvement, namely that it corresponds to restoration RGC function in borderline damaged areas but not in severely damaged ones.

Despite a greater number of phakic eyes in the control (average age 67 years), the authors addressed the potential for cataract bias by excluding patients with visual acuity below 20/30. Despite their best attempts to create a strong study design, there were shortcomings that weaken (but in my mind not seriously so) the findings: a short follow-up period of three months for the surgical group and, most importantly, the absence of a sufficient number of fields to confirm the suspected enhancement in sensitivity. Helping partially in alleviating the first issue was a secondary analysis comparing three-month surgical to the six-month follow-up of controls: surgical patients still had significantly more test locations with improved sensitivity.

Encourage clinicians to offer glaucoma patients cautious words of hope that some degree of loss may be recoverable

This report adds to a growing body of evidence demonstrating that VF progression does not have to be a one-way street in all patients. What are the clinical implications from this important study? From my perspective, it is 1) to never take any VF change for granted and to seek (re)confirmation, even in cases of improvement, despite an uphill battle with reimbursement authorities; 2) to encourage clinicians to offer glaucoma patients cautious words of hope that some degree of loss may be recoverable; and possibly 3) resort to surgery earlier in our clinical decision-making process.

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