The lateral rectus is the only extra-ocular muscle supplied by the abducens nerve (CN VI), making CN VI palsy the most common cranial nerve palsy causing ocular motility disturbance. Its sole function of abduction makes lateral rectus weakness immediately obvious as an inability to turn the affected eye outward beyond the midline. CN VI palsy produces an esotropia (eyes turn in) that is maximum on ipsilateral gaze.
| Origin | Common tendinous ring (annulus of Zinn) with two heads straddling the superior orbital fissure |
|---|---|
| Insertion | Lateral sclera 6.9 mm from the limbus |
| Nerve Supply | Abducens nerve (CN VI) — only one motor cranial nerve per muscle (unlike other rectus muscles sharing CN III) |
| Blood Supply | Lacrimal artery |
| Actions | Abduction of the eye — moves the eye toward the temple |
|---|
The lateral rectus provides the abduction force that opposes the medial rectus convergence pull, maintaining the eyes in a straight-ahead position during distance fixation. Without it, the unopposed medial rectus pulls the eye into esotropia.
CN VI palsy is the most common cause of acquired horizontal diplopia and can be a false localising sign of raised intracranial pressure (where the long intracranial CN VI course is compressed before localised pontine pathology can cause it). New onset CN VI palsy requires MRI of the brain and orbit to exclude compressive pathology. A child with new CN VI palsy and headache requires emergency assessment for posterior fossa tumour.
Not directly palpable. Assessed by inability to move the eye laterally beyond the midline.
Lateral rectus paralysis producing esotropia and horizontal diplopia requiring MRI to exclude compressive pathology before attributing to benign microvascular ischaemia.
