How to detect bilateral relative afferent pupillary defect (RAPD)?

From the Guidelines

To detect bilateral relative afferent pupillary defect (RAPD), the standard swinging-light test is not sufficient, and alternative methods such as the alternating light test with neutral density filters or pupillometry devices should be used. The swinging-light test, as described in the pediatric eye evaluations preferred practice pattern 1, is used to assess for the presence of a relative afferent pupillary defect, but it is limited to detecting unilateral defects.

When performing the alternating light test with neutral density filters, the goal is to equalize the pupillary responses between the two eyes. This is achieved by placing increasingly dense neutral density filters in front of one eye until the pupillary responses appear equal. The filter strength needed to equalize the responses quantifies the severity of the bilateral defect.

Some key points to consider when detecting bilateral RAPD include:

• Bilateral RAPD typically indicates significant bilateral optic nerve or retinal pathology with asymmetric involvement, such as bilateral optic neuritis, glaucoma, or retinal disorders with different degrees of severity between eyes.
• Early detection is crucial as bilateral RAPD often suggests serious underlying neurological or ophthalmological conditions requiring prompt evaluation.
• Pupillometry devices can objectively measure pupillary light responses in each eye independently, providing a quantitative assessment of the bilateral defect.

It is essential to note that pupillary evaluation in infants and children may be challenging due to frequent shifts in the patient’s fixation and focusing, as mentioned in the pediatric eye evaluations preferred practice pattern 1. Therefore, a thorough and careful examination is necessary to accurately detect bilateral RAPD.

From the Research

Detection of Bilateral Relative Afferent Pupillary Defect (RAPD)

To detect bilateral RAPD, several methods can be employed, including:

• The swinging flashlight test, which can be challenging in certain cases, such as dark eyes or anisocoria 2
• Infrared pupillary asymmetry (IPA) using a standard optical coherence tomography (OCT) device, which provides a rapid and practical test for RAPD detection 3
• B-mode ultrasound pupillometry, which allows for objective and quantitative assessment of RAPD 4, 5
• Automated pupillography, which can be used to confirm RAPD detection 6

Methods for RAPD Detection

The following methods have been studied for RAPD detection:

• Ultrasound pupillometry (UP) has been shown to sharply discriminate acute optic neuritis eyes and healthy controls, with an area under the curve (AUC) of 1 4
• Infrared pupillary asymmetry (IPA) has been found to have a diagnostic value with an AUC of 0.843 3
• B-mode ultrasound has been demonstrated to enable fast and easy objective quantification of RAPD 5
• The swinging flashlight test, including a magnifier-assisted version, has been evaluated for its sensitivity and specificity in detecting RAPD 6

Clinical Applications

RAPD detection can be useful in various clinical settings, including:

• Optic neuritis diagnosis and monitoring 4, 2
• Glaucoma assessment, where RAPD detection may precede apparent optic disc and visual field damage 6
• Multiple sclerosis diagnosis and monitoring, where RAPD detection may indicate subclinical optic nerve damage 4