Visual Field Testing Is Pupil Dilation a Compromise?

While pharmacological pupil dilation is advantageous for the comprehensive stereoscopic assessment of the ocular media and posterior eye,^2-6 it is accompanied by a myriad of undesirable changes in ocular function.^8,9 With this in mind, are clinicians able to dilate patients prior to visual field testing without having to compromise on perimetric accuracy? This article aims to present the most up-to-date evidence addressing this controversial topic.

there are still no clear clinical guidelines for the timing of pupillary dilation in clinical practice

Figure 1. Schematic diagram depicting ocular changes associated with pharmacological pupil dilation.

CHANGES IN VISUAL FUNCTION ASSOCIATED WITH PUPIL DILATION

With its relatively good safety profile and short-lasting effects, tropicamide 0.5% is typically the pharmacological agent of choice for routine examinations requiring a dilated pupil.^5,9 As a muscarinic antagonist, it binds to the M3 receptors of the iris sphincter and ciliary muscles, resulting in both mydriasis and cycloplegia.¹⁰ 

The resultant increase in pupil size and reduction in accommodative function results in a constellation of ocular changes such as: an increase in aberrations,^11,12 partial paralysis of accommodation,⁹ a decrease in depth of focus,^13,14 and an increase in retinal illumination¹⁵ (Figure 1).

These changes in ocular structure have the potential to affect threshold sensitivities measured during visual field testing.

Currently, the recommendation is that visual field testing should be performed with consistent natural pupils greater than 3mm in diameter16 (a numerical upper limit in pupil size has not been defined). Although several studies have reported on the relationship between pharmacological dilation and visual field sensitivity,^12,17-21 there are still no clear clinical guidelines for the timing of pupillary dilation in clinical practice.

DOES PUPIL DILATION AFFECT GLOBAL INDICES IN VISUAL FIELD RESULTS?

Mean deviation (MD) is a global index often used by clinicians to quantify the overall degree of sensitivity loss across the entire visual field.¹ From a practical perspective, it may be used to stage the level of patients with glaucoma.²² Previous studies suggest there is a small but statistically significant reduction in the MD of normal subjects following pupil dilation using tropicamide with an average change between -0.83 to -1.15 dB.^18,20 Interestingly, the effect of pupil dilation has been shown to be more pronounced in patients with existing visual field loss.²¹

A study by Mendivil in 1997²¹ examined the effect of pupil dilation on a cohort of patients with existing primary open angle glaucoma. He found MD worsened following dilation with 10% phenylephrine by an average of -3.01 dB. The degree of worsening was proportional to the predilation MD whereby patients with greater pre-existing visual field defects were more significantly affected by pupil dilation compared to those with milder defects. It is not clear whether this finding was simply an epiphenomenon with more recent computer-based modelling of measurement variability in glaucomatous visual fields showing the degree of MD variability to also be greater in patients with more advanced visual field defects.²³ 

To put these numbers into perspective, the expected test-retest variability of MD is between 1.5 and 4 dB, depending on the integrity of the visual field.²⁴ This suggests that while dilation may not cause a worsening of MD outside of expected test-retest variability in normal subjects or patients with early visual field loss, clinicians need to be careful in differentiating perimetric worsening associated with pupil dilation from true worsening, especially in patients with advanced visual field loss.

Current evidence relating to the effect of pupil dilation on other global indices, such as the pattern standard deviation (PSD), remain conflicting with some studies showing a worsening in PSD following dilation¹⁸ while others have found no significant change.²⁵ The unknown impact of this may become important in glaucoma diagnosis as PSD tends to be used as a ‘cutoff ’ to determine whether a visual field defect is considered significant²² and variability here could confound the diagnostic process.

There have been no studies to date reporting the effect of dilation on the visual field index (VFI) which is a percentage-based metric that represents the functional integrity of the visual field.

IS THERE A LOCATION-DEPENDENT EFFECT?

While global indices are useful to quickly and quantitatively assess progression of visual fields, it is also important for clinicians to consider the effect of pupil dilation on the individual threshold sensitivities across the visual field that are used to derive these global metrics. This is also relevant when doing event and trend analysis on visual fields in diseases like glaucoma, as individual or clusters of points are examined for sensitivity change.

It has been hypothesised that there is an eccentricity-dependent effect whereby peripheral locations are thought to be more affected by dilation compared to central locations.²¹ A study by Mendivil²¹ examining the effect of dilation in patients with established primary open angle glaucoma divided the 30-2 test grid into three ring-like zones, each spanning 10˚. They found the innermost zone (10˚) only showed an average change of 0.74 dB while the outermost zone (30˚) showed a significantly higher average change of 6.35 dB. This was concordant with the findings of a study examining the effect of dilation on the size of kinetic visual fields in young normal subjects, which found a reduction in isopter area following dilation.¹⁷ Interestingly, in patients with retinitis pigmentosa, the isopter area is unaffected by dilation.²⁶

WHAT’S NEW IN THE LITERATURE?

More recently, the research team at the Centre for Eye Health (CFEH) located at the University of New South Wales, Sydney, conducted a research study examining the effect of pupil dilation in normal subjects.²⁷ In addition to looking at global indices, the team performed a pointwise analysis and showed that pupil dilation results in localised sensitivity changes that fall outside of expected intra-observer variability. Furthermore, CFEH found that the area of the visual field affected varied significantly between subjects with no systematic pattern of change identified using a pattern recognition analysis software.

THE VERDICT

Based on CFEH’s recent findings, the effect of pupillary dilation on visual field testing varies considerably between individuals. The effects of pupil dilation are difficult to predict and have the potential to confound visual field interpretation at an individual level. As such, further work is required to identify the factors driving these changes and to help clinicians identify patients who may be more predisposed to changes in perimetric sensitivity associated with pupil dilation. In the meantime, clinicians should try to maintain a consistent pupil size for testing where possible, particularly in patients with existing visual field loss for the purposes of valid progression analysis.

Henrietta Wang BOptom (Hons.), BSc, FAAO graduated from the University of New South Wales in 2016. She is a Fellow of the American Academy of Optometry. During her undergraduate degree, she received numerous clinical and academic awards, including a research scholarship for her work in the Retinal Networks Laboratory. She is a staff optometrist at the Centre for Eye Health, with clinical interests in retinal disease and advanced imaging modalities. Her research interests are also in the areas of retinal disease and optimizing clinical psychophysical procedures. 

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