One of the most useful clinical methods of testing for optic nerve injury is to perform a swinging flashlights test for assessing the pupil response to light stimulation. In the swinging flashlight test a light source is alternatively directed towards the pupil of one eye and then the pupil of the fellow eye. In normal individuals the pupils exhibit an equal constriction in each eye. Both pupils show symmetrical constriction to light. However, if a pupil in one eye dilates after the other eye shows normal constriction to light, the eye with the dilating pupil is said to have an afferent pupil defect or APD.
The presence of an afferent pupil defect signified injury that mainly affects the optic nerve. Less commonly, an extensive defect of the retina can cause an afferent pupil defect. This is because the ganglion nerve fibers from the retina make up much of the optic nerve. This is particularly relevant to extensive injury to the macula where 30% to 50% of all the retinal ganglion cells are located.
The light reflex pathway consists of two the afferent pathway and the efferent pathway. In the afferent (ingoing) pathway, nerve impulses initiated by a light stimulus send impulses consecutively to the optic nerve, optic tract, optic chiasm, and ultimately the pretectal nucleus of the midbrain.
The efferent (ougoing) pathway. Nerve impulses exit the midbrain and travel to the oculomotor nerve and ciliary ganglion. These impulses result in constriction of both pupils in a normal individual.
The presence of an afferent pupil defect provides information regarding the presence, laterality, and magnitude of optic nerve injury. The magnitude of disease is reflected by the amplitude, velocity, and acceleration of the pupil. For example, pupils in individuals with early glaucoma may have a subtle amplitude, slower velocity and acceleration when compared with pupils of normal eyes.
In addition to glaucoma, injury to the optic nerve from trauma, ischemia, optic neuritis, or compression from a mass can cause an afferent pupil defect. Pupil testing is helpful for individuals who have traumatic brain injury since an afferent pupillary testing can help assess the extent of damage and serve as a baseline for progression.
Optic nerve injury testing with a swinging flashlight test is a quick and effective means of assessing optic nerve damage. It can be performed at the bedside without special equipment under routine and emergency circumstances.