Discussion

Semicircular canal occlusion was initially described for the control of intractable BPPV of the posterior semicircular canal, and has subsequently been shown to cause minimal labyrinthine trauma while preserving hearing thresholds.^{Reference Kisilevsky, Bailie, Dutt and Rutka3}

As part of their original series in 1998, Minor et al. described superior semicircular canal dehiscence syndrome, introducing a new inner-ear disorder that could be managed surgically.^{Reference Minor, Solomon, Zinreich and Zee4} Their approach initially involved resurfacing, which has evolved also to include plugging of the superior canal dehiscence via a middle fossa approach.^{Reference Ward, Carey and Minor14} The transmastoid approach soon followed, demonstrating equivalent results, but without the complications of craniotomy and temporal lobe retraction.^{Reference Agrawal and Parnes15}

The surgical approach is generally determined by surgeon experience and patient anatomy (i.e. some surgeons favour the middle fossa approach in cases of a sclerotic mastoid, low-hanging tegmen or skull base defects requiring simultaneous reconstruction).^{Reference Ward, Carey and Minor14,Reference Agrawal and Parnes15} A recent systematic review on the matter reported that patients undergoing a middle fossa approach had more complicated post-operative courses (developing more serious complications, including sensorineural hearing loss and facial paralysis), required more revision surgery and had a longer post-operative course in hospital.^{Reference Vlastarakos, Proikas, Tavoulari, Kikidis, Maragoudakis and Nikolopoulos16}

The theoretical advantage of resurfacing over plugging is that the dehiscence is protected from further erosion, preventing pulsatile transmissions from the brain and dura into the inner ear.^{Reference Vlastarakos, Proikas, Tavoulari, Kikidis, Maragoudakis and Nikolopoulos16} Resurfacing also has the advantage of allowing the physiological function of the canal to be maintained.^{Reference MacDougall, Weber, McGarvie, Halmagyi and Curthoys13} Despite these theoretical advantages, a meta-analysis comparing surgical options in superior semicircular canal dehiscence syndrome indicated better subjective patient outcomes with plugging than with capping or resurfacing.^{Reference Ziylan, Kinaci, Beynon and Kunst17}

Although there are multiple reports in the literature commenting on the subjective improvement in symptoms and hearing outcomes following semicircular plugging in superior semicircular canal dehiscence syndrome, few discuss objective vestibular function post-operatively.^{Reference Ishai, Tsang and Rutka12}

The value of electronystagmography in the assessment of post-operative vestibular function is subject to debate in posterior semicircular canal occlusion (minor changes in excitability difference between sides are more likely related to anatomical changes of the mastoid bone from surgery affecting thermal conductivity). In addition, it has limited utility in the pre- and post-operative assessment of patients with superior semicircular canal dehiscence syndrome.^{Reference Kisilevsky, Bailie, Dutt and Rutka3}

Cervical vestibular-evoked myogenic potential thresholds and amplitude responses have been previously used as part of the diagnostic investigation of superior semicircular canal dehiscence syndrome (relative lower threshold and higher amplitude responses being noted). Post-operative studies have demonstrated that the surgical treatment of superior semicircular canal dehiscence by either plugging or resurfacing could raise or normalise cervical vestibular-evoked myogenic potential thresholds.^{Reference Fiorino, Barbieri, Pizzini and Beltramello18}

Only one study has previously evaluated video head impulse testing as an objective measure of vestibular function post-operatively in patients with superior semicircular canal dehiscence syndrome following canal surgery.^{Reference Rodgers, Lin and Staeker19} In this study, the transmastoid resurfacing approach was compared with a middle fossa plugging approach. Post-operative video head impulse test gains were higher in the resurfacing group, as might be expected.

Video head impulse testing can be used to confirm change in dynamic function vestibulo-ocular reflex gain in an occluded canal. In our experience, despite these physiological changes, patients subjectively seem to demonstrate little in the way of symptoms (i.e. oscillopsia), even when bilateral occlusions are performed (i.e. for recalcitrant bilateral BPPV or bilateral symptomatic superior semicircular canal dehiscence syndrome).^{Reference Ishai, Tsang and Rutka12,Reference Carr and Rutka20} One suspects this is primarily the result of an element of central compensation, as well as the inherent redundancy in vertical canal function where unilateral occlusion has been performed, or the relative limited dynamic excursion comparatively seen in vertical versus horizontal head movements in bilateral canal occlusions.

We propose that the assessment of individual semicircular canal function using high velocity head impulse testing can reliably confirm the physiological effects of canal occlusion, even in the immediate (one-week) post-operative period,^{Reference Carr and Rutka20} and is a useful adjunct to evaluate vestibular function after canal occlusion.