What type of diplopia is associated with unilateral CN IV (trochlear) palsy?

In the case of unilateral CN IV (trochlear) palsy, the type of diplopia that occurs is binocular vertical diplopia. The trochlear nerve innervates the superior oblique muscle, which is responsible for downward, inward, and lateral movement of the eye. When this nerve is affected, the patient may have difficulty with those movements, particularly when looking downwards.

In unilateral CN IV palsy, the affected eye cannot adequately adduct and depress, resulting in the eye being higher than the unaffected eye during down-gaze. Patients often report vertical diplopia, especially when looking down, as the eye with the trochlear nerve palsy is unable to align properly with the other eye. This misalignment creates a visual disturbance, notably described as the images being one above the other (vertical diplopia).

To summarize, the characteristic clinical presentation of diplopia associated with CN IV palsy is binocular vertical diplopia, primarily due to the impairment of the superior oblique muscle's function.