Central vestibular syndromes in roll, pitch, and yaw planes: Topographic diagnosis of brainstem disorders

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Abstract
Evidence is presented for a clinical classification of central vestibular syndromes according to the three major planes of action of the vestibulo-ocular reflex (VOR): yaw, pitch, and roll. The plane-specific vestibular syndromes are determined by ocular motor, postural, and perceptual signs. Roll plane signs are: torsional nystagmus, skew deviation, ocular torsion, tilts of head, body, and perceived vertical; pitch plane signs are: upbeat/downbeat nystagmus, forward/backward tilts and falls, deviations of perceived horizontal; yaw plane signs are: horizontal nystagmus, past pointing, rotational and lateral body falls, deviation of perceived straight-ahead. The thus defined VOR syndromes allow for a precise topographic diagnosis of brainstem lesions as to their level and side. A tone imbalance in roll indicates unilateral lesions (ipsiversive at pontomedullary level, contraversive at pontomesencephalic level). A tone imbalance in pitch indicates bilateral (paramedian) lesions or bilateral dysfunction of the flocculus. It is hypothesized that signal processing of the VOR in roll and pitch is conveyed by the same rather than separate ascending pathways in the medial longitudinal fasciculus and the brachium conjunctivum. A unilateral lesion (or stimulation) of these ‘graviceptive’ pathways (which transduce input from vertical semicircular canals and otoliths) affects function in roll, whereas bilateral lesions (or stimulation) affects function in pitch. Thus, the vestibular system is able to change its functional plane of action from roll to pitch by switching from a unilateral to a bilateral mode of operation. Therefore a common integrator system is efficient (and sufficient) for both eye movements in roll and pitch (rostral interstitial nucleus of the medial longitudinal fasciculus and rostral interstitial nucleus of Cajal). Clinically this means that bilateral syndromes in roll (skew torsion) manifest in a syndrome in pitch (upbeat- or downbeat nystagmus). Pure syndromes in yaw are rare, since the small causative area covering the medial and superior vestibular nucleus is not only adjacent to but overlapped by the structures also subserving roll and pitch function. A lesion frequently results in mixed (e.g., torsional and horizontal) nystagmus. The lesional sites of yaw syndromes are restricted to the pontomedullary level because of the short distance between the vestibular nuclei and the integration center for horizontal eye movements in the paramedian pontine reticular formation. Syndromes in roll and pitch, however, may arise from brainstem lesions located in an area extending from the medulla to the mesencephalon, an area corresponding to the large distance between the vestibular nuclei and the integration centers for vertical and torsional eye movements in the rostral midbrain.