Cupulolithiasis as an Alternative Mechanism for Pseudo-spontaneous Nystagmus in Horizontal Canal Benign Paroxysmal Positional Vertigo.

Benign paroxysmal positional vertigo of the horizontal semicircular canal may present a differential diagnostic challenge. In addition to the classical positional nystagmus, a persistent nystagmus in a seated position occasionally occurs, so-called pseudo-spontaneous nystagmus (PSN), which can be mistaken for a central or peripheral spontaneous nystagmus. We report a case with cupulolithiasis of the horizontal semicircular canal presenting with horizontal PSN in a sitting position, with implications for a new pathomechanism of PSN.

Case Report

A 61-year-old patient presented to the emergency department with acute-onset vertigo accompanied by nausea, vomiting, and unsteadiness of gait since the night before. There was no hypoacusis or tinnitus. The patient reported no preexisting neurologic, internal, or otologic diseases and had no long-term medication.

The clinical examination revealed a left-beating horizontal spontaneous nystagmus with small amplitude. The amplitude increased upon left gaze and decreased upon right gaze. The nystagmus was unaffected by head movement in the pitch plane. The clinical head impulse test showed normal vestibulo-ocular reflex function bilaterally. An ICS Impulse System (GN Otometrics) was used to assess semicircular canal function. Video head impulse test results on the day of symptom onset were normal on both sides for all canals. Eye movement recording and vestibular testing were performed with infrared video-oculography (VOG) and a computerized rotational chair system (Micromedical Technologies). VOG confirmed the spontaneous nystagmus to the left with low amplitude and a slow-phase velocity of 2 deg/s ( ). The nystagmus was completely suppressed during fixation, and there was no gaze-evoked nystagmus. Horizontal and vertical saccades and smooth pursuit were unremarkable. The vestibulo-ocular reflex was symmetric during sinusoidal whole-body rotation (0.32 Hz; asymmetry, 2% counterclockwise; phase lag, 4°) with normal gain (0.87). Fixation suppression of the vestibulo-ocular reflex during sinusoidal rotation (0.04 Hz) was also within the normal range (gain, 0.09). Cranial 3-T magnetic resonance imaging and high-resolution computed tomography of the temporal bones were unremarkable.

Figure 1.

(A) Video-oculography at symptom onset showed a left-beating horizontal spontaneous nystagmus (slow-phase velocity, 2 deg/s), which was suppressed by fixation. (B) Follow-up video-oculography, after successful repositional maneuvers, documented absent spontaneous nystagmus.

During the Dix-Hallpike maneuver, there was no positional nystagmus. During the supine roll maneuver, however, a bilateral apogeotropic horizontal nystagmus could be provoked, weaker on the right side. The diagnosis of benign paroxysmal positional vertigo with pseudo-spontaneous nystagmus (PSN) due to cupulolithiasis of the right horizontal semicircular canal was made and successfully treated with the Gufoni maneuver. On the day of discharge, there was no positional nystagmus; only a subtle spontaneous nystagmus to the left was still present.

At the follow-up examination 2 weeks after onset, the patient presented completely symptom-free. Results of the clinical neurologic examination were normal, and VOG and rotational test outcomes were unremarkable without signs of spontaneous nystagmus ( ). Written informed consent was obtained on the day of first admission, and the Ethics Committee of the Medical University of Vienna confirmed that no formal vote was required.

Discussion

Two pathomechanisms are hypothesized for the development of PSN in benign paroxysmal positional vertigo. First, PSN is caused by the physiologic upward tilt (30°) of the horizontal semicircular canal in the pitch plane so that gravity and endolymph viscosity cause loose otoconia to move in the horizontal canal. Tilting the head forward 30° changes the vector of gravity, thus canceling the PSN.[1,2] However, otoconial debris moving in the canal can cause only transient positional nystagmus.

Second, in cases in which the 30° head tilt maneuver is unsuccessful, an alternative pathophysiologic mechanism has been proposed: the so-called canalith jam, in which a block of the ampullary junction causes a continuous pressure onto the cupula. However, all previous cases of PSN attributed to the canalith-jam theory showed ipsilateral canal paresis and the nystagmus could not be modified by head position change. The presence of a persistent rather than transient nystagmus, the lack of modulation of the PSN by head movements in the pitch plane, and the absence of canal paresis suggest a different etiology of the PSN in our case.

Conclusion

The persistent PSN in our patient could be explained by the attachment of otoconia to the inferior part of horizontal cupula, at a location where it could cause gravity-dependent hair cell deviation in a sitting position ( ). Cupulolithiasis has already been observed in association with PSN and is a well-established mechanism of apogeotropic positional nystagmus in horizontal benign paroxysmal positional vertigo. We suggest cupulolithiasis as a plausible explanation for PSN in our case. We performed a 3-dimensional reconstruction of a human horizontal semicircular canal, demonstrating that the sensory epithelium of the lateral canal crista extends onto the lower lateral surface of the crista. Additionally, to an eventual inclination of the horizontal semicircular canal in the roll plane, a cupulolithiasis in this inferior cupular region might cause a continuous gravity-induced deflection of the gelatinous cupula, which in turn should induce a low-amplitude persistent PSN. Head tilting in the roll plane might therefore cancel the PSN, which we could not verify after establishing this hypothesis, since the patient was already symptom-free at the follow-up examination.

Figure 2.

(A) Three-dimensional reconstruction of the utricle and horizontal canal crista with hypothetical otoconial debris causing pseudo-spontaneous nystagmus with a contralateral fast phase. (B) Histologic section of the horizontal cupula. Red arrow, gravitoinertial vector. semicircular canal (SCC).

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Supplemental material, sj-pdf-1-opn-10.1177_2473974X221089847 for Cupulolithiasis as an Alternative Mechanism for Pseudo-spontaneous Nystagmus in Horizontal Canal Benign Paroxysmal Positional Vertigo by Felix K. Schwarz, Erich Vyskocil, Bela Büki and Gerald Wiest in OTO Open: The Official Open Access Journal of the American Academy of Otolaryngology-Head and Neck Surgery Foundation