The words trochlea and trochlear (/ˈtrɒkliə/, /ˈtrɒkliər/) come from Ancient Greek τροχιλέα trokhiléa, “pulley; block-and-tackle equipment”.

[citation needed] Injury to the trochlear nerve cause weakness of downward eye movement with consequent vertical diplopia (double vision).

Weakness of intorsion results in torsional diplopia, in which two different visual fields, tilted with respect to each other, are seen at the same time.

To compensate for this, patients with trochlear nerve palsies tilt their heads to the opposite side, in order to fuse the two images into a single visual field.

The characteristic appearance of patients with fourth nerve palsies (head tilted to one side, chin tucked in) suggests the diagnosis, but other causes must be ruled out.

Patients with more extensive damage will notice frank diplopia and rotational (torsional) disturbances of the visual fields.

Infections (meningitis, herpes zoster), demyelination (multiple sclerosis), diabetic neuropathy and cavernous sinus disease can affect the fourth nerve, as can orbital tumors and Tolosa–Hunt syndrome.

The trochlear nucleus and its axons within the brainstem can be damaged by infarctions, hemorrhage, arteriovenous malformations, tumors and demyelination.

The fourth nerve is one of the final common pathways for cortical systems that control eye movement in general.

Therefore, the trochlear nerve is tested by asking the patient to look 'down and in' as the contribution of the superior oblique is greatest in this motion.

Common activities requiring this type of convergent gaze are reading the newspaper and walking down stairs.

The unique features of the trochlear nerve, including its dorsal exit from the brainstem and its contralateral innervation, are seen in the primitive brains of sharks.