Trochlear nerve palsy and double vision on looking down and in
Trochlear nerve palsy is weakness or paralysis of the superior oblique, the main muscle that lowers the eye when turned towards the nose, giving vertical double vision.
Remember
Vertical double vision when reading or going downstairs, often with the head tilt away from the weak eye, points to a weak superior oblique, the muscle supplied by the trochlear nerve.
Mnemonic
LR6 SO4
- L lateral rectus
- S superior oblique
It helps you recall that the lateral rectus is supplied by cranial nerve VI and the superior oblique by cranial nerve IV, with the rest supplied by cranial nerve III.
The trochlear nerve (cranial nerve IV) supplies a single muscle, the superior oblique. The two oblique muscles attach to the eye at an angle, so their contraction rotates the eyeball. The superior oblique turns the eye down (depression) and rotates the top of the eye towards the nose (intorsion). It lowers the eye best when the eye is already turned in towards the nose (adducted).
Reading a page and looking at the steps on the stairs both mean looking down and in. When the superior oblique is weak, the affected eye cannot follow the healthy eye as both look downwards, so the two eyes disagree vertically and one image sits above the other. That is vertical diplopia, or vertical double vision.
A palsy means weakness or paralysis of a muscle. In a trochlear nerve palsy the other muscles pull the eye out of position, so the affected eye may rest slightly higher than the other. Patients often keep the head tilted away from the affected side, which tends to reduce the double vision. The double vision is typically worse when the head tilts towards that side.
The lateral rectus, one of the four straight (rectus) muscles, moves the eye outwards and is the muscle supplied by the abducens nerve (cranial nerve VI). If it is weak, the eye rests turned inwards and the double vision is horizontal. The remaining eye muscles are supplied by the oculomotor nerve (cranial nerve III), and a palsy of that nerve would also cause ptosis (a drooping upper eyelid).
Key facts
- Nerve
- the trochlear nerve (cranial nerve IV) supplies the superior oblique and nothing else.
- Action
- the superior oblique lowers the eye, particularly when it is turned towards the nose, and rotates its top towards the nose.
- Symptom
- vertical double vision when looking down and in, such as when reading or going downstairs.
- Resting position
- the affected eye may sit slightly higher than the other.
- Head tilt
- patients often hold the head tilted away from the affected side, and the double vision is typically worse when the head tilts towards it.
- H test
- a finger is moved in an H shape in front of the patient; moving it downwards while the eye is turned towards the nose tests the superior oblique, whereas the inferior rectus, another muscle that lowers the eye, is the muscle tested when the eye is turned away from the nose.
Exam traps
- Inferior oblique weakness does not fit, because the inferior oblique lifts the eye and is supplied by the oculomotor nerve, so its weakness would cause double vision on looking up and in, rather than on reading or going downstairs.
- Loss of abduction (looking outwards) does not fit, because the lateral rectus is supplied by the abducens nerve and its loss gives horizontal double vision with the eye resting turned inwards, not vertical double vision.
- The superior rectus does contribute to intorsion, but its main action is elevation (lifting the eye), so its weakness would limit looking up and would not explain poor depression of the eye when it is turned towards the nose.
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