Relative afferent pupillary defect (RAPD; Marcus Gunn pupil)
This is a test for optic nerve disease. In normal lighting conditions, the patient’s pupils appear normal. When a bright light is shone on to them, both pupils will contract. This is called a consensual pupil response.
Learn how to prepare for pupil assessment & recording your findings in a easy way
To carry out this test, arrange very dim lighting conditions and stand slightly to the side of the patient, so that they are not tempted to focus on you, but make sure that you have a good view of the pupils of both eyes. Ask the patient to ‘relax’ their eyes and pretend that they are looking across to the far side of a field. (You must ensure that the patient has ‘relaxed’ their focus, because as well as reacting to light, the pupils will also miose when their gaze is fixed on a near object.) When you are sure that the patient is ready, swing a strong light beam into the asymptomatic eye for a second or two. You will see the pupil constrict. Without moving the light beam, observe the affected eye. Because the neural pathways for the asymptomatic eye are functioning, the brain will pick up this impulse and will send a signal to the affected eye, warning it to contract. You can see this happen. Swing the torchlight slowly on to the affected eye, by dipping the beam below the patient's nose,
This is a test for optic nerve disease. In normal lighting conditions, the patient’s pupils appear normal. When a bright light is shone on to them, both pupils will contract. This is called a consensual pupil response.
Learn how to prepare for pupil assessment & recording your findings in a easy way
To carry out this test, arrange very dim lighting conditions and stand slightly to the side of the patient, so that they are not tempted to focus on you, but make sure that you have a good view of the pupils of both eyes. Ask the patient to ‘relax’ their eyes and pretend that they are looking across to the far side of a field. (You must ensure that the patient has ‘relaxed’ their focus, because as well as reacting to light, the pupils will also miose when their gaze is fixed on a near object.) When you are sure that the patient is ready, swing a strong light beam into the asymptomatic eye for a second or two. You will see the pupil constrict. Without moving the light beam, observe the affected eye. Because the neural pathways for the asymptomatic eye are functioning, the brain will pick up this impulse and will send a signal to the affected eye, warning it to contract. You can see this happen. Swing the torchlight slowly on to the affected eye, by dipping the beam below the patient's nose,
and then shining it into the affected eye. If the eye problem is with the neural pathway, the damaged pathway will transmit the signal less efficiently and the brain will sense less light entering the eye, so causing both pupils to dilate slightly to let in more light.
Remember that the resting pupil diameter is smaller in elderly people and the response in terms of maximum dilation is smaller (Bitsios et al., 1996), so elderly people are particularly difficult to examine. It is important to check for any RAPD prior to dilating a new patient’s pupils for examination.
Document that you have done this. If there is a gross problem, this is easy to identify, but sometimes the difference between the reactions in the two eyes is very subtle as mentioned above. You should double check any findings with a more experienced colleague until you are fully competent.
If you find an RAPD, speak to the ophthalmologist; he or she may wish to verify your finding of RAPD prior to the pupil being dilated, as this is potentially very significant to the diagnosis, particularly in optic nerve disease.
Pupils equal and reacting to light and accommodation (PERLA)
When you have checked that both pupils are equal, round and reacting to light, you must check the accommodation reflex. Put the room lights on. Sit directly in front of the patient so that you can observe the pupils of both eyes. Hold out your pen (or your finger) in front of both your noses, about 10 cm from the patient’s nose. Ask the patient to look at the pen for a few seconds, then into the distance, and then back at your pen. You should be able to see both eyes converging slightly for near vision, straightening up for distance, and converging again for near. Observe the pupils of both eyes simultaneously, and you should see the pupils of both eyes miose slightly, and equally for near vision. This is called the accommodative reflex. if everything is working correctly, you can writedown ‘PERLA’ on the patient’s notes.
Recording your findings
If everything is working correctly, you can write down ‘PERLA’ on the patient's notes. If there is
an RAPD, this is recorded as "RAPD’ (specify whether the right or left eye is affected).