Testing the Optic Nerve with the Swinging Flashlight
A relative afferent pupillary defect was discovered in the right eye of this patient when the following responses were observed during the swinging-flashlight test. When light was shone into the healthy eye, there was symmetrical pupillary constriction. However, when the light was directed into the abnormal eye there was bilateral pupillary dilation. This phenomenon is due to reduced neural input reaching the pretectal region of the midbrain. A relative afferent pupillary defect is typically found in the presence of a unilateral optic neuropathy and occasionally with large macular abnormalities. It is usually present ipsilateral to the side of an injured optic nerve.
When a patient presents with blurred vision, the first and most important task is to determine whether the loss of vision is optical (i.e., refractive) or related to an ophthalmic, medical, or neurologic disorder. If the visual acuity improves when the patient looks through a pinhole, it means that the loss of vision is most likely caused by optical or refractive error. If the visual loss is not corrected by using the pinhole, the patient needs a more thorough evaluation, since a lack of improvement may indicate a more serious ophthalmic or neurologic process.
Is visual acuity a sensitive test for optic-nerve function?
Visual acuity is not always a sensitive test of optic-nerve function; visual acuity may remain normal in the setting of a profound optic neuropathy. For instance, a unilateral prechiasmal lesion can reduce visual acuity, but only when both the crossing and noncrossing fibers are affected.
What disease may be indicated by optic neuritis?
Optic neuritis, inflammation of the optic nerve, is most common in patients with multiple sclerosis. Isolated optic neuritis often heralds the onset of multiple sclerosis.
New England Journal of Medicine - Vol. 359, No. 26, December 25, 2008