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Vestibular pathways run from the eighth nerve and the vestibular nuclei through ascending fibers, such as the ipsilateral or contralateral medial longitudinal fasciculus (MLF), the brachium conjunctivum, or the ventral tegmental tract to the ocular motor nuclei, the supranuclear integration centers in the rostral midbrain, and the vestibular thalamic subnuclei. From there they reach several cortex areas through the thalamic projection. Another relevant ascending projection reaches the cortex from vestibular nuclei via vestibular cerebellum structures.
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In the majority of cases, central vestibular vertigo syndromes are caused by dysfunction or a deficit of sensory input induced by a lesion. In a small proportion of cases, they are a result of pathological excitation of various structures, extending from the peripheral vestibular organ to the vestibular cortex. Because peripheral vestibular disorders are always characterized by a combination of perceptual, ocular motor, and postural signs and symptoms, central vestibular disorders may manifest as “a complete syndrome” or with only single components. The ocular motor aspect, for example, predominates in the syndromes of upbeat or downbeat nystagmus. Lateral falls may occur without vertigo in vestibular thalamic lesions (thalamic astasia) or as lateropulsion in Wallenberg's syndrome.1,2
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Clinical Classification of Central Vestibular Disorders
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The “elementary” neuronal network of the vestibular system is the di- or trisynaptic vestibulo-ocular reflex (VOR). A useful simple clinical classification of central vestibular syndromes is based on the three major planes of action of the VOR (Fig. 5.1): yaw, roll, and pitch.2–4
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The plane-specific vestibular syndromes are determined by ocular motor, postural, and perceptual signs as follows:
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Yaw plane signs are horizontal nystagmus, past pointing, rotational and lateral body falls, and horizontal deviation of perceived straight-ahead.
Roll plane signs are torsional nystagmus, skew deviation, ocular torsion, and tilts of head, body, and perceived vertical.
Pitch plane signs are upbeat/downbeat nystagmus, forward/backward tilts and falls, and vertical deviations of perceived straight-ahead.
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The defined VOR syndromes allow for a precise topographic diagnosis of brainstem lesions as to their level and side, as follows (Fig. 5.2):
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A tone imbalance in yaw indicates lesions of the lateral medulla, including the root entry zone of the eighth cranial nerve and/or the vestibular nuclei.
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 cerebellum, especially the flocculus.
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