Extraocular muscles are one of the most specialized skeletal muscle groups in the human body. They consist of six voluntary muscles (four rectus muscles and two oblique muscles) that control eye movement, the levator palpebrae superioris (LPS) that lifts the eyelid, and three involuntary muscles (superior tarsal muscle, inferior tarsal muscle, and orbitalis muscle).
Embryologically, the muscle bodies of the extraocular muscles derive from the mesoderm, while the connective tissue components derive from neural crest cells.
QHow many extraocular muscles are there?
A
There are six voluntary muscles responsible for eye movement (medial rectus, lateral rectus, superior rectus, inferior rectus, superior oblique, and inferior oblique). Adding the levator palpebrae superioris, which elevates the eyelid, brings the total number of voluntary muscles to seven. Including the three involuntary muscles (superior tarsal, inferior tarsal, and orbitalis), there are ten muscles within the orbit.
Connecting the insertions of the four rectus muscles forms a spiral curve (Spiral of Tillaux). The distance from the corneal limbus increases in order: medial rectus (5.5 mm) → inferior rectus (6.5 mm) → lateral rectus (6.9 mm) → superior rectus (7.7 mm).
Rectus Muscles
Medial rectus (MR): Tendon length 4.5 mm (shortest). Main action is adduction. Has the shortest tendon.
Lateral rectus (LR): Contact arc 12 mm (widest). Main action is abduction1).
Superior rectus (SR): 7.7 mm from the corneal limbus (most distal). Primary action: elevation; secondary actions: intorsion and adduction1). Its innervating neurons cross in the midbrain and provide contralateral innervation.
Superior oblique (SO): The longest of the extraocular muscles. It originates from the superomedial orbit, passes through the trochlea, then runs under the superior rectus to insert posterolaterally. Primary action: intorsion; secondary actions: depression and abduction1). Innervated by the trochlear nerve (CN4), the only cranial nerve to emerge from the dorsal brainstem.
Inferior oblique (IO): Originates from the orbital floor lateral to the nasolacrimal groove. Primary action: extorsion; secondary actions: elevation and abduction1).
Oculomotor nerve (CN3): Superior branch → superior rectus and levator palpebrae superioris; inferior branch → medial rectus, inferior rectus, and inferior oblique1).
Trochlear nerve (CN4): Innervates only the superior oblique1). It is the only cranial nerve to emerge from the dorsal brainstem and innervates the contralateral superior oblique.
Abducens nerve (CN6): Innervates only the lateral rectus1).
Superior rectus innervation neurons: Cross within the midbrain and innervate from the contralateral side1).
Originates from the annulus of Zinn above, changes direction from vertical to horizontal at Whitnall’s ligament (superior transverse ligament), and inserts into the tarsal plate via the levator aponeurosis. Innervated by the superior branch of the oculomotor nerve.
QHow many muscles can be operated on at once in strabismus surgery?
A
Due to the risk of anterior segment ischemia, the principle is to operate on a maximum of two rectus muscles at a time. All four rectus muscles supply blood to the anterior part of the eye via the anterior ciliary arteries, and simultaneous surgery on multiple muscles increases the risk of ischemia.
4. Evaluation of extraocular muscles and surgical principles
Forced duction test (traction test): Used to differentiate restrictive strabismus (mechanical limitation) from paralytic strabismus (innervational disorder). In paralytic cases, the eye moves without resistance, while in restrictive cases, resistance is noted.
Anterior segment ischemia risk: Manipulating three or more rectus muscles at once can damage the anterior ciliary arteries, increasing the risk of anterior segment ischemia. A maximum of two rectus muscles should be operated on in a single surgery.
Recession: The muscle insertion is moved posteriorly (toward the posterior pole of the globe). This shortens the effective length and arc of contact, weakening the muscle’s action.
Resection: A portion of the muscle is excised to increase the effective length and arc of contact. This strengthens the muscle’s action.
6. Pathophysiology and Physiological Laws of Extraocular Muscles
Eye movements are regulated by several physiological laws.
Hering's Law
Law of equal innervation: In conjugate eye movements, equal innervation flows simultaneously to the yoke muscles (synergists).
Clinical significance: In paralytic strabismus, the secondary deviation (deviation of the healthy eye) when fixating with the paralyzed eye is greater than the primary deviation (deviation of the paralyzed eye) when fixating with the healthy eye.
Sherrington's Law
Law of reciprocal innervation: When innervation to the agonist muscle increases, innervation to the ipsilateral antagonist muscle simultaneously decreases.
Clinical significance: For example, in abducens nerve palsy (lateral rectus palsy), contracture of the ipsilateral medial rectus muscle occurs secondarily.
Donders’ law: The torsional position (cyclotorsion) of the eye is uniquely determined by the direction of gaze. There is no independent degree of freedom for ocular torsion separate from the gaze direction.
Listing’s law: Every eye position can be achieved by a single rotation from the primary position (rotation around an axis lying in Listing’s plane).
Congenital fibrosis of the extraocular muscles (CFEOM) is a representative CCDD caused by developmental abnormalities of CN3 and CN41). It is characterized by ptosis and limited upward gaze, with the eyes fixed in a downward position. CCDDs are subdivided into Duane retraction syndrome, CFEOM, Moebius syndrome, etc., depending on the cranial nerves involved1).
QWhat is Hering's law?
A
In conjugate eye movements, equal innervation is sent simultaneously to a pair of yoke muscles (e.g., right lateral rectus and left medial rectus). In paralytic strabismus, when attempting to fixate with the paralyzed eye, innervation to the yoke muscles increases, causing the yoke muscle in the healthy eye to overcontract, making the secondary deviation larger than the primary deviation. This produces important findings for diagnosing paralytic strabismus.
