A 5-year-old girl who failed a hearing test in school is brought in by her mother. The child has no family history of hearing loss, and the mother had an uneventful pregnancy. However, the child was born in another country, and did not have a newborn hearing screening. Upon examination, her ears look normal, but her audiogram shows no measurable hearing on the left side. She also has weak left facial function. The results are on the right. What is your diagnosis?
Diagnosis: Congenital Stenosis of the Internal Auditory Canal
A symmetric hearing loss in children is usually associated with anomalies of the inner ear. However, when the facial nerve function is affected, it causes concern for an abnormality in the cerebellopontine angle or in the internal auditory canal. Diagnosing facial nerve dysfunction in a very young child is challenging. Part of the problem is the strong skin turgor in children, which reduces the visibility of asymmetries usually seen in adults with skin laxity. The second problem is asking a child to do certain facial motions to look for asymmetry. Infants, in particular, may be asleep during office visits, making it difficult for the clinician to assess their facial nerve. The best strategy to examine an infant's facial nerve is by causing the child to cry briefly, preferably during the examination of the pharynx. The asymmetry in the eyelid motion and the corners of the mouth can provide clues to the presence of facial weakness.
RULING OUT POSSIBILITIES
The first step in the workup of a child with unilateral hearing loss and facial weakness is to rule out a tumor in the cerebellopontine angle. While a mass would be highly unusual in these patients, it is important to eliminate this possibility. Checking for a mass is best done with an MRI of the internal auditory canals with and without gadolinium. Unfortunately, this can only be performed with the child under general anesthesia, as it requires no motion for 40-45 minutes.
The MRI of this young patient showed a very narrow internal auditory canal. A normal internal auditory canal measures 3 mm or larger in the anterior posterior-dimension. This child's MRI, however, showed that hers measured less than 2 mm in the most medial internal auditory canal on the left side (Fig. 2). The congenital narrowing of the internal auditory canal can cause abnormalities in the development of the facial and cochlear nerves. In our practice, we usually do not order a CT scan for young children unless the diagnosis requires it or if a surgical therapy is planned. However, a CT scan of the temporal bones was performed on this patient as ordered by her outside physicians. The CT scan showed the congenitally narrowed internal auditory canal with some anomalies of the vestibule and horizontal canal (Fig. 3). The cochlea was slightly shortened but mostly normal.
UNDERSTANDING CONGENITAL STENOSIS
Congenital stenosis of the internal auditory canal is a rare anomaly that most commonly affects the cochlear nerve. A study of five patients with congenitally narrowed internal auditory canal found none had facial weakness (Otol Neurotol. 2005;26:767http://bit.ly/2tMK48l). All patients had hearing abnormalities, but only four had abnormal outer hair cell function. Superior vestibular nerve testing using galvanic stimulation showed weakness in two patients and no function in two others. One patient had normal superior vestibular nerve function. The inferior vestibular nerve showed no function in one of the patients, whereas the rest had normal function.
Another study was conducted on 69 patients with unilateral sensorineural hearing loss between the ages of 0 and 15 years (Int J Pediatr Otorhinolaryngol. 2013;77:228 http://bit.ly/2tMY81K). Of these patients, two-thirds had inner ear and/or internal auditory canal malformations. Malformations were more commonly seen in the patients under 12 months old, 85 percent of which had anomalies. Among study participants aged 1 to 15 years, 56 percent had anomalies. Nearly half of the patient cohort (46%) had cochlear nerve canal stenosis. Of the entire cohort, 13 had cochlear nerve canal stenosis alone, and 19 had cochlear nerve canal stenosis in conjunction with other malformations of the cochlea. Internal auditory canal malformation was seen in 22 subjects (32%). Of these, 14 had cochlear malformations, and five had vestibular/semicircular canal malformations in addition to the internal auditory canal anomalies. Two patients (3%) had bilateral enlarged vestibular aqueducts that were associated with Pendred syndrome. The prevalence of a narrow internal auditory canal was significantly higher in subjects with cochlear nerve canal stenosis. This is likely due to the cochlear nerve deficiency in the congenitally narrowed internal auditory canal, which leads to a smaller distal nerve and thus a narrow cochlear canal (where the cochlear nerve enters the cochlea). Interestingly, there was no correlation between the types of malformation and hearing level.
When present bilaterally, this condition creates an issue in determining a patient's cochlear implant candidacy. The primary concern is whether there is an adequate cochlear nerve that could transmit hearing information to the brain. A study by Buchman, et al., evaluated patients with cochlear nerve deficiency as diagnosed by MRI and their subsequent performance with a cochlear implant (Laryngoscope. 2011;121:1979 http://bit.ly/2tMTrVM). Even among patients with seemingly deficient or potentially absent cochlear nerves, 19 percent were able to achieve open-set speech discrimination. The mean pure tone average of the group with cochlear nerve deficiency was higher than that of the group with just inner ear anomalies, commonly enlarged vestibular aqueducts. For patients with bilateral internal auditory canal stenosis, our standard practice is to first perform a cochlear implant. If implants on both sides are unsuccessful, the patient may be enrolled to a pediatric auditory brain stem implant trial.
BONUS VIDEOS: VISUAL DIAGNOSIS
Read this month's Clinical Consultation case, then watch the accompanying videos from Hamid R. Djalilian, MD, to review the patient's imaging for yourself.
- Video 1. Axial T1-weighted post-gadolinium MRI showing the narrow IAC on the left.
- Video 2. Axial CT of the left temporal bone showing the anatomy of the IAC in the horizontal direction.
- Video 3. Coronal (vertical) CT of the left temporal bone demonstrating the facial nerve anatomy in the mastoid.
- Video 4. Sagittal (parallel to auricle from outside to inside) CT of the left temporal bone showing the IAC in cross section.
- Video 5. Axial CT of the right temporal bone showing the normal IAC. The mastoid is filled with fluid.
- Video 6. Coronal (vertical) CT of the right temporal bone showing involvement of the normal facial nerve anatomy in the mastoid.
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