In this cohort study of 402 SCD repairs via the MCF approach, we found longer AE-OT to associate with poorer symptomatic response among frank dehiscence cases despite similar operative duration and audiometric improvement. Namely, patients with longer AE-OT reported lower resolution rates of hearing loss and higher postoperative onset rates of hyperacusis, tinnitus, ear fullness, and headache.

A 2007 anatomic study (Djalilian et al.) on the middle cranial fossa of 98 non-SCDS temporal bones found the average distance between SSC dome and temporal bone outer table (SSC-OT) to be 2.1 cm [14]. More recently, a 2014 radiographic review of 202 SCDS patients (Lookabaugh et al.) found the mean distance of AE-OT to be 2.6 cm [15]. We add to the literature with the largest cohort, demonstrating a mean AE-OT of 2.7 cm among 402 SCD cases, which is similar to findings from Lookabaugh et al. Although AE is not always near the SSC dome, Lookabaugh et al. approximates the location of AE near the SSC dome.[14, 15]. Therefore, the measurements of SSC-OT by Djalilian et al. and the approximate measurements of AE-OT by Lookabaugh et al. should be comparable. However, a noticeable discrepancy emerges: the mean SSC-OT among non-SCDS cases in Djalilian et al. is markedly lower than the mean AE-OT among SCDS patients in both Lookabaugh et al. and the present study. As such, the AE-OT of SCDS patients may be longer compared to that of non-SCDS patients, although direct comparative studies are needed to ascertain this relationship.

Poorer temporal bone pneumatization is closely associated with the pathogenesis of SCDS [20, 21]. Given the observation of longer AE-OT among SCD cases, we surmise that AE-OT may implicate structural and congenital factors related to temporal bone pneumatization. We speculate that better-pneumatized mastoids may be associated with shorter AE-OT due to a more vertical and less horizontal, medially sloping orientation [22, 23]. However, this theory is mostly conjectural as very limited evidence on the clinical implications of AE-OT exists in the literature. To date, poorer temporal bone pneumatization has only been correlated with shorter mastoid height and shorter distance of jugular bulb and sigmoid sinus to internal acoustic canal, external acoustic canal, and the middle ear [24, 25].

The present study is the first to investigate the influence of AE-OT on surgical outcomes following the MCF approach. Our analyses revealed that longer AE-OT was associated with poorer symptom outcomes despite similar audiometric improvement among frank dehiscences. Specifically, the association of longer AE-OT with lower rates of OSI and lower SRS was primarily driven by lower resolution rates of hearing loss and higher postoperative onset rates of hyperacusis, amplification of internal sounds, tinnitus, ear fullness, and headache. Since the poorer symptomatic response was primarily due to the postoperative onset of symptoms rather than the lack of resolution of preoperative symptoms, we do not believe the underlying mechanism is related to inadequate repair of SCD. Furthermore, the LF-ABG effectively narrowed, and the classic auditory symptoms of third window physiology resolved at high rates regardless of AE-OT. Therefore, we surmise that the underlying mechanism may be more related to patients’ inherent anatomy, physiology, and embryology.

Important confounders that may influence AE-OT include head circumference and intracranial hypertension [26]. Patient sex may also be an important factor because males are known to have higher head circumference than females [27]. However, our analyses found the association between AE-OT and symptom outcomes to persist even after controlling for patient sex and BMI, a marker of both head circumference and intracranial hypertension. Therefore, we believe the association may be better explained by the structure and developmental intricacies of the temporal bone.

If longer AE-OT is indeed associated with poorer temporal bone pneumatization as we previously hypothesized, then cases with longer AE-OT would be expected to report less favorable symptom outcomes. Previous research has demonstrated the protective and “shock-absorbing” effect of pneumatization against trauma to the temporal bone. Specifically, studies have highlighted the association of greater pneumatization with reduced incidence of otic-capsule violation and less severe hearing impairment from temporal bone fractures [28, 29]. Increased pneumatization of the temporal bone was also shown to yield better hearing outcomes from pediatric tympanoplasty [30]. Therefore, patients with less adequately pneumatized temporal bones may be expected to be less resistant to hearing loss following the MCF approach, which may explain the lower resolution rate of hearing loss among cases with longer AE-OT.

Furthermore, the relationship between AE-OT and functional outcomes may be influenced by side effects related to temporal bone elevation. AE-OT might affect the extent to which the temporal lobe needs to be elevated, with longer AE-OT potentially necessitating a greater degree of elevation and consequently a higher risk of central nervous system side effects. This increased risk may lead to more pronounced auditory symptoms, as highlighted in this study.

Our series included very few patients with postoperative onset hyperacusis/amplification (n < 10), and these patients had longer AE-OTs (mean 2.9 cm) compared to patients without postoperative onset hyperacusis/amplification. Given the association between longer AE-OT and SCDS, the temporal bone of patients with longer AE-OT may also be less dense and insulating against acoustic energy. As such, they would be at higher risk for hyperacusis, especially after a traumatic event or an invasive procedure such as the MCF. Temporal bones that are less dense may also require a longer course of recovery following craniotomy, which may be responsible for the prolonged onset of non-specific symptoms following repair (tinnitus, fullness, and headache) among patients with longer AE-OTs. It is also possible that frank dehiscences in patients with longer AE-OT are associated with higher rates of near dehiscence in the neighboring regions of the otic capsule, hence the postoperative onset hyperacusis [31]. Ultimately, additional research on the association of AE-OT with the density and pneumatization of temporal bone is warranted. Nevertheless, surgeons should be aware of the implications of AE-OT and patients should be counseled accordingly.

Since AE-OT is neither randomizable nor manipulatable, a cohort study design would be the highest level of evidence possible, and causal inference is thus not permitted. Additionally, VEMP testing was not routinely obtained post-operatively, preventing its inclusion as a reliable measure of objective vestibular outcomes. Further investigation into the relationship between AE-OT and objective vestibular testing (VOR/vHIT, VEMP), both pre- and post-operatively, is needed. Therefore, ongoing research is needed to further characterize the mechanism through which AE-OT influences patient outcomes. Despite these limitations, we present the first analysis of the relationship between AE-OT and functional outcomes employing the largest cohort of SCD surgeries to date. Findings from this study are worthy of deliberation among the neurotology community as part of the ongoing efforts to improve the clinical outcomes of SCDS patients.