One of the functions of the pupil is regulation of the amount of light falling on the retina. Too much or too little light prevents the retina from working at peak efficiency.
Eliciting the Sign
If the amount of light shining into a pupil is reduced, both pupils dilate to gather in more light. If the intensity of the light is not reduced and the optic nerve has developed a conduction defect, less light is transmitted, a phenomenon that the brain interprets as less light getting onto the retina. The response to this light "reduction" is the same as that in the first instanceóboth pupils dilate to an appropriate amount.
The afferent pupillary defect (Marcus Gunn pupillary escape phenomenon) depends on the light-reduction response that occurs in the presence of a conduction defect in the optic nerve. In attempting to demonstrate the afferent pupillary defect in a patient, the physician shines the light into each eye separately. Both pupils respond, even though one optic nerve has a conduction defect. A difference may exist in the quality of the response of each pupil to direct light, but the difference cannot usually be appreciated clinically unless the optic nerve is severely affected. However, when the light is brought quickly from the normal pupil to the side with the conduction defect, both pupils dilate (swinging flashlight test). The brain interprets the decrease in signals from the nerve with the conduction defect as it would if the intensity of the light were reduced.
The afferent pupillary defect is more easily seen in dim illumination with distance fixation. The observer must be careful not to stimulate accommodation by standing in front of the patient or by putting a light directly in front of the pupil. Sometimes it is even valuable to quantify this sign, which can be done by use of progressive neutral density filters. Increase the neutral density filter over the normal eye as you do the swinging flashlight test until the least neutral density filter is found that neutralizes the afferent pupillary defect. If a pupil responds more slowly to light than usual, do the test more slowly to better see the redilatation, and vice versa for a pupil that responds rapidly.
A conduction defect in one optic nerve causes the afferent pupillary defect, but it does not account for unequal pupils, which is a separate entity that requires additional explanation if it occurs.
The afferent pupillary defect (APD) is seen only in unilateral optic nerve disease. It is not generally a useful sign when the chiasm is involved. It can occasionally be seen in chiasmal disease if one nerve is more involved. An APD can also occur on a retinal basis, but only when extensive destruction has occurred. Such lesions as those in macular degeneration or the large macular lesions in toxoplasmosis can cause it. A lesion such as diffuse diabetic retinopathy, with proliferation and loss of most of the retinal architecture, or a complete retinal detachment can cause an APD. Therefore, a patient who has some minor retinal disease and exhibits the APD has two diseases, one in the retina and one in the optic nerve.
APD has also been seen in cases of strabismus, but this is rare, and concomitant optic nerve disease should be seriously considered.
In a patient with a dense unilateral cataract, the afferent pupillary response may be falsely positive. If this test is repeated in a dark background environment, the test becomes negative if no other optic nerve disease coexists. The usual explanation for no APD is that even with decreased light transmission there is an increase in scattering of light that makes up for it. Sadun believes that it is due to retinal compensation over the period of time it takes the cataract to develop.
The main use of the APD is in evaluating the patient who complains about the vision in one eye but who has a normal ophthalmoscopic examination. The APD is a valuable diagnostic sign in retrobulbar neuritis, particularly in patients with relatively good vision, It can be elicited even in cases of optic neuritis with only one Snellen line difference in acuity between the two eyes.
The afferent pupillary escape phenomenon has been seen in tract lesions with a complete homonymous hemianopia. One author (O'Connor) has seen it in cases of partial homonymous hemianopias. Behr first noted a decreased contralateral pupillary response in tract lesions. The proposed reason is that there is a greater field defect in the contralateral eye with the temporal field defect.
The APD attests to the presence of the conduction defect, not to its time of onset. This defect lasts as long as there is a conduction difference between the two eyes.