Symptom: Unilateral Hearing Loss

Abstract

A 2-year-old female presented for evaluation of unilateral hearing loss discovered after failing her newborn hearing screening. The patient was born at 37 weeks gestation via C-section. She had no significant medical or family history of deafness or genetic disorders. Her auditory brainstem response testing demonstrating severe-to-profound sensorineural hearing loss (SNHL) in the left ear. On physical exam, she was found to have a grade 3 left-sided facial paralysis with complete eye closure with force, but she had a symmetric smile and symmetry at rest. She also had an incomplete cleft palate and exhibited delayed language development. The patient's ABR and audiogram demonstrated profound SNHL in her left ear. Magnetic resonance imaging (MRI) is below.Figure 1: T2-weighted axial (horizontal) MRI image of the patient's internal auditory canals bilaterally. On the right, the IAC is normal in size and nerves can be visualized within the IAC. On the left, the IAC is small and hypoplastic, and no nerves can be visualized within it. Hypoplasia, audiology.Figure 2: (A) Axial (horizontal) CT of the left temporal bone at the level of the superior vestibule showing the stenotic IAC (B) Axial (horizontal) CT of the right temporal bone demonstrating a normal caliber IAC as well as a right mastoid and middle ear effusion. Hypoplasia, audiology.Figure 3: Axial (horizontal) CT of the left temporal bone slightly inferior to Figure 2A, showing the cochlear aperture (where the cochlear nerve enters the cochlea), which appears to be closed. Hypoplasia, audiology.Figure 4: Coronal (vertical) CT of the left temporal bone demonstrating the narrowed IAC in a coronal plane. Hypoplasia, audiology.Figure 5: Maximum intensity projection (MIP) coronal (vertical) MRI image of bilateral internal auditory canals demonstrating hypoplastic left IAC. Hypoplasia, audiology.DIAGNOSIS: INTERNAL AUDITORY CANAL HYPOPLASIA The patient's age and duration of symptoms make a congenital etiology most likely. Congenital hearing loss can be attributed to many etiologies, such as genetic disorders, congenital infections, environmental toxins, or ototoxic medication exposure during gestation. For this reason, it is essential to obtain a thorough history of the prenatal, perinatal, and postnatal periods. Almost half of all cases of congenital hearing loss are due to non-genetic causes. Non-syndromic hearing loss makes up approximately 70% of all genetic hearing loss, while syndromic conditions account for about 30%.1 A detailed history and physical exam can often help to determine to which category a patient belongs, with craniofacial abnormalities, family history, and congenital exposures/illnesses being important clues. However, imaging is often obtained to help corroborate or confirm a suspected diagnosis, usually starting with a high-resolution MRI and/or CT of the internal auditory canal (IAC) and temporal bones. In this patient, MRI IAC was obtained to further evaluate the cause of her left facial paralysis and profound SNHL. MRI IAC demonstrated present but hypoplastic left vestibulocochlear and facial nerves, as well as a slightly enlarged vestibule on the left (Fig. 1). CT temporal bones demonstrated a very narrow diameter of the left IAC to 1 mm in a vertical plane, consistent with IAC hypoplasia (Figs. 2-5). Lastly, the cochlear aperture (where the cochlear nerve enters the cochlea) appeared to be closed on the left with no discernable space for the cochlear nerve to reach the cochlea. The cochlea itself appeared normal with two and a half turns and an intact modiolus. Internal auditory canal hypoplasia is a very rare cause of congenital sensorineural hearing loss.2 IAC hypoplasia accounts for 12% of all congenital temporal bone malformations.3 The normal vertical diameter of the IAC ranges from 2 to 8 mm, with 4 mm being the average size.4 IAC hypoplasia is defined as a vertical IAC diameter of less than 2 mm,5 and our patient had an IAC diameter of 1mm. IAC hypoplasia is thought to develop secondary to hypoplasia or aplasia of the vestibulocochlear nerve (VCN). During the eighth week of gestation, the mesoderm that surrounds cranial nerves VII and VIII undergoes chondrification. This cartilage then ossifies during the fifth month of gestation and becomes the bony internal auditory canal. Since the VCN predates the IAC in gestational development, IAC hypoplasia is thought to be a result of VCN hypoplasia or aplasia. Though the facial and vestibulocochlear nerves arise in close proximity, hypoplasia or aplasia of the VCN often occurs independently from facial nerve hypoplasia. One retrospective analysis of seven patients with congenital IAC stenosis found that two out of seven patients had associated facial nerve weakness. Vestibular function was also significantly diminished in four out of the seven patients.6 In a case series reviewing radiologic findings of congenital IAC stenosis in eleven patients, two of the eleven patients were found to have hypoplastic facial nerves in addition to hypoplastic vestibulocochlear nerves.7 IAC hypoplasia is also often associated with inner ear malformations and can be seen with middle and external ear abnormalities as well.3 In one review of 32 patients with cochlear nerve stenosis, 59% of these patients also had other inner ear malformations including IAC malformations, cochlear malformations, and vestibular/semicircular canal malformations.8 Treatment of patients with IAC hypoplasia depends on if it is unilateral or bilateral. In patients with bilateral IAC hypoplasia, cochlear implantation can be considered to optimize hearing and speech development. A review of 50 patients with hypoplastic or aplastic cochlear nerves who underwent cochlear implantation showed promising results, with Categories of Auditory Performance scores of five to seven.9 However, other studies have shown poorer response to cochlear implantation of patients with IAC/cochlear canal narrowing compared to children with another anomalous vestibulocochlear anatomy.10 If a patient fails to respond or has a poor outcome after cochlear implantation, auditory brainstem implantation can be considered. In patients with unilateral IAC hypoplasia, surgical treatment often does not need to be pursued if they have a contralateral normal hearing ear and the patient has normal speech and language development. In this patient with intact right-sided hearing, a CROS hearing device was used with good results. Evaluation of congenital facial paralysis is similar to the evaluation of facial paralysis or paresis that occurs later in life. It is important to distinguish whether any facial movement is present. If there is any facial nerve movement, the patient may regain full function over time and with physical rehabilitation. However, if there is a complete absence of facial nerve movement, it is less likely the patient will gain function over time and it is important to obtain facial electromyography (EMG). If facial muscle activity is absent on EMG (no fibrillation or motor unit action potential), reconstructive options such as gracilis free muscle transfer with or without cross facial nerve grafting can be considered to restore a dynamic smile. Static reconstruction options such as fascia lata sling can also be considered. To improve or create full eye closure, a gold or platinum weight can be placed in the upper eyelid. Lastly, ongoing investigations in animal models are evaluating a penetrating or implantable multichannel electrode array that aims to stimulate facial muscle movement. Results of these investigations are thus far promising and demonstrate activation in the specific facial muscles stimulated, and may become a treatment option in the future.11,12 In this patient who had a symmetric smile but incomplete eye closure, a platinum weight may be placed when she is older to create full eye closure and improve dryness. BONUS ONLINE 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. T2-weighted axial (horizontal) MRI of the temporal bones and internal auditory canals (IACs) demonstrating a normal IAC and nerves on the right and hypoplastic IAC without discernable nerves on the left. Video 2. Axial (horizontal) CT of the left temporal bones demonstrating hypoplastic left IAC and slightly enlarged left vestibule with normal appearing cochlea and semicircular canals. Video 3. Coronal (vertical) CT of the left temporal bones demonstrating a coronal view of the hypoplastic left IAC with normal appearing cochlea and semicircular canals. Video 4. Sagittal CT of the left temporal bones demonstrating sagittal reconstruction hypoplastic left IAC with a vertical diameter of 1mm. Video 5. Axial (horizontal) CT of the right temporal bones demonstrating a normal appearing IAC and inner ear with a right mastoid and middle ear effusion in the axial plane. Video 6. Coronal (vertical) CT of the right temporal bones demonstrating a normal appearing IAC and inner ear with a right mastoid and middle ear effusion in the coronal plane. Watch the patient videos online at thehearingjournal.com.