Which cranial nerve palsy is characterized by an inability to look down and in, leading to vertical diplopia?

Trochlear nerve palsy is characterized by an inability to look down and in, which is the primary movement controlled by the superior oblique muscle innervated by the trochlear nerve (cranial nerve IV). Patients with this condition often experience vertical diplopia, especially when looking down, as the affected eye is unable to properly adjust for the combined movements of adduction and depression. This results in double vision when viewing objects, particularly those positioned downwards.

In contrast, oculomotor nerve palsy primarily results in problems with eye movement in multiple directions and often presents with ptosis (drooping eyelid) and pupil involvement. Trigeminal nerve palsy affects sensation and the ability to chew, without significant impact on eye movement. Abducens nerve palsy leads to an inability to abduct the eye, predominantly affecting lateral movement without the specific diagonal movements seen in trochlear nerve palsy. Thus, the characteristic symptoms of trochlear nerve palsy align precisely with the inability to look down and in, causing the unique visual disturbance described.