What type of diplopia is experienced from unilateral CN VI palsy?

Unilateral cranial nerve VI (abducens nerve) palsy leads to horizontal and binocular diplopia. This occurs because the abducens nerve is responsible for the lateral movement of the eye; when it is affected, the eye on the same side as the lesion cannot abduct properly. As a result, when a person looks to the side opposite the affected eye, the affected eye fails to realign with the other eye, creating a misalignment. This misalignment results in the perception of two images (diplopia) that are seen only when both eyes are open, which is characteristic of binocular diplopia.

In contrast, vertical diplopia is typically associated with issues involving other cranial nerves (like CN III or IV) that control the vertical movements of the eye. Monocular diplopia usually implies an issue with the eye itself, such as a cataract or corneal irregularities, rather than a neurological cause. Thus, the specific presentation of horizontal and binocular diplopia is indicative of unilateral CN VI palsy.