How to examine RAPD

Hi,

Today we are looking at Relative Afferent Pupil Defects, or RAPD. This is also sometimes referred to as Marcus-Gunn pupil.

First we will compare its clinical appearance with that of normal pupils and also that of a complete afferent pupil defect.

To avoid pupil constriction while accommodating, ask to the patient to fix on a distant object throughout your examination.

Look for equal pupil sizes, and check again with the lights off. Anisocoria is not a feature of an afferent defect.

Now check for a reaction to light in each eye, again with the lights off. Here the normal pupils constrict briskly, then relax a little. They dilate again after the light is removed.

now swing the light from eye to eye, quickly - but pausing on each eye for around 2 seconds. In the normal patient the pupils will constrict then relax a little each time the light is swung to them.

Now a patient with a relative afferent defect. The pupils will be equal size in both light and dark. Both pupils will react to light, although sometimes a slower response is noted when light is shone on the affected side.

With the swinging light test the RAPD now becomes obvious. On the affected side, both pupils dilate when the light is swung across. Here the left side is affected.

You will miss an RAPD if you do not do the swinging light test, as it is only by comparing the relative strengths of the signals reaching the brain from the eyes that the abnormality is detected.

Finally with a complete afferent pupil defect, there is no pupil reaction to light shone on the affected side.

Due to crossing of nerve fibres at the optic chiasm, an RAPD localizes pathology to the visual pathway before the chiasm, that is the optic nerve or retina.

Some examples of pathologies causing an RAPD are Large Retinal Detachment, Central Retinal Artery or Ischaemic central retinal Vein Occlusion, Optic Nerve Ischaemia, Optic Neuritis, asymmetric glaucoma

It should be noted that an RAPD is not caused by either cataract or vitreous haemorrhage, and when associated with amblyopia is at most a mild RAPD. A Definite RAPD in these cases should prompt a look for another cause of visual loss.