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Pupillary Light Reflex Disorders

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Overview

Normally pupils constrict equally. When one pupil contracts, the other should as well, and this is called the consensual response. 

In general, the retina and optic nerve are needed to provide afferent signals, and the oculomotor nerve and its associated nuclei are needed for the efferent parts of both the direct and consensual reflexes.

Optic nerve damage

If the optic nerve of one eye is damaged, there is no afferent signal to the midbrain, so there is no efferent signal to either eye from the midbrain. If light is shone in the other eye, however, there is an afferent signal sent from that eye to the midbrain. If the efferent pathway is not damaged i.e. the oculomotor nerve is intact, then both pupils will constrict due to an intact pupillary response. In summary:

  • Same eye: direct reaction impaired, but consensual reaction preserved
  • Opposite eye: direct reaction unaffected, consensual reaction preserved

Oculomotor nerve damage

If the oculomotor nerve of one eye is damaged, then there is no efferent signal from the midbrain to that affected eye. Because the oculomotor nerve of the other eye is unaffected, the other eye will constrict and demonstrate a consensual response. This is because there is still an afferent input due to an intact optic nerve, but the efferent output to the affected eye is impaired. In summary:

  • Same eye: direct reaction impaired, consensual reaction impaired
  • Opposite eye: direct reaction unaffected, consensual reaction preserved

Relative Afferent Pupillary Defect

Overview

The afferent part of the pupillary reflex involves sending signals regarding input from the eye to the midbrain. Defects in this pathway are called relative afferent pupillary defects (RAPD). RAPDs suggest optic nerve or retinal disease. Some causes are:

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