Extraocular muscles · 15 of 24
What does the inferior rectus do?
The inferior rectus runs forward from the annulus of Zinn along the floor of the orbit to insert on the sclera about six and a half millimetres below the lower limbus. It depresses the eye, most purely when the eye is turned out, and it also extorts and adducts. It lies directly on the orbital floor, which is why a fracture of that floor catches it.
- Action
- Depression; also extorsion and adduction
- Purest test
- Looking down with the eye abducted
- Nerve
- Inferior division of the oculomotor nerve (CN III)
- Neighbour
- The orbital floor and the maxillary sinus beneath it
The mirror of the superior rectus
The inferior rectus pulls along the same angled line as the superior rectus but from below, so where the superior rectus elevates, intorts and adducts, the inferior rectus depresses, extorts and adducts. With the eye abducted it becomes a pure depressor, and that is the position it is tested in. Its fascial expansions run into the lower lid, so the lower lid follows the eye down in downgaze.
The blow-out fracture and thyroid eye disease
A blow to the eye pushes the globe back and the orbital floor gives way into the maxillary sinus, and the inferior rectus or the fat around it drops into the fracture. The eye cannot look up, the patient sees double looking up, the cheek is numb because the infraorbital nerve runs in the floor, and the eye may sink back. Blowing the nose can push air into the orbit. A trapped muscle in a child can trigger nausea and a slow pulse and is released within a day or two.
Thyroid eye disease infiltrates the inferior rectus first and most: a tight, enlarged muscle that will not let the eye look up, with double vision on upgaze, proptosis and lid retraction, and on CT a swollen muscle belly with a spared tendon.
Examine it
Entrapped in orbital floor fractures, which limits upgaze.
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