A right 4th nerve palsy causes a right hypertropia worse in down gaze, left gaze, and right head turn (all of which accentuate the weakness of the superior oblique muscle). As a result, patients with 4th nerve palsy will typically adopt a head tilt toward the contralateral shoulder and a gaze preference toward the side with the palsy (to allow more elevation from the superior rectus). For the patient above, he would be expected to adopt a LEFT head tilt and RIGHT gaze preference (achieved with LEFT face turn).
Normal vertical fusional amplitudes are about 2-3 diopters, but because patients with congenital 4th nerve palsy have had misalignment from birth, they are often able to fuse vertical deviations much larger than 3 diopters. Vertical fusional amplitudes are a very helpful test in distinguishing a longstanding congenital 4th nerve palsy that has just recently decompensated from an acute acquired 4th nerve palsy.
Because the tertiary action of the superior oblique is to abduct the eye (especially in down gaze), 4th nerve palsies can result in esotropia on down gaze which improves on up gaze. This V pattern can be seen in unilateral CN 4 palsy but is more common and more pronounced in bilateral CN 4 palsy.
