Extraocular muscles · 18 of 24
What does the superior oblique do?
The superior oblique is the longest and thinnest of the eye muscles, and the strangest. It arises at the orbital apex above the annulus, runs forward along the upper medial wall to the trochlea, a cartilage pulley on the front of the orbital roof, turns sharply back and out through it as a tendon, and inserts on the top of the globe behind the equator, under the superior rectus. Because it pulls from the trochlea in front, not from the apex behind, its actions are the opposite of what its position suggests.
- Action
- Intorsion; also depression (most in adduction) and abduction
- Purest test
- Looking down with the eye adducted
- Nerve
- Trochlear nerve (CN IV), the thinnest cranial nerve with the longest course
- Pulley
- The trochlea, on the superomedial orbital rim
Why it depresses from above
After the trochlea the tendon runs back, out and down to the top of the eyeball behind its equator. Pulling on that point from in front and medially rotates the top of the eye towards the nose, intorsion, and because the insertion is behind the equator it pulls the back of the eye up, so the front goes down. The depression is strongest when the eye is adducted, because the tendon then lines up with the visual axis, and that is the position it is tested in: look down and in, the position of reading and of walking down stairs.
The fourth nerve palsy
A fourth nerve palsy gives vertical double vision, worst looking down and in, so the patient struggles on stairs and with reading, and tilts the head away from the affected side to reduce the torsion. The affected eye sits higher, and the hypertropia increases on looking to the opposite side and on tilting the head to the same side: the Parks-Bielschowsky three-step test. The commonest causes are a congenital palsy that decompensates in adult life, with old photographs showing a lifelong head tilt, and closed head injury, where both nerves may be damaged where they cross behind the midbrain.
Examine it
A fourth nerve palsy gives vertical diplopia worst looking down and in, with a head tilt away from the palsy.
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