the DVD-associated hyperdeviation disrupts vertical orientation. To the extent that there is little binocular vision and an asymmetric DVD, one might expect the drive for vertical orientation to override, resulting in a head tilt toward the side of the fixing eye (i.e., one that is driven by a human dorsal light reflex and noncompensatory for binocular vision). Alternatively, a strong potential for fusion and stereopsis would cause the drive for binocular vision to override, resulting in a head tilt toward the side of the hyperdeviating eye (which is compensatory for binocular vision).

Ocular Tilt Reaction

The primary role of the otolithic (static) vestibular system is to maintain vertical orientation. In the ocular tilt reaction, unilateral injury to an otolith or its central pathways produces a tilt of the subjective visual vertical, necessitating a compensatory tilt of the head, torsional position of the eyes, and interpupillary axis to a new position that the central nervous system erroneously computes as vertical. Thus, injury to central graviceptive pathways from the left otolith produces greater right utricular output (as would occur physiologically with a right head tilt), causing the patient to perceive the world as tilted to the left). The ocular tilt reaction is a compensatory righting reflex consisting of head tilt, ipsiversive torsion of the eyes, and vertical divergence to restore vertical orientation (Fig. 9.5). In this setting, the head would be tilted to the left (to align the head with the perceived vertical), the upper poles of the eyes would be torsionally rotated to the left, and the right eye would be higher than the left eye (as if the horizontal axis between the two pupils also tilted to the left).

In the ocular tilt reaction, the head tilt is usually asymptomatic, because the patient perceives the tilted head and body as realigning with vertical, and not as tilted. Many patients with this condition have undoubtedly been misdiagnosed as having superior oblique palsy. In the ocular tilt reaction, however, the head tilt is not compensatory for vertical diplopia (but rather for a tilted sense of vertical). The distinction between these two disorders is facilitated by examining for subjective and objective torsion in both eyes. In the ocular tilt reaction, there is intorsion of the higher eye and extorsion of the lower eye on Double Maddox Rod testing and fundus examination, while in superior oblique palsy, these same tests show extorsion of the higher eye.70 In the ocular tilt reaction, this binocular torsion disappears in the supine position,169 where the utricles behave as if they are functionally deafferented.25 While the Bielschowsky Head Tilt test is generally negative in skew deviation, it can occasionally simulate the pattern seen in superior oblique

Fig. 9.5  Ocular tilt reaction. (a) Physiologic ocular tilt reaction in a motorcycle rider. (b) Physiologic (left) versus pathological (right) ocular tilt reaction. With permission from Brodsky et al28

palsy.7,70,182 Acute vascular brainstem stroke is the most common cause of the ocular tilt reaction in adults,16,66 whereas posterior fossa tumors seem the most common cause in childhood.28 Rare causes such as polyarteritis nodosa have also been reported.175

Photophobia, Epiphora, and Torticollis

Torticollis can be a presenting sign of a posterior fossa tumor.13,127,197 Marmor et al145 described three young children with photophobia, epiphora, and torticollis who were found to have posterior fossa tumors. In one child, the symptoms improved following surgical resection. Posterior fossa tumors could theoretically cause torticollis by irritation of the vestibular nuclear complex, dural stretch, tonsillar herniation, cyclovertical muscle paresis, or any combination thereof.145 In all probability, these conditions are attributable to some combination of the ocular tilt reaction, with regional compression of the trigeminal nerve.