The trochlear nerve (CN4) only controls the superior oblique. This muscle has a funny course, and runs through a pulley at the superior-medial wall of the eye socket. Because of this change in direction, the superior oblique muscle works mainly as an intonate, though it does perform some vertical movement, especially when the eye looks medially. Patients will often complain of a vertical diplopia, especially when looking away from the lesion. They may develop a head-tilt away from the affected eye. On cross cover testing, you’ll find a vertical hypertrophy of the affected eye that worsens when that eye is looking medially toward the nose, especially when reading.
Fourth nerve palsies can occur from DM/HTN ischemia, like all palsies. They can also be:
* Trauma (that fourth nerve pops off the dorsally and has a long course to traverse)
* Congenital palsy that decompensates with age
Thanks And Regards.