Mueller's muscle (superior tarsal muscle) is a thin smooth muscle located between the levator palpebrae superioris and the superior tarsal plate. It receives sympathetic innervation from the cervical sympathetic chain, which is why Horner syndrome (cervical sympathetic interruption) produces a 1-2 mm ptosis from Mueller's muscle paresis. It contributes to the subtle increase in palpebral fissure width during states of arousal and sympathetic activation.
| Origin | Inferior surface of the levator palpebrae superioris muscle (from its smooth muscle fibres arising near the superior fornix) |
|---|---|
| Insertion | Superior border of the superior tarsal plate of the upper eyelid |
| Nerve Supply | Sympathetic nerve fibres from the superior cervical ganglion (via the nasociliary nerve) |
| Blood Supply | Superior ophthalmic artery branches |
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It adds about 2 mm of upper eyelid elevation under sympathetic control, keeping the lid raised and widening the eye in alertness or fear.
Mueller's muscle is the target of the phenylephrine test in ptosis evaluation: instillation of 10% phenylephrine (a sympathomimetic) into the eye constricts Mueller's muscle and elevates the ptotic eyelid by 1-2 mm if Mueller's muscle is functional. A positive phenylephrine test indicates Muller-targeted surgery (Fasanella-Servat tarsoconjunctival resection, Mueller's muscle resection and conjunctivectomy) as the appropriate surgical option for mild Horner ptosis. It also guides Mullerectomy procedures in cases of mild ptosis with good levator function.
Interruption of the cervical sympathetic chain at any level from hypothalamus to the periorbital sympathetic plexus paralyses Mueller's muscle, producing 1-2 mm of upper lid ptosis with lower lid elevation (inverse ptosis) and miosis; phenylephrine reversal of the ptosis confirms Mueller's muscle involvement and localises the lesion to the postganglionic sympathetic pathway.
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