Relative Afferent Pupillary Defect
Relative Afferent Pupillary Defect (RAPD) is a condition in which a person’s pupils respond differently from one another to light stimuli shone, based on when it is shone into one eye at a time. An RAPD is due to unilateral or asymmetrical disease of the retina or optic nerve. The swinging flashlight test,otherwise known as the Marcus Gunn test, is one of the most basic eye exams that neurologists, ophthalmologists, optometrists, and primary care doctors perform when visiting most of their patients. In order to do the test, the doctor asks her/his patient to look ahead at a far point, and then shines a penlight first towards one eye, then the other, alternating quickly while observing the patient’s pupils’ response to the light. If both pupils do not show a similar response to one another, then the patient will be diagnosed with an RAPD, or Marcus Gunn pupils. The results of this simple yet very important test help doctors in the early diagnosis of many important eye related diseases, such as optic neuropathy, glaucoma, and multiple sclerosis (MS).
Conditions Leading to an RAPD
Optic nerve disorders: Unilateral or asymmetric optic neuropathies are common causes of an RAPD. If a condition is bilaterally symmetrical, it won’t lead to an RAPD because the eyes will both be affected in the same way.
Optic neuritis: Even very mild optic neuritis with no or minimal loss of vision can lead to a very severe RAPD.
Anterior ischemic optic neuropathies: These include arteritic (Giant Cell Arteritis) and non-arteritic causes. The patient complains of seeing poorly in one eye. Vision in that eye is covered by a dark shadow, often involving just the upper or lower half of vision, usually the area towards the nose.
Glaucoma: While glaucoma is normally a bilateral disease, if one optic nerve has particularly more damage, an RAPD can be seen.
Traumatic Optic Neuropathy: This refers to direct ocular trauma, orbital trauma, or incidental head trauma that can cause damage to the optic nerve as it passes through the optic canal into the cranial vault.
Optic nerve tumor: This is a rare cause, and includes primary tumors of the optic nerve (such as glioma, or meningioma) or tumors that compress the optic nerve (such as sphenoid wing meningioma, pituitary lesions, etc.).
Orbital disease: This could include compressive damage to the optic nerve from thyroid related orbitopathy (i.e. compression from enlarged extraocular muscles in the orbit), orbital tumors, or vascular malformations.
Radiation optic nerve damage: Miscellaneous optic neuropathies, such as Leber's optic neuropathy (which usually becomes bilateral eventually) and other inheritable optic neuropathies.
Optic nerve infections or inflammation: Cryptococcus can cause a severe optic nerve infection in the immunocompromised. Sarcoidosis can cause inflammation of the optic nerve. Lyme disease can affect the optic nerve.
Optic atrophy status: post papilledema;this is usually bilateral.
Surgical damage to the optic nerve: This could include damage following retrobulbar anesthesia; damage following an orbital hemorrhage related to eye, orbital, sinus, or plastic surgery; damage following neurosurgical procedures such as a pituitary tumor resection; and damage related to the migration of an orbital plate after surgical correction of a blow-out fracture.
Glaucoma Screening with RAPD Test
Glaucoma has always been the first or the second cause of blindness in the world. Approximately 2.2 million Americans are affected with glaucoma. RAPD is an important marker of asymmetric impairment of the afferent visual system, and is likely to indicate significant damage to the optic nerve in patients with glaucoma. Moreover, the magnitude of the pupillary response defect has been shown to correlate with the severity of disease. A recent systematic review found that RAPD can be detected in one-third to two-thirds of glaucomatous patients. O-Glass empowers eye care providers to detect and quantify RAPD in early stages.
RAPD test, an Important tool for Diagnosing Multiple Sclerosis (MS)
Multiple sclerosis is a chronic, often disabling disease of the central nervous system, which comprises of the brain, spinal cord, and optic nerve. Canada has the highest prevalence of MS in the world. Patients with multiple sclerosis frequently have visual symptoms. Optic neuritis is recognized clinically during the patient course of MS in up to 75% of cases, and it is the presenting feature of MS in 25% of patients. O-GLASS helps physicians detect an RAPD in the early stages of unilateral or asymmetric bilateral optic neuritis. Early diagnosis can create a great opportunity for decreasing inflammation soon enough to preserve neurological function, possibly even preventing clinical attacks of MS altogether.
RAPD Diagnosis and Challenges with the Swinging Flashlight Test
Pupils are inspected for size, equality, and regularity during an eye exam. Each pupil should constrict quickly and in the same manner as the other pupil when exposed to direct light,as well as when light is directed at the other pupil (i.e. the consensual light reflex). Using the swinging light test, physicians test and observe the pupillary response to consensual light in order to determine if there is a defect present. Normally, pupil constriction does not change as the light is swung from eye to eye. When the light is moved quickly from eye to eye, both pupils should hold their degree of constriction. But, even under the best conditions, it is hard to perform this manual test accurately. Human factors, including examiner bias, light position variability, and endpoint determination, may all influence the identification and appropriate quantification of RAPD in patients.
Moreover, other factors unique to any given individual, such as dark irises, anisocoria, small pupils, or efferent defects may make it much more difficult to detect small amounts of asymmetry in pupillary reactions. Although RAPD is known to be an important physical sign, many well-practiced doctors rarely report instances of RAPD – not because the signs are not present, but because it is difficult or impossible to detect subtle abnormalities, and so it is rarely quantified. Misdiagnosis of optic nerve disease in primary stages may lead to irreversible visual loss. In a recent study, the inter-examiner disagreement in the manual evaluation of pupillary reaction was as high as 39%. Such a high amount of disagreement justifies the need for having more appropriate test methods.
Traits that can make spotting RAPD difficult,such as dark irises (i.e. patients with a low contrast between the iris and pupil) and small pupils can make getting accurate results from the swinging flashlight test challenging. In addition, early stages of an RAPD may not be detected by a clinician’s naked eye.
When an RAPD occurs in a patient with existing anisocoria (no matter what the reason for that anisocoria is), its diagnosis will be difficult.