Relative afferent pupillary defect (RAPD) is a clinical sign detected by the swinging flashlight test, in which light is alternately shone into each eye. It is used almost synonymously with Marcus Gunn pupil. It is an important finding indicating the presence of unilateral or asymmetric damage to the retina or optic nerve (anterior to the lateral geniculate body) on one side.
RAPD is one of the most basic ophthalmic examinations performed by neurologists, ophthalmologists, and optometrists, and is useful for the early diagnosis of many important diseases such as optic neuritis and multiple sclerosis. Misdiagnosis of optic nerve disease in the early stages can lead to irreversible vision loss.
Important principles are shown below.
Bilateral and symmetric cases are not RAPD: Equal damage on both sides does not produce asymmetry, resulting in bilateral APD.
Opacities of the ocular media do not cause a positive RAPD: Lesions that block light entry, such as cataracts or vitreous hemorrhage, do not cause a positive RAPD if the pupillary afferent pathway is not directly damaged.
Lesions posterior to the optic chiasm do not generally cause a positive RAPD: However, because the optic tract contains more crossed fibers than uncrossed fibers, optic tract lesions may cause a mild RAPD in the contralateral eye.
QDoes the presence of a cataract affect the RAPD test result?
A
Opacities of the ocular media such as cataracts reduce the amount of light entering the eye, but do not directly damage the pupillary afferent pathway, so RAPD is not positive. If RAPD is positive even with severe cataract, there is a high possibility of underlying optic nerve or retinal disease.
RAPD is not a “symptom” perceived by the patient, but a “sign” detected by the physician through examination. What the patient perceives are symptoms caused by the underlying disease, which commonly include the following.
Decreased visual acuity: Occurs in optic neuritis, ischemic optic neuropathy, etc.
Eye pain or pain with eye movement: Seen in about 60% of optic neuritis cases.
Visual field defects: Central scotoma, centrocecal scotoma, etc.
Color vision abnormalities: Particularly dulling of red (characteristic of optic neuritis).
Clinical Signs (Findings Confirmed by Physician Examination)
Pupillary dilation during the swinging flashlight test: The pupil dilates (or the degree of constriction is reduced) despite light being shone into the affected eye. This is the essential finding of a positive RAPD.
Diminished direct light reflex: On the affected side, the speed of pupillary constriction slows and the response amplitude decreases.
Anisocoria does not occur: Because the abnormality is in the afferent pathway, the direct and consensual responses are nearly equal under binocular open conditions, so resting anisocoria does not occur.
Differentiation from efferent pathway disorders: In efferent pathway disorders such as oculomotor nerve palsy, the affected eye is constantly dilated and does not constrict when light is shone. In RAPD, both eyes constrict when the healthy eye is illuminated (because the efferent pathway is normal).
Retinal detachment: Macular detachment or involving two or more quadrants.
Intraocular tumors: Metastatic lesions, melanoma.
Amblyopia: Severe cases (visual acuity 20/400 or worse).
Optic nerve causes are the most common. Representative diseases are listed below.
Demyelinating optic neuritis: Peak age 15–45 years, more common in women.
MOG-IgG-associated optic neuritis (MOGAD): Ocular pain with eye movement 86%, optic disc edema 86%. Visual loss is severe but recovery is good (poor prognosis with visual acuity 20/200 or worse in 5–14%) 2).
Anti-AQP4 antibody-positive optic neuritis: Approximately 10% of idiopathic optic neuritis. Female-to-male ratio 9:1. Steroid-resistant and poor prognosis.
Metastatic optic neuropathy: Orbital metastasis from breast cancer, etc. Optic nerve metastasis accounts for 4.5% of ocular metastases. Onset occurs on average 4.5–6.5 years after primary diagnosis 1).
Orbital compartment syndrome: Positive RAPD is an indication for emergency decompression. In a case of postpartum lateral rectus hematoma, positive RAPD prompted lateral canthotomy and hematoma evacuation, with visual acuity improving from no light perception (NPL) to 6/6 3).
Choroidal tuberculoma: Intraocular lesions associated with miliary tuberculosis can also cause RAPD 4).
Drug-induced optic neuropathy: Caused by ethambutol, etc. Contrast-enhanced MRI shows no enhancement of the optic nerve.
QWill RAPD be positive even if both eyes have the same degree of retinal disease?
A
RAPD does not appear in symmetric bilateral retinal disease. If there is no difference in light input between the left and right eyes, pupillary dilation does not occur during the swinging flashlight test. It is important to understand that RAPD is a test that detects “left-right difference (asymmetry).“
Can be performed with a single penlight. No special equipment is required, and it is simple, quick, and of high diagnostic value.
Procedure details:
Perform in a dimly lit room after dark adaptation.
Have the patient fixate on a distant target.
Shine a bright penlight (or handheld slit lamp) from below alternately into each eye for about 2 seconds.
Observe only the pupil of the eye being illuminated.
Normal: When quickly moving to the other eye, adequate constriction is similarly observed.
RAPD positive: When light is moved to the affected eye, the light reflex does not occur and both eyes dilate (or the degree of constriction is clearly reduced).
Note: It is important to direct the light source at the same angle (preferably from the front) for both eyes. Oblique illumination can easily cause false positives.
The inter-examiner disagreement rate for manual assessment is as high as 39%. Detection of subtle differences between the eyes becomes difficult in cases of dark iris color, anisocoria, small pupils, or efferent pathway disorders.
Neutral density (ND) filter method: Perform the swinging flashlight test with an ND filter placed in front of the healthy eye, and quantify the RAPD by the filter density at which it disappears. This is also useful for evaluating treatment response.
Subjective grading: Grading based on the initial constriction and subsequent redilation of each pupil. It is considered to have quantification accuracy equivalent to the ND filter method.
Objective and quantitative evaluation using infrared video devices is possible, compensating for the limitations of manual assessment. A mechanical frame that completely isolates each eye is used, with full-color LED stimulation and real-time high-resolution camera recording of both anterior segments. Detailed information such as dilation velocity and pupil area, which cannot be measured with the naked eye, can be obtained. Devices available in Japan include the Iriscorder Dual C-10641 (dual wavelength selection of blue 470 nm and red 635 nm, simultaneous binocular recording) and the ET-200 (two cameras, three-color multi-color LED).
Differentiation from efferent pathway disorders (oculomotor nerve palsy, traumatic mydriasis) is most important. In efferent pathway disorders, the affected eye is always dilated, and even with indirect light stimulation of the healthy eye, the affected eye does not constrict. In RAPD, both eyes constrict when the healthy eye is stimulated, which is a key difference.
Tonic pupil (Adie pupil): Diminished to absent light reflex, preserved near response, and tonic reaction. Shows denervation supersensitivity to 0.125% pilocarpine.
QCan the degree of RAPD be quantified?
A
Quantification is possible using the ND filter method. It is expressed by the filter density at which RAPD disappears when placed in front of the healthy eye (e.g., 0.3 log unit), and can also be used to monitor treatment progress. Digital pupillography allows objective quantification using multiple parameters such as dilation velocity and pupil area.
RAPD-positive plus visual loss is an indication for emergency decompression. Lateral canthotomy and inferior cantholysis are performed. If the hematoma is localized, image-guided surgical drainage may be necessary.
QIs there a medication to treat RAPD itself?
A
RAPD is not a disease itself but a “sign” reflecting the degree of optic nerve or retinal damage; therefore, there is no specific treatment for RAPD itself. The principle is to diagnose the underlying cause and provide corresponding treatment (e.g., steroid pulse therapy for optic neuritis).
Afferent pathway: Retina → optic nerve → optic chiasm (bifurcates to ipsilateral and contralateral sides) → leaves the main visual pathway before the lateral geniculate body → pretectal nucleus in the midbrain → bilateral Edinger-Westphal nuclei (EW nuclei)
Efferent pathway: EW nucleus → oculomotor nerve → ciliary ganglion → short ciliary nerves → sphincter pupillae muscle → miosis
If the amount of pupillary constriction when stimulating one eye is less than when stimulating the other eye, it indicates a difference in visual input between the two eyes. Since the direct and consensual responses are nearly equal, anisocoria does not occur under binocular viewing. The difference in response between the affected and healthy sides becomes apparent only with alternating stimulation.
In the optic chiasm, crossing fibers (approximately 53%) outnumber non-crossing fibers. Lesions of the optic tract can cause mild RAPD in the contralateral eye. In homonymous hemianopia due to optic tract damage, RAPD occurs in the eye with temporal visual field loss (nasal retina = temporal visual field = crossing fibers).
The supranuclear fibers to the Edinger-Westphal nucleus for near response run more ventrally than the afferent fibers for the light reflex. The ratio of neurons for light reflex to accommodation in the ciliary ganglion is 3:97, so even if the light reflex is impaired, pupillary constriction due to near response is often preserved.
Pathophysiology of Each Causative Disease (Overview)
Digital pupillography using infrared video devices eliminates the limitations of traditional manual testing (examiner bias, subjectivity, reproducibility) and provides objective quantification, recording, and improved follow-up quality for RAPD. With the development of personal computer-based systems, precise pupillometry, previously only possible at specialized institutions, is becoming available in general clinical settings.
Elucidation of MOG-IgG-Associated Disease (MOGAD)2)
MOGAD is being established as a disease entity independent of MS and anti-AQP4 antibody-positive NMOSD. Optic neuritis is the most common initial symptom in adults, and headache precedes attacks in about half of cases. Persistent MOG-IgG positivity may be a predictor of relapse, but transient positivity carries a low risk of relapse. Optimal long-term immunotherapy has not been established, and international prospective studies are ongoing.
Hurley DJ, Murphy R, Farrell S. Spontaneous postpartum lateral rectus haemorrhage. BMJ Case Rep. 2022;15:e248133.
Jojo V, Singh P, Samanta RP, Ahmad R. Unilateral Choroidal Granuloma and a Pupillary Abnormality in a Case of Miliary Tuberculosis: A Dilemma for the Physician. Cureus. 2022;14(9):e28713.
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