The superior oblique is a fusiform (spindle-shaped) muscle belonging to the extraocular group of muscles. It originates near the nose. Along with the other extraocular muscles, it performs the role of controlling eye movements.
Its primary, secondary, and tertiary actions are internal rotation (looking toward the nose), depression (looking downward), and abduction (looking away from the nose), respectively. This muscle’s movement of the eye downward is most effective when the eye is abducted. Downward movement of the eye also receives support from the lateral rectus, another extraocular muscle.
Another key role of the superior oblique muscle is to provide visual stability. It resists the eye's tendency to rotate itself involuntarily when looking downwards or upwards. It also causes inward torsion, which maintains the position of the eye towards the midline of the face.
The fourth cranial (trochlear) nerve supplies only this muscle and it supports the eye’s ability to depress downward. People with fourth nerve palsy — which can occur from birth or as the result of trauma — have weakened downward eye movement, which can make it look like their eye floats upward. This can result in blurry or doubled vision. Fourth nerve palsy can be treated by glasses, surgery, or may resolve on its own over time.