PP1503 - Isolated Bilateral Posterior Canal Dysfunction: A New Clinical Entity?

Abstract:
Isolated bilateral posterior semicircular canal dysfunction was observed in three older female patients who presented with dizziness, imbalance, and oscillopsia. Additional findings included subjective and/or objective high-frequency hearing loss and down-beating nystagmus with fixation removed (2/3 patients). Other portions of the vestibular examination were unremarkable. This clinical pattern may be due to bilateral peripheral vestibular dysfunction or a vestibular-cerebellar lesion disrupting bilateral posterior canal pathways. Video head impulse testing (vHIT) may provide diagnostically revealing information in the presence of down-beating nystagmus and or elderly patients with dizziness and related symptoms.

Summary:

Introduction:
The video head impulse test (vHIT) can effectively measure angular vestibular-ocular reflex function of all six semicircular canals. The authors have noticed a clinical pattern of isolated bilateral posterior semicircular canal (P-SCC) dysfunction, with the horizontal and superior canals spared. Three clinical cases of patients with bilateral posterior canal dysfunction will be presented. Two clinical questions will be answered with a review of the medical literature:
(1) What other clinical factors (e.g., age and hearing loss) are associated with isolated abnormal bilateral P-SCC?
(2) Is this clinical entity consistent with central or peripheral vestibular etiology?

Case Presentations:
Three female patients, ages 63, 65, and 74, presented to our clinic with symptoms of continuous dizziness, gait instability, and intermittent oscillopsia. Associated otologic symptoms included subjective high-frequency hearing loss. VHIT revealed isolated P-SCC dysfunction with corrective saccades and reduced VOR gain, with normal function of bilateral superior and horizontal canals. Additionally, two patients demonstrated down-beating nystagmus (with fixation removed) throughout eccentric gaze or positional testing. Other portions of videonystagmography (VNG) testing were normal. Two out of three patients had unremarkable recent magnetic resonance imaging (MRI) with and without contrast; one patient did not have available neuroimaging.

Results

What are other clinical factors (e.g., age and hearing loss) associated with isolated abnormal P-SCC?
The research of Jimenez and Fernandez (2014) and Lerchundi et al. (2020) revealed a correlation between bilateral and symmetrically reduced P-SCC vHIT gain with increased age (mean age: 73 years old; prevalence: 71-90 years old). While unilateral P-SCC dysfunction often occurs with idiopathic sudden sensorineural hearing loss, the pattern of bilateral P-SCC may be linked to presbycusis (Lerchundi et al., 2019; Li et al., 2020; Pogson et al., 2016).

Is this clinical entity consistent with central or peripheral vestibular etiology?
The correlation of bilateral P-SCC dysfunction to presbycusis may suggest an age-related cochlear and degenerative vestibular process. This may be due to the proximity of the cochlear and inferior vestibular nerve. Additionally, the presence of down-beating nystagmus can occur due to the tonic imbalance of vestibular input from bilateral P-SCCs at the level of the peripheral system (Lerchundi et al., 2020). However, the presence of down-beating nystagmus may also indicate a central site of lesion due to suspected disruption of P-SCCs projections to the brainstem tegmentum (Wong, 2007). In the setting of bilateral vestibular involvement without revealing neuroimaging, a central etiology cannot be ruled out at this time. It is noteworthy that the presentation of down-beating nystagmus only with fixation removed adds more support for the tonic imbalance of peripheral vestibular input of the P-SCCs.

Discussion/

Conclusions:
Although current medical literature links this unique clinical presentation to increased age, presbycusis, and down-beating nystagmus, the mechanism is poorly understood. Additional research is necessary to understand the pathogenesis and site of lesion. Given the current evidence and the cases presented, it may be prudent to perform vHIT (all semicircular canals) on older patients with symptoms of imbalance and oscillopsia, high-frequency sensorineural hearing loss, or when down-beating nystagmus is observed.