DEFINITIONS

When the vestibular apparatus is damaged by any pathology, a number of typical clinical manifestations arise. Symptoms and signs of pathology are determined by the site of the lesion and not by the etiology. This in part compounds the diagnosis for the clinician. Nonetheless, the specific diagnosis is highly relevant in terms of appropriate management. The term peripheral vestibular disorders refers to pathology in the vestibular labyrinth and/or cranial nerve VIII, whereas pathology affecting the central nervous system pathways at or above the vestibular nuclei are referred to as central vestibular syndromes.

For the purposes of this chapter, vertigo refers to an illusion of movement, whereas dizziness is a lay term variously covering a plethora of synonyms, including giddiness, light-headedness, falling, “swimminess,” and mental disorientation. Ataxia is an inability to coordinate muscular movements; the terms falls and unsteadiness are self-explanatory. Oscillopsia refers to the rhythmic oscillation of the visual environment, often spontaneously as a consequence of a central eye movement disorder or in response to motion, as a consequence of bilateral vestibular failure.

EPIDEMIOLOGY

Dizziness is an extremely common symptom, both in primary care and at the tertiary level. One in four healthy subjects in the community reports symptoms of dizziness, with significant effects on their daily living.¹ By the age of 70, 36% of women and 29% of men have balance problems, whereas by the ages of 88 to 90, 45% to 50% of the population suffer symptoms of balance dysfunction.²

In the community, many cases of vestibular dysfunction resolve spontaneously, without recourse to medical care, although each year 5 per 1000 patients consult their general practitioners because of symptoms that are classified as vertigo, and a further 10 per 1000 are seen for dizziness or giddiness.³ In a tertiary setting, dizziness is associated with significant morbidity, and in the older population, falls and mortality are common sequelae.⁴ Vestibular symptoms after head/whiplash injury are the commonest cause of failure to return to work, and two thirds of patients in a tertiary neuro-otological clinic suffer psychiatric symptoms in association with vestibular pathology.⁵

VESTIBULAR ANATOMY AND PHYSIOLOGY

The internal ear is a minute membranous structure within the bony labyrinth, buried in the temporal bone. Within the internal ear, the cochlea is the acoustic end-organ receptor, whereas, of the five vestibular end-organs, one lies within each of the three semicircular canals and one each in the utricle and saccule within the vestibule. The vestibular sensory epithelium is composed of type 1 and type 2 hair cells, covered with a gelatinous membrane that, in the saccule and utricle, contains calcium carbonate-rich crystals termed otoconia (Fig. 27-2).

Figure 27-2 The vestibular end-organ and vestibular sensory epithelia.

A force parallel to the surface of the sensory epithelium provides the maximal stimulus. Thus, the horizontal anterior and posterior semicircular canals are stimulated by angular acceleration in the three planes of space but are insensitive to gravity or head position.⁶ The saccule, which lies approximately vertically, senses vertical linear head acceleration and gravity; the utricle, which is oriented approximately horizontally, senses horizontal linear head motion and head position in space.

Physiologically, the vestibular apparatus can be considered in two halves, the right labyrinth and the left labyrinth, which are perfectly balanced and work in parallel. For example, when the head is turned to the right, the right horizontal semicircular canal increases its firing rate, whereas the left decreases its firing rate. This asymmetry in neural input is transmitted (1) to the vestibular nuclei and the cerebellum, which controls the amplitude and timing of movements, and (2) via the vestibular nuclei and the thalamus to the parietoinsular vestibular cortex. From birth, the vestibular, visual, and proprioceptive inputs associated with every type of movement are monitored, integrated, and stored in a “data bank,” which is considered to be the reticular formation of the brainstem.⁷ Subsequently, each movement generates signals that are then compared with the information in the “data bank.” Integration of movement-induced neural asymmetry with other sensory input and comparison with the “data bank” template allow for awareness of head and body position in space, together with the generation of compensatory oculomotor (vestibulo-ocular reflex) and motor (vestibulospinal) activity. In addition to the motor control, the extensive convergence of vestibular and autonomic afferent information in the brainstem and cerebellum allows for coordination of appropriate motor and autonomic responses during movement or changes in posture.

Thus, if there is any mismatch of the sensory input to the existing template, the patient senses disorientation, may develop an abnormal eye movement, frequently feels off balance, and may develop nausea lvomiting and other autonomic symptoms (Fig. 27-3). The classic physiological example of such a mismatch is motion sickness.⁸ However, any pathological lesion that results in a change in, for example, vestibular input to the central nervous system, as may occur in Meniere’s disease or vestibular neuritis, produces similar symptoms of disorientation, nausea, vomiting, and malaise as a consequence of the change in the vestibular signal, with no corresponding changes in visual and proprioceptive inputs. In addition, connections at various levels of the central vestibular system with the locus ceruleus, the limbic system, and other brain regions that control affective responses, mood, and arousal may underlie the observed overlap between psychiatric and vestibular disorders.^9,10

Figure 27-3 Visual-vestibular mismatch. An active movement leads to stimulation of the sensory organs whose messages are compared with a multisensory pattern of expectation calibrated by earlier experience of motions (central store-data bank). The pattern of expectation is prepared either by the efference copy signal, which is emitted parallel to and simultaneously with the motion impulse, or by vestibular excitation during passive transportation in vehicles. If concurrent sensory stimulation and the pattern of expectation are in agreement, self-motion is perceived while “space constancy” is maintained. If, for example, there is no appropriate visual report of motion, as a result of the field of view being filled with stationary environmental contrasts (reading in the car), a sensory mismatch occurs. With repeated stimulation, motion sickness is induced through summation; the repeated stimulation leads to a rearrangement of the stored pattern of expectation, however, so that a habituation to the initially challenging stimulation is attained within a few days. An acute unilateral labyrinthine loss causes vertigo, because the self-motion sensation induced by the vestibular tone imbalance is contradicted by vision and the somatosensors.

(Reprinted from Brandt T: Vertigo: Its Multisensory Syndromes. London: Springer, 2002, p 5, Fig. 1.2. Reprinted with kind permission of Springer Science and Business Media.)

AGING AND THE VESTIBULAR SYSTEM

Histopathological age-related changes in the human vestibular sensory organs include progressive hair cell degeneration, otoconial degeneration in the otolith organs, and decreasing numbers of vestibular nerve fibers,^11,12 and age-dependent changes in both caloric and rotational test responses have been demonstrated.^13,14 These changes alone are unlikely to generate vestibular symptoms, as they are symmetrical, and dizziness in elderly people is probably more multifactorial in origin.¹⁵ Thus, although older patients may be subject to the same common balance disorders as are younger patients, they have more problems with chronic disequilibrium and falls, and vertigo has been reported to rise with advancing age in parallel with the incidence of hearing loss.¹³ Correct diagnosis, prevention, and rehabilitation are particularly important in treating this group of patients.¹⁶

PERIPHERAL VESTIBULAR DISORDERS

In most cases, the characteristic symptoms and signs of vestibular deafferentation abate, and the patient is rendered asymptomatic over a period of 2 weeks to several months. In this regard, the vestibular system has been shown to be extremely adaptable.¹⁸ The processes, which bring about the resolution of vestibular symptoms, are collectively known as cerebral compensation and are attributed to cerebral plasticity (Fig. 27-4; Table 27-3).

Figure 27-4 Natural history of peripheral vestibular dysfunction and symptoms.

TABLE 27-3 Vestibular Compensation

The structures subserving compensation for vestibular dysfunction are unknown, but it has been shown that brainstem, cerebellar, and cortical structures are involved; the cerebellum is key to this recovery phenomenon,^17,19,20 in addition to the requirement for all sensory inputs, including vision, somatosensory afferents, and remaining labyrinthine input.^21–23 Furthermore, integrity of both the vestibular nerve²⁴ and the central vestibular connections²⁵ is required.

The physiological mechanisms on which compensation depends include physical activity^26,27 and vision²⁸ (Fig. 27-5). Moreover, Fetter and coworkers²⁹ demonstrated that occipital lobectomy before labyrinthectomy impaired compensational recovery, and Schaefer and Meyer in 1973³⁰ also demonstrated that transsection of the cervical cord that led to loss of proprioception delayed vestibular compensation.

Figure 27-5 Graph of clinical state of recovery in restrained versus unrestrained animals after unilateral vestibular neurotomy.

Chronic Unilateral Vestibular Deafferentation

This situation is seen clinically in the presence of a slowly growing tumor, such as a vestibular schwannoma. The patient may report merely some mild instability or no sense of imbalance, although a unilateral peripheral abnormality may be detected on formal vestibular function testing. This relative lack of symptoms may be explained on the basis of such slow loss that no significant acute vestibular asymmetry occurs. Alternatively, adaptation occurs simultaneously with the loss, so that symptoms do not manifest.

Bilateral Vestibular Hypofunction

If there is sequential loss of unilateral function with a period of 1 week or more between each event, then the clinical presentation is that of two episodes of acute unilateral loss, as outlined previously. If, however, both vestibular labyrinths are lost together, there may be no acute vertiginous symptoms, in the absence of any vestibular asymmetry. The long-term effects of bilateral vestibular failure are the same, irrespective of whether the loss has been simultaneous or sequential. The clinical features include the following:

Sense of imbalance when standing or walking, especially on uneven surfaces (e.g., sand) or in the absence of vision (e.g., at night).

Bobbing oscillopsia: that is, vertical bouncing or blurring of vision when the patient walks, runs, or moves, with degradation of visual acuity as a result of loss of the vestibulo-ocular reflex.

An inability to stand or walk when both vision and proprioception are removed (e.g., when standing on a foam pad with eyes closed or when attempting to walk across a foam pad with eyes closed).

In these cases, the cervico-ocular reflex has been implicated in recovery of function³¹; other authorities have suggested that slippage of the retinal image in bilateral vestibular failure may be compensated for by central visual mechanisms, as occurs in congenital nystagmus³² and oculomotor palsies.³³ In general, patients with bilateral vestibular function recover significantly, although a proportion remain handicapped by oscillopsia and instability.

Compensation/Decompensation

The majority of cases of a unilateral peripheral vestibular deficit recover by means of cerebral compensation. However, some patients do not recover spontaneously and require vestibular rehabilitation with physiotherapy. The basis of physical therapy intervention relies on a structured approach in promoting recovery with visual, proprioceptive, and vestibular stimulation by means of a standard or customized range of exercises. A number of factors that predispose to failure of compensation (Fig. 27-6) or decompensation from a previously recovered state (Fig. 27-7) have been identified. There is some evidence that there exists a critical period in which stimuli must be provided to the adaptive mechanisms and recalibration of the vestibular function must begin, or else the rate of recovery and perhaps the ultimate degree of recovery may decrease.³⁴ However, Shepard and colleagues³⁵ did not identify duration of symptoms, or age, as a negative prognostic factor of a vestibular rehabilitation program, although financial compensation, head injury, and severe postural control abnormalities have all been reported to indicate poor outcome.

Figure 27-6 Factors that predispose to failure of compensation. CNS, central nervous system.

Figure 27-7 Vestibular decompensation.

Failure of compensation may follow an acute episode of vertigo and vomiting leaving the patient constantly disorientated and disabled, so that he or she cannot function occupationally or socially in an effective manner. Decompensation is often seen in a patient who has had a single acute episode of vertigo and vomiting with recovery over a period of 6 weeks to 6 months and then subsequent relapses, episodes usually becoming progressively less severe and more infrequent with the passage of time. Notwithstanding this, for the nonexpert in the field, such a presentation may be misdiagnosed as a recurrent condition or a new illness, as opposed to a previous labyrinthine event with repeated decompensation.

DIAGNOSIS OF VESTIBULAR DISORDERS

The diagnosis of vertigo is critically dependent on a clear history that includes the following:

The exact character of the symptom (dizziness, vertigo, ataxia).

The duration of both the illness and individual episodes.

The presence of associated symptoms: cochlear, neurological, or general medical (Fig. 27-8).

Figure 27-8 Diagnosis of vertigo.

Vertigo of less than 1 minute’s duration is most commonly associated with benign paroxysmal positional vertigo (BPPV), whereas acute rotational vertigo of several hours’ duration is most commonly associated with migraine and Meniere’s disease. Vertigo lasting several days is common in viral vestibular neuritis and in ischemic and brainstem labyrinthitis. Pathology involving the labyrinth and cranial nerve VIII is commonly associated with hearing loss and/or tinnitus, whereas vertigo arising in the central vestibular pathways is most commonly associated with disordered eye movements. In order to make a correct neuro-otological diagnosis, a clinical examination of the vestibular and oculomotor systems is key and requires a clear understanding of vestibular and oculomotor pathology, together with regular clinical practice at examination.³⁶

COMMON PERIPHERAL VESTIBULAR DISORDERS