Reverse Relative Afferent Pupillary Defect (RAPD)
Background of RAPD
A relative afferent pupillary defect (RAPD) also known as a Marcus Gunn pupil, is a critically important ophthalmological examination finding that defines a defect ( pathology) in the pupil pathway on the afferent side. An RAPD is relative to the fellow eye and occurs because of the bilateral and equal innervation of the pupils in normal individuals. The RAPD manifests as a difference in pupillary light reaction between the two eyes. The test requires two eyes but only one working pupil. Patients do not have anisocoria.
A RAPD is seen in unilateral or bilateral but asymmetric lesions of the prechiasmal optic nerve starting from the retina but can occur anywhere in the afferent pupillary pathway including the optic tract and the pretectal afferent fibers in the dorsal midbrain.
In a normal patient without afferent pupillary disease, shining a light in either eye will produce constriction in both eyes equally. In the case of an afferent pupillary pathway lesion, the light response is tested individually in each eye, and then the light swings between the two eyes in order to detect an RAPD. The normal pupillary response is characterized by equal constriction of the pupils in both eyes when the light stimulus is applied to each eye individually and no dilation or pupillary escape when the light swings from one eye to the other. The pathologic response that characterizes the RAPD includes the following: 1) the light reaction causes pupil constriction in both eyes when the light shines in the normal eye, and (2) dilatation of the pupils in both eyes when the light stimulus is rapidly transferred from the normal eye to the pathologic eye. The RAPD is a critically important sign in patients with unexplained visual loss because it is an objective finding of afferent pupillary dysfunction.
Conditions Leading to a RAPD
Occur in lesions affecting the visual pathway in front of the lateral geniculate body including:
Lesions of the Anterior Optic Pathway
- Lesions of the optic nerve, regardless of the cause of optic neuropathy (e.g., optic neuritis, glaucoma, compression, infection etc.)
- Lesions of the optic chiasm
- Lesions of the optic tract
- Lesions of the pretectum
Lesions of the Retina/Posterior Segment
- Large retinal detachments
- Ischemia (e.g., ischemic central retinal vein occlusion or central retinal artery occlusion)
- Dense macular lesions (chorioretinal scar)
Reverse RAPD and its usage
Testing for a RAPD requires two eyes, but only one functioning pupil. Where one eye has an efferent pupillary defect (e.g., iris posterior synechiae, trauma, third nerve palsy, or pharmacological mydriasis), a ‘reverse RAPD’ test can be employed.Reverse RAPD, or reverse testing for RAPD, utilizes the swinging flashlight test while evaluating the normal, unaffected pupil for dilation. This finding is present in every RAPD, but most examiners are used to only observing the affected pupil during the swinging flashlight test. In fact, both pupils dilate in every RAPD, but in the reverse RAPD, the clinician will be observing the dilation of the unaffected pupil rather than the affected pupil (in the setting of a patient with both an afferent and efferent pupillary defect).
Figure 1 shows the right pupil has both an afferent pupillary defect and a fixed pupil from an efferent pupillary defect. When the light is shone onto the left pupil, the left pupil constricts, but the right pupil is fixed and therefore cannot constrict. When the light swings from the abnormal right pupil with the afferent (and efferent) pupillary defect to the normal left pupil, the left pupil constricts, but when the light swings from the normal left pupil back to the abnormal right pupil, the examiner observes the left pupil dilating. This is the right RAPD by reverse testing.
A reverse RAPD should be evaluated in every patient with an efferent pupillary defect. For example, in a patient with a cranial nerve (CN) III palsy with a dilated pupil, one of the main diagnostic considerations is possible aneurysm of the posterior communicating artery (PComm). In this setting, a CN III palsy is potentially life-threatening, and urgent vascular imaging (e.g., computed tomography (CT) and a CT angiogram of the brain) may be necessary. However, if this same patient with a CN III palsy also has an RAPD (by reverse testing), then the localization of the lesion moves from the posterior communicating artery to the orbital apex. Some clinicians may neglect to check for a RAPD by reverse testing because they are misled into believing that the fixed and dilated pupil precludes assessment of that pupil’s afferent response.
Figure 2 shows the swinging flashlight test in a patient with a right RAPD and a non reactive right pupil. In this instance, the left pupil should be watched the whole time the examination is conducted to identify if there is an RAPD.
In summary, clinicians should be aware that the RAPD can be tested in any patient who has two eyes and at least one working pupil. The reverse RAPD is present every time because both pupils dilate when the light swings from the normal pupil to the abnormal pupil but in a reverse RAPD the clinician is observing the normal pupil when the light swings from uhe naormal eyeto ahe fafected eye.
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