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Imaging of Hearing Loss
Otolaryngol Clin N Am 41 (2008) 157–178
Imaging of Hearing Loss Michele B. St. Martin, MD, MBAa, Barry E. Hirsch, MDb,* a
Department of Otolaryngology, University of Florida, PO Box 100264, 1600 SW Archer Road M228, Gainesville, FL 32610, USA b Department of Otolaryngology, Division of Otology/Neurotology, University of Pittsburgh, 203 Lothrop Street, Suite 500, Pittsburgh, PA 15213, USA
Imaging of hearing loss Diagnostic imaging plays a critical role in the evaluation and management of hearing loss. The initial evaluation consists of obtaining a history that should address pertinent otologic information. The rapidity of onset, progressive nature, and presence of unilateral or bilateral symptoms influence the diagnostic possibilities. A history of trauma, otorrhea, pain, associated vertigo, exposure to noise or ototoxic medications, previous ear disease or surgery, and a family history of hearing loss are all pertinent to achieving a diagnosis. The physical examination is critical in determining the patency of the external auditory canal, the integrity and appearance of the tympanic membrane and aeration of the middle ear space, or the presence of otorrhea and tympanic membrane perforation. Evidence of a retraction pocket or cholesteatoma should be identified. Compromise of facial nerve function can occur anywhere along its course from the cortical brain, brainstem, through the CP angle and internal auditory canal, the otic capsule, middle ear, remainder of the temporal bone, and distal to its exit at the stylomastoid foramen. The presence of hearing loss and facial paresis, synkinesis, or paralysis always warrants further investigation with imaging. The use of a tuning fork helps to identify the nature and degree of hearing loss, especially if a conductive component is present. Unless there is an obvious source of hearing loss, such as seen with a cerumen impaction, an audiogram is always obtained. An accurate audiogram identifies the nature and degree of hearing loss and whether one or both ears are involved. Hearing can be categorized as normal, showing a conductive
* Corresponding author. E-mail address: [email protected] (B.E. Hirsch). 0030-6665/08/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.otc.2007.10.007
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hearing loss, sensorineural hearing loss, or mixed loss (conductive and sensorineural). The modalities of imaging that are most pertinent to the evaluation of hearing loss are computed tomography (CT) and magnetic resonance imaging (MRI), depending on the type, degree, and location of the hearing loss. In general, the external auditory canal, the middle ear space including the ossicles, the mastoid and petrous air cell system, and the otic capsule are best visualized with CT using bone algorithm and windowing techniques. Suspicion of retrocochlear pathology (audiometric identification of unilateral reduced word recognition scores) usually reflects involvement of the eighth cranial nerve along the pathway from the cochlea through the internal auditory canal, cerebellopontine angle, and brainstem.
Imaging of conductive hearing loss Conductive hearing loss (CHL) is most commonly caused by pathology of the external or middle ear and on occasion the inner ear. Sometimes the cause is obvious and imaging studies are not required before intervention. Imaging can be valuable in cases in which the cause is uncertain, however. External ear Conductive hearing loss attributable to external ear pathology is related to blockage of the external auditory canal, which can occur because of cerumen impaction, inflammation, foreign body, neoplasm, congenital or acquired stenosis, exostoses, or osteomas. High-resolution CT of the temporal bones is the most appropriate imaging modality to evaluate external ear disorders that result in hearing loss. Cerumen appears isointense to soft tissue and does not cause associated bony erosion. Inflammatory changes attributable to otitis externa appear as thickening of the soft tissues of the external auditory canal; bony erosion is not seen unless there is associated necrotizing otitis externa. External auditory canal cholesteatoma also appears as a soft tissue mass filling a portion of the canal; this process is distinguished from cerumen by localized bony erosion adjacent to the mass (Fig. 1). The erosion can be smooth or irregular, rendering it difficult to distinguish from malignant neoplasm of the external auditory canal (EAC) [1]. Squamous cell carcinoma and basal cell carcinoma account for most of the primary malignancies of the EAC. These tumors appear as soft tissue masses within the EAC usually associated with bony erosion (Fig. 2). Often the bony changes appear irregular or lytic in nature. Stenosis of the EAC may be attributable to soft tissue plug or bony atresia. Congenital atresia is discussed in the following section on congenital hearing loss. Acquired stenosis from infection, trauma, or surgery usually appears as a soft tissue narrowing within the EAC. Physical examination reveals narrowing or complete closure of the external canal. The medial extent
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Fig. 1. Axial CT reveals an erosive mass (arrow) just lateral to the tympanic membrane along the anterior EAC, representing EAC cholesteatoma.
of the scarring process often cannot be determined by examination. CT imaging identifies the presence or absence of middle ear involvement, which critically helps in the planning of operative management (Fig. 3). Exostoses or osteomas represent bony outgrowths in the EAC. Exostoses are usually multiple and are seen in patients who have a history of frequent exposure to cold water. Exostoses are typically bilateral, multiple, nonpedunculated, and arise medially in the bony EAC. In contrast, osteoma is
Fig. 2. Axial CT of the temporal bone showing basal cell carcinoma filling the left external auditory canal. Note irregular erosion (arrowheads) of the posterior bony EAC.
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Fig. 3. Coronal CT of the temporal bone showing acquired EAC stenosis on the right. The soft tissue plug extends medially to the tympanic membrane but does not involve the middle ear space.
usually unilateral, solitary, pedunculated, and arises more laterally in the bony EAC [1]. Middle ear/inner ear A wide range of pathologic conditions cause CHL in the middle or inner ear. CHL of middle ear origin may be attributable to trauma, acute or chronic otitis media, cholesteatoma, fenestral otosclerosis, and benign or malignant neoplasm. Recently, conductive hearing loss attributable to the presence of a functional third window in the inner ear in addition to the round and oval windows has been described. Dehiscence of the superior semicircular canal (SCC) can cause a conductive hearing loss similar to that seen with otosclerosis. When special audiometric tests show acoustic reflexes to be present at a lower threshold and especially if vestibular symptoms exist, recent experience verifies that imaging studies for underlying pathology in CHL should include evaluation of the bony labyrinth [2–4]. Similar to CHL attributable to disease of the external ear canal, the best imaging study to assess CHL of middle or inner ear origin is the high resolution temporal bone CT. When specifically evaluating for a dehiscent SCC, direct coronal images or coronal reformations from less than 1 mm acquisition data are ideal. The Stenver plane (perpendicular to the SCC) and Po¨schl plane (parallel to the SCC) may also be used for equivocal cases, but are typically not necessary for diagnosis [5]. Temporal bone trauma may cause conductive hearing loss because of ossicular discontinuity or hemotympanum. It is common for temporal bone fractures to be associated with ossicular chain disruption, although radiologic evidence of a fracture need not be present for this to occur. Longitudinal fractures are more often associated with CHL. The most common abnormality of the ossicular chain following temporal bone trauma is disarticulation of the incudostapedial joint with dislocation of the incus (Fig. 4).
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Fig. 4. Axial CT of the temporal bone revealing dislocation of the incus on the right, following temporal bone trauma. Note the widened distance (arrow) between the head of the malleus and body of the incus in the epitympanum.
Suspicion of otosclerosis is raised when patients present with new-onset unilateral or bilateral gradually progressive hearing loss in the presence of normal physical examination of the external auditory canal, tympanic membrane, and middle ear. On rare occasion, hyperemia of the mucosa of the promontory can be seen on otoscopy (Schwartze sign). Stapes reflex testing shows absence of the acoustic reflex. Imaging is not routinely obtained. When obtained, though, imaging often demonstrates demineralization at the fissula ante fenestram indicating otosclerosis of the stapes footplate. In the presence of additional sensorineural loss (mixed hearing loss) cochlear otosclerosis is manifested by a halo of lucency surrounding the otic capsule demonstrated on noncontrast bone windowed CT imaging. Inflammatory or infectious processes, such as acute and chronic otitis media (OM) or cholesteatoma, may cause conductive hearing loss because of the presence of fluid or soft tissue in the middle ear space. Acute otitis media appears as opacification of the middle ear and mastoid air cells on high-resolution computed tomography (HRCT) of the temporal bones. Unless there is a fulminant infection with coalescence of mastoid air cells, bony changes are not seen. In the setting of chronic otitis media and CHL, particular attention should be paid to the status of the ossicular chain and the scutum on CT. Erosion of the incudostapedial joint may be an additional cause of CHL in a patient who has chronic OM. Scutal erosion seen on coronal images, or the presence of soft tissue in Prussak space, is suspicious for cholesteatoma formation (Fig. 5). It can be difficult to differentiate between cholesteatoma and fluid or inflammatory tissue secondary to OM on the CT, as all appear as a soft tissue density. The presence of bony erosion alerts
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Fig. 5. Coronal CT of the temporal bone showing acquired cholesteatoma on the left. Soft tissue density is seen in Prussak space, and the scutum is eroded (arrow).
the clinician to the possibility of cholesteatoma. MRI can help to distinguish between fluid, thickened mucosa, and cholesteatoma. Fluid and cholesteatoma both appear bright on T2 images; however, both primary and recurrent cholesteatomas have been found to show increased signal on diffusion weighted imaging, which can distinguish them from fluid [6,7]. Neoplasms of the middle ear include benign entities, such as paraganglioma (glomus tympanicum or jugulotympanicum), schwannoma, adenoma, hemangioma, or malignant processes, such as squamous cell carcinoma, basal cell carcinoma, rhabdomyosarcoma, lymphoma, and carcinoid tumor. Glomus jugulare tumors originate from the glomus body of the jugular bulb and may extend into the hypotympanum and mesotympanum, causing CHL. The typical appearance on HRCT is of an erosive process at the jugular foramen, with permeative bony destruction at the jugular spine (Fig. 6). MRI shows intense enhancement with gadolinium and the typical ‘‘salt and pepper’’ imaging characteristics of paraganglioma attributable to methemoglobin and flow voids. Glomus tympanicum may also cause CHL, but appears on HRCT as a smooth soft tissue density in the mesotympanum without involvement of the jugular foramen. Other benign neoplasms of the middle ear may expand, but not erode, bone. This is the key imaging feature that distinguishes benign from malignant neoplasms.
Imaging of sensorineural hearing loss Imaging of sensorineural hearing loss is often unnecessary in symmetric downward sloping patterns consistent with presbycusis. Sudden or asymmetric sensorineural loss requires diagnostic imaging to evaluate for retrocochlear pathology, however. Recent advances in MR techniques have revealed findings in the inner ear in many of these patients also, potentially increasing the role of MRI in diagnosis of asymmetric and sudden hearing loss beyond identification of CPA masses.
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Fig. 6. Axial CT image of the temporal bone showing a right glomus jugulare tumor. Note the permeative destruction of bone (arrow) centered around the jugular bulb.
Sudden sensorineural hearing loss (SSNHL) is most commonly defined as a loss of at least 30 decibels in three or more contiguous frequencies occurring over 72 hours or less. This entity may be idiopathic or the result of various conditions, including tumor of the internal auditory canal (IAC) or cerebellopontine angle (CPA), labyrinthitis, anterior inferior cerebellar artery (AICA) infarct, multiple sclerosis, pachymeningitis, rupture of an inner ear membrane, viral infection, or rarely compression by a vascular loop. Imaging of SSNHL has traditionally been performed to rule out CPA masses, but increasing technological sophistication of MRI has allowed the detection of inner ear abnormalities also. MRI should be performed in all cases of SSNHL with uncertain cause. Some authors have advocated the use of screening T2 fast spin echo MRI for SSNHL to limit the time needed to scan and to contain costs, with the rationale that IAC/CPA tumors are hypointense relative to CSF on T2-weighted images [8]. Limiting the study in this manner renders the physician unable to detect abnormalities within the IAC or inner ear that may only be seen on T1 images or following administration of contrast medium, however. Additional tumors or processes, such as lipoma, epidermoid tumors, and cholesterol granuloma, require additional specific sequences for differentiation. A commonly used definition of asymmetric sensorineural hearing loss is a difference of 15 or more decibels at two or more frequencies, or a difference of 15% or greater in word recognition scores [9]. Traditionally, auditory brainstem response was used to evaluate patients who had asymmetric hearing loss for the presence of acoustic neuroma. MRI has been shown to be more sensitive for the detection of retrocochlear pathology, however [10,11]. Imaging of asymmetric sensorineural hearing loss is performed
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with the same techniques and for the same reasons as for sudden sensorineural hearing loss. Extra-axial lesions The most commonly encountered mass in the IAC and CPA is acoustic neuroma, or vestibular schwannoma, which compose approximately 85% of CPA masses [12]. Vestibular schwannomas are believed to arise from the area of Scarpa ganglion in the IAC. Typically the tumor originates from the inferior or superior vestibular branch of the eighth cranial nerve at the Obersteiner-Redlich zone. Approximately 10% to 20% of patients who have vestibular schwannoma present with sudden hearing loss. Vestibular schwannoma is best visualized on MRI; however, high-resolution temporal bone CT with contrast may be performed on those patients in whom MRI is contraindicated. Typical MRI findings include an ovoid mass contained within the IAC, or a ‘‘lightbulb-shaped’’ mass extending from the IAC into the CPA (Fig. 7). Vestibular schwannoma may be isolated to the CPA without IAC extension, but this is unusual [13]. The IAC may be enlarged compared with the contralateral side. The mass is isointense on T1 precontrast images and, if small, enhances uniformly with gadolinium. Lesions larger than 1 cm usually have irregular nonenhancing (‘‘cystic’’) regions. Hemorrhage into the tumor may rarely occur. Vestibular schwannomas produce a hypointense filling defect surrounded by brighter CSF signal on T2-weighted images. T2 images may be particularly helpful in delineating the lateral extent of the tumor [14]. This information is most useful in treatment planning. Tumor extending into the lateral fundus,
Fig. 7. Axial T1-weighted MRI of the IAC with gadolinium showing the typical ‘‘light bulb’’ shape of an acoustic neuroma (arrowhead) on the left. The tumor enhances intensely with contrast.
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inferior to the transverse crest, poses greater difficulty in surgical access through a middle fossa approach (Fig. 8). Bilateral vestibular schwannomas confirm the diagnosis of neurofibromatosis type II. Some 3% to 8% of CPA masses are meningiomas [12]. Meningiomas appear similar to vestibular schwannomas in the CPA; however, a slightly different morphology of the tumor distinguishes them from acoustic neuroma. Both tumors enhance with gadolinium, but meningiomas have a more broad-based dural attachment in the posterior fossa and a ‘‘dural tail’’ may be seen (Fig. 9). Meningiomas may also extend into the IAC although they are usually located eccentrically to the porus. Other rare causes of SSNHL attributable to CPA mass may include metastatic lesions to the IAC, particularly from breast or lung carcinoma, lymphoma, or melanoma (Fig. 10A, B) [15]. With the advent of newer imaging techniques and stronger magnet strength, abnormalities of the inner ear or eighth cranial nerve are more readily identified in sudden or asymmetric SNHL. Infectious or autoimmune labyrinthitis is demonstrated on MRI by enhancement of the membranous labyrinth following administration of contrast (Fig. 11). Contrast enhancement is believed to occur because of breakdown of the blood–labyrinthine barrier secondary to inflammation [16]. Enhancement is diffusely seen throughout the cochlea and/or vestibule. The disease may progress to cause fibrosis or ossification of the membranous labyrinth, which can be demonstrated on T2-weighted images as a reduction in fluid volume (Fig. 12) [16]. Evaluation for labyrinthitis ossificans before cochlear implantation can be performed with CT or MRI (Fig. 13). One should keep in mind, however, that fibrosis may not always be obvious on CT [16].
Fig. 8. Coronal T1-weighted MRI with gadolinium of the IAC showing an intracanalicular left acoustic neuroma. The enhancement of the tumor can be seen to extend beyond the dark transverse crest (arrow) at the lateral end of the IAC.
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Fig. 9. Axial T1-weighted MRI of the IAC with gadolinium in a patient who has a large left CPA meningioma. Note the typical ‘‘dural tail’’ (arrow) extending along the posterior border of the tumor.
An intracochlear or intralabyrinthine schwannoma may also cause enhancement of the inner ear; however, the enhancement is intense and discrete (Fig. 14) as opposed to mild and diffuse [14]. Finally, radiation, especially in combination with ototoxic chemotherapeutic agents, may cause ischemic changes in the cochlea leading to hearing loss. This condition is
Fig. 10. (A) Axial head CT performed to evaluate complaints of headache, hearing loss, vertigo, and facial paralysis in a patient who has a history of lung carcinoma. Bone windows revealed extensive lytic destruction of the left IAC extending into the left petrous apex (arrows). (B) T1-weighted axial MRI with gadolinium of the same patient, showing a large enhancing mass filling the left IAC, extending into the left cerebellopontine angle that is responsible for the erosion seen in A.
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Fig. 11. Axial T1-weighted MRI of the IAC with gadolinium in a patient who has Cogan syndrome. Note enhancement of the cochlea and vestibule (arrow) on the left because of autoimmune labyrinthitis.
manifested on MRI as cochlear and labyrinthine enhancement with gadolinium [16]. Recent studies have demonstrated that MRI can detect intracochlear hemorrhage or elevated protein concentration. In particular, three-dimensional (3D) fluid attenuated inversion recovery (FLAIR) sequences show increased signal intensity in the inner ear without contrast in the case of
Fig. 12. High-resolution axial T2-weighted MRI of the IAC in a patient who has right-sided labyrinthitis ossificans. The bright fluid in the normal left cochlea (arrow) is absent on the affected right side (arrowhead).
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Fig. 13. Axial CT of the temporal bone in a patient who has right-sided labyrinthitis ossificans. The view is at the level of the fundus of the right IAC, showing complete ossification of the right cochlea (arrowhead) and near-total ossification of the right labyrinth.
increased protein concentration or hemorrhage [17]. Imaging findings may lag behind clinical symptoms. 3D FLAIR has also been reported to show increased signal in the labyrinth in the setting of SNHL attributable to mumps [18]. Acute or subacute hemorrhage may be seen as increased signal within the cochlea or vestibule on precontrast T1-weighted images, because of presence of methemoglobin [16]. Mild contrast enhancement may or may not be seen, and appearance on T2 sequences is variable. The finding of
Fig. 14. Axial T1-weighted MRI with gadolinium of the IAC in a patient who has a right cochlear schwannoma. The tumor (arrow) enhances discretely with contrast and follows the shape of the basal turn of the cochlea.
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intracochlear hemorrhage underscores the need to obtain precontrast T1weighted imaging, lest increased T1 signal be mistaken for enhancement. Eighth nerve enhancement in the absence of tumor may be found on MRI (Fig. 15). This finding is typically the result of vestibular neuritis. Such enhancement can be mistaken for a small vestibular schwannoma, however, and may be responsible for the low rate of false-positive MRI findings in asymmetric or sudden hearing loss. Other potential causes of enhancement within the IAC other than tumor include vascular loop, arachnoiditis, very small arteriovenous malformation, or edema of cranial nerve VIII [19,20]. Neurosarcoidosis may cause a leptomeningitis involving the IAC that appears nodular, similar to the appearance of leptomeningeal carcinomatosis (Fig. 16). Granulomatous meningitis of bacterial, fungal, or tuberculous origin may also be seen, but the meningeal enhancement in the IAC is typically more linear (Fig. 17). Superficial siderosis has been described as a cause of sensorineural hearing loss [21–23]. The pattern of loss is typically retrocochlear. This condition is caused by repeated subarachnoid hemorrhage and hemosiderin deposition over the cerebellar hemispheres and the eighth cranial nerves. MRI reveals a rim of hypointensity over the cerebellum and brainstem on T2-weighted images [24]. Intra-axial lesions In rare instances, sudden hearing loss can be the sole presenting symptom of AICA infarction. MRI obtained to evaluate sudden hearing loss should include the central auditory pathways for this reason. Reports of this
Fig. 15. Coronal T1-weighted MRI of the IAC with gadolinium in a patient who has right vestibular neuritis. The right vestibulocochlear nerve (arrowheads) is seen to enhance in a linear fashion in the IAC following contrast administration.
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Fig. 16. Axial T1-weighted MRI of the IAC with gadolinium showing enhancement of the right seventh and eighth cranial nerves (arrowheads) in the CPA because of neurosarcoidosis.
phenomenon in the literature usually involve infarcts of the lateral pons [25– 27]. Additionally, hearing loss may be the first presenting symptom of multiple sclerosis in 3% to 5% of patients [28]. Demyelinating plaques may be seen in the central auditory pathway at the level of the pons or medulla (Fig. 18). In addition to leptomeningitis, neurosarcoidosis may cause nodular parenchymal brainstem lesions leading to hearing loss. Abscesses or granulomas within the central auditory pathway may also be seen.
Fig. 17. Coronal T1-weighted MRI of the IAC with gadolinium showing bilateral meningeal enhancement of the IACs (arrows) in a patient who has meningitis.
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Fig. 18. Axial T2-weighted MRI of the IAC in a patient who has multiple sclerosis. T2weighted axial image through the pons reveals a small focus of increased signal (arrow) near the brachium pontis, at the intrapontine genu of the facial nerve. The brachium pontis is a classic location for demyelinating plaques in multiple sclerosis.
Temporal bone trauma Sensorineural hearing loss seen as a consequence of temporal bone trauma may be the result of a fracture of the otic capsule, perilymphatic fistula, or cochlear concussion. To evaluate for fracture of the otic capsule, a high-resolution temporal bone CT should be obtained. Evidence of otic capsule disruption is invariably associated with profound sensorineural hearing loss in the affected ear (Fig. 19) [29]. Perilymphatic fistula may be seen on MRI as increased signal adjacent to the oval window on T2weighted images, or as air density within the labyrinth on CT [30]. Clinical suspicion usually outweighs imaging studies when making a decision to explore for a suspected perilymphatic fistula, however. Cochlear concussion is not detectable with current imaging modalities. Cochlear otosclerosis Cochlear otosclerosis typically manifests as an increase in the bone conduction thresholds in the setting of preexisting conductive hearing loss attributable to fenestral otosclerosis. The patient shows a pattern of mixed hearing loss on the audiogram; the sensorineural component may progress over time. High-resolution temporal bone CT reveals evidence of radiolucent areas within the otic capsule representing otospongiosis (Fig. 20). This finding differs from Paget disease in that it is limited to the otic capsule.
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Fig. 19. Axial CT of the temporal bone showing a left temporal bone fracture that violates the otic capsule. The fracture line may be seen extending from the posterior fossa (arrow) into the basal turn of the cochlea.
Congenital hearing loss Conductive hearing loss Congenital conductive hearing loss may be caused by lateral chain fixation, ossicular malformation, congenital stapes fixation, aural atresia, oval or round window atresia, or a cochlear conductive loss attributable to inner
Fig. 20. Axial CT of the temporal bone in a patient who has bilateral cochlear otosclerosis. Areas of abnormal lucency are seen surrounding the cochlea within the otic capsule on both sides.
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ear anomaly. The imaging study of choice to evaluate for most of these entities is HRCT of the temporal bones, because most causes of congenital conductive hearing loss are due to bony anomalies within the middle or inner ear. Congenital aural atresia is often associated with ossicular chain anomalies, indicating that multiple causes of conductive hearing loss exist in such patients. Inner ear abnormalities are less frequent. A spectrum of findings may be seen clinically and on imaging studies. The EAC may be stenotic or truly atretic; stenosis or atresia may be membranous, bony, or a combination of the two. Otic capsule abnormalities may or may not be present. Of particular importance is evaluation of the ossicular chain. Often the malleus and incus are fused into one malformed ossicle that may or may not articulate with a stapes superstructure (Fig. 21). The status of the middle ear space should thus be determined. The course of the facial nerve is often abnormal in these patients also; the second genu and mastoid segment are frequently displaced anteriorly and laterally posing a risk for injury during reconstructive surgery. Careful attention should be paid to the facial nerve on preoperative imaging to alert the surgeon to potential intraoperative difficulties. Several classification systems exist that attempt to identify good surgical candidates. The system proposed by Jahrsdoerfer forms the basis on which most grading systems are derived. Points are given for the status of various features on preoperative imaging and clinical examination, such as presence of a stapes, oval window, middle ear cleft, course of the facial nerve, presence of a malleus/incus complex, pneumatization of the mastoid cavity, presence of an incudostapedial joint, round window, and the appearance of the external ear [31,32].
Fig. 21. Axial CT of the temporal bone showing fusion of the malleus and incus into a single malformed ossicle (arrow).
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Congenital ossicular abnormalities, lateral chain fixation, and stapes fixation may occur in the absence of congenital stenosis and atresia. The typical history is of a child who has stable congenital conductive hearing loss and a normal external ear and EAC. Imaging findings may indicate the reason for the hearing loss. Disconnection between the incus and stapes can be seen on coronal reformats of the temporal bone CT. Fusion of the malleus and incus gives an appearance similar to the malformed malleus/incus complex seen in aural atresia. Also, lateral chain fixations may appear as a bony bridge connecting the head of the malleus or body of the incus to the surrounding bone of the attic (Fig. 22). In the case of congenital stapes fixation, the CT may appear normal, or may reveal thickening of the footplate [14]. The scan should also be evaluated for a wide IAC with an enlarged connection to the entrance of the cochlear nerves (habenula perforata). This association of findings in males suggests an X-linked inheritance pattern of stapes fixation that when addressed surgically is prone to a profuse leak of perilymph (stapes gusher). Certain malformations of the inner ear cause a conductive hearing loss pattern. Oval window and round window atresia are rare and may be seen on coronal sections of the temporal bone CT. Certain inner ear anomalies cause a cochlear conductive loss. Finding a displaced facial nerve overlying the oval window area may preclude operative intervention. Sensorineural hearing loss Congenital sensorineural hearing loss is the most common birth defect in the United States, with an incidence of approximately 1:1000 live births. With the advent of universal newborn screening in many states, the
Fig. 22. Axial CT of the temporal bone that reveals a bony bridge (arrow) connecting the malleus to the lateral wall of the attic.
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diagnosis of congenital hearing loss is made at an early age. Imaging of congenital sensorineural loss is frequently performed in an attempt to determine an underlying cause. HRCT and MRI have been used in this set of patients, with advantages and disadvantages to each. The HRCT reveals many types of bony inner ear malformations and takes less time to perform, requiring a shorter period of sedation. The MRI provides better visualization of the membranous labyrinth and the ability to evaluate for cochlear nerve hypoplasia or aplasia. In the setting of congenital sensorineural hearing loss, the most common CT abnormality is a large vestibular aqueduct (LVA), defined as measuring greater than 1.5 mm in diameter [33,34]. This disorder may be unilateral or bilateral, and the patient may experience fluctuating hearing loss throughout childhood, with sudden loss possibly precipitated by head trauma. HRCT and T2-weighted MRI reveal the anomaly (Fig. 23A, B). LVA may occur in isolation, or be associated with other inner ear anomalies. Mondini deformity is defined as a normal basal turn of the cochlea with incomplete partition between the middle and apical turns [35]. This malformation may be associated with LVA, enlarged or absent vestibule, or deformities of the semicircular canals (Fig. 24). The common cavity deformity consists of a single cavity representing both cochlea and vestibule, with or without the presence of one or more semicircular canals. Cochlear hypoplasia and aplasia, respectively, are revealed as either a smaller-than-normal cochlea with fewer than two and a half turns, or complete absence of the cochlea. Alternatively, labyrinthine aplasia (Michel aplasia), with no inner ear structures whatsoever, may be seen. HRCT successfully identifies these bony abnormalities of the otic capsule. MRI is needed to identify hypoplasia or aplasia of the cochlear nerve, however, which have important implications for cochlear implantation. If a narrow IAC is seen on HRCT it should
Fig. 23. Axial CT of the temporal bone (A) and T2-weighted MRI (B) showing large vestibular aqueducts. (A) Unilateral left LVA (arrow). (B) Bilateral LVAs (arrows).
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Fig. 24. Axial CT of the temporal bone showing left-sided Mondini deformity. There is an incomplete partition between the middle and apical turns (arrowhead), associated with a large vestibular aqueduct (arrow).
raise the index of suspicion for cochlear nerve hypoplasia or aplasia, and MRI of the IACs should be obtained [36]. CT and current MRI technology are unable to identify congenital abnormalities of the membranous labyrinth, such as Scheibe dysplasia. Inner ear anomalies may be associated with syndromic hearing loss. CHARGE syndrome is associated with abnormalities of the middle and inner ear. Common findings include absent semicircular canals, cochlear dysplasia, cochlear nerve atresia, absence of the cochlear nerve aperture, aplasia of the oval or round window, LVA, and anomalous facial nerve course, along with ossicular chain abnormalities [37]. Branchiootorenal syndrome leads to cochlear hypoplasia or dysplasia, and can be associated with LVA, ossicular chain abnormalities, and cochlear nerve hypoplasia [38]. X-linked mixed deafness can be syndromic or nonsyndromic, and is associated with a dilated lateral IAC and an abnormally patent connection between the IAC and the cochlea, leading to a CSF gusher if stapedectomy is attempted [39].
Summary A wide range of pathologies involving the external, middle, and inner ear contribute to conductive and sensorineural hearing loss. HRCT of the temporal bone and MRI are the preferred imaging modalities to evaluate the ear structures for causes of hearing loss, with the specific type of hearing loss and location of defect dictating which type of imaging is preferred. In general, the external auditory canal, middle ear space, mastoid, petrous apex,
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and otic capsule are best visualized with CT, whereas suspicion of retrocochlear pathology warrants MRI.
References [1] Remley KB, Swartz JD, Harnsberger HR. The external auditory canal. In: Swartz JD, Harnsberger HR, editors. Imaging of the temporal bone. New York: Thieme; 1998. p. 16–46. [2] Cox KM, Lee DJ, Carey JP, et al. Dehiscence of bone overlying the superior semicircular canal as a cause of an air-bone gap on audiometry: a case study. Am J Audiol 2003;12(1): 11–6. [3] Minor LB, Carey JP, Cremer PD, et al. Dehiscence of bone overlying the superior canal as a cause of apparent conductive hearing loss. Otol Neurotol 2003;24(2):270–8. [4] Minor LB. Clinical manifestations of superior semicircular canal dehiscence. Laryngoscope 2005;115(10):1717–27. [5] Branstetter BF 4th, Harrigal C, Escott EJ, et al. Superior semicircular canal dehiscence: oblique reformatted CT images for diagnosis. Radiology 2006;238(3):938–42. [6] Stasolla A, Magliulo G, Parrotto D, et al. Detection of postoperative relapsing/residual cholesteatomas with diffusion-weighted echo-planar magnetic resonance imaging. Otol Neurotol 2004;25(6):879–84. [7] Vercruysse JP, De Foer B, Pouillon M, et al. The value of diffusion-weighted MR imaging in the diagnosis of primary acquired and residual cholesteatoma: a surgical verified study of 100 patients. Eur Radiol 2006;16(7):1461–7. [8] Daniels RL, Shelton C, Harnsberger HR. Ultra high resolution nonenhanced fast spin echo magnetic resonance imaging: cost-effective screening for acoustic neuroma in patients with sudden sensorineural hearing loss. Otolaryngol Head Neck Surg 1998;119(4):364–9. [9] Ruckenstein MJ, Cueva RA, Morrison DH, et al. A prospective study of ABR and MRI in the screening for vestibular schwannomas. Am J Otol 1996;17(2):317–20. [10] Levine SC, Antonelli PJ, Le CT, et al. Relative value of diagnostic tests for small acoustic neuromas. Am J Otol 1991;12(5):341–6. [11] Wilson DF, Talbot JM, Mills L. A critical appraisal of the role of auditory brain stem response and magnetic resonance imaging in acoustic neuroma diagnosis. Am J Otol 1997; 18(5):673–81. [12] Hasso AN, Smith DS. The cerebellopontine angle. Semin Ultrasound CT MR 1989;10(3): 280–301. [13] Swartz JD, Harnsberger HR. The middle ear and mastoid. In: Swartz JD, Harnsberger HR, editors. Imaging of the temporal bone. New York: Thieme; 1998. p. 47–169. [14] Swartz JD, Harnsberger HR. The otic capsule and otodystrophies. In: Swartz JD, Harnsberger HR, editors. Imaging of the temporal bone. New York: Thieme; 1998. p. 240–317. [15] Yuh WT, Mayr-Yuh NA, Koci TM, et al. Metastatic lesions involving the cerebellopontine angle. AJNR Am J Neuroradiol 1993;14(1):99–106. [16] Hegarty JL, Patel S, Fischbein N, et al. The value of enhanced magnetic resonance imaging in the evaluation of endocochlear disease. Laryngoscope 2002;112(1):8–17. [17] Sugiura M, Naganawa S, Teranishi M, et al. Three-dimensional fluid-attenuated inversion recovery magnetic resonance imaging findings in patients with sudden sensorineural hearing loss. Laryngoscope 2006;116(8):1451–4. [18] Otake H, Sugiura M, Naganawa S, et al. 3D-FLAIR magnetic resonance imaging in the evaluation of mumps deafness. Int J Pediatr Otorhinolaryngol 2006;70(12):2115–7. [19] Arriaga MA, Carrier D, Houston GD. False-positive magnetic resonance imaging of small internal auditory canal tumors: a clinical, radiologic, and pathologic correlation studyOtolaryngol Head Neck Surg 1995;113(1):61–70. [20] Maeta M, Saito R, Nameki H. False-positive magnetic resonance image in the diagnosis of small acoustic neuroma. J Laryngol Otol 2001;115(10):842–4.
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[21] Pribitkin EA, Rondinella L, Rosenberg S, et al. Superficial siderosis of the central nervous system: an underdiagnosed cause of sensorineural hearing loss and ataxia. Am J Otol 1994;15(3):415–8. [22] Dodson KM, Sismanis A, Nance WE. Superficial siderosis: a potentially important cause of genetic as well as non-genetic deafness. Am J Med Genet A 2004;130(1):22–5. [23] Vibert D, Hausler R, Lovblad KO, et al. Hearing loss and vertigo in superficial siderosis of the central nervous system. Am J Otolaryngol 2004;25(2):142–9. [24] Kale SU, Donaldson I, West RJ, et al. Superficial siderosis of the meninges and its otolaryngologic connection: a series of five patients. Otol Neurotol 2003;24(1):90–5. [25] Lee H, Ahn BH, Baloh RW. Sudden deafness with vertigo as a sole manifestation of anterior inferior cerebellar artery infarction. J Neurol Sci 2004;222(1–2):105–7. [26] Rajesh R, Rafeequ M, Girija AS. Anterior inferior cerebellar artery infarct with unilateral deafness. J Assoc Physicians India 2004;52:333–4. [27] Yi HA, Lee SR, Lee H, et al. Sudden deafness as a sign of stroke with normal diffusionweighted brain MRI. Acta Otolaryngol 2005;125(10):1119–21. [28] Drulovi B, Ribari-Jankes K, Kosti V, et al. Multiple sclerosis as the cause of sudden ‘‘pontine’’ deafness. Audiology 1994;33(4):195–201. [29] Brodie HA, Thompson TC. Management of complications from 820 temporal bone fractures. Am J Otol 1997;18(2):188–97. [30] Nakashima T, Sone M, Teranishi MA, et al. Imaging of a congenital perilymphatic fistula. Int J Pediatr Otorhinolaryngol 2003;67(4):421–5. [31] Jahrsdoerfer RA, Yeakley JW, Aguilar EA, et al. Grading system for the selection of patients with congenital aural atresia. Am J Otol 1992;13(1):6–12. [32] Yeakley JW, Jahrsdoerfer RA. CT evaluation of congenital aural atresia: what the radiologist and surgeon need to know. J Comput Assist Tomogr 1996;20(5):724–31. [33] Valvassori GE, Clemis JD. The large vestibular aqueduct syndrome. Laryngoscope 1978; 88(5):723–8. [34] Robson CD. Congenital hearing impairment. Pediatr Radiol 2006;36(4):309–24. [35] Mondini C. Minor works of Carlo Mondini: the anatomical section of a boy born deaf. Am J Otol 1997;18(3):288–93. [36] Sennaroglu L, Saatci I, Aralasmak A, et al. Magnetic resonance imaging versus computed tomography in pre-operative evaluation of cochlear implant candidates with congenital hearing loss. J Laryngol Otol 2002;116(10):804–10. [37] Morimoto AK, Wiggins RH 3rd, Hudgins PA, et al. Absent semicircular canals in CHARGE syndrome: radiologic spectrum of findings. AJNR Am J Neuroradiol 2006; 27(8):1663–71. [38] Ceruti S, Stinckens C, Cremers CW, et al. Temporal bone anomalies in the branchio-oto-renal syndrome: detailed computed tomographic and magnetic resonance imaging findings. Otol Neurotol 2002;23(2):200–7. [39] Phelps PD, Reardon W, Pembrey M, et al. X-linked deafness, stapes gushers and a distinctive defect of the inner ear. Neuroradiology 1991;33(4):326–30.
Imaging of Hearing Loss Michele B. St. Martin, MD, MBAa, Barry E. Hirsch, MDb,* a
Department of Otolaryngology, University of Florida, PO Box 100264, 1600 SW Archer Road M228, Gainesville, FL 32610, USA b Department of Otolaryngology, Division of Otology/Neurotology, University of Pittsburgh, 203 Lothrop Street, Suite 500, Pittsburgh, PA 15213, USA
Imaging of hearing loss Diagnostic imaging plays a critical role in the evaluation and management of hearing loss. The initial evaluation consists of obtaining a history that should address pertinent otologic information. The rapidity of onset, progressive nature, and presence of unilateral or bilateral symptoms influence the diagnostic possibilities. A history of trauma, otorrhea, pain, associated vertigo, exposure to noise or ototoxic medications, previous ear disease or surgery, and a family history of hearing loss are all pertinent to achieving a diagnosis. The physical examination is critical in determining the patency of the external auditory canal, the integrity and appearance of the tympanic membrane and aeration of the middle ear space, or the presence of otorrhea and tympanic membrane perforation. Evidence of a retraction pocket or cholesteatoma should be identified. Compromise of facial nerve function can occur anywhere along its course from the cortical brain, brainstem, through the CP angle and internal auditory canal, the otic capsule, middle ear, remainder of the temporal bone, and distal to its exit at the stylomastoid foramen. The presence of hearing loss and facial paresis, synkinesis, or paralysis always warrants further investigation with imaging. The use of a tuning fork helps to identify the nature and degree of hearing loss, especially if a conductive component is present. Unless there is an obvious source of hearing loss, such as seen with a cerumen impaction, an audiogram is always obtained. An accurate audiogram identifies the nature and degree of hearing loss and whether one or both ears are involved. Hearing can be categorized as normal, showing a conductive
* Corresponding author. E-mail address: [email protected] (B.E. Hirsch). 0030-6665/08/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.otc.2007.10.007
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hearing loss, sensorineural hearing loss, or mixed loss (conductive and sensorineural). The modalities of imaging that are most pertinent to the evaluation of hearing loss are computed tomography (CT) and magnetic resonance imaging (MRI), depending on the type, degree, and location of the hearing loss. In general, the external auditory canal, the middle ear space including the ossicles, the mastoid and petrous air cell system, and the otic capsule are best visualized with CT using bone algorithm and windowing techniques. Suspicion of retrocochlear pathology (audiometric identification of unilateral reduced word recognition scores) usually reflects involvement of the eighth cranial nerve along the pathway from the cochlea through the internal auditory canal, cerebellopontine angle, and brainstem.
Imaging of conductive hearing loss Conductive hearing loss (CHL) is most commonly caused by pathology of the external or middle ear and on occasion the inner ear. Sometimes the cause is obvious and imaging studies are not required before intervention. Imaging can be valuable in cases in which the cause is uncertain, however. External ear Conductive hearing loss attributable to external ear pathology is related to blockage of the external auditory canal, which can occur because of cerumen impaction, inflammation, foreign body, neoplasm, congenital or acquired stenosis, exostoses, or osteomas. High-resolution CT of the temporal bones is the most appropriate imaging modality to evaluate external ear disorders that result in hearing loss. Cerumen appears isointense to soft tissue and does not cause associated bony erosion. Inflammatory changes attributable to otitis externa appear as thickening of the soft tissues of the external auditory canal; bony erosion is not seen unless there is associated necrotizing otitis externa. External auditory canal cholesteatoma also appears as a soft tissue mass filling a portion of the canal; this process is distinguished from cerumen by localized bony erosion adjacent to the mass (Fig. 1). The erosion can be smooth or irregular, rendering it difficult to distinguish from malignant neoplasm of the external auditory canal (EAC) [1]. Squamous cell carcinoma and basal cell carcinoma account for most of the primary malignancies of the EAC. These tumors appear as soft tissue masses within the EAC usually associated with bony erosion (Fig. 2). Often the bony changes appear irregular or lytic in nature. Stenosis of the EAC may be attributable to soft tissue plug or bony atresia. Congenital atresia is discussed in the following section on congenital hearing loss. Acquired stenosis from infection, trauma, or surgery usually appears as a soft tissue narrowing within the EAC. Physical examination reveals narrowing or complete closure of the external canal. The medial extent
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Fig. 1. Axial CT reveals an erosive mass (arrow) just lateral to the tympanic membrane along the anterior EAC, representing EAC cholesteatoma.
of the scarring process often cannot be determined by examination. CT imaging identifies the presence or absence of middle ear involvement, which critically helps in the planning of operative management (Fig. 3). Exostoses or osteomas represent bony outgrowths in the EAC. Exostoses are usually multiple and are seen in patients who have a history of frequent exposure to cold water. Exostoses are typically bilateral, multiple, nonpedunculated, and arise medially in the bony EAC. In contrast, osteoma is
Fig. 2. Axial CT of the temporal bone showing basal cell carcinoma filling the left external auditory canal. Note irregular erosion (arrowheads) of the posterior bony EAC.
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Fig. 3. Coronal CT of the temporal bone showing acquired EAC stenosis on the right. The soft tissue plug extends medially to the tympanic membrane but does not involve the middle ear space.
usually unilateral, solitary, pedunculated, and arises more laterally in the bony EAC [1]. Middle ear/inner ear A wide range of pathologic conditions cause CHL in the middle or inner ear. CHL of middle ear origin may be attributable to trauma, acute or chronic otitis media, cholesteatoma, fenestral otosclerosis, and benign or malignant neoplasm. Recently, conductive hearing loss attributable to the presence of a functional third window in the inner ear in addition to the round and oval windows has been described. Dehiscence of the superior semicircular canal (SCC) can cause a conductive hearing loss similar to that seen with otosclerosis. When special audiometric tests show acoustic reflexes to be present at a lower threshold and especially if vestibular symptoms exist, recent experience verifies that imaging studies for underlying pathology in CHL should include evaluation of the bony labyrinth [2–4]. Similar to CHL attributable to disease of the external ear canal, the best imaging study to assess CHL of middle or inner ear origin is the high resolution temporal bone CT. When specifically evaluating for a dehiscent SCC, direct coronal images or coronal reformations from less than 1 mm acquisition data are ideal. The Stenver plane (perpendicular to the SCC) and Po¨schl plane (parallel to the SCC) may also be used for equivocal cases, but are typically not necessary for diagnosis [5]. Temporal bone trauma may cause conductive hearing loss because of ossicular discontinuity or hemotympanum. It is common for temporal bone fractures to be associated with ossicular chain disruption, although radiologic evidence of a fracture need not be present for this to occur. Longitudinal fractures are more often associated with CHL. The most common abnormality of the ossicular chain following temporal bone trauma is disarticulation of the incudostapedial joint with dislocation of the incus (Fig. 4).
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Fig. 4. Axial CT of the temporal bone revealing dislocation of the incus on the right, following temporal bone trauma. Note the widened distance (arrow) between the head of the malleus and body of the incus in the epitympanum.
Suspicion of otosclerosis is raised when patients present with new-onset unilateral or bilateral gradually progressive hearing loss in the presence of normal physical examination of the external auditory canal, tympanic membrane, and middle ear. On rare occasion, hyperemia of the mucosa of the promontory can be seen on otoscopy (Schwartze sign). Stapes reflex testing shows absence of the acoustic reflex. Imaging is not routinely obtained. When obtained, though, imaging often demonstrates demineralization at the fissula ante fenestram indicating otosclerosis of the stapes footplate. In the presence of additional sensorineural loss (mixed hearing loss) cochlear otosclerosis is manifested by a halo of lucency surrounding the otic capsule demonstrated on noncontrast bone windowed CT imaging. Inflammatory or infectious processes, such as acute and chronic otitis media (OM) or cholesteatoma, may cause conductive hearing loss because of the presence of fluid or soft tissue in the middle ear space. Acute otitis media appears as opacification of the middle ear and mastoid air cells on high-resolution computed tomography (HRCT) of the temporal bones. Unless there is a fulminant infection with coalescence of mastoid air cells, bony changes are not seen. In the setting of chronic otitis media and CHL, particular attention should be paid to the status of the ossicular chain and the scutum on CT. Erosion of the incudostapedial joint may be an additional cause of CHL in a patient who has chronic OM. Scutal erosion seen on coronal images, or the presence of soft tissue in Prussak space, is suspicious for cholesteatoma formation (Fig. 5). It can be difficult to differentiate between cholesteatoma and fluid or inflammatory tissue secondary to OM on the CT, as all appear as a soft tissue density. The presence of bony erosion alerts
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Fig. 5. Coronal CT of the temporal bone showing acquired cholesteatoma on the left. Soft tissue density is seen in Prussak space, and the scutum is eroded (arrow).
the clinician to the possibility of cholesteatoma. MRI can help to distinguish between fluid, thickened mucosa, and cholesteatoma. Fluid and cholesteatoma both appear bright on T2 images; however, both primary and recurrent cholesteatomas have been found to show increased signal on diffusion weighted imaging, which can distinguish them from fluid [6,7]. Neoplasms of the middle ear include benign entities, such as paraganglioma (glomus tympanicum or jugulotympanicum), schwannoma, adenoma, hemangioma, or malignant processes, such as squamous cell carcinoma, basal cell carcinoma, rhabdomyosarcoma, lymphoma, and carcinoid tumor. Glomus jugulare tumors originate from the glomus body of the jugular bulb and may extend into the hypotympanum and mesotympanum, causing CHL. The typical appearance on HRCT is of an erosive process at the jugular foramen, with permeative bony destruction at the jugular spine (Fig. 6). MRI shows intense enhancement with gadolinium and the typical ‘‘salt and pepper’’ imaging characteristics of paraganglioma attributable to methemoglobin and flow voids. Glomus tympanicum may also cause CHL, but appears on HRCT as a smooth soft tissue density in the mesotympanum without involvement of the jugular foramen. Other benign neoplasms of the middle ear may expand, but not erode, bone. This is the key imaging feature that distinguishes benign from malignant neoplasms.
Imaging of sensorineural hearing loss Imaging of sensorineural hearing loss is often unnecessary in symmetric downward sloping patterns consistent with presbycusis. Sudden or asymmetric sensorineural loss requires diagnostic imaging to evaluate for retrocochlear pathology, however. Recent advances in MR techniques have revealed findings in the inner ear in many of these patients also, potentially increasing the role of MRI in diagnosis of asymmetric and sudden hearing loss beyond identification of CPA masses.
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Fig. 6. Axial CT image of the temporal bone showing a right glomus jugulare tumor. Note the permeative destruction of bone (arrow) centered around the jugular bulb.
Sudden sensorineural hearing loss (SSNHL) is most commonly defined as a loss of at least 30 decibels in three or more contiguous frequencies occurring over 72 hours or less. This entity may be idiopathic or the result of various conditions, including tumor of the internal auditory canal (IAC) or cerebellopontine angle (CPA), labyrinthitis, anterior inferior cerebellar artery (AICA) infarct, multiple sclerosis, pachymeningitis, rupture of an inner ear membrane, viral infection, or rarely compression by a vascular loop. Imaging of SSNHL has traditionally been performed to rule out CPA masses, but increasing technological sophistication of MRI has allowed the detection of inner ear abnormalities also. MRI should be performed in all cases of SSNHL with uncertain cause. Some authors have advocated the use of screening T2 fast spin echo MRI for SSNHL to limit the time needed to scan and to contain costs, with the rationale that IAC/CPA tumors are hypointense relative to CSF on T2-weighted images [8]. Limiting the study in this manner renders the physician unable to detect abnormalities within the IAC or inner ear that may only be seen on T1 images or following administration of contrast medium, however. Additional tumors or processes, such as lipoma, epidermoid tumors, and cholesterol granuloma, require additional specific sequences for differentiation. A commonly used definition of asymmetric sensorineural hearing loss is a difference of 15 or more decibels at two or more frequencies, or a difference of 15% or greater in word recognition scores [9]. Traditionally, auditory brainstem response was used to evaluate patients who had asymmetric hearing loss for the presence of acoustic neuroma. MRI has been shown to be more sensitive for the detection of retrocochlear pathology, however [10,11]. Imaging of asymmetric sensorineural hearing loss is performed
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with the same techniques and for the same reasons as for sudden sensorineural hearing loss. Extra-axial lesions The most commonly encountered mass in the IAC and CPA is acoustic neuroma, or vestibular schwannoma, which compose approximately 85% of CPA masses [12]. Vestibular schwannomas are believed to arise from the area of Scarpa ganglion in the IAC. Typically the tumor originates from the inferior or superior vestibular branch of the eighth cranial nerve at the Obersteiner-Redlich zone. Approximately 10% to 20% of patients who have vestibular schwannoma present with sudden hearing loss. Vestibular schwannoma is best visualized on MRI; however, high-resolution temporal bone CT with contrast may be performed on those patients in whom MRI is contraindicated. Typical MRI findings include an ovoid mass contained within the IAC, or a ‘‘lightbulb-shaped’’ mass extending from the IAC into the CPA (Fig. 7). Vestibular schwannoma may be isolated to the CPA without IAC extension, but this is unusual [13]. The IAC may be enlarged compared with the contralateral side. The mass is isointense on T1 precontrast images and, if small, enhances uniformly with gadolinium. Lesions larger than 1 cm usually have irregular nonenhancing (‘‘cystic’’) regions. Hemorrhage into the tumor may rarely occur. Vestibular schwannomas produce a hypointense filling defect surrounded by brighter CSF signal on T2-weighted images. T2 images may be particularly helpful in delineating the lateral extent of the tumor [14]. This information is most useful in treatment planning. Tumor extending into the lateral fundus,
Fig. 7. Axial T1-weighted MRI of the IAC with gadolinium showing the typical ‘‘light bulb’’ shape of an acoustic neuroma (arrowhead) on the left. The tumor enhances intensely with contrast.
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inferior to the transverse crest, poses greater difficulty in surgical access through a middle fossa approach (Fig. 8). Bilateral vestibular schwannomas confirm the diagnosis of neurofibromatosis type II. Some 3% to 8% of CPA masses are meningiomas [12]. Meningiomas appear similar to vestibular schwannomas in the CPA; however, a slightly different morphology of the tumor distinguishes them from acoustic neuroma. Both tumors enhance with gadolinium, but meningiomas have a more broad-based dural attachment in the posterior fossa and a ‘‘dural tail’’ may be seen (Fig. 9). Meningiomas may also extend into the IAC although they are usually located eccentrically to the porus. Other rare causes of SSNHL attributable to CPA mass may include metastatic lesions to the IAC, particularly from breast or lung carcinoma, lymphoma, or melanoma (Fig. 10A, B) [15]. With the advent of newer imaging techniques and stronger magnet strength, abnormalities of the inner ear or eighth cranial nerve are more readily identified in sudden or asymmetric SNHL. Infectious or autoimmune labyrinthitis is demonstrated on MRI by enhancement of the membranous labyrinth following administration of contrast (Fig. 11). Contrast enhancement is believed to occur because of breakdown of the blood–labyrinthine barrier secondary to inflammation [16]. Enhancement is diffusely seen throughout the cochlea and/or vestibule. The disease may progress to cause fibrosis or ossification of the membranous labyrinth, which can be demonstrated on T2-weighted images as a reduction in fluid volume (Fig. 12) [16]. Evaluation for labyrinthitis ossificans before cochlear implantation can be performed with CT or MRI (Fig. 13). One should keep in mind, however, that fibrosis may not always be obvious on CT [16].
Fig. 8. Coronal T1-weighted MRI with gadolinium of the IAC showing an intracanalicular left acoustic neuroma. The enhancement of the tumor can be seen to extend beyond the dark transverse crest (arrow) at the lateral end of the IAC.
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Fig. 9. Axial T1-weighted MRI of the IAC with gadolinium in a patient who has a large left CPA meningioma. Note the typical ‘‘dural tail’’ (arrow) extending along the posterior border of the tumor.
An intracochlear or intralabyrinthine schwannoma may also cause enhancement of the inner ear; however, the enhancement is intense and discrete (Fig. 14) as opposed to mild and diffuse [14]. Finally, radiation, especially in combination with ototoxic chemotherapeutic agents, may cause ischemic changes in the cochlea leading to hearing loss. This condition is
Fig. 10. (A) Axial head CT performed to evaluate complaints of headache, hearing loss, vertigo, and facial paralysis in a patient who has a history of lung carcinoma. Bone windows revealed extensive lytic destruction of the left IAC extending into the left petrous apex (arrows). (B) T1-weighted axial MRI with gadolinium of the same patient, showing a large enhancing mass filling the left IAC, extending into the left cerebellopontine angle that is responsible for the erosion seen in A.
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Fig. 11. Axial T1-weighted MRI of the IAC with gadolinium in a patient who has Cogan syndrome. Note enhancement of the cochlea and vestibule (arrow) on the left because of autoimmune labyrinthitis.
manifested on MRI as cochlear and labyrinthine enhancement with gadolinium [16]. Recent studies have demonstrated that MRI can detect intracochlear hemorrhage or elevated protein concentration. In particular, three-dimensional (3D) fluid attenuated inversion recovery (FLAIR) sequences show increased signal intensity in the inner ear without contrast in the case of
Fig. 12. High-resolution axial T2-weighted MRI of the IAC in a patient who has right-sided labyrinthitis ossificans. The bright fluid in the normal left cochlea (arrow) is absent on the affected right side (arrowhead).
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Fig. 13. Axial CT of the temporal bone in a patient who has right-sided labyrinthitis ossificans. The view is at the level of the fundus of the right IAC, showing complete ossification of the right cochlea (arrowhead) and near-total ossification of the right labyrinth.
increased protein concentration or hemorrhage [17]. Imaging findings may lag behind clinical symptoms. 3D FLAIR has also been reported to show increased signal in the labyrinth in the setting of SNHL attributable to mumps [18]. Acute or subacute hemorrhage may be seen as increased signal within the cochlea or vestibule on precontrast T1-weighted images, because of presence of methemoglobin [16]. Mild contrast enhancement may or may not be seen, and appearance on T2 sequences is variable. The finding of
Fig. 14. Axial T1-weighted MRI with gadolinium of the IAC in a patient who has a right cochlear schwannoma. The tumor (arrow) enhances discretely with contrast and follows the shape of the basal turn of the cochlea.
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intracochlear hemorrhage underscores the need to obtain precontrast T1weighted imaging, lest increased T1 signal be mistaken for enhancement. Eighth nerve enhancement in the absence of tumor may be found on MRI (Fig. 15). This finding is typically the result of vestibular neuritis. Such enhancement can be mistaken for a small vestibular schwannoma, however, and may be responsible for the low rate of false-positive MRI findings in asymmetric or sudden hearing loss. Other potential causes of enhancement within the IAC other than tumor include vascular loop, arachnoiditis, very small arteriovenous malformation, or edema of cranial nerve VIII [19,20]. Neurosarcoidosis may cause a leptomeningitis involving the IAC that appears nodular, similar to the appearance of leptomeningeal carcinomatosis (Fig. 16). Granulomatous meningitis of bacterial, fungal, or tuberculous origin may also be seen, but the meningeal enhancement in the IAC is typically more linear (Fig. 17). Superficial siderosis has been described as a cause of sensorineural hearing loss [21–23]. The pattern of loss is typically retrocochlear. This condition is caused by repeated subarachnoid hemorrhage and hemosiderin deposition over the cerebellar hemispheres and the eighth cranial nerves. MRI reveals a rim of hypointensity over the cerebellum and brainstem on T2-weighted images [24]. Intra-axial lesions In rare instances, sudden hearing loss can be the sole presenting symptom of AICA infarction. MRI obtained to evaluate sudden hearing loss should include the central auditory pathways for this reason. Reports of this
Fig. 15. Coronal T1-weighted MRI of the IAC with gadolinium in a patient who has right vestibular neuritis. The right vestibulocochlear nerve (arrowheads) is seen to enhance in a linear fashion in the IAC following contrast administration.
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Fig. 16. Axial T1-weighted MRI of the IAC with gadolinium showing enhancement of the right seventh and eighth cranial nerves (arrowheads) in the CPA because of neurosarcoidosis.
phenomenon in the literature usually involve infarcts of the lateral pons [25– 27]. Additionally, hearing loss may be the first presenting symptom of multiple sclerosis in 3% to 5% of patients [28]. Demyelinating plaques may be seen in the central auditory pathway at the level of the pons or medulla (Fig. 18). In addition to leptomeningitis, neurosarcoidosis may cause nodular parenchymal brainstem lesions leading to hearing loss. Abscesses or granulomas within the central auditory pathway may also be seen.
Fig. 17. Coronal T1-weighted MRI of the IAC with gadolinium showing bilateral meningeal enhancement of the IACs (arrows) in a patient who has meningitis.
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Fig. 18. Axial T2-weighted MRI of the IAC in a patient who has multiple sclerosis. T2weighted axial image through the pons reveals a small focus of increased signal (arrow) near the brachium pontis, at the intrapontine genu of the facial nerve. The brachium pontis is a classic location for demyelinating plaques in multiple sclerosis.
Temporal bone trauma Sensorineural hearing loss seen as a consequence of temporal bone trauma may be the result of a fracture of the otic capsule, perilymphatic fistula, or cochlear concussion. To evaluate for fracture of the otic capsule, a high-resolution temporal bone CT should be obtained. Evidence of otic capsule disruption is invariably associated with profound sensorineural hearing loss in the affected ear (Fig. 19) [29]. Perilymphatic fistula may be seen on MRI as increased signal adjacent to the oval window on T2weighted images, or as air density within the labyrinth on CT [30]. Clinical suspicion usually outweighs imaging studies when making a decision to explore for a suspected perilymphatic fistula, however. Cochlear concussion is not detectable with current imaging modalities. Cochlear otosclerosis Cochlear otosclerosis typically manifests as an increase in the bone conduction thresholds in the setting of preexisting conductive hearing loss attributable to fenestral otosclerosis. The patient shows a pattern of mixed hearing loss on the audiogram; the sensorineural component may progress over time. High-resolution temporal bone CT reveals evidence of radiolucent areas within the otic capsule representing otospongiosis (Fig. 20). This finding differs from Paget disease in that it is limited to the otic capsule.
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Fig. 19. Axial CT of the temporal bone showing a left temporal bone fracture that violates the otic capsule. The fracture line may be seen extending from the posterior fossa (arrow) into the basal turn of the cochlea.
Congenital hearing loss Conductive hearing loss Congenital conductive hearing loss may be caused by lateral chain fixation, ossicular malformation, congenital stapes fixation, aural atresia, oval or round window atresia, or a cochlear conductive loss attributable to inner
Fig. 20. Axial CT of the temporal bone in a patient who has bilateral cochlear otosclerosis. Areas of abnormal lucency are seen surrounding the cochlea within the otic capsule on both sides.
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ear anomaly. The imaging study of choice to evaluate for most of these entities is HRCT of the temporal bones, because most causes of congenital conductive hearing loss are due to bony anomalies within the middle or inner ear. Congenital aural atresia is often associated with ossicular chain anomalies, indicating that multiple causes of conductive hearing loss exist in such patients. Inner ear abnormalities are less frequent. A spectrum of findings may be seen clinically and on imaging studies. The EAC may be stenotic or truly atretic; stenosis or atresia may be membranous, bony, or a combination of the two. Otic capsule abnormalities may or may not be present. Of particular importance is evaluation of the ossicular chain. Often the malleus and incus are fused into one malformed ossicle that may or may not articulate with a stapes superstructure (Fig. 21). The status of the middle ear space should thus be determined. The course of the facial nerve is often abnormal in these patients also; the second genu and mastoid segment are frequently displaced anteriorly and laterally posing a risk for injury during reconstructive surgery. Careful attention should be paid to the facial nerve on preoperative imaging to alert the surgeon to potential intraoperative difficulties. Several classification systems exist that attempt to identify good surgical candidates. The system proposed by Jahrsdoerfer forms the basis on which most grading systems are derived. Points are given for the status of various features on preoperative imaging and clinical examination, such as presence of a stapes, oval window, middle ear cleft, course of the facial nerve, presence of a malleus/incus complex, pneumatization of the mastoid cavity, presence of an incudostapedial joint, round window, and the appearance of the external ear [31,32].
Fig. 21. Axial CT of the temporal bone showing fusion of the malleus and incus into a single malformed ossicle (arrow).
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Congenital ossicular abnormalities, lateral chain fixation, and stapes fixation may occur in the absence of congenital stenosis and atresia. The typical history is of a child who has stable congenital conductive hearing loss and a normal external ear and EAC. Imaging findings may indicate the reason for the hearing loss. Disconnection between the incus and stapes can be seen on coronal reformats of the temporal bone CT. Fusion of the malleus and incus gives an appearance similar to the malformed malleus/incus complex seen in aural atresia. Also, lateral chain fixations may appear as a bony bridge connecting the head of the malleus or body of the incus to the surrounding bone of the attic (Fig. 22). In the case of congenital stapes fixation, the CT may appear normal, or may reveal thickening of the footplate [14]. The scan should also be evaluated for a wide IAC with an enlarged connection to the entrance of the cochlear nerves (habenula perforata). This association of findings in males suggests an X-linked inheritance pattern of stapes fixation that when addressed surgically is prone to a profuse leak of perilymph (stapes gusher). Certain malformations of the inner ear cause a conductive hearing loss pattern. Oval window and round window atresia are rare and may be seen on coronal sections of the temporal bone CT. Certain inner ear anomalies cause a cochlear conductive loss. Finding a displaced facial nerve overlying the oval window area may preclude operative intervention. Sensorineural hearing loss Congenital sensorineural hearing loss is the most common birth defect in the United States, with an incidence of approximately 1:1000 live births. With the advent of universal newborn screening in many states, the
Fig. 22. Axial CT of the temporal bone that reveals a bony bridge (arrow) connecting the malleus to the lateral wall of the attic.
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diagnosis of congenital hearing loss is made at an early age. Imaging of congenital sensorineural loss is frequently performed in an attempt to determine an underlying cause. HRCT and MRI have been used in this set of patients, with advantages and disadvantages to each. The HRCT reveals many types of bony inner ear malformations and takes less time to perform, requiring a shorter period of sedation. The MRI provides better visualization of the membranous labyrinth and the ability to evaluate for cochlear nerve hypoplasia or aplasia. In the setting of congenital sensorineural hearing loss, the most common CT abnormality is a large vestibular aqueduct (LVA), defined as measuring greater than 1.5 mm in diameter [33,34]. This disorder may be unilateral or bilateral, and the patient may experience fluctuating hearing loss throughout childhood, with sudden loss possibly precipitated by head trauma. HRCT and T2-weighted MRI reveal the anomaly (Fig. 23A, B). LVA may occur in isolation, or be associated with other inner ear anomalies. Mondini deformity is defined as a normal basal turn of the cochlea with incomplete partition between the middle and apical turns [35]. This malformation may be associated with LVA, enlarged or absent vestibule, or deformities of the semicircular canals (Fig. 24). The common cavity deformity consists of a single cavity representing both cochlea and vestibule, with or without the presence of one or more semicircular canals. Cochlear hypoplasia and aplasia, respectively, are revealed as either a smaller-than-normal cochlea with fewer than two and a half turns, or complete absence of the cochlea. Alternatively, labyrinthine aplasia (Michel aplasia), with no inner ear structures whatsoever, may be seen. HRCT successfully identifies these bony abnormalities of the otic capsule. MRI is needed to identify hypoplasia or aplasia of the cochlear nerve, however, which have important implications for cochlear implantation. If a narrow IAC is seen on HRCT it should
Fig. 23. Axial CT of the temporal bone (A) and T2-weighted MRI (B) showing large vestibular aqueducts. (A) Unilateral left LVA (arrow). (B) Bilateral LVAs (arrows).
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Fig. 24. Axial CT of the temporal bone showing left-sided Mondini deformity. There is an incomplete partition between the middle and apical turns (arrowhead), associated with a large vestibular aqueduct (arrow).
raise the index of suspicion for cochlear nerve hypoplasia or aplasia, and MRI of the IACs should be obtained [36]. CT and current MRI technology are unable to identify congenital abnormalities of the membranous labyrinth, such as Scheibe dysplasia. Inner ear anomalies may be associated with syndromic hearing loss. CHARGE syndrome is associated with abnormalities of the middle and inner ear. Common findings include absent semicircular canals, cochlear dysplasia, cochlear nerve atresia, absence of the cochlear nerve aperture, aplasia of the oval or round window, LVA, and anomalous facial nerve course, along with ossicular chain abnormalities [37]. Branchiootorenal syndrome leads to cochlear hypoplasia or dysplasia, and can be associated with LVA, ossicular chain abnormalities, and cochlear nerve hypoplasia [38]. X-linked mixed deafness can be syndromic or nonsyndromic, and is associated with a dilated lateral IAC and an abnormally patent connection between the IAC and the cochlea, leading to a CSF gusher if stapedectomy is attempted [39].
Summary A wide range of pathologies involving the external, middle, and inner ear contribute to conductive and sensorineural hearing loss. HRCT of the temporal bone and MRI are the preferred imaging modalities to evaluate the ear structures for causes of hearing loss, with the specific type of hearing loss and location of defect dictating which type of imaging is preferred. In general, the external auditory canal, middle ear space, mastoid, petrous apex,
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and otic capsule are best visualized with CT, whereas suspicion of retrocochlear pathology warrants MRI.
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