The utriculo-ocular motor pathways - physiologic and pathologic ocular tilt reaction: OTR Diagram Pathologic EOMs Labelled (Figure 3)

A skew deviation is a non-paralytic vertical ocular misalignment that is due to imbalance in the utriculo-ocular motor pathways. While vestibular jerk nystagmus is a consequence of static semicircular canal pathway imbalance (e.g., left-beating nystagmus due to acute right vestibular hypofunction from vestibular neuritis), an ocular tilt reaction, which includes skew deviation, head tilt, and ocular counter-roll, is a consequence of static utricle pathway imbalance. Anatomy of the utricle-ocular motor pathways Beginning caudally with the right utriculo-ocular motor pathway: the right labyrinth contains the linear acceleration detectors (otoliths), including the saccule and utricle. The utricle responds not only to linear translation, but also to head tilt. Physiologic ocular tilt reaction (OTR) (Figure 1) • A motorcyclist going around a tight curve to the right will tilt the head and body to the right, and this tilt will excite the right utricle and result in an increase in the firing rate. • The major component of the physiologic OTR will be a large head tilt to the left in an effort to reflexively reorient the head back to gravitational vertical. • These utricular afferents will synapse in the right vestibular nucleus (thought to mainly project to medial and superior vestibular subnuclei1) in the medulla. • The utriculo-ocular motor fibers which began on the right side then decussate at the level of the pontomedullary junction and ascend as part of the left medial longitudinal fasciculus (MLF). • Afferents travel via the MLF to their respective cyclovertical-acting ocular motor nuclei in the left midbrain (Figure 2): o Right eye elevates and incycloducts. Elevation from contraction of right superior rectus (SR, decussating fibers from left 3rd nucleus). Incycloduction from contraction of the right superior oblique (SO, decussating fibers from left 4th nucleus) o Left eye depresses and excycloducts. Depression from contraction of left inferior rectus (IR, fibers from left 3rd nucleus). Excycloduction from contraction of left inferior oblique (IO, fibers from left 3rd nucleus) • Finally, some of the utricle afferents ascending to the left MLF will synapse in the left INC, which is responsible for vertical and torsional gaze-holding. This ensures that the eyes will stay in their new position (without drifting back to their initial straight ahead position) while the head tilt is maintained. • While the head tilt is the major component of the physiologic OTR, the skew (elevation in one eye and depression in the other) and ocular counterroll (cycloduction of both eyes in the same direction) are quite small