Extraocular muscles

Musculi externi bulbi oculi

Also known as: EOM, extrinsic eye muscles, rectus muscles, oblique muscles, lateral rectus, medial rectus, superior rectus, inferior rectus, superior oblique, inferior oblique, LR6SO4R3

Six skeletal muscles — four recti and two obliques — that rotate the eyeball about three axes so the two eyes move together (conjugate gaze) and keep images on both foveae.

Show on body map Eye — Horizontal Section

Overview

Number of muscles moving the globe6 (4 recti, 2 obliques)

Orbit–visual axis angle≈23°

Recti insertions from limbus (M, I, L, S)≈5.5, 6.5, 6.9, 7.7 mm

The four recti (superior, inferior, medial, lateral) arise from the common tendinous ring (annulus of Zinn) around the optic canal and the central part of the superior orbital fissure, run forwards and insert into the sclera anterior to the equator, at increasing distances from the limbus (medial ≈5.5 mm, inferior ≈6.5 mm, lateral ≈6.9 mm, superior ≈7.7 mm — the spiral of Tillaux). The superior oblique arises from the body of the sphenoid superomedial to the optic canal, runs forward to the trochlea (a fibrocartilaginous pulley on the frontal bone), then turns posterolaterally to insert behind the equator on the superolateral globe. The inferior oblique is the only one arising from the anterior orbit — the orbital floor (maxilla) lateral to the nasolacrimal canal — and passes posterolaterally beneath the inferior rectus to insert behind the equator on the inferolateral globe.

Because the orbital axis diverges ≈23° from the visual axis, the superior and inferior recti pull medially as well as vertically, so they also adduct and rotate (tort) the eye; the obliques, inserting behind the equator and pulling from in front, abduct. Each muscle's pure vertical action is therefore tested clinically in the position where it is aligned with the visual axis: SR and IR in abduction; SO and IO in adduction (the "H" test).

Eye movements are almost always conjugate and obey Hering's law of equal innervation to yoke muscles (e.g. right lateral rectus and left medial rectus for right gaze) and Sherrington's law of reciprocal innervation of antagonists. A paralysed muscle produces diplopia that is maximal in the direction of that muscle's action.

Location: Within the orbital cone, surrounded by orbital fat; the recti form a muscular cone around the optic nerve.

Parts & subdivisions

Lateral rectus (CN VI)
Origin: common tendinous ring (spanning the superior orbital fissure). Action: abduction. Palsy: eye deviated medially (esotropia), horizontal diplopia worse on looking to the affected side. CN VI has a long intracranial course and is a classic "false localising sign" of raised intracranial pressure.
Medial rectus (CN III)
Origin: common tendinous ring. Action: adduction (largest and strongest rectus). Key to convergence in the near reflex. Internuclear ophthalmoplegia (MLF lesion, e.g. multiple sclerosis) causes failure of adduction on lateral gaze with nystagmus of the abducting eye.
Superior rectus (CN III — superior division)
Primary action: elevation (maximal in abduction); secondary: intorsion and adduction.
Inferior rectus (CN III — inferior division)
Primary action: depression (maximal in abduction); secondary: extorsion and adduction. Entrapped in orbital floor blow-out fractures.
Superior oblique (CN IV)
Primary action: intorsion; secondary: depression (maximal in adduction) and abduction. Palsy: vertical diplopia worst when looking down and in (reading, descending stairs); patients tilt the head away from the affected side to compensate.
Inferior oblique (CN III — inferior division)
Primary action: extorsion; secondary: elevation (maximal in adduction) and abduction.
Levator palpebrae superioris (CN III)
Not a mover of the globe but develops with the superior rectus; elevates the upper eyelid.

Anatomy

Origin: Recti: common tendinous ring (annulus of Zinn). Superior oblique: body of sphenoid superomedial to the optic canal. Inferior oblique: orbital surface of the maxilla, lateral to the nasolacrimal canal.
Insertion: Recti: sclera anterior to the equator (spiral of Tillaux). Obliques: sclera posterior to the equator on the lateral side of the globe.
Action: MR — adduction; LR — abduction; SR — elevation, intorsion, adduction; IR — depression, extorsion, adduction; SO — intorsion, depression, abduction; IO — extorsion, elevation, abduction.
Innervation: LR6SO4R3: lateral rectus — abducens nerve (VI); superior oblique — trochlear nerve (IV); all others — oculomotor nerve (III): superior division (superior rectus, levator), inferior division (medial and inferior recti, inferior oblique).
Arterial supply: Muscular branches of the ophthalmic artery (lateral and medial), plus the lacrimal and infraorbital arteries; the anterior ciliary arteries travel with the recti to the anterior segment.
Histology: Unusually rich in small motor units (innervation ratio as low as ≈1:10) and contain both singly- and multiply-innervated fibres, allowing extremely precise, fatigue-resistant control. Organised into an orbital (pulley) layer and a global layer.

Function

Move the eyes conjugately (saccades, smooth pursuit, vestibulo-ocular and optokinetic movements)
Converge the eyes for near vision (medial recti)
Align both foveae on the target to allow single binocular vision and stereopsis
Counter-roll the eyes (torsion) during head tilt

Clinical significance

Oculomotor (III) palsy

Eye "down and out" (unopposed LR and SO), complete ptosis, and — if parasympathetic fibres are involved — a dilated, unreactive pupil. A painful pupil-involving III palsy suggests compression by a posterior communicating artery aneurysm (pupillomotor fibres run superficially); a pupil-sparing palsy is typical of diabetic microvascular infarction of the nerve core.

Strabismus and amblyopia

Misalignment of the eyes (squint) in childhood leads the brain to suppress the deviating eye's image, causing amblyopia ("lazy eye") if not treated during the critical period (up to ≈7–8 years) by glasses, patching or surgery.

Thyroid eye disease

Graves orbitopathy: autoimmune inflammation and enlargement of extraocular muscles (inferior and medial recti most often) and orbital fat, causing proptosis, lid retraction, restrictive diplopia and, in severe cases, compressive optic neuropathy.

Mnemonics

LR6SO4R3

Lateral rectus — VI, superior oblique — IV, the rest — III.

RAD SIN

Recti ADduct, Superiors INtort; obliques abduct, inferiors extort.

Compiled from standard anatomy and physiology references — see the Special Senses sources. For education only; not medical advice.