In this discussion paper the authors pose four questions for the clinician diagnosing and monitoring glaucoma, and supply evidence-based answers. Worldwide, the most common functional test used to diagnose and monitor glaucoma is static automated perimetry, most typically with a grid of test points spaced six degrees apart, such as in the 24-2 or 30-2 patterns of the Humphrey Visual Field (VF) Analyzer (Carl Zeiss Meditec Inc.). Structural damage is traditionally assessed by a fundus examination of the disc that is augmented in many clinics by fundus photographs and / or optical coherence tomography (OCT), which typically includes a scan of the disc. The first question the authors pose is: “When do you perform a 10-2 (2-degree grid) VF test?” They argue the best answer is: anyone you would do, or have done, a 24-2 (6-degree grid) VF on should have both a 24-2 and a 10-2 VF within the first two visits. It is currently clear that local arcuate damage near fixation can occur very early in the glaucomatous process. This damage is more common in the inferior macular region or superior paracentral VF. The extent to which the 24-2 contains abnormal points associated with early, local glaucomatous damage close to fixation will depend upon the location and width of the defect. Abnormal 10-2 VFs are found in many eyes with 24-2 mean deviation (MD) values better than −6 dB, a common definition of mild glaucoma. The authors schematically illustrate an anatomic basis for this damage. The axons from the retinal ganglion cells (RGC) in the inferior region of the macula enter the disc in its most vulnerable region, the temporal portion of the inferior quadrant. In general, the temporal halves of the superior and inferior quadrants are the locations of the largest loss in circumpapillary retinal nerve fibres (cpRNFL).
The authors call these regions the inferior vulnerability zones (IVZ) and superior vulnerability zones (SVZ). The portion of the IVZ associated with the macula was termed the macular vulnerability zone (MVZ). The authors suggest that clinicians should perform 10-2 examinations or their equivalent, even in eyes in which the 24-2 indicates a central abnormality. The finer 2-degree grid is invaluable to more accurately define the extent of damage and to track changes over time, and it is essential for comparisons to OCT abnormalities. The second question the authors pose is: “When do you perform an OCT scan of the macula?” The authors argue that, if you are performing an OCT test, then it should include both the macula and disc, either as a single scan or as two scans, one centred on the macula and the other on the disc. However, while it is common clinical practice to obtain an OCT scan of the disc, many clinicians do not obtain a scan of the macular region for patients with glaucoma or suspected glaucoma. This is a mistake since the OCT disc scan, as typically performed, can miss damage seen on a cube scan of the macula. It is also important to obtain macular scans to avoid missing non-glaucomatous macular damage, which may contribute to the patient’s complaints and / or VF abnormalities. Some of the most common conditions missed are epiretinal membranes, macular oedema or holes, and age-related macular degeneration. The third question the authors pose is: “How do you know if the VF and OCT tests agree?” The poor answer is: “I use summary statistics such as 24-2 MD and global or quadrant average of RNFL thickness.” It is much better to topographically compare abnormal regions on the OCT to abnormal regions on the VF.
The generally held belief that structural damage precedes functional damage, which is often quoted in papers and talks, can be misleading. The extent to which structural and functional measures agree depends upon a number of factors including: the structural and functional tests used; the measures used to define an abnormal test; and the relative levels of the baseline values when the eye was healthy. Finally, the fourth question the authors pose is: “When do you look at OCT images?” The authors argue that, at a minimum, the clinician should be directly examining an image of the cpRNFL, and this image should be sufficiently large and with sufficient resolution so that local damage can be seen, and the segmentation evaluated. In summary, the authors recommend that all patients with glaucoma, or suspected glaucoma, should undergo VF testing that includes more points in the macula, such as the 10-2 pattern, the G-programme, or a 24-2 hybrid, as well as OCT testing that includes scans of the macula. For more details, the authors suggest further studying of their lectures available at:
http://hoodvisualscience.psych.columbia.edu/
