Dr. Vossough is a third-year Resident in the Department of Radiology at the Hospital of the University of Pennsylvania, Philadelphia, PA.

Hearing loss is classified into conductive, sensorineural, and mixed types. Disorders of the auditory pathways cause sensorineural hearing loss (SNHL). The damage to the auditory pathway may take place in the inner ear structures (sensory hearing loss), or in the auditory nerves or central auditory pathways in the brainstem or thalamus (neural hearing loss). It is estimated that approximately 17 million Americans suffer from some degree of SNHL. ¹ The prevalence of unilateral SNHL of 45 decibels or worse among school children in the United States is 3 in 1000. ²

Sensorineural hearing loss can be variously classified as hereditary versus nonhereditary, congenital and early versus delayed, unilateral versus bilateral, sudden versus progressive, or isolated versus associated with a syndrome. A simple classification of the causes of SNHL is shown in Table 1. Radiologists are often asked to evaluate the complex inner ear structures and auditory pathways in search of a cause for SNHL or as part of evaluation of potential candidates for cochlear implantation. This article serves as a brief overview of the imaging findings in patients with SNHL.

Anatomy of inner ear

The inner ear is composed of the osseous labyrinth within which the membranous labyrinth is located. The osseous labyrinth, also known as the otic capsule, is composed of the vestibule, cochlea, semicircular canals, vestibular aqueduct, and cochlear aqueduct (Figure 1). The vestibule is a central cavity in the inner ear to which the cochlea, semicircular canals, and vestibular aqueduct are connected. The vestibule itself contains two membranous sacs called the utricle and saccule, and is separated from the middle ear cavity by the round and oval windows. The stapes endplate fills the oval window and transmits sound waves to the inner ear. The normal cochlea has 2 ¹ /2 to 2 ³ /4 (basal, middle, and apical) turns around a central axis called the modiolus (Figure 1). Within each turn, a thin interscalal septum divides the cochlear, transforming it into separate compartments. The vestibular aqueduct contains the membranous endolymphatic duct, which connects to a small blind-ended sac called the endolymphatic sac. The endolymphatic sac is situated between the posterior surface of the petrous bone and the dura matter of the posterior fossa. The internal auditory canal (IAC) extends from the labyrinth to the cerebellopontine angle (CPA) and contains the seventh and the eighth cranial nerves (Figure 2). The eighth nerve itself is composed of three branches. The superior vestibular and inferior vestibular branches occupy the posterior half of the IAC. The cochlear branch is located in the anteroinferior part of the IAC, whereas the facial nerve occupies the anterosuperior portion. The normal cochlear branch of the eighth nerve should have approximately the same diameter as the facial nerve in the IAC.

Imaging technique

Direct thin-section computed tomography (CT) scanning in both the axial and coronal planes is usually required for optimal demonstration of temporal bone anatomy and pathology. The axial scan plane is kept parallel to the infra-orbitomeatal line to minimize radiation dose to the lens of the eye. Direct coronal images are obtained either with the patient supine in a hanging-head position or a prone position with the neck extended. The axial images are obtained from the top of the petrous apex to the inferior tip of the mastoid, and the coronal images are obtained from the anterior margin of the petrous apex to the posterior margin of the mastoid. Contiguous slices are obtained with a slice thickness of at least 1 to 1.5 mm. Conventional sequential acquisition is preferred, but a spiral technique can also be used if a low pitch of 1:1 is applied. A small field of view of approximately 12 cm is used, and each ear is reconstructed separately. Images are reconstructed using a bone algorithm. Intravenous contrast is not required unless the patient is unable to undergo magnetic resonance imaging (MRI), in which case contrast-enhanced CT is sought as an alternative to MRI. In these cases, the postcontrast images should be reconstructed using a soft-tissue reconstruction algorithm. CT gas cisternography is rarely, if ever, used today.

Selection of the optimal MRI sequence for imaging of the inner ear structures and IAC depends on the clinical situation and the age of the patient. Traditionally, high-field contrast-enhanced imaging in axial and coronal planes has been regarded as the gold standard for MRI of the IAC and inner ear structures when one is in search of tumors, hemorrhage, or inflammatory processes. Precontrast T1-weighted images are also necessary to differentiate enhancing lesions from inherently T1 bright lesions such as lipomas or hemorrhage. However, these sequences are not able to accurately depict the complex anatomy of the membranous labyrinth and cranial nerves in detail due to poor spatial resolution and thick slices (3 mm). With the increased availability of faster and stronger gradients, three-dimensional Fourier transform (3DFT) fast-spin-echo T2-weighted sequences have allowed very high-resolution imaging of the labyrinth for congenital anomalies and evaluation of the IAC by utilizing the inherent contrast of the fluid-filled inner ear structures. ³ Also, a variety of steady-state gradient-echo techniques such as 3DFT fast imaging with steady-state precession (FISP), true-FISP, 3D-gradient-recalled acquisition in the steady state (GRASS), and 3DFT constructive interference in the steady state (CISS), which utilize ultra-thin slices, have been used in the study of the inner ear structures and basal cisterns. ^4-6 These sequences can provide highly T2*-weighted images with submillimeter slice thickness, which are invaluable for evaluation of the small fluid-filled spaces of the inner ear and IAC (Figure 2). These techniques approach or exceed the resolution of CT of the temporal bone and can be acquired in a reasonable amount of time.

Some authors have advocated that a combination of contrast-enhanced T1-weighted imaging and a gradient-echo sequence, such as 3DFT-CISS, should become the gold standard for MRI of the temporal bone. ^7-11 It should be noted, however, that lesions in the brainstem, thalamus, or temporal lobe may also cause SNHL; therefore, routine images of the brain must be acquired in addition to dedicated inner ear and IAC sequences. Noncontrast high-resolution, fast-spin-echo, T2-weighted MRI has been advocated as a screening tool for acoustic schwannomas in asymptomatic populations, although the overall utility of this practice is yet to be determined. ¹² Although noncontrast high-resolution MRI is useful in the evaluation of congenital anomalies of the inner ear and mass lesions of the IAC, it is not able to detect some labyrinthine pathologies, such as labyrinthitis or labyrinthine schwannomas, which rely on contrast enhancement for diagnosis. ^13-15 All of the above sequences may be acquired utilizing a standard head coil, but use of a dedicated phased-array surface coil, such as a temporomandibular joint coil, can be extremely useful in increasing the signal-to-noise ratio and improving imaging quality.

Congenital abnormalities of the inner ear

Cochlear dysplasias

Congenital abnormalities of the inner ear can be limited to the membranous labyrinth or involve the osseous labyrinth. CT can detect abnormalities of only the osseous labyrinth, while MRI may be helpful in delineating gross abnormalities of the membranous labyrinth. Only 2% to 20% of patients with congenital SNHL have bony anomalies detectable by CT. ^16,17 Nevertheless, even MRI cannot show all the minute structures or abnormalities of the membranous labyrinth.

Developmental failures at various stages of differentiation of the otic placode give rise to different recognizable abnormalities in the inner ear. The most severe form of inner ear malformation is complete bony and membranous aplasia of the labyrinth, also known as Michel's aplasia. ¹⁸ This anomaly is extremely rare and constitutes only 1% of osseous inner ear malformations. ¹⁶ CT will show total absence of the inner ear labyrinth (Figure 3A). A second form of osseous malformation of the cochlea is the common cavity malformation in which there is failure of differentiation of the embryonic otocyst into a separate vestibule and cochlea (Figures 3B and 3C). This disorder comprises one-fourth of cochlear malformations. On imaging, there is a large fluid-filled cavity in the inner ear with no internal architecture, representing the common vestibule and cochlea. ⁶ Cochlear hypoplasia, in which only a small bud of cochlea arises from the vestibule, represents 15% of cochlear malformations. The term "Mondini malformation" has been inappropriately used as a generic term to describe all congenital malformations of the cochlea or inner ear. The classic Mondini malformation, however, is actually an incomplete partition of the cochlea, which results in formation of only 1 ¹ /2 turns in the cochlea (Figure 3D). ¹⁶ The basal turn is present, but the middle and apical turns are fused together secondary to the lack of a bony septum. There is also lack of interscalal septa in the cochlea. The Mondini malformation is the most common cochlear dysplasia, comprising 55% of all cases. Malformations of the semicircular canals are frequently associated with cochlear dysplasias.

Large endolymphatic duct and sac

Enlarged endolymphatic duct and sac is being increasingly diagnosed as perhaps the most common radiographically identifiable inner ear anomaly. ^17,19,20 In this disease, hearing is usually present at birth and decreases in a stepwise fashion during childhood and adolescence. ²¹ Onset of hearing loss often follows trauma or other activities that may increase cerebrospinal fluid pressure. On CT, the manifestation of this sydrome is enlargement of the vestibular aqueduct, through which the endolymphatic duct passes to reach the endolymphatic sac. Enlargement of the vestibular aqueduct is defined as a diameter >1.5 mm in its mid-segment (Figure 4). ²² MRI is highly accurate in depicting an enlarged endolymphatic duct and can also demonstrate an enlarged endolymphatic sac lying against the posterior surface of the petrous bone. ^20,23,24 The normal endolymphatic sac is not often seen on routine MRI. More than half of all patients with this disease may have associated vestibulocochlear abnormalities. ^25-27

Acquired abnormalities of the inner ear

Trauma

Temporal bone fractures have traditionally been divided into longitudinal and transverse fractures. However, there can be considerable overlap in clinical findings depending on exactly which structures in the temporal bone are involved by the fracture. ^28,29 Longitudinal fractures comprise 80% to 90% of temporal bone fractures and typically involve the tympanic cavity, resulting in conductive hearing loss from ossicular chain disruption or tympanic membrane rupture. Longitudinal fractures rarely involve the labyrinth. Transverse fractures comprise 10% to 20% of fractures; they often involve the otic capsule and may result in SNHL by damaging the inner ear structures or disrupting the eighth nerve (Figure 5). ^30,31

Neoplastic disorders

Acoustic schwannomas are benign slow-growing neoplasms of the nerve sheath of the eighth cranial nerve. They more commonly involve the vestibular branches of the eighth nerve and hence the often-used term, vestibular schwannoma. These tumors represent 60% to 91% of all IAC and CPA tumors. ^32,33 The diagnostic hallmark of vestibular schwannoma is an avidly enhancing mass in the IAC and CPA (Figure 6A). It has been shown that ultrahigh resolution FSE T2 or CISS MRI can diagnose virtually all of these neoplasms without the need for gadolinium contrast. ^34,35 However, some authors argue that using this approach is probably not justified since other alternative diagnoses may be missed due to the lack of contrast material. ^3,36 Nevertheless, high-resolution FSE T2-weighted MRI has proven to be much more sensitive than auditory brainstem evoked response testing and has been used in screening for acoustic schwannomas as a cost-saving alternative to contrast-enhanced MRI. ^12,34,37,38 The vast majority of acoustic schwannomas occur in the IAC or CPA. However, a small percentage arise from within the vestibule and cochlea and are known as labyrinthine schwannomas (Figure 6B). They appear as intensely enhancing masses in the labyrinth, and if particularly small, can be difficult to differentiate from labyrinthitis.

Meningiomas are the second most common CPA tumors and account for 5% to 10% of all masses in this region. They show calcification in a quarter of cases, often have a broad dural margin and dural tail, and demonstrate adjacent hyperostosis. ^39,40 If a meningioma extends from the CPA into the IAC, differentiation from an acoustic neuroma can, nevertheless, be quite difficult.

Endolymphatic tumors are rare papillary adenomatous tumors of the retrolabyrinthine temporal bone that can occur sporadically or in association with von Hippel-Lindau syndrome. ^41,42 On CT, these tumors are seen as locally destructive lesions centered over the posteromedial temporal bone (Figure 7A). On MRI, these tumors demonstrate heterogeneous high signal intensity from the presence of blood products and often have solid and protein or blood-filled cystic components (Figure 7B). ^41,43 These tumors are frequently hypervascular and enhance with contrast.

Other neoplastic and cystic disorders that can occur in the IAC and CPA include lipoma, epidermoid, dermoid, arachnoid cyst, lymphoma, or metastasis.

Labyrinthitis

Labyrinthitis can result from many different etiologies, including viral, bacteria, and spirochetal infections of the inner ear; autoimmune diseases; and trauma. Labyrinthitis can also occur secondarily from an extension of infection from the middle ear or from meningitis. ³⁰ Patients often present with SNHL and vertigo. On MRI, labyrinthitis is seen as gadolinium enhancement of the cochlea, vestibule, or semicircular canals (Figure 8). ^44-46 Therefore, acquisition of contrast MR images is essential in patients with suspected labyrinthitis. The main MRI differential diagnosis of labyrinthitis is labyrinthine schwannoma, which is a rare entity. Labyrinthitis causes faint and sometimes diffuse enhancement of the membranous labyrinth. ³⁰ On the other hand, labyrinthine schwannomas are small masses that intensely enhance with gadolinium and are seen as small filling defects on high-resolution FSE T2 or CISS images (Figure 6B). ^31,47,48 A delayed complication of labyrinthitis is postinflammatory fibro-osseous obliteration of the labyrinth, also known as labyrinthitis ossificans. ^31,49 Fibrous replacement of the membranous labyrinth can be seen as decreased signal intensity on ultra-thin-section T2-weighted MRI. In more advanced stages of the disease, CT can demonstrate ossification of the labyrinth. ^30,31 Labyrinthitis ossificans can cause technical challenges for cochlear implantation. ⁵⁰

Other etiologies

Labyrinthine hemorrhage can be caused by trauma, coagulopathy, labyrinthitis, or tumor fistulization, and is seen as increased signal on unenhanced T1-weighted MR images. ^30,45 Perilymphatic fistula is an abnormal communication between the middle and inner ear and an important cause of fluctuating SNHL and vertigo in children and adults. Perilymphatic fistula can be congenital or be the result of trauma, infection, neoplasm, or surgery. It is estimated that 6% of children with SNHL have perilymphatic fistula. ⁵¹ A presumptive diagnosis of perilymphatic fistula can be made by demonstrating air within the inner ear labyrinth (Figure 9). ⁵² There is also a high association of inner and middle ear abnormalities. ⁵³ Pneumolabyrinth or other associated abnormalities are not frequently visualized on imaging studies, and patients may require surgery for diagnosis of perilymphatic fistula. ⁵⁴

A number of otodystrophies can involve the inner ear structures and cause SNHL. Paget's disease of bone, fibrous dysplasia, osteogenesis imperfecta, osteopetrosis, and otosclerosis (otospongiosis) can all involve the osseous labyrinth. Retrofenestral otosclerosis, which primarily involves the cochlea, is often seen in conjunction with fenestral otosclerosis involving the stapes and oval window and causes mixed hearing loss. ⁵⁵ Early in this disease, there is demineralization and rarefaction of bone, and hence the term otospongiosis may be a more appropriate description (Figure 10). ^55,56 Sclerotic changes may appear later in the course of the disease.

Sensorineural hearing loss can result from disorders of the central auditory pathways in the brainstem or thalamus. A variety of ischemic, inflammatory, traumatic, demyelinating, or neoplastic disorders can involve the central pathways and cochlear nuclei in the brainstem (Figure 11). ³⁰ Superficial siderosis of the central nervous system is a rare disease resulting in the accumulation of hemosiderin pigment in the meninges, the brain surface, the spinal cord and the cranial nerves. ⁵⁷ The pigment is deposited as a result of chronic bleeding in the subarachnoid space and can lead to SNHL, along with ataxia and other cranial nerve deficits. ⁵⁸ Lesions of the auditory cortex in the temporal lobes rarely produce SNHL and often only cause problems with higher intellectual processing of auditory information. Nevertheless, knowledge of the anatomy is important so that pertinent findings can be reported.

Cochlear implantation

Cochlear implants are electronic auditory prostheses used to rehabilitate patients with profound or severe SNHL who have lost the hair cells in the cochlea. The microphone and transducer part of the implant is worn externally over the ear and electrodes are passed into the scala tympani of the basal turn of the cochlea via a variety of surgical procedures. They provide a direct electrical stimulation of the residual spiral ganglion cells of the cochlear nerve by bypassing the destroyed hair cells. Hence, success of cochlear implantation is dependent on the presence of a functional cochlear nerve that can transmit the impulses to the brain. ⁵⁹ Before cochlear implantation, imaging evaluation of the ear is mandatory to determine the following: which type of cochlear device to use; which side to implant; when the surgery should be performed; cochlear patency; round window niche access; and the degree of mastoid aeration. ⁵⁰ Since imaging is pivotal in demonstrating contraindications for cochlear implantation, radiologists should be familiar with them. High-resolution FSE T2-weighted MRI and CISS can demonstrate hypoplasia or acquired atrophy of the cochlear nerve, especially in oblique sagittal views (Figure 12). ⁵⁹ A normal cochlear nerve should be approximately as large as the facial nerve. The presence of a narrow internal auditory canal (<2 mm) is associated with absence or severe hypoplasia of the cochlear nerve (Figure 12C). ^60,61 The presence of mastoiditis would predispose the implant to infection and failure. The finding of cochlear fibrosis or ossification may change the choice of which cochlear implant model is used or may alter the method of insertion. The finding of cochlear enhancement may prompt cochlear implantation before the potential development of cochlear ossification. It has been recommended that both CT and MRI be performed in cochlear implant candidates as they can provide complementary information. ^50,59,62

Conclusion

It has been estimated that MRI can determine the cause of hearing impairment in approximately 30% of patients with SNHL. ⁶³ However, despite the great advances in imaging technology, there are a number of congenital and acquired abnormalities of the inner ear that do not have any radiologic manifestations. Many of the membranous labyrinthine dysplasias and metabolic, toxic, infectious, or idiopathic causes of SNHL do not demonstrate any abnormality on CT or MRI. ^30,31 Nevertheless, imaging is an indispensable tool for the evaluation of congenital and acquired causes of SNHL. Knowledge of the various pathologies of the inner ear and central auditory pathways can aid in better consultation and management of patients with SNHL.