Type of squint seen with 6th cranial nerve palsy: March 2007

Ans. B: Uncrossed diplopia The abducens nerve/ abducent nerve/sixth cranial nerve/sixth nerve /VI is a "somatic efferent" nerve that controls the movement of a single muscle, the lateral rectus muscle of the eye. Complete interruption of the peripheral sixth nerve causes diplopia (double vision), due to the unopposed action of the medial rectus muscle. The affected eye is pulled medially. In order to see without double vision, patients will rotate their heads so that both eyes are looking sideways. On formal testing, the affected eye cannot abduct past the midline - it cannot look sideways, toward the temple. Paial damage to the sixth nerve causes weak or incomplete abduction of the affected eye. The diplopia is worse on attempted lateral gaze, as would be expected (since the lateral gaze muscle is impaired). Isolated sixth nerve palsies in children are assumed to be due to brain tumors until proven otherwise. The most common overall cause of sixth nerve impairment is diabetic neuropathy. The long course of the abducens nerve between the brainstem and the eye makes it vulnerable to injury at many levels: