Light intensity between 50 lux and 300 lux best reveals afferent pupillary defect.
Light intensity between 50 lux and 300 lux best reveals afferent pupillary defect. Photo: Caroline B. Pate, OD. Click image to enlarge.

Devised almost a century ago, the swinging flash-light test is still a workhorse assessment technique conducted to look for relative afferent pupillary defects (RAPD). Since it remains the first-in-line test when suspecting an afferent pupillary pathway abnormality upon eye examination, researchers from one new study wanted to standardize the light intensity required to elicit the pupillary defect during this test.

To do so, the study included 20 healthy controls (aged 14 to 50) and 31 individuals with clinically diagnosed RAPD (aged 12 to 72). All participants were measured for monocular pupillary light reflexes three times using one second ling light pulses, following with three seconds of darkness at eight different light intensities (6.4 lux to 1200 lux) using objective pupillometry.

The median defect score decreased from baseline values of 1.58 for right eye pathology and 0.45 for left eye pathology to saturation values of 1.18 and 0.98, respectively, at intensities between 56.9 lux and 300.5 lux in 54.8% of cases. The remaining defect scores of the 45.2% of cases were similar to that of controls, constant with light intensity at 1.23 and 0.87 for the right and left pathology, respectively.

From these results, the study authors came up with three main points. First, that pupillary light reflex magnitude increased up to 25 lux of light intensity, beyond this point saturating both cases and controls. The second is that the direct reflexes were reduced in cases’ affected eye relative to the fellow eye, but still with unremarkable consensual reflexes. Finally, low light intensities corresponded with exaggerated relative afferent pupillary defect scores, while these scores were reduced in high light intensity in more than 50% of patients with unilateral neuro-ophthalmic pathology.

Related to this last point, the authors note that the light intensity used when testing may determine if an afferent pupillary defect is detected or not, as well as its grade in a clinical evaluation. Since no cases displayed a trend of increasing defect score with light intensity, perhaps lower light intensity is better to detect defects than higher intensities. However, keep in mind that magnitude of pupillary light reflexes are typically smaller for low than high intensities.

Despite this idea, the ability to detect meaningful change in pupillary response from the steady-state pupillary hippus may limit a practitioner’s ability to identify the defect, especially when made without pupillometry. The signal-to-noise ratio of affected eyes in this study were much smaller than for controls and unaffected eyes, prompting the researchers to add that the low light intensities likely do not produce a high enough signal for easy defect detection, even with higher intensities masking manifestation.

As such, the authors contend that “the ‘sweet spot’ for detecting the defect may thus lie at intermediate light intensities […] the optimal light intensities may range from 50 to 300 lux for the estimation of relative afferent pupillary defect in the clinic.” These levels would be ideal for individuals who do not display change in defect scores with light intensity.

They add, “It is also recommended that pupillary responses be tested at multiple light intensities—say, 25 lux, 300 lux and 1000 lux—for identifying putative changes in the severity of the pupillary defect with light intensity in patients with unilateral neuro-ophthalmic pathology.”

Negi R, Raviselvan M, Yarravarapu D, et al. Effect of light intensity on the relative afferent pupillary defect in unilateral neuro-ophthalmic pathology. Optom Vis Sci. August 29, 2023. [Epub ahead of print].