Understanding skew deviation and a new clinical test to differentiate it from trochlear nerve palsy

SummarySkew deviation is a vertical strabismus caused by a supranuclear lesion in the posterior fossa. Because skew deviation may clinically mimic trochlear nerve palsy, it is sometimes difficult to differentiate the 2 conditions. In this review we compare the clinical presentations of skew deviation and trochlear nerve palsy and examine the pathophysiology that underlies skew deviation. We then describe a novel clinical test—the upright–supine test—to differentiate skew deviation from trochlear nerve palsy: a vertical deviation that decreases by ≥50% from the upright to supine position suggests skew deviation and warrants investigation for a lesion in the posterior fossa as the cause of vertical diplopia.