Discussion

The cochlear nerve canal is also called the cochlear aperture, bony cochlear nerve canal and cochlear fossette. There is also some variation across the literature in what is considered a normal versus narrow diameter of the cochlear nerve canal. Despite the variation, the literature consistently reports that patients with SNHL are more likely to have narrower cochlear nerve canals than patients with normal hearing, and a significantly narrowed cochlear nerve canal is often associated with cochlear nerve hypoplasia or aplasia.^{Reference Adunka, Jewells and Buchman4,Reference Miyasaka, Nosaka, Morimoto, Taiji and Masaki5,Reference Kono10} Wilkins et al. also found that there is a statistically significant relationship between the degree of hearing loss and the degree of stenosis. In addition, more severe hearing loss is also correlated with cochlear nerve canal stenosis plus additional inner ear anomalies, and these structural abnormalities may occur more often than previously appreciated.^{Reference Wilkins, Prabhu, Huang, Ogando and Kenna3}

The most common malformation of the semicircular canal bony labyrinth is a lateral semicircular canal anomaly, which was also seen in our study.^{Reference Satar, Mukherji and Telian6,Reference Breheret, Brecheteau, Tanguy and Laccourreye11} As reported in previous literature, complete semicircular canal aplasia is very rare, and we only depicted it in 4 out of 265 ears (1.5 per cent) in our study.^{Reference Satar, Mukherji and Telian6} Complete semicircular canal aplasia has been universally associated with ‘CHARGE’ syndrome^{Reference Morimoto, Wiggins, Hudgins, Hedlund, Hamilton and Mukherji12} as demonstrated in one of our patients and has been reported to occur in Goldenhar syndrome,^{Reference Wineland, Menezes, Shimony, Shinawi, Hullar and Hirose13} Noonan syndrome^{Reference Satar, Mukherji and Telian6} and persistent petrosquamosal sinus,^{Reference Goetz14} but few case studies have reported it to occur in isolation, without an associated syndrome or other inner ear anomaly. Breheret et al. reported one such case of isolated bilateral complete semicircular canal aplasia in a non-syndromic adult patient presenting with unilateral hearing loss.^{Reference Breheret, Brecheteau, Tanguy and Laccourreye11}

This study found that there was significant correlation between cochlear nerve canal stenosis and semicircular canal abnormalities (p = 0.00), and that the most frequent semicircular canal abnormality associated with cochlear nerve canal stenosis was lateral semicircular canal dilatation. The correlation between cochlear nerve canal stenosis and cochlear anomalies was less statistically significant. Previous literature provides conflicting results with regard to this.^{Reference Yi, Lim, Kang, Park, Park and Lee15–Reference Yu, Mukherji, Carrasco, Pillsbury and Shores17} Yi et al.^{Reference Yi, Lim, Kang, Park, Park and Lee15} found that the most common associated inner ear anomaly in cochlear nerve canal stenosis was internal auditory canal stenosis whereas Wilkins et al.^{Reference Wilkins, Prabhu, Huang, Ogando and Kenna3} reported cochlear anomalies, in particular hypoplasia of the modiolus and cochlear turn abnormalities, as the most common associated anomalies. In this study, although we found that the same percentage of patients (26 per cent) had semicircular canal or cochlear anomalies in the setting of cochlear nerve canal stenosis, the non-significant correlation may be because more cases of cochlear anomalies were seen without cochlear nerve canal stenosis (31 ears versus 17 ears with cochlear nerve canal stenosis). Similarly, when semicircular canal abnormalities and cochlear anomalies occur together, there was no significant correlation between cochlear nerve canal stenosis and these conditions (p = 0.06).

Most of the patients with semicircular canal anomalies and cochlear nerve canal stenosis presented with SNHL without any other otology symptoms such as imbalance or otalgia. Previous studies also showed similar clinical findings, and genetic and molecular testing has been advocated in the literature because semicircular canal anomalies are currently known to be associated with certain genetic abnormalities such as ‘CHARGE’ and Goldenhar syndrome, among others.^{Reference Breheret, Brecheteau, Tanguy and Laccourreye11–Reference Wineland, Menezes, Shimony, Shinawi, Hullar and Hirose13,Reference Yu, Mukherji, Carrasco, Pillsbury and Shores17} Therefore, besides aiming to treat SNHL, physicians should screen patients for any relevant genetic and molecular testing to ensure holistic management of the patients.

Though there is a strong correlation between cochlear nerve canal stenosis and semicircular canal abnormalities, the most common anomaly found accompanying the cochlear nerve canal stenosis was incomplete partition type II, which was also the most common cochlear anomaly with or without other inner ear malformations found in this study population. This finding is concordant with previous literature, which stated that incomplete partition type II is the most common cochlear malformation and accounts for more than 50 per cent of all cochlear deformities.^{Reference Joshi, Navlekar, Kishore, Reddy and Kumar16} Thus, it is important to identify this anomaly while reporting high-resolution CT temporal bone in cochlear implantation candidates due to its common occurrence. Our study does not include the post-cochlear implantation outcomes. Nevertheless, previous studies have demonstrated good surgical and functional outcomes in patients with incomplete partition type II.^{Reference Sefein, Younes, Omara, Hamada, Sami and EL Rouby18} However, post-operative performance is reported to be poor in total semicircular canal aplasia, isolated incomplete partition, cochlear hypoplasia and the common cavity, all of which predominantly affect development rather than being related to labyrinthine anatomy alone.^{Reference Buchman, Copeland, Yu, Brown, Carrasco and Pillsbury19}

Another interesting finding in our study was that there were nine ears with semicircular canal abnormalities without cochlear nerve canal stenosis or cochlear anomalies. The most common type of semicircular canal anomaly was lateral semicircular canal aplasia. Embryologically, the otic vesicle splits into the pars superior (dorsal part) and pars inferior (ventral part) at the sixth week of foetal development.^{Reference Breheret, Brecheteau, Tanguy and Laccourreye11} The pars superior becomes the utricle and then the semicircular canal by week eight of foetal development and is phylogenetically older than the pars inferior. The pars inferior forms the vestibular saccules and cochlear canal and is fully formed by week nine of foetal development.^{Reference Satar, Mukherji and Telian6,Reference Breheret, Brecheteau, Tanguy and Laccourreye11} Based on the inner ear malformations classification established by Sennaroglu and Saatci,^{Reference Sennaroglu and Saatci1} deformities result from sudden arrest of development during these successive stages, which is why the lateral canal is the most frequently involved as it is the last to form among the three semicircular canals. Given the sequence of the inner ear embryogenesis, canal aplasia with a normal cochleovestibular system seems chronologically impossible, as the membranous vestibular labyrinth forms before the definitive formation of the cochlea.^{Reference Satar, Mukherji and Telian6,Reference Breheret, Brecheteau, Tanguy and Laccourreye11} It is postulated that there may be a more selective, probably genetic disorder of semicircular canal development or ossification in the pars superior that may be responsible for this finding and that cochlear and vestibular development may rely on independent mechanisms without interaction.^{Reference Satar, Mukherji and Telian6,Reference Breheret, Brecheteau, Tanguy and Laccourreye11}

Hearing loss that is associated with semicircular canal dysplasia without cochlear anomalies is most likely due to anomalous membranous labyrinth development, which is not radiologically detectable.^{Reference Yu, Mukherji, Carrasco, Pillsbury and Shores17} In addition, Sennaroglu and Saatci^{Reference Sennaroglu and Saatci1} found that semicircular canal aplasia was never detected in the absence of other malformations, and Yu et al.^{Reference Yu, Mukherji, Carrasco, Pillsbury and Shores17} reported that isolated semicircular canal abnormalities always occurred in a context of congenital malformation.

One limitation of the present study is that the patient database and clinical information was obtained retrospectively without a predetermined definition about the information desired. Therefore, some relevant information may not have been recorded and thus we could not rule out the possible association of all the nine ears with isolated semicircular canal abnormalities with any clinical syndrome(s) that may not be evaluated clinically at the time of the study.