Peripheral Vestibular Nystagmus
Patients with peripheral vestibular nystagmus typically present with a sudden, sometimes dramatic, onset of disequilibrium 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 (lasting 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, for example when riding in a fast-moving car or boat.
Peripheral vestibular nystagmus results from dysfunction of the end organs (eg, semicircular canals, otolithic structures, or vestibular nerve). End-organ damage is usually unilateral or asymmetric (except in cases of toxicity) and disrupts the normally symmetric vestibular afferent input to the ocular motor nuclei. Asymmetric afferent input from the semicircular canals generates vestibular nystagmus. The characteristics of peripheral vestibular nystagmus reflect the combined effects of decreased afferent input from all 3 semicircular canals.
Table 9-1 Clinical Characteristics of Peripheral Vestibular Nystagmus
Input from the horizontal semicircular canal is routed to the contralateral abducens nucleus (see Chapters 1 and 8). Therefore, stimulating the horizontal canal generates contralateral horizontal gaze. Conversely, a lesion of the horizontal canal results in ipsilateral gaze. Therefore, a lesion of the horizontal canal results in a tonic gaze deviation toward the side of the lesion, followed by a corrective saccade away from the side of the lesion (ie, nystagmus beats away from the lesion). For example, a left-sided vestibular lesion reduces input from the left horizontal semicircular canal, resulting in a leftward tonic gaze deviation, followed by a corrective saccade away from the side of the lesion. Thus, a left-sided lesion would produce leftward slow phases and right jerk nystagmus.
The anterior semicircular canals generate upgaze, and the posterior semicircular canals generate downgaze. Therefore, a unilateral vestibular lesion that affects the anterior and posterior canals on one side has no net effect on vertical gaze. However, the anterior and posterior canals generate contralateral torsion. Therefore, a unilateral vestibular lesion causes tonic ipsilateral torsion, followed by a corrective torsional saccade away from the side of the lesion.
Combining the effects of decreased afferent input from all 3 semicircular canals on 1 side, a unilateral peripheral vestibular lesion generates a combined horizontal–torsional nystagmus with jerk phases directed away from the side of the lesion. The nystagmus follows Alexander’s law, which states that 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. Asymmetric otolithic input may result in a skew deviation or an ocular tilt reaction (see Chapter 8).
A characteristic feature of peripheral vestibular nystagmus is the ability of visual fixation to damp the nystagmus. The clinician can evaluate the effect of visual fixation on nystagmus during direct ophthalmoscopy by temporarily covering the contralateral fixating 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 patterns:
Acute monophasic disorder secondary to a (presumed viral) vestibular neuronitis.
Recurrent vestibular dysfunction, usually associated with auditory symptoms (eg, tinnitus and hearing loss). This disorder, exemplified by Ménière disease, is usually progressive, although there may be long symptom-free intervals.
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 determine on which side the semicircular canals are dysfunctional. Once the side is identified, repositioning treatments such as the Epley maneuver can remove the otoconia from that semicircular canal and provide lasting remission, although recurrence is not unusual.
Toxic vestibular dysfunction, primarily caused by 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 (ie, vestibular hypofunction without asymmetry).
A very large cerebellopontine angle tumor (eg, vestibular schwannoma or meningioma) may produce Bruns nystagmus, which is a combination of gaze-evoked and peripheral vestibular nystagmus. Initially, because 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, a large-amplitude, low-frequency, ipsilesional gaze-evoked nystagmus is noted, whereas in contralateral gaze (away from the side of the lesion), a small-amplitude, high-frequency, contralesional vestibular nystagmus is observed.
Baloh RW. Clinical practice. Vestibular neuritis. N Engl J Med. 2003;348(11):1027–1032.
Fife TD, Tusa RJ, Furman JM, et al. Assessment: vestibular testing techniques in adults and children: report of the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology. Neurology. 2000;55(10):1431–1441.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.