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.Skew deviation is a vertical strabismus caused by supranuclear lesions. It is often associated with ocular torsion and head tilt, which together constitute the ocular tilt reaction. [1][2][3][4] Skew deviation is often the initial manifestation of diseases that affect the brainstem, cerebellum, or peripheral vestibular system. [4][5][6][7][8][9][10] Because both skew deviation and trochlear nerve palsy may result from intracranial lesions or trauma, and because some skew deviations may clinically mimic trochlear nerve palsy, differentiating these 2 conditions can be challenging. Understanding skew deviation remains difficult, partly because it requires knowledge of the underlying anatomy and pathophysiology. In this review, we first compare the clinical presentation of skew deviation versus trochlear nerve palsy. We then focus on the pathophysiologic mechanism of skew deviation and examine some current evidence that shows that skew deviation results from an imbalance of the utriculo-ocular pathway. We then present a novel upright-supine test that can be used clinically to differentiate skew deviation from trochlear nerve palsy at the bedside.
Clinical PresentationSkew deviation was first induced experimentally in animals by Magendie 11 and later Hertwig, 12 who produced skew deviation in cats by sectioning the middle cerebellar peduncle. It was first observed in humans with cerebellar tumors by Stewart and Holmes in 1904. 13 Skew deviation is a vertical misalignment of the visual axes caused by a disturbance of supranuclear inputs as a result of lesions in the brainstem, cerebellum, or peripheral
CIHR Author ManuscriptCIHR Author Manuscript CIHR Author Manuscript vestibular system (ie, the inner ear and its afferent projections). The vertical misalignment may be comitant or incomitant. Rarely, it alternates with eye position (eg, right hypertropia on right gaze and left hypertropia on left gaze). 14,15 Skew deviation often is associated with other neurologic signs and may be part of the ocular tilt reaction, which consists of a triad of skew deviation, ocular torsion, and head tilt. [1][2][3]10 In ocular tilt reaction, the pathologic head tilt is ipsilateral to the hypotropic eye, and the ocular torsion is such that the upper poles of both eyes rotate in the same direction as that of the head tilt (ie, the h...