Inferior Oblique Overaction in a Congenital 4th Nerve Palsy

60-yo-man complaining of intermittent oblique diplopia. There was a left hypertropia that worsened in down gaze, right gaze and in left head tilt. There was a large vertical fusional amplitude in addition to a longstanding rightward head tilt, and on examination there was left inferior oblique overaction. These features were highly suggestive of a congenital left 4th nerve palsy. Video shows left congenital 4th nerve palsy with left inferior oblique overaction seen in right gaze. This is not primary inferior oblique overaction, but is secondary to a superior oblique palsy. The positive head tilt test is consistent with a left SO palsy, while in primary IO overaction, it would be negative