________________________________________________________________________ Dizziness and Vertigo
Anterior Inferior Cerebellar Artery Distribution The anterior inferior cerebellar artery supplies the lateral pons and branches off to form the internal auditory artery, which irrigates the cochlea and vestibular labyrinth. The labyrinth is especially vulnerable to ischemia, because the internal auditory artery is an end-artery with minimal collaterals. Isolated acute audio-vestibular loss may herald an impending anterior inferior cerebellar artery territory infarction. Auditory artery occlusion causes loss of auditory and vestibular function [47; 51]. Pontine syndrome is a partial pontine ischemia produced by anterior inferior cerebellar artery occlusion, characterized by vertigo, tinnitus, and hearing loss [47; 53]. STRUCTURAL ABNORMALITIES AND LESIONS Malformations or lesions that affect the vestibular system can cause dizziness or vertigo, as can any process that leads to acute vestibular asymmetry. The brain may adapt to this asymmetry over time to prevent or diminish vertigo, but imbalance may persist when vestibulo-cerebellar pathways are impacted. For example, sudden-onset vertigo is more likely with stroke, but less likely with a slow-growing acoustic neuroma (a vestibular tumor) because central adaptation to the asymmetry is gradual and imperceptible [15]. Superior Canal Dehiscence Syndrome Superior canal dehiscence syndrome is caused by an abnormal opening (dehiscence) in the thin bony roof of the superior semicircular canal. This is thought to be related to congenital thinning of the temporal bone and occurs in 0.1% of the population. Development of the bony opening renders the membranous labyrinth unusually susceptible to changes in sound and pressure. In addition to dizziness and sensitivity to loud noises, patients with superior canal dehiscence syndrome experience autophony, whereby one’s voice, breathing, or bodily sounds are excessively audible [57]. The dizziness/vertigo induced by sound and pressure stimuli can be associated with auditory symptoms of conductive or mixed hearing loss [58]. Chiari Malformation A Chiari malformation is defined as descent of the cerebellar tonsils ≥5 mm past the foramen magnum [18; 47]. This occurs when the lower part of the cerebellum does not fully migrate rostrally during fetal development. Compression of the cer- ebellum from this condition may lead to occipital headaches, vertigo, nystagmus, and ataxia in early childhood or later in life [15]. Patients also commonly describe occipital and neck pain provoked by physical activity or maneuvers that increase intracranial pressure [18]. Symptomatic Chiari malformations should be referred for neurosurgical evaluation and considered for posterior fossa decompression. Asymptomatic Chiari malformations can be discovered incidentally during MRI for dizziness from a dif- ferent etiology. Asymptomatic patients should be spared the risks of surgery [18].
Vertebrobasilar Artery Territory (Posterior Circulation) Infarcts or Strokes
Around 25% of all ischemic events occur in the vertebrobasilar artery territory, with dizziness/vertigo the sole presentation in up to 33% of patients [53]. Inferior cerebellar and small brainstem infarctions are an increasingly recognized cause of isolated vertigo, and transient isolated vertigo is a common manifestation of vertebrobasilar artery ischemia. Vertebrobasi- lar artery transient ischemic attacks (TIAs) typically last eight minutes on average [51; 53]. Several syndromes are associated with TIAs, ischemia, or infarcts in vertebrobasilar artery ter- ritories (e.g., the brainstem, cerebellum) [47]. Numerous arteries extend from the posterior inferior cerebellar artery to supply the medial, lateral, and dorsal aspects of the medulla. Vertigo is more frequent in posterior inferior cerebel- lar artery strokes, and these are the most common cerebellar infarcts [54]. Dizziness/vertigo is one of the most common symptoms of cerebellar ischemic stroke. A large prospective study showed 11% of patients with cerebellar infarction had isolated vertigo, of whom 96% had an infarct in the medial branch of the posterior inferior cerebellar artery [55]. Medial Medullary Infarcts Medial medullary infarctions usually cause a vertiginous labyrinthian syndrome that mimics peripheral vestibulopathy. Severe vertigo and nystagmus are prominent, and some patients describe feeling as if being pulled to the ipsilateral side by a magnet [52]. Lateral Medullary Infarcts The vestibular nuclei are more vulnerable to ischemia than other structures in the posterior fossa, and ischemia of the lateral medulla (including the vestibular nuclei) may be a com- mon mechanism of isolated vascular vertigo [51]. Wallenberg syndrome describes a lateral medullary infarction that presents with a range of focal neurologic signs [53]. Vestibulo-cerebellar symptoms/signs are almost always present, because lateral medullary infarcts involve the vestibular nuclei and their connections. The symptoms/signs include dizziness or disequilibrium; acute vertigo, nausea, or vomiting; ipsilateral facial pain, numbness, and ataxia (falling to the side of the infarct); and decreased contralateral pain and thermal sensitiv- ity. The nystagmus has horizontal and rotational components [47; 52; 53]. Dorsal Medullary Infarcts Less prevalent than medial or lateral medullary infarction, dorsal medullary infarcts typically cause dizziness/vertigo, imbalance, and ocular motor abnormalities. This is because the dorsal medulla also contains the vestibular nuclei and other structures that relay and process vestibular and ocular motor signals [56].
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Book Code: ANCCTX2027
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