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Letter: Internal Auditory Canal Variability: Anatomic Variation Affects Cisternal Facial Nerve Visualization.

To the Editor: We read with great interest the paper by Cohen et al,1 entitled “Internal Auditory Canal Variability: Anatomic Variation Affects Cisternal Facial Nerve Visualization,” reporting relevant data on the high anatomic variability of the internal auditory canal and its impact on surgical procedures. Interindividual anatomic variability represents a relevant point in different challenging skull base procedures, such as acoustic neuromas surgery. Whenever opening the internal auditory canal (IAC) is required, in translabyrinthic or retromastoid approaches, anatomic landmark evaluation may ensure safer procedures, eventually preserving cranial nerve function. The authors quantitatively evaluated the variability of this anatomic district, carefully highlighting the relevance of IAC position and orientation within the petrous temporal bone. Morphological valuations and measurements were conducted on preoperative magnetic resonance imaging (MRI) images, properly reporting anatomicsurgical correlations. Different radiological parameters have been standardized to systematically collect data and to perform the statistical analysis.1 According to our skull base surgery institutional series, merging neurosurgical and craniofacial expertise and experiences, we do agree with the authors on how this high variability can impact the surgical exposure of IAC, therefore the visualization of vestibulocochlear and facial nerves. However, it is our professional opinion that local anatomy may influence the surgical exposure even more in translabyrinth approaches. As the authors stated, the IAC angle slightly influences facial nerve visualization during a retrosigmoid approach, since the working corridor is oblique to the IAC, regardless of its orientation. Conversely, further factor should be considered whenever opening the IAC is part of the procedure. Furthermore, cranial nerves may be displaced in the case of skull base tumors. Preoperative radiological evaluation and recognition of their position with respect to the tumor would tremendously facilitate conducting safer and faster procedures. This also matches the current trend in identifying the preferred surgical trajectory in both spinal and brain surgeries, to further minimize the surgical invasiveness. Nowadays, hearing preservation is progressively accepted as a primary outcome in elective surgeries for acoustic neuroma on patients with preoperative functional hearing.2,3 This is even more relevant in the case radiosurgery may represent an alternative treatment.4 The relationship between the IAC and inner ear structures, such as the labyrinth, may highly affect the chances for hearing preservation in canal opening. Several studies have reported the preoperative risk estimation of hearing injury in drill-assisted IAC posterior wall opening,5-8 confirming the high relevance of this topic for skull base surgeons worldwide. Relationships between the meatus and the labyrinth, and appropriate distances such as the one between the posterior meatal lip of the IAC and the projection of the vestibular structures on its posterior meatal wall, should also be carefully considered in the preoperative planning.4 Although facial nerve exposure and its direct visualization can be considered as mandatory in these surgeries, hearing preservation should be always pursued as well, definitely representing a non-negligible functional outcome. Finally, it is our opinion that Cohen et al provided a valuable contribution to a wide scientific audience, proponing new parameters to be considered in the preoperative planning in many different skull base surgeries involving the opening of the IAC. Their use in future case series and prospective studies would result in a more standardized way to collect and to report data. Lastly, in the age of minimally invasive surgeries, these types of contribution have to be applauded, eventually fostering the interest in investigations on these topics and the relevance of functional outcome evaluation in our patients. Multidisciplinary collaborations between different medical specialists, such as neurosurgeons, craniofacial surgeons, otorhinolaryngoiatry, neurologists, and radiologists have to be considered the preferred way to pursue even better results in improving our results.