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MANAGING OPEN ANGLE GLAUCOMA

Rate of change calculations become more reliable when a greater number of visual field analyses are obtained. The
time it takes to detect progression depends on the speed of progression of the glaucoma and the intervals at which
visual fields are being tested. It has been demonstrated that it could take as long as 5 years to detect someone dete-
riorating very quickly if fields are only being done annually. Increasing testing frequency to every 4 months short-
ens that time frame to less than two years. Table 7 demonstrates the number of visual fields needed to detect mild/
moderate/fast progressing visual fields at 1/year, 2/year and 3/year testing intervals in reliable field takers. In the
scenario of an unreliable visual field taker, the length of time required to detect progression when fields are done
annually increases from 13 to 30 years in slow progression and 6 to 13 years in fast progression. 57

Table 7: Length of time required to detect different rates of visual field progression at different annual testing frequencies
(in low variability (reliable) field takers)

Length of time (years) to visual field progression
at intervals of:
VF/year 2 VF/year 3 VF/year
slow (-0.25 dB/year) 13 6.5 4.3
moderate (-0.5dB/year) 9 4.5 3
fast (-1.0dB/year) 6 3 2
adapted from Chauhan 2008 57

CLINICAL RECOMMENDATIONS FOR FREQUENCY OF AVF ASSESSMENT:
• It is recommended that at least 6 reliable fields are obtained in the first 18 to 24 months to establish a solid
baseline and identify rapidly-progressing glaucoma (-2.0dB MD or 6 to 8% VFI per year).

• In the instance of a very poor (highly variable) visual field taker the number of visual fields should be
increased.

MONITORING FOR PROGRESSION USING STRUCTURAL ANALYSIS
It is evident that structural changes to the ONH and RNFL occur before detectable functional loss using the tests that
we have available today. The scanning laser ophthalmoscope (HRT) has over 20 years of use in clinical care and
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longitudinal studies, and is able to provide reliable measurements of rim area and RNFL change. However, despite its
ability to provide high quality progression data, it is not an instrument found in many eye care provider’s offices.
In the last decade, there has been significant improvement in the quality of the objective measurements of ONH,
RNFL, and macular RGC parameters. Specifically, the advent of SD-OCT has revolutionized glaucoma imaging.
Current versions of SD-OCT demonstrate low variability (≤3%) for ONH, RNFL and macular imaging. It has been
suggested that SD-OCT can detect a change as small as 5μm in the average RNFL thickness with event-based analy-
sis. 49,215 If an adequate number of exams are available, very slow rates of change can potentially be quantified with
trend-based analysis. 238,301 In order to improve detection of progression using SD-OCT it is important to use high
quality scans (see earlier review of common imaging artifacts).
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Clinical Recommendation for monitoring for structural progression:
• Having high quality scans, looking at multiple parameters (optic nerve, RNFL and macular data) and a
higher frequency of test taking will improve the reliability of detecting progression using the SD-OCT. 302

CORRELATING STRUCTURE AND FUNCTION WITH PROGRESSION
The agreement between SD-OCT and SAP in detecting progression has to date been poor. SD-OCT seems to do
a better job at detecting early disease while SAP appears to be better in advanced disease. The limitation of SD-
OCT in advanced disease is likely related to the floor effect reached at an RNFL thickness of approximately 50μm,
representing residual glial and vascular tissue. At this point the instrument is no longer able to discern change,
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given that there is little viable RNFL remaining. For this reason, in advanced glaucoma macular retinal ganglion cell
analysis may be superior to RNFL analysis. At this stage, functional (AVF) assessment also becomes more helpful
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than RNFL analysis. 289,302 The logarithmic scale of AVF analysis masks loss in early disease, but amplifies it in more

CANADIAN JOURNAL of OPTOMETRY | REVUE CANADIENNE D’OPTOMÉTRIE VOL. 79 SUPPLEMENT 1, 2017 45