Visual Field Monitoring:

Standard automated perimetry (SAP)

The most common type of visual field testing flashes a white dot of different intensity and sizes on a white (achromatic) background.   The light tests 24 degrees of the vision as most early glaucoma defects show within this area (the total extent of the vision is 110 degrees).

By modulating the size and intensity of the light to the faintest light the patient is able to discern, the visual threshold is determined.  SAP testing is most useful when a visual field defect had been found and the doctor wishes to observe the defect for progression. Progression can occur be the defect either getting denser or larger or merging without other defects. Because of normal fluctuations in test taking, at least three contiguous points must have worsened from previous testing, and this worsening must be verifiable on two subsequent tests to validate the progression.

Double Frequency Testing (DFT)

Another modality-double frequency testing flashes two images a microsecond apart in the same portion of the visual field. The patient perceives this as a single not doubled image. This double frequency image is uniquely processed by the M-ganglion. The M-ganglion, in contradistinction to other cell types of the nerve fiber layer, is only present in small numbers. The M-ganglion cell, being in such short supply, is much more sensitive to even a small loss of its cells. Hence, the double frequency stimuli have been found to be more sensitive in picking up early glaucoma field loss than white on white testing. Numerous clinical studies have confirmed the increase sensitivity of double frequency screening versus white on white testing in early glaucoma.

We find the DFT test to be a valuable early detection tool. Whereas the standard white on white is often normal in patients with ocular hypertensive and suspected early cupping, the double frequency testing will often manifest a defect in these cases. And it can be quite reassuring if this test is normal in the presence of questionable nerve findings.

Zeiss Cirrus OCT

Randomized Clinical Trials

The key for glaucoma treatment is early detection. The earlier one detects it the less aggressive one needs to be in terms of IOP lowering the better the long term prognosis.  The randomized clinical trials have born this out.

It is important to know what each study was designed to ask, and what answers or conclusion were found.  Care must be taken not to interpolate too much from the original study design.  

So, your first task when you have a patient in front of you for the first time-- with all their test results and having completed your exam-- is to decide how this patient best fits into the glaucoma spectrum and which study is most relevant.

Please be aware that at the time most of these studies were initiated, prostaglandins were not yet available. Also, visual field progression in some studies was not controlled for cataract progression (most notably in the CITGS study).  For this and other reasons, the study conclusions--while immensely valuable-- should not be relied upon as the last word on what you should be doing for your glaucoma patients.

OHTS—Ocular Hypertension Treatment Study (ocular hypertensives)

The goal of the trial was to lower intraocular pressure 20%. This was achieved by placing patients on Timoptic® eye drops.

Patients had pressure readings of 24-32 in the study eye with fellow eye being above 21.  After 5 years, 10% of control (no treatment) eyes developed glaucoma.  In treated eyes (target was 20% reduction which was achieved using timolol maleate), only 5% of patients developed glaucoma.   Glaucoma was defined by either visual field changes or changes in the optic nerve appearance as judged by stereo-photography, or both. In this study, optic nerve progression was found to be the most sensitive way to detect glaucoma.  Another interesting conclusion the study was that corneal thickness was related to the risk of having glaucoma. Patients with thin corneas were more at risk for glaucoma than patients with thick corneas.

EMGT—Early Manifest Glaucoma Trial   (newly diagnosed  high and normal pressure Glaucoma) The study evaluated both high and normal tension glaucoma patients with a mean IOP of 21. The 24-2 Humphrey visual field test averaged minus 4.00 decibels.

Patients were treated with a combination of topical beta-blockers (betaxolol) and argon laser trabeculoplasty (ALT) and averaged a lowering of 5.1 mm or 25%.  No target pressure was set.

With treatment, progression occurred in 45% of the treated eyes versus 62% of control eyes.  Also treated eyes had a delayed onset of glaucoma vs. untreated eyes.  The study’s authors concluded that the study shows that each additional one mm of IOP reduction is associated with a 10% reduced risk of disease progression.  

One school of thought holds that while this 10% per 1 point relationship may be valid for a certain range of pressure lowering,   at lower pressures there are diminishing returns.  The putative protective value of further lowering beyond a certain level is more debatable; and moreover one needs to consider the effects on patient quality of life from additional medications or incisional surgery, ocular surface problems, difficulties with compliance, or other issues.  (Singh, K., Limitations in the assessment of the relationship between IOP and POAG, Ocular Surgery News Special Supplement, May 1, 2007, page 5-8.)

The EMGT found that visual field deterioration and optic disc hemorrhages were the most sensitive indicators of glaucomatous progression.  It also conclude that elevated IOP but not IOP fluctuation, is a strong risk factor for progression. 

CIGTS-- Collaborative Intervention Glaucoma Treatment Study (primary open angle glaucoma) CIGTS was designed to determine the long-term effect of treating newly diagnosed primary open angle glaucoma with trabeculectomy compared to medication.  Unlike the EMGT study, patients with normal tension glaucoma are excluded from this study.   

Think of this as two studies in one.  Aggressive target pressures were set based upon the degree of visual field loss and intraocular pressures, using a complex formula.  Patients (not eyes) were randomized with either medical or incisional surgery.  At the time of the study, prostaglandins were not yet available. 

Patients in this group had a mean deviations on 24-2  Humphrey visual field of  minus 5.00 decibels.   In the surgery group (trabeculectomy) pressure reduction to mean of 14.2 (average IOP drop of 46%) was achieved; the medical group achieved reductions to 17.5 mm (average IOP drop of 38%).  75% of medically controlled patients required 2 or more medicines to reach the target IOP.  After 5 years, there was no difference between the surgically controlled the medically controlled group on visual field testing.   The study however did not adjust for the higher rate of cataract formation in the surgery vs. the medical group.

Another conclusion was that variability of IOP readings between visits is an independent risk factor for progression (unlike the EMTG study).

CITGS showed us that patients with mild initial damage do perfectly well at somewhat higher pressures (mean reduction from 25 to 17) as long as the IOP is lowered at least 35%.  Aggressive percentage reductions whether through medical, laser or surgery (between 36 to 48%) allowed more stability of the visual field. 

However filtration surgery was shown to result in higher rates of cataract surgery and more local eye symptoms than the medication group. These findings have led to the conclusion that initial trabeculectomy is not recommended at this time.
 

The CITGS investigators themselves recently concluded that the most valuable lesson from the  CIGTS thus far is that the judicious use of medical therapy is effective in stabilizing vision, visual acuity and the quality of life, at least in the short term.  (Lichter, et al., The Investigators Perspective on the Collaborative Initial Glaucoma Treatment Study, Arch Ophthal 126 (1), 122.)   In addition, “The CITGS is a work in progress, and final opinion about the study and its impact on ophthalmic practice requires longer term follow-up.”   (Pasquale, L., Optimizing Therapy for Newly Diagnosed Open-angle glaucoma, Arch Ophthal, 126 (1), 125.)

CNTGT – Collaborative Normal Tension Glaucoma Trial

This group studied patients with normal tension glaucoma and was designed to determine if aggressive IOP lowering slows optic nerve progression and visual field loss. Patients had untreated IOP of less than 24, with 9 of 10 readings equal or less than 21.

A significant lowering of intraocular pressure (30% of more from baseline) significantly arrested progression of field loss in 80% of patients.  However, 20% of patients will continue to progress despite this level of pressure lowering. Subgroups of patients more likely to have progression were woman, blacks, migraine sufferers, and those with disk hemorrhages. CNTGT showed that visual progression in normal tension glaucoma is more variable than in primary open angle glaucoma with only half of the untreated eyes experiencing visual field progression by 5-7 years.

AGIS

AGIS taught us that patients with advanced damage (mean deviation -10.0 decibels) do best with an IOP in the low-normal range. To achieve these low pressures (13.0 mm of Hg) filtering surgery must be considered as an option if medicines and laser trabeculoplasty do not lower pressures into this target range.