Patients with peripheral vestibular nystagmus typically present with a sudden, sometimes dramatic, onset of dysequilibrium with vertigo, nausea, and vomiting (Table 9-1). Patients often recognize that their symptoms are worsened by particular head movements or postures. Oscillopsia, tinnitus, and hearing loss may also occur. After the acute phase of peripheral vestibular loss, which typically lasts days, patients experience a slow period (weeks to months) of gradually waning symptoms. Even patients who become asymptomatic may experience discomfort months to years later, when their vestibular system is challenged, as when riding in a fast-moving car or boat.
Peripheral vestibular nystagmus occurs in patients with dysfunction of the end organ (semicircular canals, otolithic structures, vestibular nerve). End-organ damage, which is usually unilateral or at least asymmetric (except in cases of toxicity), disrupts the otherwise symmetric vestibular afferent inputs to the neural integrator, which stabilizes eye position in eccentric locations. The output of the neural integrator is routed to the contralateral paramedian pontine reticular formation. This loss of tonic symmetry produces a directional bias in eye position. A reduction in input from a left-sided vestibular lesion, for instance, produces a leftward bias, which then induces a corrective saccade away from the side of the lesion. Thus, a left-sided lesion would produce leftward slow phases and right jerk nystagmus.
Peripheral vestibular nystagmus related to vestibular neuropathy typically disrupts output from all 3 semicircular canals and the otolithic organs, producing a mixed horizontal-torsional pattern of nystagmus that changes depending on the direction of gaze. This follows Alexander’s law: The nystagmus is more pronounced when gaze is directed toward the side of the fast-beating component. Depending on the severity of the lesion, the nystagmus may be evident in primary position. A skew deviation may also be present due to disruption of peripheral vestibular structures. Nystagmus that is purely vertical or torsional almost always signifies a central lesion (see the following section).
A characteristic feature of peripheral vestibular nystagmus is the ability of visual fixation to dampen the nystagmus. The effect of visual fixation on nystagmus can be evaluated during direct ophthalmoscopy by temporarily covering the contralateral fixing eye. Other methods for enhancing vestibular nystagmus include vigorous head shaking, hyperventilation, mastoid vibration, and the Valsalva maneuver.
Peripheral vestibular dysfunction, often accompanied by nystagmus, usually occurs in 1 of 4 clinical settings. The first is an acute, monophasic disorder that occurs secondary to a (presumed viral) vestibular neuronitis. The second is a recurrent form of vestibular dysfunction that is usually associated with auditory symptoms (tinnitus and hearing loss). This disorder, exemplified by Ménière disease, is usually progressive, although typically there are long symptom-free intervals. The third clinical setting is a paroxysmal dysfunction of the vestibular system that produces vertigo in response to certain postures of the head. This disorder, known as benign, paroxysmal, positional vertigo (BPPV), develops because of free movement of otoconia particles (calcium carbonate crystals normally contained within the utricle and saccule), which act as foreign debris within a semicircular canal. The Dix-Hallpike maneuver, during which the patient’s head is turned 45° to the right or left and lowered below the horizontal plane of an examining table to induce symptoms, can be used to diagnose which side and semicircular canal are dysfunctional. Once that is determined, repositioning treatments such as the Epley maneuver can remove the otoconia from that semicircular canal. The Epley procedure for the right ear, for example, begins in the right Dix-Hallpike position, with the head 45° to the right and below the horizon; the head is then turned slowly (over minutes) 180° to the patient’s left. These patients often enjoy a remission after a bout of BPPV, but it is not uncommon for patients to be intermittently plagued by this disorder. A fourth clinical setting for the occurrence of peripheral vestibular dysfunction is a toxic etiology, primarily the use of aminoglycosides (but also other medications such as chemotherapeutics). Systemic ototoxins typically produce head movement–related oscillopsia and decreased vestibular ocular reflex (VOR) gain bilaterally with little or no nystagmus (vestibular hypofunction without asymmetry).
Some patients with cerebellopontine angle tumors (usually acoustic neurinoma or meningioma) may experience Bruns nystagmus, which is a combination of gaze-evoked and peripheral vestibular nystagmus. Initially, as the vestibular nerve is affected, the eyes drift toward the side of the lesion, with a corrective fast phase in the opposite direction. As the lesion enlarges, the ipsilateral brainstem is compressed, causing problems in maintaining ipsilateral eccentric gaze; thus, as the patient looks to the side of the lesion, large-amplitude, lower-frequency gaze-evoked nystagmus is noted, whereas in contralateral gaze, small-amplitude, high-frequency vestibular nystagmus is seen.
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