Pupil

• Pupil margin is positioned more anteriorly than the iris root and gets support from the lens

• Slightly off-centre: inferonasal
• Varies in size from 1 to 8mm in diameter

• Physiological anisocoria (less than 0.4mm difference) is present in 25% of the population and may change sides!

• Physiological tremor in pupil known as hippus (or pupillary athetosis)
• Mydriasis occurs in low light, excitement or fear
• Miosis occurs in illumination, convergence/accommodation and sleep

• The strength of constriction increases with light intensity up to 9 log units above the scotopic threshold and then levels off

• The pupil cannot respond to stimuli of greater than 5Hz frequency
• Pupillary dilatation:
• Reduced depth of field
• Reduced depth of focus
• Increased Stiles-Crawford effect (see below)
• Reduced diffraction of light
• Increased chromatic aberration

• Stiles-Crawford effect: light entering at the edge of the pupil is less effective at stimulating photoreceptors than light entering centrally because it meets the receptors obliquely rather than head on

• Pupil size is associated with:
• Iris colour: blue irises are larger than brown
• Autonomic nervous system: fatigue, exercise etc.

Light reflexes

• Only one functioning pupil is needed to test the anterior visual pathways
• The latency of the pupil light reaction is 0.2 to 0.5s
• Afferent impulses travel from the retina through the optic nerve, chiasma and tracts

• Fibres leave the tract and synapse in the pretectal olivary nucleus near the superior colliculus

• Each pretectal nucleus receives only an ipsilateral input

• Axons of the pretectal nerve cells synapse with the parasympathetic nuclei (Edinger-Westphal nuclei) of the oculomotor nerve on BOTH sides of the midbrain

• Preganglionic fibres travel with the oculomotor nerve ( through its inferior division to the inferior oblique) to the ciliary ganglion and synapse

• Postganglionic fibres travel in the short ciliary nerves to the sphincter pupillae muscle and both pupils constrict

• Since both Edinger-Westphal nuclei are innervated, an unilateral optic nerve lesion will not affect the indirect/consensual response in the ipsilateral eye

• Pupillary reflexes originate in the brainstem and are therefore unaffected by cortical lesions

• Causes of efferent pupillary defect (fixed dilated pupil)

• Brain stem lesions at the level of the superior colliculus and red nucleus, typically accompanied by so-called long tract signs

• Third nerve lesions, usually compressive (but 20% of microvascular palsies involve the pupil)

• May be false localising sign in raised ICP causing uncal herniation
• Ciliary ganglion or short ciliary nerve lesions
• Iris damage due to surgery or grossly elevated IOP

• Mydriatic drugs (pilocarpine will not constrict such a pupil but will constrict a pupil affected by a neurological lesion)

Mydriasis

• Contraction of the dilator pupillae (innervated by alpha-1-adrenergic receptors)

• Stimulated by noradrenergic sympathetic nerves passing through the ciliary ganglion

Miosis

• Stimulation and contraction of sphincter pupillae 
• Stimulated by cholinergic postganglionic parasympathetic fibres from the ciliary ganglion

Horner’s syndrome

• Interruption of the sympathetic chain in the head and neck
• Causes:

• Central brain stem lesions (ie. first-neurone: the descending reticulospinal fibres within the medulla oblongata eg. lateral medullary syndrome -see below)

• Preganglionic lesions (second neurone: travels from T1 exit over apex of lung and around first rib up to carotid bifurcation)

• Postganglionic lesions (third neurone: travels from superior cervical ganglion to eye along the internal carotid and within the cavernous sinus)

• No anhidrosis since the postganglionic sympathetic fibres controlling facial sweating follow the external carotid artery rather than the internal. They separate from the fibres to the pupil approximately at the level of the carotid bifurcation

• Anisocoria (especially in low light conditions) due to failure of normal dilation by the affected pupil

Classic drugs tests for Horner’s

• Cocaine will not dilate a Horner’s pupil since increasing adrenergic activity by preventing reuptake has no effect on an interrupted neural pathway

• Hydroxyamphetamine (increases adrenaline release from postganglionic nerve terminals) will dilate a first or second neurone Horner’s but not a third order neurone lesion (since there’s no noradrenaline to release)

• Adrenaline, apraclonidine and phenylephrine will dilate a post-ganglionic Horner’s due to denervation hypersensitivity

• Note: there is no test to distinguish a second from a first-order lesion

Light-near dissociation

• When the near response is greater than the response to light
• Implies a diminished light response
• Causes
• Retinal lesions
• Optic nerve lesions
• Pineal lesions
• Pretectal lesions eg. Sylvian aqueduct syndrome, Argyll-Robertson

• Third nerve lesions: secondary to aberrant regeneration of neurones to the iris sphincter normally supplying medial rectus

• Ciliary ganglion lesions (as above for third nerve lesions), eg. Adie’s

Argyll-Robertson pupil

• Small pupil
• Unresponsive to light
• Good near response
• Does not dilate with atropine
• Physostigmine increases constriction
• Associated with neurosyphilis
• Lesion site unknown
• Absent tendon reflexes and iris atrophy are common

Adie pupil

• Sluggish light reaction: may present with blurred vision due to mydriasis
• Strong near response (with light-near dissociation)

• Supersensitive to cholinergics eg. pilocarpine, due to “post-denervation phenomenon”