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Glaucoma Dialogue IGR 16-3

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Comment by Shamira Perera 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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This study supports our long-held suspicions about the enhancement of visual field that occurs after surgical reduction of intraocular pressure (IOP) in eyes with glaucomatous optic neuropathy.

This mimics the observed retinal nerve fiber layer changes following IOP reduction alongside reversal of optic disc cupping. Clinically, it would be interesting to see if this improvement is noticeable or useful to the patient at this stage of field loss. We know that in more advanced disease the improvements are sustained and distinctly observable. This study showed no clear relationship between the degree of IOP lowering and the improvement in visual field sensitivity. Previous studies have been inconclusive on this point and no biologically plausible theory exists to explain how this might occur.

The fact that only 30% had an improvement in Pattern Standard Deviation despite the dramatic IOP lowering to 10mmHg certainly suggests that the number of potentially salvageable retinal ganglion cells (RGCs) vary from patient to patient irrespective of the stage of disease. Unfortunately, we know little about this group of RGCs. Notably, a similar number of patients in each group had worsening eyes or worsening points indicating some RGCs will deteriorate and atrophy inevitably. Newer methods of imaging with spectral domain Optical Coherence Tomography (OCT) that concentrate on the ganglion cell complex may be able to map the patterns of visual field improvement to structural changes. It would seem logical that the last affected areas would be more likely to improve, but that would need a more technical analysis.

Further research could use automated pointwise linear regression of the visual field on the two groups. The Progressor software (Medisoft, Ltd, Leeds, UK) is a sophisticated tool to characterize the changes and how they evolve with time, but it requires more than five visual fields.

The profile of the IOP changes in both groups would have been useful to know, as it is likely that the flatter IOP after surgery would be beneficial to the recovery of the RGCs. In the absence of this, the authors, attribute the mechanism to be via an improvement in ocular perfusion pressure. The authors should be commended on their robust methodology and analysis which addresses the learning effect and the authors acknowledge that some of the visual improvement may reside in an improved ocular surface.

Whilst longer longitudinal studies would help expound the relationship between IOP and VF enhancement, the authors have opened up the potential role for using localized changes in visual sensitivity as a potential biomarker for salvageable RGC function. Current options of the Pattern Electro Retinogram are impractical for routine use and OCT is reportedly less sensitive for early disease. This biomarker may be used to test the response to IOP lowering of various modalities.

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