Reduce the ambient light and ask the patient to fixate on the far wall. When pupillary function is normal, pupils are isocoric (equally sized) and react equally to light. Size is measured in millimetres and the normal pupil ranges from 1-8 mm. Note the shape and size of the pupils in ambient bright light. This may provide helpful clues as to the cause of pupillary abnormalities, particularly where there is an underlying neurological cause. See also the separate Examination of the Eye article. There is a secondary sympathetic effect modulated by adrenergic receptors in the Edinger-Westphal nucleus which are inhibited by the direct action of sympathetic amines. The sympathetic fibres then travel with the trigeminal nerve through the superior orbital fissure to the ciliary muscle. Postsynaptic neurons travel down all the way through the brain stem on each side and finally exit through the cervical sympathetic chain, travel over the lung apices, and ascend to the superior cervical ganglia with the carotid artery, then onwards as a plexus around the internal carotid artery, passing through the cavernous sinus. The sympathetic input then comes from the hypothalamus with the first synapse at the ciliospinal centre at C8-T1 level. During sleep the pupils are partially constricted but still react to light. The pathway begins in the cortex, which exerts a modulatory effect on constriction which is lost during drowsiness and sleep but increased during intense concentration and arousal. Pupillary dilatation is controlled by the sympathetic system and is efferent only. Short ciliary nerves then innervate the iris sphincter and muscles of accommodation. They travel in the superficial part of the oculomotor nerve via the cavernous sinus and the superior orbital fissure to synapse in the ciliary ganglia. From each Edinger-Westphal nucleus, preganglionic parasympathetic fibres exit with the oculomotor nerve. Each pretectal nucleus has two pupillary motor outputs, one to the Edinger-Westphal nucleus on its own side and one to the other side. The efferent limb for pupillary constriction comes from the pretectal nucleus via the Edinger-Westphal nucleus (also in the midbrain) to the ciliary sphincter muscle of the iris. The afferent limb is made up of the retina, the optic nerve and the pretectal nucleus in the midbrain, all on the same side. The pathway for pupillary constriction for each eye has an afferent limb taking sensory information to the midbrain, and two efferent limbs (one to each eye).
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