Diagnostic imaging
Both MRI and CT play a central role in the diagnosis of pediatric hearing disorders.
Imaging is used not only to determine the etiology but also to plan the optimal treatment
[3]
[4].
The objectives of imaging in the case of hearing loss in early childhood are [2]
-
Identification of possible structural causes of the hearing loss
-
Search for additional anomalies that can help to diagnose an underlying syndrome
-
Evaluation of whether a hearing-improving ear surgery or cochlear implant (CI) would
be promising
-
Determination of anatomical risk factors in connection with surgical planning
CT is the only established imaging method for the diagnosis of CHL. In contrast, the
optimal imaging modality for SNHL is a topic of discussion in the literature [5]
[6]
[7].
MRI and CT provide complementary information: The external auditory canal, middle
ear, and the bony labyrinth including the internal auditory canal can be evaluated
with CT. MRI allows direct evaluation of the cochlear nerve (CN) and the fluid compartments
of the inner ear, as well as assessment of the brain structures and thus the diagnosis
of cerebral comorbidities.
MRI and CT together offer the greatest reliability for the diagnostic imaging of sensorineural
hearing loss and should therefore be used in combination in the ideal case.
CT
A dedicated high-resolution technique is needed to achieve an appropriate level of
detail regarding the petrous bone. Imaging is performed using fine collimation and
a submillimeter slice thickness. Oblique coronary and oblique sagittal reconstructions
are generated from the primary axial images [8]. One disadvantage of CT is the use of ionizing radiation in the pediatric population.
However, performing a low-dose CT examination of the temporal bone to reduce the radiation
dose can be problematic since high image quality with the best possible resolution
is needed to evaluate the tiny structures [8]. However, in the case of isotropic imaging, it is usually possible to position the
head of the child on an incline so that the radiosensitive ocular lenses are outside
the path of primary X-ray beam.
At our facility, CT is performed as an axial spiral acquisition at 120 kV and 200
mA on a 128-slice scanner (Ingenuity, Philips Amsterdam, The Netherlands) with a collimation
of 0.625 mm and a slice thickness of 0.67 mm.
MRI
While MRI provides significantly worse visualization of the bony structures of the
inner ear, it is clearly superior to CT in the evaluation of the auditory nerve due
to the excellent soft-tissue contrast. Optimal visualization of the CN requires a
high-resolution, thin-slice, T2-weighted sequence. This sequence also provides detailed
information about the fluid-filled labyrinth and can visualize relevant anomalies
of the inner ear. In general, MR images are acquired on the axial plane. A 3 D acquisition
method allows generation of almost isometric voxels, thus allowing a diagnostically
important secondary reconstruction perpendicular to the course of the CN in the region
of the internal auditory canal.
At our facility, we use 3D-SPACE (Sampling Perfection with Application optimized Contrast
using different flip angle Evolution) sequences on the axial plane with 0.25 × 0.25 × 0.35
(interpolated from 0.5 × 0.5 × 0.4 mm) voxel size (3T, Prisma Fit; Siemens, Erlangen,
Germany). Special 3D gradient echo sequences, like CISS (constructive interference
in steady state), are also suitable for visualizing the structures of the ear on MRI.
In addition, to evaluate the brain and in particular to rule out malformations, at
least one T2-weighted sequence of the entire cranium should be acquired. Contrast
administration is only necessary in the case of justified suspicion of a schwannoma
in the case of a predisposing primary disease, e. g., neurofibromatosis 2.
Due to the long duration of the examination and the necessity for vibration-free images,
sedation is often needed in children up to preschool age.
Conductive hearing loss
The most common cause of acquired conductive hearing loss in infants and toddlers
is acute otitis media. Due to fluid collections in the tympanum, the mobility of the eardrum and the auditory
ossicles is limited, thus affecting the transfer of sound to the inner ear. This type
of CHL is characterized by intermittent mild to moderate hearing loss that is usually
reversible after the infection resolves and does not require any diagnostic imaging.
Imaging is only indicated in the case of clinical suspicion of a complication of otitis
media, e. g., in the case of peripheral facial paresis, signs of meningitis, or symptoms
of sinus vein thrombosis, and the onset of rotational vertigo or indications of acute
involvement of the inner ear.
Persistent acquired CHL is an indication for cross-sectional imaging. It can be caused
by chronic otitis media with perforation of the eardrum, cholesteatoma, or trauma.
A cholesteatoma is a collection of keratinized squamous epithelium and is often referred to as “skin
at the wrong location”. The disruption in sound transmission can be the result of
the mass effect or erosion of the ossicles [9]. In children, it is necessary to differentiate between congenital and acquired types.
Congenital cholesteatomas, which are very rare, develop from residual embryonic epithelial
cells, which can be found everywhere in the temporal bone, e. g., in the middle ear,
but also in the eardrum, in the external auditory canal, and in the mastoid. They
can be associated with atresia of the external auditory canal but can also occur in
otherwise healthy children. If the diagnosis is made too late, irreversible destruction
of the auditory ossicles and the surrounding bony structures can occur.
Congenital cholesteatomas can occur at any location in the petrous bone.
Due to the pathogenesis, acquired cholesteatomas can be divided into retraction pocket
cholesteatomas and non-retraction pocket cholesteatomas. Retraction pocket cholesteatomas
occur as a result of hypoventilation and inflammatory processes of the middle ear
resulting in retraction of the eardrum in the direction of the middle ear. Non-retraction
pocket cholesteatomas can occur secondary to the migration of epithelial cells from
the external auditory canal through a perforated eardrum (inflammatory, post-traumatic,
or iatrogenic) into the tympanum.
On CT, a cholesteatoma presents with soft tissue attenuation, typically in connection
with bone erosion in the region of the ossicles, the scutum, or the tympanum ([Fig. 1a]). However, a lack of erosion does not exclude a cholesteatoma. Therefore, reliable
differentiation between a cholesteatoma and granulation tissue, effusions, and other
soft-tissue lesions in a chronically inflamed middle ear or mastoid is not always
possible based solely on CT.
Fig. 1 Conductive hearing loss, CT. a 2-year-old boy, congenital cholesteatoma: tympanic cavity and parts of the mastoid
are filled with soft tissue, ossicles, tympanic cavity wall and mastoid cells are
destroyed (arrow). b 4-year-old boy, 1 year after petrous bone fracture, coronary reconstruction: dislocation
of the incudomalleolar joint- the malleus head (arrowhead) is clearly medial to the
incus (arrow), the socket is empty. c 1-year-old boy: Hypoplasia of the external auditory canal, atresia of the tympanic
membrane (arrow), non-ventilated, hypoplastic tympanic cavity.
Diffusion-weighted (DWI) MRI sequences have proven to be helpful for the differentiation
between cholesteatomas and non-cholesteatoma soft tissue since cholesteatomas have
diffusion restriction. The specificity of diffusion restriction in cholesteatomas
greater than 5 mm in size is up to 100 % in the literature [10]. The typical pronounced artifacts on conventional echoplanar DWI sequences at the
base of the skull can be significantly reduced by using alternative read-out strategies,
like TSW-DWI, non-EPI (HASTE) DWI or RESOLVE-DWI [11]
[12].
On CT, cholesteatomas are seen as tissue with soft tissue attenuation, typically with
bone erosion. If the latter is absent, the detection of diffusion restriction can
confirm the diagnosis.
In the case of head trauma with petrous bone fracture, fractures or luxations in the
region of the auditory ossicles can occur. The pathology is often initially not able
to be detected due to the secretory otitis media. Therefore, in the case of persistent
CHL after trauma, a follow-up CT examination should be performed.
Rare congenital causes of CHL are malformations of the outer and middle ear. This includes hypoplasia or atresia of the external
auditory canal, the eardrum, or the tympanum and malformations of the auditory ossicles
([Fig. 1c]). Such malformations can be isolated or can occur in combination and often as part
of a syndrome. The inner ear and the internal auditory canal are typically not affected
in these patients due to their different embryological origin.
Sensorineural hearing loss
Pediatric SNHL can be unilateral or bilateral and is usually congenital. The prevalence
of congenital SNHL is 1–2 per 1000 live births [13]. Unilateral SNHL is significantly more frequently associated with malformations
of the inner ear than in the case of a bilateral presentation [14].
The cochlea is the primary hearing organ. The modiolus is a central bony pillar around which
the cochlea turns 2.5 times and that can be seen as a structure with bone density
on CT and as a hypoechoic structure on MRI ([Fig. 2a, b]). The spiral lamina projects from the modiolus and divides the cochlea into an upper
(scala vestibuli) and a lower (scala tympani) compartment. The scala tympani is the
target site for cochlear implantation (CI).
Fig. 2 Normal anatomy and internal structure of the cochlea: Both CT a and MRI b show the modiolus (arrow) and the interscalar septum (arrowheads) well. The lamina
spiralis is more visible on MRI than on CT due to the high soft-tissue contrast (long
arrow). c The oblique sagittal reconstruction allows the CDL to be determined by measuring
the distance between the center of the round window and the opposite boundary of the
cochlea.
The goal of imaging is to rule out cochlear anomalies that could limit the success
of CI. Therefore, the presence of the 2.5 turns should be evaluated and the required
length of the electrode must be determined. The cochlear duct length (CDL) is calculated
from the CT scan using a simplified formula ([Fig. 2c]).
Moreover, particularly after meningitis, sclerosis of the cochlea, which can be identified
based on the hypointense signal on MRI and calcifications on CT, must be ruled out.
Masses in the cochlea, e. g., intracochlear schwannomas, can very rarely be detected
on MRI.
The CT report should explicitly describe the morphology of the cochlea, the number
of turns, and the presence of the inner septation, the spiral lamina, and the modiolus
[15].
Imaging findings when diagnosing sensorineural hearing loss
Brain
Hearing disorders can be caused by brain anomalies that are often caused by a prenatal
infection. These often result in structural changes in the brain that can be seen
on MRI, e. g., leukoencephalopathy, calcifications, or gyration disorders ([Fig. 3]). The diagnosis of such pathologies helps with the etiological workup of hearing
loss and can also provide prognostic information regarding the success of CI since
this largely depends on the subsequent cooperation of the patient, particularly during
the rehabilitation phase.
Fig. 3 Congenital CMV infection, SNHL,1-year-old girl, MRI T2 weighting: Severe gyration
disorder with pachygyri temporal (short arrows), polymicrogyri frontal (arrowheads),
and marked leukoencephalopathy (asterisk).
Vestibulocochlear nerve
Evaluation of the vestibulocochlear nerve and especially the cochlear nerve requires
particular attention since aplasia of the cochlear nerve is a contraindication for
implantation of a cochlear implant. In contrast, hypoplasia is only a relative contraindication
but affects the prognosis.
It is recommended to generate parasagittal reconstructions perpendicular to the course
of the cochlear nerve from the thin-slice isovoxel MRI sequences. The cochlear nerve
is seen in the anterior lower quadrant of the internal auditory canal (IAC). In the
normal case, the vestibulocochlear nerve is approximately twice as thick as the facial
nerve. After it divides into the vestibular and cochlear portions, the cochlear nerve
is at least as thick as the overlying facial nerve ([Fig. 4a, b]). If the caliber is smaller, hypoplasia must be assumed [8]. The cochlear nerve canal (CNC) can be easily visualized on CT ([Fig. 4c]).
Fig. 4 Top row: Inconspicuous cochlear nerve on the left (arrows) a MRI CISS sequence transverse, b reconstruction perpendicular to the internal auditory canal, c CT shows a normal width of the CNC (> 1.9 mm, double arrow). Bottom row: Aplasia
of the right cochlear nerve: It does not show up on MRI in the transverse slice d, nor in the oblique sagittal reconstruction e (arrowheads). f CT shows a bony occlusion of the CNC (black arrow) = “isolated cochlea”, additionally
hypoplasia of the IAC.
Aplasia of the cochlear nerve can be missed in transverse slices, particularly in
the case of a hypoplastic internal auditory canal (IAC). In parasagittal reconstructions,
this can be diagnosed with great certainty by visualizing only 3 instead of 4 nerve
structures ([Fig. 4d, e]).
However, the finding is not always so definitive. A narrow IAC or artifacts, e. g.,
due to movement or dental braces, can limit the ability to evaluate MRI images. CT
can also be helpful in the evaluation of cochlear nerve deficiency (CND): If the CNC
is narrower than 1.9 mm at the widest location, cochlear nerve deficiency is highly
probable [16]. If the CNC is completely occluded by a bony protrusion, it is referred to as an
isolated cochlea and is always associated with a CND ([Fig. 4f]).
Internal auditory canal
Hypoplasia of the IAC can also be indicative of a cochlear nerve deficiency, but the
correlation is significantly less pronounced than in the CNC [16]
[17] so that this finding is less relevant in practice. Aplasia can occur, for example,
as part of Michels syndrome (see below).
Inner ear
The inner ear begins developing already in the third week of pregnancy. In the eighth
week, the cochlea is fully developed. The vestibulum is fully developed in the 11th
week and the semicircular canals in the 19th to the 22nd week. The bony labyrinth is fully developed in the 23 rd week [18].
The earlier a problem occurs in the embryonic period the more severe the deformity,
resulting in varying degrees of differentiation of the labyrinth.
According to Sennaroglu [19], inner ear malformations can be classified as follows:
-
Aplasia of the labyrinth (Michel deformity): This is the most severe type of inner
ear malformation and is the result of a developmental problem in the third gestational
week. It is characterized by the complete absence of the inner ear structures and
the cochlear nerve. The IAC is hypoplastic or aplastic. Additional anomalies in the
region of the middle ear and the base of the skull and relating to the course of vessels
are often seen.
-
Cochlear aplasia: This anomaly occurs approximately in the fourth gestational week.
The cochlea is completely absent, and the region appears sclerotic. The vestibulum
and semicircular canals can be hypoplastic, dilated, or normal. The finding must be
differentiated from inflammatory ossification based on the absence of the promontorium.
-
Common cavity: If the problem occurs at the start of the fifth gestational week, there
is no differentiation between the vestibulum and the cochlea. They form a common cavity.
-
Incomplete partition type I (IP-I): In the case of a developmental problem at the
end of the fifth gestational week, the vestibulum and cochlea are normal in size and
differentiated from one another, but the internal architecture is absent. The cochlea
is an empty cyst and there is no identifiable modiolus.
-
Incomplete partition type II (IP-II, Mondini malformation): If the problem occurs
in the seventh gestational week, the basal turn and the modiolus are present. However,
the middle and apical turns cannot be differentiated from one another and the interscalar
septum and spiral lamina are absent. The vestibulum and vestibular aqueduct are often
dilated. IP-II is the most common form of cochlear malformation (50 %) [20] and is often seen as part of complex malformations ([Fig. 5]). In contrast to the malformations described so far, IP-II is not an absolute contraindication
to CI.
Fig. 5 12-year-old girl with branchio-oto-renal syndrome and progressive mixed hearing loss:
Incomplete partition type II (Mondini malformation), a CT, b MRI. Lack of differentiation between middle and apical convolutions (short arrows),
preserved modiolus (thin arrows). Massively dilated aquaeductus vestibuli (asterisk)
in the sense of an LVAS.
X-linked deafness (perilymphatic gusher phenomenon) is classified as incomplete partition
type III (IP-III). As a result of an atypical connection between the internal auditory
canal and the basal turn of the cochlea, the intracranial pressure is transferred
to the endolymph. Imaging shows an absent modiolus and a bulbous dilation of the fundus
of the IAC that has a broad connection to the basal turn of the cochlea. The cochlea
has a corkscrew-like appearance ([Fig. 6]). Clinical signs of the gusher phenomenon include pronounced hearing impairment
in the low-frequency range and hearing loss at a very young age (also see Section
6).
Fig. 6 5-year-old boy with X-linked hearing loss a CT, b MRI: Incomplete partition type III: The basal cochlear turn has a wide connection
to the internal auditory canal (asterisk), resulting in a “corkscrew-like” image impression
of the cochlea. The modiolus is missing.
Dysplasia and aplasia of the equilibrium organ do not play an immediate role in hearing
loss but often occur as part of complex malformation syndromes or inner ear malformations,
e. g., in CHARGE syndrome, and should trigger an intensive search for malformations
in the region of the cochlea.
Large vestibular aqueduct syndrome (LVAS) refers to dilation of the bony aqueduct
(visible on CT) or the endolymphatic duct (visible on MRI). The latter arises from
the posterior vestibulum and runs through the petrous bone to the endolymphatic sac,
which ends in the epidural space of the posterior cranial fossa.
Normally, the aqueduct is not wider than the posterior semicircular canal and has
a diameter < 1.5 mm [21]. It is considered dilated when the middle third is ≥ 1 mm on axial slices or the
opening measures ≥ 2 mm [22]
[23] ([Fig. 5]). A dilated aqueduct can be an isolated occurrence, but in most cases (approx. 85 %)
it is combined with other malformation or occurs as part of a malformation syndrome
[20].
LVAS manifests as sensorineural hearing loss or mixed, typically fluctuating and/or
progressive, hearing loss in childhood. It is caused by a transfer of intracranial
pressure to the labyrinth resulting in damage to the hair cells. Hearing can still
be normal at birth. The first symptoms often appear suddenly, either spontaneously
or in connection with mild head trauma [20]. There is a significant correlation between the extent of dilation of the aqueduct
and the degree of hearing loss [24]. Physical activity resulting in significant fluctuations in pressure, e. g., jumping
on a trampoline and headers in soccer, should therefore be avoided.
There were various attempts in the past to take into account the complexity of inner
ear malformations and develop a feasible classification system [19]. The most recent classification is the INCAV system with the letters indicating
the various parts of the inner ear: Internal auditory canal (I), cochlear Nerve (N), Cochlea (C), vestibular Aqueduct (A), and Vestibulum (V) [25] ([Table 1]). As a result of this structured approach, malformations of the inner ear structures
can be viewed individually, with a category and a number for categorization being
assigned to every ear structure. Zero “0” means that the inner ear component is normal.
Higher numbers indicate the severity of the malformation. This classification is performed
individually for each ear. Thus, I0N0C0A0V0 indicates a normal finding. I3, N3, and
C6 are absolute contraindications for CI. These patients can be easily identified
at a glance. The classification can be used for MRI and CT. On CT a narrow CNC as
a surrogate for a CND is classified as “CX”.
Table 1
INCAV system for classifying inner ear malformations according to Adibelli et al.
2017 [25].
|
INCAV classification (MRI)
|
0
|
1
|
2
|
3
|
4
|
5
|
6
|
|
Internal auditory canal (I)
|
Normal
|
Dilated
|
Narrow
|
Atresia
|
|
|
|
|
Cochlear nerve (N)
|
Normal
|
Thickened
|
Hypoplasia
|
Aplasia
|
|
|
|
|
Cochlea (C)
|
Normal
|
IP-II
|
IP-III
|
Hypoplasia
|
IP-I
|
Common cavity
|
Aplasia
|
|
Vestibular aqueduct (A)
|
Normal
|
Dilated
|
|
|
|
|
|
|
Vestibulum (V)
|
Normal
|
Malformation of the semicircular canal
|
Dilated
|
Hypoplasia
|
|
Common cavity
|
Aplasia
|
External auditory canal, tympanum, and mastoid
CT evaluation of the bony structures is decisive for surgical CI planning also in
the case of SNHL. A partial mastoidectomy is performed in order to place the CI electrode.
Therefore, prior evaluation of the pneumatization of the mastoid is important. Visualization
of the tympanic course of the facial nerve is important since anomalies in this region
can result in serious surgical complications as a result of injury to the facial nerve.
Multiplanar reconstructions perpendicular to the tympanic course of the nerve are
suitable for this. The caudal bony boundary of the facial canal can be effectively
evaluated ([Fig. 7]).
Fig. 7 Tympanic course of the facial nerve: An oblique coronal reconstruction perpendicular
to the course of the nerve allows the assessment of the bone cover a: Normal findings. The facial canal is covered by bones in the caudal direction (arrow),
b: The nerve hangs far into the tympanic cavity (thick arrow), the bony cover is missing.
It continues to be important to identify anomalies and bony protrusions into the round
window through which the electrode is inserted into the cochlea in the case of CI.
Anatomical variants that are important for surgical planning
Some anatomical variants are extremely important for the planning of cochlear implantation
and must be mentioned in the report. These are, for example, an atypical course of
the carotid artery, a high-riding or dehiscent jugular bulb, or a protrusion of the
sigmoid sinus into the mastoid ([Fig. 8a]).
Fig. 8 Variations of anatomy important for surgical planning, CT, a: Protrusion of the sigmoid sinus into the petrous bone (short arrow) and of the bulbus
venae jugularis into the tympanic cavity (long arrow), b: Atypical connection between the basal turn and the internal auditory canal visible
only in the oblique reconstruction (small image) (arrow), c: for comparison, normal findings – complete bony separation of the basal turn and
the IAC (arrow).
An atypical connection between the basal cochlear turn and the internal auditory canal
can occur not only in connection with X-linked hearing loss but also as an isolated
anatomical variant. During surgery, this can result in a massive loss of spinal fluid
in terms of a gusher phenomenon, particularly in the case of opening of the footplate,
e. g., in the case of suspected otosclerosis. In spite of sufficient surgical closure,
sensorineural hearing loss can occur. Persistence of the CSF leak results in deafness
and poses a risk of meningitis.
Since this anomaly is not always visible on axial CT scans, Varadarajan et al. [26] recommend oblique multiplanar reconstructions along an axis between the apical cochlear
turn and the center of the IAC ([Fig. 8b, c]).
Hearing-improving surgical procedures in addition to CI are available. These include
various implantable hearing systems that use sound conduction via bone (BoneBridge,
OSIA, BAHA). An interdisciplinary conference with phoniatricians, pediatric audiologists,
surgically active ENT physicians, and specialized radiologists has become established
for selecting the suitable surgical method, surgical planning, and subsequent rehabilitation
and for ensuring mutual understanding between the individual disciplines. These interdisciplinary
conferences are now required by quality management when certifying CI centers.
Summary
High-resolution CT and MRI play an important role in the evaluation of hearing loss
in children. CT is the method of choice for the evaluation of bony anomalies in the
region of the middle ear and the labyrinth. It is important for diagnosis and surgical
planning with regard to both conductive hearing loss and sensorineural hearing loss.
MRI provides information about the membranous labyrinth, the vestibulocochlear nerve,
and the brain, which is essential for the diagnosis and treatment of sensorineural
hearing loss. MRI and CT in combination provide the highest possible diagnostic accuracy.
The goal of imaging is to visualize the anatomy and to identify malformations causing
the hearing loss. At the same time, contraindications for CI must be ruled out and
anatomical variants that could lead to surgical complications must be identified.
An understanding of the embryogenesis and anatomy of the inner ear in connection with
standardized classification systems for inner ear malformations is important for describing
and interpreting CT and MRI images. An interdisciplinary case conference with phoniatricians,
pediatric audiologists, surgically active ENT physicians, and specialized radiologists
has become established for selecting the suitable type of treatment and discussing
surgically relevant findings and is the standard at certified CI centers.
