Pupillary reflex

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In medicine, there are two pupillary reflexes-

• The pupillary light reflex is the reduction of pupil size in response to light.
• The pupillary accommodation reflex is the reduction of pupil size in response to an object coming close to the eye.

Both these reflexes affect both eyes, even if only one eye is stimulated.

The pattern of pupillary response to light can help determine which of the cranial nerves is damaged. There are two types of response assessed for each eye:

• Direct pupillary reflex: whether each pupil constricts with light shone into that eye
• Consensual pupillary reflex: whether each pupil constricts with light shone into the other eye

Mechanism

The reflex pathway consists of retinal ganglion cells, which convey information from the photoreceptors to the optic nerve which connects to the pretectal nucleus of the high midbrain. It bypasses the lateral geniculate nucleus and the primary visual cortex. From the pretectal nucleus neurons send axons to neurons of the Edinger-Westphal nucleus whose axons run along both the left and right oculomotor nerves. Oculomotor nerve axons synapse on ciliary ganglion neurons whose axons innervate the constrictor muscle of the iris.

Test for nerve damage

Emergency room physicians often assess the pupillary reflex because it is useful for gauging brain stem function. Normally, pupils react (constrict) equally. Lack of the pupillary reflex or an abnormal pupillary reflex can be caused by optic nerve damage, oculomotor nerve damage, brain death and depressant drugs, such as barbiturates. The optic nerve is responsible for the afferent limb of the pupillary reflex, or in other words, senses the incoming light. The oculomotor nerve is responsible for the efferent limb of the pupillary reflex; in other words, it drives the muscles that constrict the pupil.

Normally, each pupil should constrict with light shone into either eye. On testing each reflex for each eye, several patterns are possible.

• Optic nerve damage on one side
  • The ipsilateral (on the side with the damage) direct reflex is lost
  • The contralateral (on the other side) direct reflex is intact
  • The ipsilateral consensual reflex is intact (because light shone into the opposite eye can signal to the brain, causing constriction of both pupils via the normal oculomotor nerves
  • The contralateral consensual reflex is lost (because light shone into the eye on the damaged side cannot signal to the brain)
• Oculomotor nerve damage on one side
  • The ipsilateral direct reflex is lost
  • The contralateral direct reflex is intact
  • The ipsilateral consensual reflex is lost (because light shone into the opposite eye can signal the brain, causing attempted constriction of both pupils, but constriction fails on the damaged side)
  • The contralateral consensual reflex is intact (because light shone into the damaged eye can still signal to the brain via the normal optic nerve, causing attempted constriction of both pupils, which works on the other side via the normal oculomotor nerve on that side)

Pupillary Accommodation Reflex

The pupillary accommodation reflex reduces the size of the pupil when an object is close to the eye. A smaller pupil produces a sharper image on the retina. There is also a separate accommodation reflex which changes the shape of the lens so as to focus the image on the retina.