TROCHLEAR NUCLEUS (C.N. IV)
The trochlear nucleus lies ventral to the cerebral aqueduct at levels #8 (rostral pons or isthmus) and #9 (caudal midbrain; inferior collicular level). The nucleus lies on top of our old friend the MLF. Axons from this nucleus pass DORSALLY around the aqueduct and DECUSSATE immediately caudal to the inferior colliculi. The trochlear nerve (which is quite thin) then winds around the cerebral peduncle and eventually innervates the SUPERIOR OBLIQUE (SO4). This is the only cranial nerve to emerge from the dorsal aspect of the brain stem.
While at first glance it appears that contraction of the superior oblique turns the eye down and out, the rest of the story (Paul Harvey would love this) is slightly more complicated. If you are interested, read on! The vector diagram resolves the arrow RB into effective components. Vector RA depresses the eye around the lateral axis. Vector RC abducts the eye around the vertical axis and intorts (medial rotation) the eye around the anteroposterior axis. Therefore, vector RB acts to depress, abduct and intort the eye.
When the eye is in the primary position, the superior oblique lies medial to the A-P axis of the globe. However, when the eye is adducted, the line of pull of the tendon of the superior oblique is parallel to the A-P axis of the globe. In this position, none of the actions of the muscle are dissipated in the other actions (abduction and intorsion). Hence the clinical test for the strongest action of the superior oblique is to ask the patient to look in (medially) and then down.
A 4th nerve lesion causes atrophy of the superior oblique muscle. When looking down and in (medially) with the bad eye there will be DIPLOPIA. The false image will lie below the true image (vertical diplopia) and will be somewhat oblique (torsional diplopia). The weakness of downward movement of the affected eye, most markedly when the eye is turned inward, results in the patient complaining of difficulty in especially reading or going downstairs.
The weakness of the superior oblique in the primary position (looking straight ahead) results in the bad eye being slightly extorted and elevated due to the unopposed action of the inferior oblique. This will result in torsional and vertical diplopia. For instance, if the LEFT superior oblique is paralyzed, the LEFT eye is extorted and elevated. In order to get rid of the torsional part of the double vision, the patient will tilt their head to the side OPPOSITE the paralyzed muscle, that is to the RIGHT. This causes reflex (from the otoliths) intorsion of the normal RIGHT eye (on side of head tilt) so that the vertical axis of the two eyes become parallel (the eye associated with the paralyzed superior oblique is already extorted by the unopposed inferior oblique). To alleviate the vertical diplopia, the patient will also FLEX his/her chin when tilted to the RIGHT. In this position the patient will have to elevate the normal RIGHT eye in order to look straight ahead. The bad (LEFT) eye is already elevated and when the two eyes are located at the same vertical (up-down) position in the socket, the vertical diplopia is ameliorated.
REMEMBER, LESION OF TROCHLEAR NERVE (after it has crossed the midline) = HEAD TILTED AWAY FROM PARALYZED MUSCLE; HEAD ALSO FLEXED IN THIS POSITION. HOWEVER, IF LESION IS IN THE TROCHLEAR NUCLEUS, HEAD TILT = TOWARDS THE LESION