Pupillary Disorders


Pupillary abnormalities range from benign isolated findings to harbingers of significant, even life‐threatening, conditions. A complete understanding of the neuroanatomy underlying the innervations to the antagonistic pupillary sphincter and dilator muscles is essential to detecting, and discerning the importance of, a particular pupillary abnormality. The sphincter muscle receives neuronal input from the parasympathetic division of the autonomic nervous system, whereas the dilator muscle receives input from the sympathetic division. Afferent input carrying light information to the brain is transmitted by retinal ganglion cell axons via the optic nerves, chiasm, and tracts. The parasympathetic innervation to the pupil originates in the Edinger‐Westphal nucleus in the midbrain. The sympathetic innervation to the pupil consists of a three‐neuron pathway originating in the hypothalamus. Important pupillary disorders include physiologic anisocoria, relative afferent pupillary defect, amaurotic pupil, Argyll Robertson pupils, cranial nerve III palsy, pharmacologic mydriasis, tonic pupil, and Horner syndrome.

Key Concepts:

  • Examination of the pupil is an important part of the physician's neurological and ophthalmological evaluation of a patient.

  • The size of the pupil is controlled by the actions of antagonistic muscles located within the iris: the pupillary sphincter and dilator.

  • Constriction of the pupils is mediated by the parasympathetic nervous system, whereas dilation of the pupils is mediated by the sympathetic nervous system.

  • Pupillary constriction to light occurs via an involuntary reflex arc.

  • Retinal ganglion cell axons involved in carrying light information for the pupillary light reflex exit the optic tract (proximal to the lateral geniculate nucleus) to synapse in the midbrain pretectal nucleus.

  • The parasympathetic output controlling pupillary constriction originates in the Edinger‐Westphal nucleus in the midbrain and terminates at the pupil sphincter muscle.

  • Pupillary dilation involves a ‘three‐neuron’ sympathetic pathway, which originates in the posterolateral hypothalamus and terminates at the pupil dilator muscle.

  • A relative afferent pupillary defect is caused by a decreased amount of light information from the affected eye reaching the pretectal nuclei in the midbrain and is most commonly a sign of ipsilateral optic nerve pathology.

  • Damage to cranial nerve III can result in injury to the parasympathetic fibres travelling to the pupillary sphincter muscle, thus disrupting the efferent arc of the pupillary light reflex, resulting in deficient pupillary constriction on the involved side.

  • Horner syndrome, characterised by miosis, ptosis, and facial anhidrosis, results from disruption of the oculosympathetic pathway.

Keywords: pupil; pupillary disorders; anisocoria; miosis; mydriasis; Horner syndrome; afferent pupillary defect; cranial nerve III palsy; sympathetic; parasympathetic

Figure 1.

A normal left eye. The pupil is the round dark area in the middle of the eye. The iris surrounds the pupil and is brown in this person.

Figure 2.

Parasympathetic pathway for pupil constriction to light. Light entering one eye (orange arrows, bottom right) leads to stimulation of retinal ganglion cells whose axons project through the (ON). Some of these axons cross at the (OC) to travel within the contralateral (OT) whereas others continue to travel through the ipsilateral optic tract. The fibres then leave the optic tract, bypass the (LGN), travel through the brachium of the (SC), and synapse at the (PTN). Interneurons then connect bilaterally to the cranial nerve (III Nuc) at the (EW). The Edinger‐Westphal nuclei then issue parasympathetic fibres, which travel within (CN III) ipsilaterally and synapse at the (CG). Post‐ganglionic fibres then travel from the ciliary ganglion to synapse at the pupil sphincter muscle causing pupil constriction. Light entering one eye causes both pupils to constrict.

Figure 3.

Sympathetic pathway for pupil dilation. The oculosympathetic pathway is a ‘three‐neuron’ pathway. The first‐order neuron travels from the posterolateral hypothalamus through the brainstem and cervical spinal cord to synapse in the intermediolateral cell column (ciliospinal centre of Budge) at spinal cord level C8‐T2. Axons of the second‐order neuron exit the spinal cord via C8, T1, and T2 nerve roots and ascend to join the paravertebral sympathetic chain and synapse in the superior cervical ganglion. Axons of the third‐order neuron ascend along the internal carotid artery and eventually into the orbit to reach the pupil dilator muscle to cause pupil dilation.

Figure 4.

Large unreactive pupil due to a right third nerve palsy. (a) This patient had complete third nerve palsy on the right and a partial third nerve palsy on the left. Note the bilateral eyelid drooping (ptosis), right much more than left. (b) A large pupil on the right and deviation of the right eye downward and outward are seen after manual elevation of both eyelids. This is typical of complete third nerve palsy. (c) The large pupil on the right does not constrict despite a bright light directed at the right eye, again typical for a complete third nerve palsy.

Figure 5.

Left Horner syndrome due to carotid dissection. (a) Horner syndrome manifested by slight ptosis and miosis (small pupil) on the left. (b) (MRI), axial T1‐weighted with fat saturation, through the neck of the patient in (a) demonstrating high signal around the left carotid artery (arrow), consistent with internal carotid artery dissection.



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How to Cite close
Walsh, Ryan D, and Liu, Grant T(May 2012) Pupillary Disorders. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0024016]