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
is a third-year Resident in the Department of Radiology at the
Hospital of the University of Pennsylvania, Philadelphia,
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.
The prevalence of unilateral SNHL of 45 decibels or worse among
school children in the United States is 3 in 1000.
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
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
/2 to 2
/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.
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.
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.
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
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
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.
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.
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
Congenital abnormalities of the inner ear
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.
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.
This anomaly is extremely rare and constitutes only 1% of osseous
inner ear malformations.
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.
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
/2 turns in the cochlea (Figure 3D).
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.
In this disease, hearing is usually present at birth and decreases
in a stepwise fashion during childhood and adolescence.
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).
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.
The normal endolymphatic sac is not often seen on routine MRI. More
than half of all patients with this disease may have associated
Acquired abnormalities of the inner ear
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.
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
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.
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.
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.
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.
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
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.
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.
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).
These tumors are frequently hypervascular and enhance with
Other neoplastic and cystic disorders that can occur in the IAC
and CPA include lipoma, epidermoid, dermoid, arachnoid cyst,
lymphoma, or metastasis.
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
Patients often present with SNHL and vertigo. On MRI, labyrinthitis
is seen as gadolinium enhancement of the cochlea, vestibule, or
semicircular canals (Figure 8).
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.
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).
A delayed complication of labyrinthitis is postinflammatory
fibro-osseous obliteration of the labyrinth, also known as
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.
Labyrinthitis ossificans can cause technical challenges for
Labyrinthine hemorrhage can be caused by trauma, coagulopathy,
labyrinthitis, or tumor fistulization, and is seen as increased
signal on unenhanced T1-weighted MR images.
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
A presumptive diagnosis of perilymphatic fistula can be made by
demonstrating air within the inner ear labyrinth (Figure 9).
There is also a high association of inner and middle ear
Pneumolabyrinth or other associated abnormalities are not
frequently visualized on imaging studies, and patients may require
surgery for diagnosis of perilymphatic fistula.
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.
Early in this disease, there is demineralization and rarefaction of
bone, and hence the term
may be a more appropriate description (Figure 10).
Sclerotic changes may appear later in the course of the
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).
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
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.
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
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.
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.
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).
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).
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.
It has been estimated that MRI can determine the cause of
hearing impairment in approximately 30% of patients with SNHL.
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.
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.