Teaching Case

A 2-year-old female patient was sent with a history of profound congenital bilateral sensorineural hearing loss detected through audiometric assessment and associated with right central facial palsy. Currently in speech therapy, which has been unsuccessful during the past 7 months.

Diagnostic brain MRI study was carried out using constructive interference steady state sequence (CISS) acquisitions (Figure 1) under sedative induction. Postprocessing multiplanar and volumetric images at the level of both cerebellopontine angles (Figure 2) showed absence of the anatomical course of cisternal and canalicular portions of both vestibulocochlear nerves, as well as the right facial nerve compared to the left side (Figure 3), this abnormality associated with ipsilateral internal auditory canal (IAC) atresia (Figure 4). Remember that MRI is the gold standard for identifying structural causes of sensorineural hearing loss pathology.

The prevalence of sensorineural hearing loss (SNHL) is approximately 1.5-6 out of every 1,000 births [1], positioning unilateral vestibulocochlear nerve aplasia as one of the main causes of congenital deafness.

Its bilateral presentation is even more singular (<15%) [2] in association with aplasia of the facial nerve (29.2%) and atresia of the internal auditory canal (IAC), which can manifest in isolation or as a syndromic feature in up to 30% of patients (BOR and CHARGE sequences / Goldenhar syndrome) [1].

Since vestibulocochlear nerve (VCN) aplasia is relatively common, its diagnosis has been considered decisive in the pre-surgical planning of candidates for cochlear implantation before the first year of life. The delay in its diagnosis entails diverse repercussions, which mainly impact the intellectual and social development of the individual in the long term [3].

Due to its higher contrast resolution, MRI is the optimal modality to depict subtle abnormalities along the cisternal and canalicular course of the vestibular and cochlear nerves, as well as abnormalities in the auditory cortex, by using specific sequences.

Moreover, MRI complements the information acquired through a CT scan, and, in the case of pediatric patients with sensorineural hearing loss, it may be the only examination required to reach a final diagnosis obviating the exposure to ionizing radiation.

Brief checklist for inner ear and intracranial abnormalities seen on MRI [4]:

• Heavily T2-weighted sequence in the axial plane (FIESTA, CISS, SPACE) - Used for imaging abnormalities in the inner ear (vestibule, scala vestibuli, scala tympani, interscalar septum, central modiolus and VCN).
• Heavily T2-weighted sequence in the oblique sagittal plane (obtained perpendicular to the course of the nerves in the IAC) - Specific for depiction of internal auditory canal permeability and measurement of intracanalicular segments of vestibular and cochlear nerves.
• T1WI+C, FLAIR, DWI/ADC, T2*, Supplementary brain imaging acquisitions in case of intracranial findings - Such as tumoral lesions, haemorrhage, brainstem and hearing cortex abnormalities.

In summary, cochlear nerve deficiency (CND) is one of the many causes of sensorineural deafness, including aplasia and hypoplasia within the spectrum of nerve abnormalities easily evaluated on MRI, and it is bilateral in about 15% of patients with profound bilateral SNHL.

There is a significant association between the degree of hearing loss and the different types of malformation of the auditory nerve. According to Casselman classification, patients with Type 1 (tolerance ≥120db) have a worse therapeutic outcome and a worse prognosis [1,5]. It is important to highlight that unilateral facial palsy unrelated to perinatal injury is a rare condition but is often seen in association with unilateral aplasia of the VCN [6].

For treatment planning it is crucial to depict patients with cochlear nerve aplasia as these patients will not respond to cochlear implantation and require brainstem implants [3].

Written informed patient consent for publication has been obtained.