One test which my neurologist, ophthalmologist, and primary care doctor each conduct during every office visit is the “swinging flashlight test.” You know the one. The doctor asks you to look ahead then shines a penlight first toward one eye, then the other, alternating quickly to observe your pupils’ response to light.
I strangely enjoy this test because I know that my pupils will show something unique. Something which proves that I have damage to my optic nerve. My pupils show a Relative Afferent Pupillary Defect (RAPD) or Marcus Gunn Sign.
What does the doctor look for during the “swinging light test”?
The pupils (the black centers of the eyes which dilate or constrict in response to light) are inspected for size, equality, and regularity. Did you know that the pupils will constrict or dilate when you look at objects far or near? They do, which is kinda cool.
More importantly, each pupil should constrict quickly and equally during exposure to direct light and to light directed at the other pupil (the consensual light reflex). Using the swinging light test, the doctor can test and observe the pupillary response to consensual light in order to determine if there is a defect present.
Normally, the 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.
What is a Relative Afferent Pupillary Defect?
The Afferent Pupillary Defect (APD) or Relative Afferent Pupillary Defect (RAPD) is an abnormal and unequal response in the pupils of the eyes when exposed to light. It basically demonstrates that one optic nerve transmits a different message to the brain than the other one. Testing for RAPD is a good way to implicate or rule out optic nerve damage such as is caused by optic neuritis.
My temporarily blinding case of optic neuritis in 2000 left my right eye impaired. It doesn’t register light in the same way as my left eye as the optic nerve has permanent damage. When the doctor shines the light in my left eye (the “good” eye), both pupils will constrict. This is normal. When the doctor quickly moves the light to my right eye (the “bad” eye), my pupils begin to dilate since the brain thinks that less light is coming in. This shows that there is damage to the corresponding optic nerve.
What is interesting is that my neurologist told me that my right pupil will even continue to dilate somewhat. In the swinging light test, the difference in pupil response gives the “relative” indication of functioning of each eye and optic nerve. If both optic nerves are dysfunctional, then there will be no relative defect detected.
Of various degrees of afferent pupillary defects, here are four examples.
- No Relative Afferent Pupillary Defect: Both pupils constrict equally without evidence of pupillary re-dilation with the “swinging flashlight test,” except possibly for “hippus.” Hippus refers to non-rhythmic fluctuations in pupillary size when there is a steady illumination.
- Mild Relative Afferent Pupillary Defect: The affected pupil shows a weak initial constriction, followed by dilation to a greater size.
- Moderate Relative Afferent Pupillary Defect: The affected pupil shows a stable or unchanged level of constriction, followed by dilation to a greater size.
- Severe Relative Afferent Pupillary Defect: The affected pupil shows an immediate dilation to a greater size.
So what is Marcus Gunn Pupil?
Marcus Gunn Pupil or Marcus Gunn Sign is the term given to the relative afferent pupillary defect in which the affected pupil paradoxically dilates to the light stimulus. This can result in what is known as a “dynamic anisocoria” and is caused by damage along the 2nd cranial nerve. In my case, my right pupil continues to dilate and becomes larger than the left. This is the “dynamic anisocoria” (where pupils become different sizes) and is called the Marcus Gunn Pupil.
Anisocoria is a condition in which the pupils are of different sizes at all times. See video example here. It is an efferent defect rather than an afferent defect and is related to the 3rd cranial nerve. A future post may discuss efferent or parasympathetic dysfunctions.
What are possible causes of Marcus Gunn Pupil?
- Optic Nerve Disorders: optic neuritis, optic nerve infections or inflammations, glaucoma, optic nerve tumor, optic neuropathy, orbital disease
- Retinal Causes: ischemic retinal disease, retinal detachment, sever macular degeneration, intraocular tumor, retinal infection
- Cerebral vascular disease
If I show the Marcus Gunn Sign, do I have of MS?
As mentioned above, there are a number of potential causes for a relative afferent pupillary defect. Damage to the optic nerve is the most common cause and damage to the optic nerve is very common in multiple sclerosis. But that does not mean that anybody who shows this sign has or might have MS. If you are concerned about any unusual symptoms or signs you notice, then contact your doctor to discuss the possible causes and treatments recommended.