Congenital SNHL Pathologies

Congenital hearing loss is defined as hearing loss present at birth and is generally divided into genetic and nongenetic forms. Congenital SNHL etiologies include membranous labyrinth dysplasias, cochlear/vestibular aplasia/dysplasia, enlarged vestibular aqueduct, IAC atresia/stenosis, and perilymphatic fistula. Genetic anomalies cause approximately half of the cases of congenital SNHL ; of these approximately 75%–80% demonstrate an autosomal recessive inheritance, 15%–20% demonstrate an autosomal dominant inheritance, and 1%–2% demonstrate an X-linked inheritance. Approximately 30% of inherited forms of hearing loss are syndromic, and the remaining 70% are considered nonsyndromic.

Although the cause is unknown in 25%–40% of cases, intrauterine toxin exposure, infections, and perinatal insults account for many cases of pediatric SNHL. Cytomegalovirus (CMV) infection is the most common environmental cause of prelingual hearing loss in the United States, implicated in approximately 10% of infants with congenital hearing loss and 34% of children with moderate to severe late-onset idiopathic hearing loss. Among children with clinically apparent congenital CMV infection, the prevalence of SNHL is ∼30%.

Cross-sectional imaging is an integral tool in the clinical evaluation of congenital SNHL. Otolaryngologists utilize CT and/or MRI to identify potential etiologies and abnormalities that may predict hearing loss progression or prognosis, to define TB anatomy and the central auditory pathway and to identify additional intracranial abnormalities that may require further workup and/or intervention. Abnormalities on CT and/or MRI are found in 20%–50% of children with SNHL and correlate with the degree of hearing loss. Although traditionally bone algorithm CT is the modality of choice in demonstrating inner ear dysplasias in children with SNHL, dual-technique imaging with high-resolution TB CT and MRI has been show to identify a substantially larger number of abnormalities in children being evaluated for cochlear implantation than either technique alone.

The spectrum of inner ear anomalies ranges from complete aplasia to subtle dysplasia. Sennaroglu and Saatci proposed a classification for cochleovestibular malformations that included, in order of decreasing severity: labyrinthine aplasia, cochlear aplasia, common cavity deformities, cystic cochleovestibular malformations (incomplete partition type-1), cochleovestibular hypoplasia, and incomplete partition type-2 (IP-II), each of which is thought to result from an insult occurring at a progressively later stage of development.

Labyrinthine aplasia (Michel deformity) is the complete absence of the cochlea, vestibule, and semicircular canals (SCCs). In cases of labyrinthine aplasia, there is SNHL from birth without the option of cochlear implantation in the affected ear. The bony labyrinth is featureless, and the petrous apex may be hypoplastic or absent, and the IAC is small. The middle ear may be normal in the mild form to abnormal with fused ossicles in the more severe form. In these cases, the lateral wall of the inner ear is flat and the facial nerve geniculate ganglion is displaced posterior to its normal expected location. There is no normal high-signal-intensity fluid in the membranous labyrinth on high-resolution T2W MR. Labyrinthine aplasia occurs when there is arrest of the otic placode development at the third gestational week. It may be associated with Klippel-Feil syndrome and/or thalidomide exposure ( Fig. 8.2 ).

Labyrinthine aplasia. Axial bone algorithm thin-section CT of the right temporal bone through the petrous apex demonstrates dense bone without inner ear formation ( arrow ), consistent with labyrinthine aplasia.

Cochlear aplasia occurs in late third week arrest, with absent cochlea, dysmorphic vestibule, and SCCs. The vestibule and IAC may be dilated. In these cases, the cochlear promontory is flat and the facial nerve labyrinthine segment, geniculate ganglion, and anterior tympanic portions are located at the expected location of the cochlea. The EAC, middle ear, ossicular chain, bony vestibular aqueduct, and endolymphatic duct are normal in size ( Fig. 8.3 ).

Cochlear aplasia. Axial thin-section bone algorithm CT (A) and correlating axial cerebrospinal fluid bright constructive interference in steady state MRI (B) through the inner ear structures demonstrates a dysplastic dilated vestibule ( arrow ) without anterior and medial cochlear formation, consistent with cochlear aplasia.

If there is an arrest in the fourth week after differentiation of the otic placode into the otocyst, a common cavity deformity occurs and an ovoid single inner ear cavity is formed. The SCCs are usually absent but may be present and dysplastic. The IAC size is concordant with the size of the common cavity and enters the cavity at the center portion; however, the modiolus is absent ( Fig. 8.4 ). Oblique sagittal T2W images perpendicular to the IAC are important to assess the presence or absence of the cochlear nerve if cochlear implantation is planned ( Fig. 8.5 ).

Common cavity deformity. Axial thin-section cerebrospinal fluid bright CISS image through the right temporal bone demonstrates a common cavity deformity with an ovoid single inner ear cystic structure ( arrows ) of a combined dysplastic cochlea and vestibule.

Cochlear nerve aplasia. Oblique thin-section cerebrospinal fluid bright MRI sections through the midportion of the bilateral internal auditory canals (IACs) demonstrates a normal IAC on the right (A) with four nerves seen in cross-section, with the facial nerve identified ( arrow ) superiorly and anteriorly. The cochlear nerve is seen anteriorly and inferiorly, inferior to the facial nerve, and the superior and inferior vestibular nerves are seen posteriorly within the IAC. The correlating small IAC on the left (B) shows a single nerve in cross-section ( arrow ), consistent with the facial nerve in this pediatric patient with normal bilateral facial motion but left congenital SNHL.

A cystic cochleovestibular anomaly results if there is arrest in the fifth gestational week. A “figure 8”-shaped dysplastic and cystic cochlea and vestibule with SCC dysplasia is seen. There are no internal features, and no modiolus is identified in these cases. The vestibule is cystic and dilated, and the SCCs are variable in shape and degree of dilatation. The IAC is normal or small in size with a normal facial nerve; however, the cochleovestibular nerves are deficient or absent. The EAC, middle ear, ossicular chain, bony vestibular aqueduct, and endolymphatic duct are normal in size.

SCC dysplasia is a spectrum of congenital deformities. The embryologic insult occurs at 6–8 weeks of gestation. The SCCs begin as disk-shaped evaginations arising from the vestibular appendage in the sixth gestational week. The central portion of each disk is resorbed and replaced by the mesenchyme, which results in the formation of the characteristic SCC. Failure of formation of one of these disks results in the absence of the involved SCC, whereas incomplete absorption of the central portion of the disk results in a dysplastic or pocket-shaped SCC. The superior SCC is the first to form, followed by the posterior SCC and then the lateral SCC. The most common and least severe dysplasia in this spectrum is a common cavity formed by the lateral SCC and the vestibule ( Fig. 8.6 ). On axial CT images, subtle SCC abnormalities may be indicated by an absent or small bony island between the vestibule and the lateral SCC ( Fig. 8.7 ). Normally, the transverse diameter of this bony island measures between 2.6 and 4.8 mm. As an internal standard for comparison, the osseous bony island between the vestibule and inside the loop of the lateral SCC should be wider from right to left than the vestibule width from right to left. In these cases, there may be associated oval window atresia (OWA) and the cochlea may be affected with incomplete partition of the apical and middle turns. There may also be ossicular anomalies. In addition to SNHL, there may be CHL due to this OWA and ossicular chain anomalies. It can be sporadic or associated with congenital syndromes (e.g., CHARGE, Alagille, Waardenburg, Crouzon, Apert). In CHARGE syndrome (coloboma, heart defects, choanal atresia, retardation of growth and development, and ear abnormalities), for instance, there is absence of the SCC bilaterally, dysmorphic vestibule, and cochlear anomalies with absence of the cochlear nerve canal.

Lateral semicircular canal (SCC) dysplasia. Axial cerebrospinal fluid bright thin-section MRI through the IACs and vestibules demonstrates a cystic vestibule, consistent with a right lateral SCC dysplasia with no osseous island or lateral SCC formation ( arrow ).

Mild lateral semicircular canal (SCC) dysplasia. Axial thin-section cerebrospinal fluid bright MRI (A) through the left cochlea and vestibule ( arrow ), correlating axial thin-section bone algorithm CT through the dysplastic cochlea ( B , arrow ), and thin-section CT through the mildly dysplastic vestibule ( C , arrow ) demonstrate abnormal dysplasia of the cochlea and vestibule, with cystic changes, and abnormal architecture of the inner ear structures, in this case of mild lateral SCC dysplasia. The osseous island inside the inner ring of the lateral SCC in (C) should be wider from right to left than the vestibule width from right to left. In this case, the vestibule ( arrow ) is wider than the osseous island, consistent with mild lateral SCC dysplasia.

Large endolymphatic sac anomaly (often used when found on MRI), or large vestibular aqueduct syndrome (often used when found on CT), is seen when there is arrest in the seventh gestational week ( Fig. 8.8 ). This is the most common inner ear malformation seen in children with nonsyndromic hearing loss and is bilateral in 90% of cases. The prevalence of enlarged vestibular aqueducts in children with SNHL is estimated to be 10%–15%. There are often associated other inner ear anomalies, such as IP-II, vestibular enlargement, and SCC dysplasias. It is a familial lesion with autosomal recessive inheritance. Patients are able to hear at birth, but their hearing deteriorates over the early years of life. The typical clinical presentation is a child or teenager with progressive SNHL. There may be posttraumatic potentiation of SNHL. It is the most commonly missed cause of congenital deafness. Rarely, patients have distal renal tubular acidosis resulting in varying degrees of metabolic acidosis. There is also an association with Pendred syndrome, which is severe SNHL with thyroid pathology.

Large endolymphatic sac anomaly (LESA) or large vestibular aqueduct syndrome (LVAS). Axial thin-section bone algorithm CT of the right (A) and left (B) temporal bones demonstrates bilateral LVAS ( arrows ) extending posterior from the vestibules (LVAS is wider than the internal standard of the lateral semicircular canal). The correlating axial thin-section cerebrospinal fluid (CSF) bright MRI of the right (C) and left (D) temporal bones demonstrates the corresponding CSF-like bright LESA bilaterally ( arrows ).

The criteria to determine an enlarged vestibular aqueduct (in which the endolymphatic sac lies) are variable. On CT, a bony vestibular aqueduct diameter measuring greater than 1.5 mm in the transverse dimension at the midpoint or an opercular measurement greater than 2 mm is generally considered to be the defining characteristics. The vestibular aqueduct is usually found at the level of the vestibule and lateral SCC, so it is easy to simply compare the diameter of the duct/sac with the lateral and posterior SCC width, which should be larger than the aqueduct ( Fig. 8.8A and B ). There is no relationship between the size of the endolymphatic sac and the severity of the SNHL. The aqueduct itself is best evaluated with CT, whereas the sac is best seen as bright signal intensity on thin-section T2W MRI. In greater than 75% of cases, there is an associated cochlear dysplasia. The apical turn of the cochlea in these cases is dysmorphic with modiolar deficiency ( Fig. 8.9 ). High-resolution T2W MRI may be able to distinguish more subtle abnormalities of scalar chamber asymmetry with the more anterior scala vestibuli larger than the more posterior scala tympani. Approximately 50% of cases have associated vestibular and/or SCC anomalies. In patients with sudden hearing loss, studies have shown wider vestibular aqueducts in the affected ear as compared with controls. The endolymphatic sac can show enhancement, which may be due to inflammation of the endolymphatic tissue or venous engorgement. It is hypothesized that a wide vestibular aqueduct may be associated with insufficient maturation of the inner ear. The congenital “fragile” inner ear may receive abnormal pressure transmission through the vestibular aqueduct.

Cochlear dysplasia. Axial thin-section bone algorithm CT images of the right (A) and left (B) temporal bones at the level of the cochleas and vestibules demonstrate a normal right cochlea ( A , arrow ) and a dysplastic left cochlea ( B , arrow ), with less than two turns to the cochlea itself.

Cochlear nerve deficiency (CND) is noted in 12%–18% of pediatric patient ears with SNHL. It is usually congenital and refers to the absence or reduction in caliber of the cochlear nerve. Because cochlear implants are generally contraindicated in CND, it is important to identify this condition in children being considered for implantation. There will be an associated narrowing of the IAC on CT (IAC diameter of <4 mm), which is theorized to occur because the IAC width depends on the presence of the vestibulocochlear nerve cells to form normally. In general, pediatric patients with narrow IACs on CT perform worse after implantation than those with normal-caliber IACs, presumably because the cochlear nerve is likely to be absent or small when the IAC is narrow. With CND, CT may also show a stenotic or absent bony cochlear canal (normally between 1.4 and 3.0 mm). The osseous anatomy must be assessed with caution, as cochlear nerve-deficient ears may demonstrate normal caliber of the bony cochlear canal in ≤23% of cases and normal-sized IACs in ≤73% of cases. Heavily T2W or CSF bright MRI sequences, such as SPACE, CISS, and FIESTA, can be used to identify the cochlear nerve, which should be located in the anterior inferior quadrant of the IAC, as well as the intracanalicular segment of the facial nerve (anterior and superior) and the superior and inferior divisions of the vestibular nerve posteriorly within the IAC ( Fig. 8.5A ). Although CND can be seen in isolation, it can occur in conjunction with aplasia of the vestibular nerve (complete absence of the eighth cranial nerve) ( Fig. 8.5B ).

Rarely, it has been reported that an intrameatal AICA loop can cause asymmetric SNHL because of neurovascular compression of the cochlear nerve. The study by Gorrie et al. found a statistically significant association between AICA loops that ran between the facial and vestibulocochlear nerves and hearing loss but found no statistically significant association between AICA loops that made no contact with the nerve, ran adjacent to the nerve, or displaced the nerve.

Acquired SNHL Pathologies

Acquired SNHL etiologies include tumors, infectious/inflammatory processes, trauma, autoimmune/immune disorders, and degenerative/idiopathic processes. Tumors can cause SNHL, including vestibular (acoustic) schwannomas (60%–90%), meningiomas (3%–6%), epidermoids (3%–6%), schwannomas of cranial nerves 5, 7, 9, 10, 11, and 12, lymphoma, leukemia, metastases, and meningeal carcinomatosis.

VS is the most common tumor of the IAC/CPA, accounting for 60%–90% of tumors in this region. In sporadic cases, the incidence is highest in patients in the fifth through the seventh decades. However, in patients with neurofibromatosis type II, patients commonly present in the first 2 decades with bilateral VSs ( Fig. 8.10 ) along with other cranial nerve schwannomas, meningiomas, and ependymomas of the central nervous system. In addition to SNHL, patients can have symptoms of tinnitus, dysequilibrium, and/or decreased speech discrimination, because of the mass effect of the tumor on the cochlear and vestibular divisions of cranial nerve 7.

Bilateral vestibular schwannomas. Axial postcontrasted fat-saturated T1W MRI through the level of the bilateral internal auditory canals (IACs) shows bilateral enhancing lesions of the cerebellopontine angle/IACs consistent with bilateral schwannomas in this patient with known neurofibromatosis type-II (better referred to as the MISME syndrome, or multiple inherited schwannomas, meningiomas, and ependymomas; because patients with neurofibromatosis type-II do not get neurofibromas, it is another poorly named thing in medicine).

On CT, VSs are isoattenuating compared with the brain parenchyma and can be difficult to delineate without contrast material enhancement. Lateral extension of the VS from the CPA into the IAC results in widening of the porus acusticus ( Fig. 8.11 ). The spherical cisternal component forms an acute angle with the petrous bone. Hemorrhage and calcification are rare in untreated tumors. Enhancement is typically avid and homogenous and better seen on MRI. On T1W MRI, VSs are usually isointense or mildly hypointense compared with brain parenchyma and hyperintense compared with CSF. On T2W MRI, they are mildly hyperintense compared with brain parenchyma and isointense to hypointense compared with CSF. If large, the tumors may have a heterogeneous internal architecture with cystic components ( Fig. 8.11 ). Deformity and mass effect on the adjacent structures can result in facial numbness if the trigeminal nerve is compressed, cerebellar signs if the lower cranial nerves are affected, or hydrocephalus if there is compression of the fourth ventricle. Heavily T2W or CSF bright MRI sequences can outline the tumor relative to adjacent structures ( Fig. 8.12 ). There may be decreased labyrinthine signal intensity, which may be due to increased protein content.

Vestibular schwannoma. Axial T1W postcontrasted MRI through the cerebellopontine angle/internal auditory canal (IAC) demonstrates a large enhancing lesion with internal cystic foci extending into the IAC with widening of the porus acusticus (the opening of the IAC) ( arrows ), consistent with a vestibular schwannoma. Although these are often referred to as vestibular schwannomas, the schwannoma may be arising from any of the four nerves within the IAC; therefore it is important to check for possible enhancing extension along the labyrinthine segment of the facial nerve to distinguish a facial nerve schwannoma from a vestibulocochlear nerve schwannoma. Facial nerve schwannomas within the IAC may present clinically with sensorineural hearing loss, as the facial nerve interestingly is less sensitive to internal derangement than the cochlear nerve is to external compression.

Internal auditory canal (IAC) vestibular schwannoma. Axial thin-section cerebrospinal fluid (CSF) bright (T2 FSE, SPACE, CISS, or FIESTA) MRI through the left IAC shows a well-defined filling defect within the normally bright CSF of the IAC ( arrow ), consistent with a vestibular schwannoma.

The VSs are slow-growing lesions, with growth rates ranging from 0.2 mm to a few millimeters per year. Surgical resection for complete cure is performed, if feasible. Of the three main surgical approaches (retrosigmoid, middle cranial fossa, and translabyrinthine), the translabyrinthine approach is reserved for patients with poor hearing or complete hearing loss, because it leads it to hearing sacrifice. When the VS is “impacted,” extending laterally into the cochlear aperture, there is a decreased chance of hearing preservation after surgery. Stereotactic radiosurgery is an option in high-risk patients, those with bilateral tumors, and those with residual tumors after initial treatment.

The VSs can also arise from Schwann cells within the membranous labyrinth, described as an intralabyrinthine schwannoma (you may have noticed on the right if you were looking closely at Fig. 8.6 ). This benign tumor appears on MRI as a soft tissue density lesion within the high-signal inner ear fluid and shows focal enhancement. It appears as a focal enhancing mass within the membranous labyrinth on contrast-enhanced T1WI MRI and as a filling defect on high-resolution T2W MRI. These can be subdivided according to location within the inner ear structures: intracochlear, vestibulocochlear, transmodiolar, transmacular, and transotic. Intralabyrinthine schwannomas result in progressive hearing loss.

Meningiomas are also found in the CPA but are most commonly found eccentric to the medial opening of the IAC, the porus acusticus ( Fig. 8.13 ). Possible IAC extension of a meningioma usually does not result in widening of the porus acusticus as it does with VSs. These meningiomas are characteristically broad based along the petrous wall with a hemispheric morphology and form an obtuse angle at the bone-tumor interface. Meningiomas are hyperattenuating or isoattenuating compared with brain parenchyma and may be calcified on CT. A helpful imaging feature is the adjacent bone, which may be sclerotic or hyperostotic ( Fig. 8.13A ). On MRI, meningiomas are isointense or slightly hypointense compared with gray matter on T1W MRI and isointense or hypointense compared with gray matter on T2W MRI. Typically, meningiomas show homogeneous enhancement unless there is internal calcification or microcystic change. A “dural tail” of dural enhancement extending from the margins of the lesion is often seen and may extend into the IAC itself ( Fig. 8.13D ). Surface CSF cleft and flow voids from marginal pial vessels may be seen, often described as a CSF/vascular cleft between the extraaxial meningioma and the adjacent normal brain parenchyma ( Fig. 8.13B ).

Cerebellopontine angle (CPA) meningioma. Axial thin-section bone algorithm CT image (A) through the left CPA/internal auditory canal (IAC) demonstrates a focal region of hyperostotic change ( arrow ) anterior to the IAC porus acusticus. Axial thin-section cerebrospinal fluid (CSF) bright SPACE MRI (B) demonstrates a CSF/vascular cleft ( arrow ) between the extraaxial lesion and the brain parenchyma. The axial T1W precontrast MRI (C) shows the lesion to be isointense compared with brain parenchyma ( arrow ), and the corresponding axial thin-section postcontrasted T1W MRI (D) with fat saturation shows avid homogeneous enhancement with an enhancing dural tail ( arrow ) extending along the anterior wall of the left IAC, consistent with a CPA meningioma.

An endolymphatic sac tumor (ELST) is a locally invasive papillary cystadenomatous tumor of the endolymphatic sac in a retrolabyrinthine location. These lesions are centered in the fovea of the endolymphatic sac along the posterior surface of the petrous TB. Larger lesions (greater than 3 cm) can spread to involve the middle ear, CPA cistern, and/or jugular foramen. On CT, this lesion shows intratumoral bone spicules with a moth-eaten, lytic appearance of the invaded bone. A thin rim of calcification along the posterior margin of the tumor is common ( Fig. 8.14A ). A vascular catheter or CT angiogram may exhibit enlarged distal vessels from the ascending pharyngeal and occipital arteries feeding the vascular tumor. Flow voids can be seen when the tumor is greater than 2 cm in size. On T1W MRI, lesions greater than 3 cm and along the margin when the lesion is less than 3 cm may show high-signal foci, reflecting blood products ( Fig. 8.14B ). The majority of ELSTs (80%) have these foci of increased signal intensity. Low-intensity foci in the tumor matrix may represent hemosiderin staining. These lesions show heterogeneous enhancement ( Fig. 8.14C ).

Endolymphatic sac tumor (ELST). Axial thin-section bone algorithm CT image (A) through the right cerebellopontine angle/internal auditory canal demonstrates abnormal permeative changes ( arrow ) to the posterior wall of the petrous apex. Correlating axial thin-section T1W precontrasted MRI (B) shows peripheral abnormal bright T1W signal intensity ( arrow ) consistent with blood products, whereas the corresponding axial thin-section T1W postcontrasted MRI (C) with fat saturation shows avid heterogeneous enhancement ( arrow ), consistent with an ELST.

Virtually all patients with ELSTs will have SNHL. Other symptoms include facial nerve palsy, pulsatile tinnitus, and vertigo. Hearing loss and ELST are frequently associated with von Hippel-Lindau syndrome (VHL) and should be considered when screening individuals at risk for VHL and when monitoring patients with an established diagnosis of VHL. Although most cases are sporadic, ∼7% ELSTs may be seen in the setting of VHL. Bilateral ELSTs are seen in VHL. Audiologic evaluation and MRI should allow early detection and enhance management of hearing loss in these patients. Many patients with VHL have hearing loss without radiographic evidence of an ELST. Whether it is caused by an ELST that is too small to be detected by MRI or is produced by some other etiology is still unknown.

Labyrinthine ossificans (LO) is also a potential cause of acquired SNHL ( Fig. 8.15 ). There are three stages to labyrinthitis (vestibulocochlear neuritis). In the acute stage, MRI may show inner ear enhancement. In the intermediate stage, fibrous tissue results in loss of the fluid signal on heavily T2W MRI (e.g., SPACE, CISS, or FIESTA) ( Fig. 8.15C and D ). However, there is higher T2 signal than as seen with an intralabyrinthine schwannoma. CT may appear normal in these early stages. In the late stage, the inner ear structures show bone attenuation on CT ( Fig. 8.15A and B ). The most extensive cases are seen as a complication of meningitis. TB trauma (e.g., a unilateral fracture with a perilymphatic fistula or postoperatively after stapedectomy or other inner ear surgery) and autoimmune inner ear disease have also been attributed to the development of LO. The scala tympani of the basal turn is the most common region of cochlear ossification. Patients present acutely with profound SNHL, vertigo, and tinnitus. LO can complicate or may preclude cochlear implantation.

Labyrinthine ossificans (LO). Axial thin-section bone algorithm right (A) and left (B) CT images show bilateral increased density within the vestibules and lateral semicircular canals (SCCs) ( arrows ). The correlating axial thin-section cerebrospinal fluid (CSF) bright MRI sections on the right (C) and left (D) show correlating loss of the expected CSF-like bright signal intensity within the bilateral vestibules and lateral SCCs, consistent with LO.

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