The superior oblique is the longest and thinnest extra-ocular muscle, redirected by the trochlear pulley at the medial orbital wall, which changes its direction from forward-medial to backward-lateral before inserting on the posterior superior sclera. Its trochlear pulley redirection explains its CN IV innervation name (trochlear = pulley). Trochlear nerve palsy produces superior oblique paralysis causing vertical diplopia with the affected eye elevated and rotated, corrected by head tilt away from the affected side.
| Origin | Orbital apex (sphenoid body, above the optic foramen) |
|---|---|
| Insertion | After passing through the trochlear pulley at the medial orbit, inserts on the superior posterior sclera |
| Nerve Supply | Trochlear nerve (CN IV) |
| Blood Supply | Ophthalmic artery |
| Actions | Depression of the adducted eye (primary action); Internal rotation (intorsion); Abduction of the eye |
|---|
The trochlear pulley redirects the superior oblique force through approximately 90 degrees, converting a forward pull into a backward-lateral-depressing force. This geometry makes it the primary depressor of the adducted eye and explains why CN IV palsy produces maximum vertical diplopia when looking down and toward the nose.
The Parks-Bielschowsky three-step test localises the specific paretic extra-ocular muscle producing vertical diplopia: hypertropia worse on contralateral gaze and ipsilateral head tilt indicates superior oblique palsy. Bilateral superior oblique palsy from head trauma produces a large V-pattern esotropia with bilateral excyclotorsion (outward rotation of the upper poles of both eyes) visible on fundoscopy. Superior oblique tuck (shortening) or inferior oblique weakening are performed surgically.
Not accessible to external palpation. Assessed by cover test and three-step test.
CN IV palsy producing vertical diplopia with head tilt to the opposite shoulder for compensation, confirmed by the Parks-Bielschowsky three-step test.
