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Relative Afferent Pupillary Defect (RAPD)

A frequent ophthalmic dilemma is to decide if a patient's complaint of decreased vision is real or imagined.  This is frequently seen in the setting of a normal looking nerve and retina particularly the macula.  It is important to recognize true disease and identify which part of the eye is causing the vision loss so proper diagnostic tests can be instituted.  If the initial ocular exam appears normal, testing for a relative afferent pupillary defect (RAPD) is a reliable way to implicate or rule out optic nerve disease.  In the presence of known macular disease of a significant degree this test is negative.  If the patient has a positive RAPD, then you can be sure he has optic nerve disease in addition to his macular disease and this optic nerve disease is most likely the reason for a new complaint of decreased vision in an eye previously compromised by macular disease.

The usual pupillary response to direct light is that both pupils contract equally if there is nothing wrong with either pupil; that is, the muscle and stroma of the iris.  If you move the light quickly from one eye to the other, both pupils should hold their level of contraction.  If you are too slow moving the light so that neither eye is "dazzled", then the initial response from the first eye is lost and even in the normal state both pupils dilate somewhat.  The RAPD is different.  When you shine a hand light in the good eye, both pupils constrict.  Then shifting it to the other eye, an injured optic nerve will also transmit light but to a lesser and slower degree.  As a result, when the light is moved from the good to the bad eye the brain interprets this as a decrease of light being presented.  The brain's response is to dilate both pupils to let in more light.  This dilation response is in both eyes, despite only one eye being affected.  This is the essence of the RAPD.

The degree of the RAPD is not related to the degree of visual loss between the two eyes.  Some physicians feel differently and measure this using neutral density filters.  These filters are increased in front of the normal eye until the RAPD is equalized and abolished.  I do not find this feature to be clinically useful.  Even if the RAPD response decreases over the course of the illness, it is not a good prognosticator for the degree of recovery.  I have seen an RAPD present with even one line of Snellen acuity difference between the eyes.

The important feature is that a positive RAPD test is a sign of optic nerve disease.  It does not confirm the acute nature of the complaint, but only that there is a different function between the two optic nerves.  It could be acute or could have occurred a long time before.

It is a valuable clinical sign that should be used properly and not extrapolated to the point of irrelevance.

 

 graphic 1

Room Light

OD 20/20

OS 20/20

  graphic 2

Bright Light OD = Both pupils constrict

  graphic 3

Move light rapidly to OS,
both eyes stay constricted

Normal pupillary response to rapid shift of bright light from one eye to the other indicates equal optic nerve function.

 

 

Room Light

OD 20/20

OS 20/200

  graphic 5

Bright light OD = Both pupils constrict

graphic 6 

Move light rapidly to OS,
both eyes dilate equally

Positive RAPD left eye confirms optic nerve disease is cause of decreased visual acuity.


Thomas J. Walsh, M.D.