Case Report: Could Hennebert's Sign Be Evoked Despite Global Vestibular Impairment on Video Head Impulse Test? Considerations Upon Pathomechanisms Underlying Pressure-Induced Nystagmus due to Labyrinthine Fistula

Abstract

We describe a case series of labyrinthine fistula, characterized by Hennebert's sign (HS) elicited by tragal compression despite global hypofunction of semicircular canals (SCs) on a video-head impulse test (vHIT), and review the relevant literature. All three patients presented with different amounts of cochleo-vestibular loss, consistent with labyrinthitis likely induced by labyrinthine fistula due to different temporal bone pathologies (squamous cell carcinoma involving the external auditory canal in one case and middle ear cholesteatoma in two cases). Despite global hypofunction on vHIT proving impaired function for each SC for high accelerations, all patients developed pressure-induced nystagmus, presumably through spared and/or recovered activity for low-velocity canal afferents. In particular, two patients with isolated horizontal SC fistula developed HS with ipsilesional horizontal nystagmus due to resulting excitatory ampullopetal endolymphatic flows within horizontal canals. Conversely, the last patient with bony erosion involving all SCs developed mainly torsional nystagmus directed contralaterally due to additional inhibitory ampullopetal flows within vertical canals. Moreover, despite impaired measurements on vHIT, we found simultaneous direction-changing positional nystagmus likely due to a buoyancy mechanism within the affected horizontal canal in a case and benign paroxysmal positional vertigo involving the dehiscent posterior canal in another case. Based on our findings, we might suggest a functional dissociation between high (impaired) and low (spared/recovered) accelerations for SCs. Therefore, it could be hypothesized that HS in labyrinthine fistula might be due to the activation of regular ampullary fibers encoding low-velocity inputs, as pressure-induced nystagmus is perfectly aligned with the planes of dehiscent SCs in accordance with Ewald's laws, despite global vestibular impairment on vHIT. Moreover, we showed how pressure-induced nystagmus could present in a rare case of labyrinthine fistulas involving all canals simultaneously. Nevertheless, definite conclusions on the genesis of pressure-induced nystagmus in our patients are prevented due to the lack of objective measurements of both low-acceleration canal responses and otolith function.

Possible pathomechanisms accounting for both sound/pressure-induced eye movements and video head impulse test data in superior canal dehiscence

“…On the contrary, the same condition could allow low-acceleration endolymphatic flows, such as sound pressure waves dissipating in the vestibular partition (accounting for low-frequency conductive hearing loss) and sound-induced endolymphatic movements (resulting in Tullio phenomenon). A similar functional dissociation pattern between high (impaired) and low-frequencies (spared) canal VOR has been implied in other conditions accounting for a third window mechanism, such as labyrinthine fistula [14]. In fact, it has been observed how pressure-induced nystagmus (comparable to sound-evoked eye movements) aligning with the plane of the eroded canals could be evoked despite a global vestibular impairment on vHIT, suggesting a possible residual activity for canal afferents encoding low-velocity inputs [14].…”

“…A similar functional dissociation pattern between high (impaired) and low-frequencies (spared) canal VOR has been implied in other conditions accounting for a third window mechanism, such as labyrinthine fistula [14]. In fact, it has been observed how pressure-induced nystagmus (comparable to sound-evoked eye movements) aligning with the plane of the eroded canals could be evoked despite a global vestibular impairment on vHIT, suggesting a possible residual activity for canal afferents encoding low-velocity inputs [14]. The hypothesis that a loss of sensitivity for high-acceleration head movements does not prevent the occurrence of vestibular signs driven by low-frequency canal afferents is also supported by experimental studies on animal models of SCD.…”

Vestibular assessment in sudden sensorineural hearing loss: Role in the prediction of hearing outcome and in the early detection of vascular and hydropic pathomechanisms

“…In particular, paroxysmal PN despite SC impairment on vHIT might imply a selective damage for type I hair-cells and irregular canal afferents (measured by vHIT) sparing the activity of type II hair-cells and regular fibers encoding cupular displacements which generates nystagmus ( 103 – 105 ). On the other hand, it may be assumed a different recovery time for damaged end-organs/afferents following acute labyrinthine injury, resulting in faster restoration for low-frequency responses compared to higher-frequency VOR ( 106 , 107 ). Nevertheless, a possible role of the residual function of irregular afferents in the genesis of paroxysmal PN could not be excluded, as these patients did not develop a complete PSC loss on vHIT.…”

Posterior Semicircular Canal Dehiscence with Vestibulo-Ocular Reflex Reduction for the Affected Canal at the Video-Head Impulse Test: Considerations to Pathomechanisms

“…A high-resolution CT (HRCT) scan of temporal bones with parasagittal reconstructed images along the PSC plane (Stenver plane) represents the gold standard for diagnosis [2,24]. Conversely, horizontal SCD (HSCD) is generally caused by an erosive process of the middle ear, which usually masks symptoms and signs due to a TMWM [4,28,29]. Nevertheless, HSCD has been anecdotally detected in cases of normal middle ear status [30,31].…”