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Anisocoria
Anatomy
1. Overview
Exam pearl
- Clinical Analogy: Think of the pupil as a camera aperture controlled by two opposing motors: the sympathetic system (dilator - accelerates aperture opening) and the parasympathetic system (sphincter - closes it). Anisocoria can occur when one camera's motor is faulty. Your job as the examiner is to identify WHICH motor is broken and WHERE in the circuit the fault lies - peripheral or central, preganglionic or postganglionic.
2. Overview & Definitions
- Definition: Anisocoria is unequal pupil size; a difference of ≥0.4 mm is a commonly used operational criterion.
- There is no universal clinically significant threshold; physiologic anisocoria is usually about 0.4 mm and seldom exceeds 0.8 mm.
- Prevalence: About 10–20% of individuals may have physiological anisocoria, usually around 0.4 mm and seldom greater than 0.8 mm.
- New-onset anisocoria warrants clinical assessment to determine whether reassurance or further investigation is needed.
Pupillary Anatomy & Innervation Recap
| Structure | Innervation | Neurotransmitter | Function |
|---|---|---|---|
| • Iris Sphincter (0.75–1 mm in radial width) | • Parasympathetic CN III | • Acetylcholine; M3 is a muscarinic receptor subtype. | • Miosis (constriction) |
| • Iris Dilator (thin radial slips) | • Sympathetic (C8–T2 → superior cervical ganglion → postganglionic fibers to the iris dilator) | • Noradrenaline; alpha-adrenergic receptor | • Mydriasis (dilation) |
| • Ciliary muscle | • Parasympathetic CN III | • Acetylcholine | • Accommodation |
| Structure | Innervation | Neurotransmitter | Function |
|---|---|---|---|
| • Iris Sphincter (0.75–1 mm in radial width) | • Parasympathetic CN III | • Acetylcholine; M3 is a muscarinic receptor subtype. | • Miosis (constriction) |
| • Iris Dilator (thin radial slips) | • Sympathetic (C8–T2 → superior cervical ganglion → postganglionic fibers to the iris dilator) | • Noradrenaline; alpha-adrenergic receptor | • Mydriasis (dilation) |
| • Ciliary muscle | • Parasympathetic CN III | • Acetylcholine | • Accommodation |
Sympathetic Pathway - 3 Neuron Arc (Classic for Horner)
- 1st order (central): Hypothalamus → descends uncrossed through midbrain and pons → ipsilateral ciliospinal centre of Budge (C8–T2) in the intermediolateral cell columns of the spinal cord
- 2nd order (preganglionic): Exits spinal cord at T1, hooks over lung apex, under subclavian artery, up carotid sheath → superior cervical ganglion (SCG)
- 3rd order (postganglionic): SCG → plexus along internal carotid artery → cavernous sinus → ophthalmic division (CN V1); fibers to the dilator pupillae travel via the nasociliary and long ciliary nerves; sympathetic fibers also supply Müller muscle and inferior tarsal muscle
- Sudomotor fibres travel predominantly along the external carotid artery - their sparing or involvement helps localise lesions; third-order Horner usually has no facial anhidrosis, apart from possible limited anhidrosis near the ipsilateral brow
- Parasympathetic Pathway - CN III
- Edinger-Westphal nucleus (midbrain dorsal) → parasympathetic fibres run on SURFACE of CN III (peripherally placed - vulnerable to compression)
- Synapse at ciliary ganglion (in orbital cone) → short ciliary nerves → ~93% ciliary muscle/~7% sphincter
- Afferent limb: Retinal ganglion cells → optic nerve → chiasm → tract → pretectal nucleus (bilateral) → bilateral EW nuclei
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Keep reading Anisocoria
13 more sections are included with the Clinical plan.
- Diagnostic Approach
- HORNER SYNDROME (Oculosympathetic Paresis)
- Third Nerve Palsy & Pupil Involvement
- Adie Tonic Pupil
- Argyll Robertson Pupil
- Pharmacological Pupil - Dilation & Constriction