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The inferior oblique muscle is one of six thin, extraocular muscles responsible for eye movement. Arising from the maxillary bone on the inner floor of the eye socket, the inferior oblique muscle courses forward to attach to the sclera of the eye at a point between two muscles that connect to the inferior and lateral poles of the eye, the inferior and lateral recti. The primary action of this muscle is external rotation of the eye, in which the superior pole of the eye is rotated counterclockwise. In addition, the muscle also lifts the eye up and turns the eye outward. The inferior division of the third cranial nerve, the oculomotor nerve, provides the nerve supply to the muscle.
Strabismus refers to imbalances in the actions of the extraocular muscles with associated deviations in the alignment of the eyes. “Overaction” of the inferior oblique muscle can coexist with other childhood ocular misalignments, either eyes that cross or eyes that turn outward. Excessive action of this muscle produces an over-elevation of the eye when the patient turns the eye inward. Surgery to weaken the muscle can alleviate this condition.
If damage occurs to the oculomotor nerve, the patient can experience weakness of this muscle. Consequently, the eye will rotate in the clockwise direction, producing a tilted image. In order to avoid double vision, the patient will tilt his head toward the side with the inactive inferior oblique muscle. When the patient attempts to look up and inward, the affected eye does not look up as much as it should. Treatments for the palsy include weakening of the opposing superior oblique muscle on the same eye or weakening of the superior rectus muscle on the opposite eye.
Oculomotor damage impacts not only the inferior oblique but also the superior rectus, the inferior rectus, and the medial rectus muscles. These muscles move the eye up, down, and inward, respectively. When there is an impaired nerve supply to all of these muscles, the eye will stay in an outward and downward position. The eye will be unable to move inward or upward past the midline.
Damage to the fourth cranial nerve, the trochlear nerve, disrupts the nerve supply of the superior oblique muscle. Palsy of the superior oblique muscle will create a clinical picture that resembles an overactive inferior oblique muscle. The patient will typically tilt his head away from the affected side. Marked double vision will occur if the patient tilts his head toward the affected side.