7 Central Vestibular Disorders

The two fundamental goals of the vestibular system are stabilization of gaze and postural control during movements. The peripheral vestibular system detects linear and angular accelerations and transmits the information to the brainstem via the vestibular nerves. The central vestibular system integrates vestibular, visual, and proprioceptive inputs for the conscious awareness of the body’s position in its environment, gaze stabilization, and posture during locomotion.1 In health, the function of the vestibular system is subconscious and not apparent. It is only when there is disease or excessive stimulation that symptoms of dizziness and disorientation occur. C. Miller Fisher defined dizziness as a false and conflicting sensation of motion or position.² The sensation is not only false but is unusual and has no normal counterpart.

The evaluation of the patient with central vestibular dysfunction requires a meticulous understanding of the patient’s complaint, the temporal profile of symptoms, associated neurologic symptoms, a complete review of medical history and medication exposures. This, combined with medical, neurologic, vestibular, and auditory examinations, allows the formulation of a differential diagnosis. The clinical diagnosis can then be established by laboratory studies, serial monitoring of the course of the illness, and response to therapy. The treatment options of central vestibular disorders are more limited than those for peripheral diseases. The critical part is the correct diagnosis and direct treatment of the central cause; for example, treating migraine headaches to minimize vestibular symptoms. When the central disease is stabilized, vestibular rehabilitation as used for peripheral vestibular disease can augment the treatment.

Peripheral causes of vertigo are more common and are characterized by episodic, fluctuating symptoms. Central causes of vertigo are more likely to produce prolonged, continuous symptoms of nonspecific dizziness than true spinning vertigo. Neurotologic examination and features of nystagmus with and without visual fixation facilitate differentiation of central from peripheral vestibular mechanisms. However, there is considerable overlap for each nystagmus parameter and the evaluation must always be systematic and complete. This chapter discusses the anatomy and physiology of the central vestibular system, associated diseases, and management.

Anatomy of the Central Vestibular System

The key components of the central vestibular system are the vestibular nuclei complex in the brainstem. These nuclei lie in the floor of the fourth ventricle in the pontine area bound laterally by the restiform body, ventrally by the nucleus and spinal tract of the trigeminal nerve, and medially by the pontine reticular formation and abducens nucleus. A schematic diagram of the central vestibular system is shown in Fig. 1–7 in Chapter 1. The vestibular nuclei are divided into four subgroups: the lateral, medial, superior, and inferior nuclei. They receive primary afferent signals from the vestibular nerve in addition to multisensory afferents from the contralateral vestibular system, reticular formation, cerebellum, spinal cord, and neck muscular and bony structures. The efferent signals include the lateral vestibulospinal tract, the medical vestibulospinal tract, the vestibuloculomotor tract, the otolithocular tract, the vestibulospinocerebellar tract, and the vestibulothalamic-cortical tract. The major tracts are the vestibulospinal, the vestibuloculomotor, and their connection with the cerebellum. These tracts control posture and head movements during active and passive motion.

The topography of secondary fibers projecting from the vestibular nuclei is complex. A practical and simplified approach is that the superior and medial nuclei mediate the vestibulo-ocular reflex (VOR) and the medial and lateral nuclei mediate vestibulospinal reflex (VSR). Fibers ascend in the medial longitudinal fasciculus (MLF) to brainstem oculomotor nuclei for coordination of the VOR. Fibers descending directly from the lateral nucleus and crossed fibers via the MLF from the medial nucleus mediate the lateral and medial VSR, respectively. The VSR controls upper and lower body tone and postural responses. There is overlap between the two reflexes mediated via the medial nucleus. In addition, all vestibular nuclei receive additional inputs from the commissural fibers, cerebellum, and reticular formation.3

The vestibulocerebellum is a critical part of the central vestibular system and exerts continual inhibitory influence via the floccular-nodular efferents. These project through the inferior cerebellar peduncle to the vestibular nuclei, as shown in Fig. 1–9 in Chapter 1. There are also direct efferents from the fastigial nuclei to the brainstem reticular formation and from the vermis to vestibular nuclei (mostly lateral), shown in Fig. 1–11, to control voluntary and involuntary eye movements and vestibular reflexes, respectively.

Tertiary-order vestibulocortical fibers project from the ventral posterior lateral nucleus of the thalamus to the frontal, temporal, and parietal lobes. It has been argued that because cortical vestibular function involves integration of multimodal visual and somatosensory information, it is difficult to identify a single, purely vestibular, cortical area. The vestibular cortical areas appear to be important for perceptual judgments of the head and body position in space and direction of motion.⁴

Migraine and Vestibular Migraine

Migraine is a common disorder, with a prevalence of 6 to 20% in men and 10 to 30% in women, most of whom have not sought medical care.⁵ Patients often do not offer a positive response to questions of previous migraine because they have attributed headaches to a sinus or tension etiology. Migraine has important impact on the quality of life and carries a significant economic burden in the costs of treatment and lost productivity. Dizziness is a frequent and often overlooked migraine symptom.

The understanding of the mechanism of migraine has advanced rapidly in the past decade. The theory of vasoconstriction-vasodilatation put forward in the 1960s has been replaced by neurotransmitters and neurogenic inflammation as the mechanism. Brainstem aminergic nuclei, which are involved in autonomic regulation of cerebral blood flow and antinociception, excite efferent neurons in the trigeminocervical complex, resulting in release of many neuropeptides. These include serotonin, calcitonin gene-related peptide (CGRP), and neurokinin A.⁶ The result is sterile inflammation and central pain sensitization. Fibers releasing CGRP have also been identified in the walls of inner ear blood vessels and the sensory epithelium of the cristae of the semicircular canals. Serotonergic and CGRP-containing fibers have also been identified in the vestibular nuclei.^7,8 It has been hypothesized that asymmetric serotonin and CRGP release to the vestibular nuclei could be the mechanism of migraine vertigo.⁶

The 2004 International Headache Society (HIS) diagnostic criteria for migraine without aura require at least five attacks of headache lasting 4 to 72 hours with either nausea or photophobia.⁹ The headache should have two of the following characteristics: unilateral location, pulsatile quality, moderate to severe in intensity, or aggravation by routine physical activity. In basilar-type migraine, a subtype of migraine, dizziness is part of an array of aura symptoms, which can include diminished hearing or tinnitus, dysarthria, visual loss, diplopia, or disturbances of sensory function. It is crucial to appreciate that dizziness of all forms is very frequent in migraine, but only a few percent of these patients meet the criteria for basilar-type migraine. A classification of definite and probable migraine vertigo has been proposed.^10,17

As with migraine headache, vestibular migraine is more common in women, by a 3:1 ratio. There is an increased incidence of motion sickness, often beginning in childhood and predating headaches.¹² Vertigo and motion intolerance are the common vestibular symptoms and occur in 25% of patients.^10,11 Vertigo is more common and episodic, lasting minutes to hours. Motion intolerance typically has a gradual onset and persists for days to weeks, intermixed with episodes of vertigo.13 It is critical to realize that headaches and vestibular symptoms do not occur simultaneously in the majority of these patients. A careful history of past headache, migraine trigger factors as precipitants of dizziness, photophobia, phonophobia, auras during dizziness, and family history of headache are very important in the evaluation of all patients with dizziness.

Benign paroxysmal vertigo of childhood affects primary school-aged children and produces sudden, brief attacks of vertigo. It may cause the child to cry in fear and hold onto others during an attack. Although these spells typically do not include headache, they are thought to be a childhood variant of migraine. Children who experience these attacks often will have migraine later in life and frequently have family members with migraine.

Recurrent vestibular dysfunction and vestibular Meniere’s disease can also be forms of migraine. Migraine can mimic typical Meniere’s disease, with fluctuating low-frequency hearing loss, tinnitus, and vertigo. Migraine is more common in patients with Meniere’s disease than in the general population.¹⁴ Neurologic symptoms accompanying a typical Meniere’s episode may be masked by the intensity of the vertigo or may not be volunteered by the patient. Of patients initially diagnosed with vestibular Meniere’s disease, only 21% developed typical Meniere’s disease.¹⁵ Migraine can rarely lead to fixed injury of the inner ear, analogous to the rare occurrence of retinal or cerebral injury in migraine.¹⁶

Few studies have focused on specifically treating migraine vertigo. Headache hygiene and trigger factor avoidance, combined with appropriately selected migraine prophylaxis, seems appropriate.¹⁷ Calcium channel blockers, such as Verapamil, and carbonic anhydrase inhibitors, such as acetazolamide and topiramate, may be particularly useful in basilar-type and vestibular migraine.¹⁸ Abortive treatment with subcutaneous or oral sumatriptan was very effective in treating either headache or vertigo.¹⁹ Patients with migrainous motion intolerance may benefit from migraine prophylaxis, vestibular rehabilitation, and vestibular suppressant medications.²⁰

Migraine-associated fluctuating low-frequency hearing loss is not uncommon, with the range of occurrence reported between 10% and 38%^21,22 and is believed to be due to various mechanisms. Severe isolated sensorineural hearing loss is uncommon in migraine. In a series of 13 cases of sudden hearing loss attributed to migraine, all patients had other neurologic symptoms, such as visual aura, amaurosis, vertigo, or paresthesias.²³ Actual infarction of the cochlea is not necessary for hearing loss. Transient reduced blood flow has produced hair cell loss in animal models. As with the vestibular sensory epithelium, the cochlea is innervated by CGRP-containing fibers. This may be the pathogenesis of migraine-related auditory symptoms, such as hearing loss and phonophobia.

Other central nervous system (CNS) diseases may mimic vestibular migraine. The diagnosis and treatment of migraine inner ear symptoms require careful attention to the neurotologic history, exam, course, and response to treatment. Any atypical features should lead to investigation for other etiologies. Certain medical problems include both headache and dizziness, which should be considered in the presence of central vestibular symptomatology. Substance exposure can produce both headaches and dizziness symptoms. These symptoms, among others, can occur after exposure to toxins such as carbon monoxide or organic solvents. Virtually all medicines have some reported incidence of headache and dizziness as adverse side effects; common examples are oral contraceptive agents and antihypertensive agents.

Traumatic Brain Injury

Traumatic brain injury is both common and widely variable, both in severity and prognosis. Brain injury is classified as severe, moderate, or mild, based on the initial Glasgow Coma Scale score, the duration of unconsciousness, the presence of posttraumatic amnesia, and the presence of focal neurologic findings. Diffuse axonal damage and dysfunction are the primary pathologic findings in mild traumatic brain injury (MTBI). This damage is mostly localized to the poles of the frontal and temporal lobes. Skull radiographs exclude skull fractures but do not evaluate brain injury. Computed tomography (CT) of the brain likely excludes relevant intracranial lesions. The majority of injuries are MTBI. In MBTI, patients often report a constellation of physical, cognitive, emotional, and behavioral symptoms; headache and dizziness are the two most commonly reported symptoms. These patients should be evaluated for benign paroxysmal positional vertigo (BPPV), and a CT scan of the temporal bone should be done if there is hearing loss to rule out inner ear dysfunction. Most patients with MBTI recover over weeks to months without residua. During this recovery period, however, these symptoms can cause significant distress. Some patients with MBTI have persistent symptoms; this clinical course is termed postconcussive syndrome. A variety of premorbid, injury-related, and postmorbid neuropathologic and psychological factors contribute to the development of postconcussive syndrome. Physical, psychological, and cognitive therapy and vestibular rehabilitation help with dizziness in motivated patients.

The incidence of strokes in the United States is about 700,000 events per year. It is the most common cause of disability and the third most common cause of death among adults.24 Although posterior fossa stroke accounts for only 2% of all strokes, as many as 25% of elderly patients with vascular risk factors that present with first ever acute vertigo will have a cerebellar infarction. In Fisher’s²⁵ series of 112 basilar artery occlusions, one fourth of patients had isolated episodes of dizziness prior to infarction. This could occur hours to months before the infarction, but was rarely more than 6 weeks. Ischemic posterior circulation stroke is most often caused by large vessel disease (50%), but vertebral artery dissection accounts for more than 20% of lower brainstem infarctions.²⁶ Other causes of stroke are cardiac embolism (30%), small vessel disease (20%), and less common etiologies such as endocarditis, coagulopathy, hematologic disorders, vasculitis, or substance abuse. The major untreatable risk factors for stroke are age, gender, race, and family history. The major treatable risk factors are hypertension, diabetes, smoking, and hyperlipidemia.

The peripheral and central vestibular and auditory structures are supplied by the vertebrobasilar (posterior) circulation. The basilar artery supplies the ventral brainstem by paramedian and short circumferential small penetrating vessels. The cerebellum and dorsolateral brainstem are supplied by three paired, long circumferential arteries: the superior cerebellar artery (SCA), the anterior inferior cerebellar artery (AICA), and the posterior inferior cerebellar artery (PICA).

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