Hamad Hejazi 1, Salwa Faizan 2, Murtaza Sheikh 3
1Newcastle upon Tyne Hospitals NHS Foundation Trust
2Malaysian Allied Health Sciences Academy
3South Tees NHS Foundation Trust
The relative afferent pupillary defect (RAPD) is taught early in most medical careers. It is often tested in acute examinations, yet often the swinging flashlight test that elicits it can be very technique-dependent. In clinical settings where it can be most helpful, for example acute medical units, acute stroke units and emergency departments, it is frequently performed under conditions that work against it. Conditions like bright ambient light, a rushed single pass, and pupils examined once rather than swung repeatedly can affect the utility of this helpful neuro-ophthalmic test. This results in a sign that could either be missed, or worse falsely picked up as present when it is absent. RAPD clearly weighs very heavily on the differential for unilateral optic nerve or severe retinal pathology. Therefore, looking at the gap between how the test is taught and how it ends up being performed at the bedside can be very useful for clinicians (1).
To start, let’s look at the most common technique errors that can lead to a false positive for RAPD. The first error is to do with lighting. Illumination that is too bright drives the pupil to its maximum constriction. This can mask a genuine but modest defect. Alternatively, a beam too dim, may fail to elicit a measurable constriction at all. Another common error is to linger the light on one eye before swinging, instead of using a standardised pause. A three-second interval validated by Bell and colleagues, introduces variability that undermines both detection and grading (2). Finally, an uneven swing can bleach one retina more than the other. This produces an apparent defect in cases where they may be none. This can be avoided by the clinician using a smooth, brisk, symmetrical swing between eyes.
Hippus can also lead to confounding errors in the swinging light test. Hippus is defined as the normal oscillation of pupil size under steady illumination. This may be mistaken for a positive RAPD by the examiner if they are unfamiliar with hippus’ rhythm. The cornea can also reflect light that may obscure the pupil margin hence leading to inconsistent results. If the examiner angles the light source slightly above the visual axis level, they may avoid this. These errors recur across grades of seniority, and hence they reflect a gap in the standardised technique of this examination (3).
Clinicians may also be trapped by bilateral and symmetric disease. The swinging light test is very useful at assessing asymmetry between the afferent nerve pathways in the right and left eye, but can sometimes hide simultaneous and equal bilateral optic nerve or retinal pathology. Bilateral ischaemic optic neuropathy or a toxic or nutritional optic neuropathy, will not produce an RAPD even though both eyes are affected (4). Within the acute clinical setting, a negative swinging flashlight test should never be read as “no optic nerve pathology” in isolation.
Efferent pupillary abnormalities can also lead to issues for examiners. Anisocoria can be caused by trauma, mydriatic agents, or a third nerve palsy. This may lead to the affected pupil as being unreliable, and the test itself can be abandoned by many examiners because of this. Teaching around the reverse technique, where examiners may observe the reactive, unaffected pupil for paradoxical dilation as the light swings towards the affected eye is worthwhile for clinicians. This allows an RAPD to still be identified in this setting. This is important because trauma and periorbital swelling often coexist with suspected optic nerve injury in acute settings (5).
Paediatric populations have unique issues with the swinging light test. Patients may not stay still, fixate or cooperate, which may lead to confounding through miosis. A child who is not adequately fixing at distance will constrict due to convergence regardless of afferent status. A couple ways to help out with this are using a distant target rather than the examiners face, and testing before the child gets restless. In children, an RAPD may be a red flag for time-critical conditions like retinoblastoma or optic pathway glioma which makes technique all the more important.
This piece absolutely does not argue to abandon the swinging light test. It remains a very useful hallmark of the neuro-ophthalmic examination for clinicians in the acute setting. If the technique is adequate and the test is interpreted with adequate context, it is a rapid and low-cost way to rule out or rule in pathology. It is one of the most helpful signs available outside of a formal neuro-ophthalmology clinic. This article argues that the swinging light test should be viewed as a technique-dependent test rather than a binary finding. It encourages clinicians to document exactly how the RAPD was elicited. The RAPD, when looked for inadequately, can not be presumed to be absent.
References
1. Penney DC, Rushetsky ZM, Penney DW. Abnormal pupils at the bedside: rapid recognition of neurologic and systemic emergencies in acute care settings. Int J Emerg Med. 2026;19:73. https://doi.org/10.1186/s12245-026-01195-0
2. Bell RA, Waggoner PM, Boyd WM, Akers RE, Yee CE. Clinical grading of relative afferent pupillary defects. Arch Ophthalmol. 1993;111(7):938-42. https://doi.org/10.1001/archopht.1993.01090070056019
3. Kawasaki AK. Diagnostic approach to pupillary abnormalities. Continuum (Minneap Minn). 2014;20(4 Neuro-ophthalmology):1008-22. https://doi.org/10.1212/01.CON.0000453306.42981.94
4. Simakurthy S, Tripathy K. Marcus Gunn pupil. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026. https://www.ncbi.nlm.nih.gov/books/NBK557675/
5. Ramamoorthy T, Ayyan S, Deb AK. Diagnostic value of point-of-care ultrasound-guided assessment of relative afferent pupillary defect in adult ocular trauma patients presenting to the emergency department: a prospective cohort study. J Ultrasound Med. 2024;43(7):1343-51. https://doi.org/10.1002/jum.16458