Light Reaction Examine the patient while she is sitting, with moderate background illumination. Inspect and compare the pupils while your flashlight is held vertically, pointing up, below the eye, and just in front of the patient's cheek.
Then ask the patient to look at the far wall and direct a bright light at first one and then the other eye. Be careful not to illuminate the two eyes at the same time.
Both pupils should react to the light in either eye with equal speed and to the same degree. This tests the second cranial nerve and other parts of the visual pathways (afferent) and the pupillary fibers of the third cranial nerve (efferent).
Direct and Consensual Light Reaction A direct pupil response to light means that the right pupil constricts when the right eye is exposed to a bright light. A consensual response means that the right pupil constricts when the left eye is exposed to a bright light, and vice versa.
If the right eye has a normal direct response and an abnormal consensual response, the lesion is in the afferent pathway of the left eye. If the right eye has an abnormal direct response and a normal consensual response, the lesion is in the afferent pathway of the right eye. If the right eye has neither a direct nor a consensual response, the lesion is in the efferent pathway on the right (third nerve palsy). A lesion at the apex of the right orbit involving both efferent and afferent limbs of the reflex may also abolish direct and consensual responses for the right eye.
Near Reaction If the light reflex works, so will the near reflex. If one or both pupils do not react to light, ask the patient to look at the far wall and then at the tip of your pencil, which should be 5-10 cm in front of his nose. If his visual acuity is seriously compromised, ask the patient to look at the tip of his own finger held in front of his nose.
Look for two things: bilateral constriction of the pupils as the patient changes his gaze from far to near and convergence of the eyes. Accommodation also occurs. This increases curvature of the lens and is a result of contraction of the ciliary muscle and loosening of the suspensory ligament.
If the right eye has a sluggish direct pupillary response to light, relative to the direct left response, but has a brisk near response, the lesion is in the right afferent limb of the light reflex.
Afferent Pupil Defect, Swinging Flashlight Test, and Marcus Gunn Pupil
Take, for example, the case of a patient with retrobulbar neuritis of the right optic nerve. His vision is 20/80 right, 20/20 left; peripheral fields are full; and the retina, optic nerve head, and media are all normal. Examine his pupil responses in dim illumination. Shine a strong light into his left eye only. Both pupils constrict as expected. Shield the left eye from light and shine the light into the right eye only. The right pupil seems to constrict momentarily and then dilates widely, as does the left. Swing the light back to the left eye only and again both constrict.
The high-intensity light-induced impulses from the left eye have their normal influence on the two pretectal and Edinger-Westphal nuclei. The efferent systems (third nerves) are normal, so both pupils constrict. However, when the light is directed into the right eye, the pupil changes from the constriction of a normal consensual light reaction to the relative dilatation of a low-intensity direct reaction. This low-intensity response is caused by diminished conduction through the diseased afferent system of the right eye. The left pupil also dilates as the consensual response in this eye is determined by the afferent system on the right.
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