Isolated Internal Auditory Canal Diverticula: A Normal Anatomic Variant Not Associated with Sensorineural Hearing Loss

Discussion

Previously published histopathologic studies of IAC diverticula, alternatively known as IAC cupping, demonstrated that these are well-marginated concavities along the anteroinferior wall of the IAC fundus with normal bone around the periphery of the concavity.^1,2 The cases of isolated IAC diverticula in this study correlate well with the published histology, being characterized on CT by intact cortical, trabecular, and labyrinthine bone adjacent to the concavity and, in most cases with volumetric 3D T2-weighted imaging, by normal CSF extending into the diverticulum (Fig 1). Historically, IAC diverticula have been referred to as normal variants, with a warning not to confuse them with pathology.^1,2 Despite the evidence in the literature characterizing the benign nature of these diverticula, a recent publication has found an association between these lesions and SNHL.⁴ In the current study, we did not identify a statistically significant association with SNHL or word recognition score loss. These findings corroborate our anecdotal clinical observation and prior pathology-based studies that showed isolated IAC diverticula (ie, those not associated with cavitary otosclerosis) should be considered benign normal variants.

The etiology of IAC diverticula or cupping remains uncertain. Possible explanations include developmental variations in the interface between the 14 periotic enchondral growth plates and the adjacent sites of secondary bone formation of the surrounding skull base. Considering that the IAC extends across the transition zone between primary enchondral bone and secondary bone formation, variations in this transition may explain the consistent location of the diverticulum in the anteroinferior margin of the IAC fundus. Bast and Anson,⁵ in their seminal temporal bone textbook, The Temporal Bone and the Ear, claimed that residual cartilaginous tissue, such as that seen in fissula ante fenestram, can be found elsewhere within the otic capsule and may explain the distribution of otosclerosis involving the labyrinthine bone. If the anteroinferior margin of the IAC fundus represents one of these residual cartilaginous rests, then its resorption may result in the concavity observed in this and other studies. Persistence of the cartilaginous rest might also serve as a focus of pericochlear otosclerosis, contributing to the appearance of what has been reported in the literature as cavitary otosclerosis involving this segment of the IAC fundus.^6⇓–8 Otospongiotic bone superimposed on a pre-existing congenital diverticulum might also explain the difference in diverticular size between the otospongiotic and nonotospongiotic groups.

The observed association between IAC diverticula and SNHL reported by Pippin et al⁴ could have arisen due to the potentially confounding effects of the inherent selection bias present when retrospectively examining a population of temporal bone CTs because these examinations have a much higher probability of being performed in patients with hearing loss. This inherent selection bias is exemplified by 88% of our patients with unilateral or bilateral IAC diverticula having either clinical concerns for or a reported history of hearing impairment in 1 or both ears at the time of their temporal bone CT scans. Our study design attempts to address the inherent selection bias issue by examining the subset of patients with unilateral IAC diverticula and without otosclerosis and by using the contralateral ear as an internal control. Because cochlear otosclerosis is defined as an SNHL due to involvement of the cochlear capsule, eliminating cases with cochlear otosclerosis should eliminate this as a potential confounder that could otherwise artificially increase the prevalence of SNHL in the study population.⁸ Using these criteria, we found that there is no statistically significant correlation of IAC diverticula with SNHL or decline in word recognition scores. Diverticula were shown to be present more often and were larger in patients with concurrent otosclerosis, but in patients without otosclerosis, though the IAC diverticula varied in size, there did not appear to be a pattern of progression with age. This observation, in combination with the benign-appearing osseous margins, as demonstrated on temporal bone histology (Fig 2), further supports the notion that IAC diverticula are congenital rather than acquired and do not progress with time.

This normal variant can usually be distinguished from cavitary otosclerosis, which classically involves the anteroinferior wall of the IAC fundus and occurs in conjunction with pericochlear and fenestral otosclerosis (Fig 3).^6⇓–8 An anterior wall IAC concavity with density greater than that in the adjacent CSF and with ill-defined bony margins should raise concern for cavitary otosclerosis. Patients with cavitary otosclerosis will almost always have radiographic evidence of fenestral and/or retrofenestral otosclerosis in addition to audiometric evidence of a conductive or mixed hearing loss. Thus, finding a focal lucency in the anterior wall of the IAC should raise suspicion for cavitary otosclerosis if these findings are also present.

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Fig 3.

Right temporal bone axial (A and D), Pöschl (B and E), and coronal (C and F) views of an IAC diverticulum (upper row) versus cavitary otosclerosis (lower row). Note the well-demarcated margins of a lucent IAC diverticulum (white arrow) compared with the ill-defined spongiotic bone composing the otosclerotic focus along the anterior wall of the IAC (black arrow), with indistinct bony margins between this lesion and the basal turn of the cochlea (E and F). Also note the presence of fenestral otosclerosis (white arrowhead in D and F).

IAC diverticula are relatively common (approximately 5% prevalence), and cavitary otosclerosis involving the anteroinferior wall of the IAC fundus is comparatively quite rare.^4,6,8 Discerning the difference between the two is clearly of clinical importance. In profound SNHL secondary to pericochlear otosclerosis, the presence of cavitary otosclerosis may predispose to electrode array extrusion during cochlear implant insertion.⁷ The absence of concurrent otosclerosis, as was seen in most (54/82, 66%) of our patients, should be reassuring that such a finding does not represent the sequelae of disease. Furthermore, the presence of a well-marginated lucency in the anteroinferior wall of the IAC fundus in the absence of surrounding otospongiotic bone should heavily favor an isolated IAC diverticulum over cavitary otosclerosis. Correlation with clinical presentation, surgical history, and audiometric findings also remains important in assessing the potential importance of this finding. In the absence of other temporal bone derangement or findings to support an alternative diagnosis, an isolated well-demarcated lucency in the anteroinferior wall of the IAC fundus likely represents a normal variant and is unlikely to be of clinical importance. Conversely, attributing SNHL to the presence of these diverticula could lead a clinician to overlook other treatable causes of SNHL, which could, in turn, lead to permanent hearing loss.

Our study is limited by its retrospective approach, the lack of a true healthy control group, minor variations due to the use of CT scanners with different slice intervals despite a common manufacturer, and the use of a single neuroradiologist for interpretation. The population size in relation to the power of the study and the observed effect sizes raises the question of whether we would have been able to observe the difference asserted by an even smaller previous study.⁴ However, this remains the largest study of this radiographic entity to date, and the findings are in agreement with historical observations.^1,2 Furthermore, the observed differences in frequency-specific bone conduction thresholds (ie, SNHL) and word recognition between the involved ear and contralateral control ear were small (median difference ranged between 0 and 5 dB in bone conduction thresholds; median word recognition score difference, 0%), suggesting that even if differences were found to be statistically significant in a study with much larger patient numbers, this small difference would still not be considered clinically relevant (Table 2). We have attempted to address some of the inherent weaknesses of a retrospective analysis and lack of true healthy controls with our case selection and use of internal controls. Based on our limited number of cases in which volumetric CT and MR imaging were available for review, further research comparing high-resolution MR imaging with audiometric data would be useful in corroborating our findings. Future studies might also attempt to objectively demonstrate which features best distinguish these IAC diverticula from cavitary otosclerosis in a population of patients with otosclerosis.