Congenital Temporal Bone
Anomalies: An Embryologic
Approach
Brandt RJ, Girard BT, Guerin SJ
Dartmouth-Hitchcock Medical Center
#783
Disclosures
• The authors have no disclosures
Objectives
1. To understand the basic anatomy and
embryologic development of the temporal
bone.
2. To correlate different stages of arrest in
embryologic development with common
congenital temporal bone abnormalities.
Selected rare temporal bone malformations
will be covered.
Normal Temporal Bone Anatomy
PSCC= Posterior
Semicircular Canal
IAC= Internal
Auditory Canal
FN= Facial Nerve
Upper: Axial CT demonstrates
normal inner and middle ear
structures of the temporal bone.
Right: 3D reconstructed image of the
temporal bone showing a normal
cochlea with 2 ½ turns (arrows).
There are well developed lateral,
posterior and superior semicircular
canals.
Temporal Bone-Normal Anatomy
Right: Coronal image through the temporal bone. The ossicular chain,
including the stapes (S) and incus (I) are normal. The stapes footplate
articulates with the oval window. EAC=External Auditory Canal, TM=Tympanic
Membrane, IAC = Internal Auditory Canal, * = Lateral Semicircular Canal.
Left: 3D image of the cochleovestibular system en face demonstrating the
normal course of the facial nerve (blue), coursing just inferior to the lateral
semicircular canal (*). Orange-IAC, Yellow-Cochlea, Green- Vestibule/SCCs.
Embryologic Development-Overview
Left: Graphic of a 5 week old embryo depicting the early inner ear (otic pit)
and external/middle ear structures (branchial arches 1 and 2). The auricular
hillocks located on the first and second arches will develop into the auricle
(pinna).
Right: Coronal graphic through the branchial arches showing the relationship
of the branchial grooves with the pharyngeal pouches.
External/Middle Ear Development
• Early in development,
the branchial groove
elongates to appose the
first pharyngeal pouch.
These structures, including
a small interspersed layer
of mesenchyme will form
the tympanic membrane.
• The first branchial
groove forms the external
auditory canal.
• The first pharyngeal
pouch forms the
Eustachian tube.
• The first pharyngeal
pouch will envelop the
primitive ossicles,
ultimately forming the
middle ear cavity.
External Ear
Development
Embryologically, the external and
middle ears are derived from
branchial arches 1 and 2, and so
there is a high association of
concomitant congenital defects. The
external ear hillocks begin to
develop around 5 weeks gestational
age, becoming a fully formed auricle
by the forth month of gestation.
Several congenital syndromes
affect the 1st and 2nd branchial
arches. In hemifacial microsomia,
there is characteristic asymmetric
facial hypoplasia involving the
mandible, maxilla and external ear to
a varying degree. Goldenhar
syndrome includes these facial
abnormalities with additional organ
and spine malformations.
3D Surface rendered CT of a patient showing
a normal external ear. The embryologic Hillocks
(numbered) develop from the ectoderm of the
first and second branchial arches into the
normal ear superstructure.
External Ear Malformations
3D reconstructed images of the skull (left) and superficial soft tissues (right) in a
patient with hemifacial microsomia. This disease is thought to be due to an early
vascular insult affecting branchial arch development and is the second most
common facial developmental anomaly after facial clefts. There is dysplasia of the
right mandibular condyle (*) with absence of the madibular fossa (short arrow) and
atresia of the external auditory canal (long arrow). The zygomatic arch is also
underdeveloped. Malformation of the auricle, or microtia, is demonstrated on the
surface rendered image.
External Ear Malformations
6 year old male with Goldenhar
syndrome.
Upper: Axial CT of the right temporal
bone demonstrates a hypoplastic middle
ear cavity (arrow) without ossicle
formation. Complete EAC atresia is
noted. The cochlea (arrowhead) has the
normal 2 ½ turns. The remaining inner
ear structures were also normal.
Lower: Coronal CT shows an abnormal
course of the facial nerve (short arrow),
which usually is seen inferior to the
lateral semicircular canal (arrowhead).
External ear malformations are
associated with an aberrant course of the
facial nerve.
External Ear Malformations
17 year old male with a history of Crouzon Syndrome. 3D reconstructed image (left)
demonstrates characteristic midface hypoplasia (arrows) with prognathia of the
mandible. There is an abnormal downsloping of the external auditory canal. Soft
tissue atresia of the right external auditory canal is shown on the axial CT (right).
Crouzon syndrome can be associated with middle and external ear abnormalities.
External Ear Malformations
Patient with cervical vertebral body fusion abnormalities (KlippelFeil Syndrome) and hearing loss. Klippel-Feil syndrome is associated
with various inner, middle and external ear anomalies.
Axial CT (left) and coronal CT (right) demonstrate absence of the
external auditory canal (*) and a small dysplastic middle ear cavity
(arrows). The ossicles are atretic. The vestibule was mildly dysplastic.
Incidentally, the facial nerve had an aberrant branch arising from the
tympanic portion (FN-2).
Middle Ear
• The ossicles develop from
mesenchyme of the first and
second branchial arches.
• The first arch forms the
bodies of malleus and incus.
• The crura of stapes, lenticular
and long processes of the incus,
and manubrium of the malleus
form from the second arch.
• Ossicular chain abnormalities
commonly occur with external
ear defects due to their common
origin.
• The footplate of stapes and
annular ligament are derived
from the otic vesicle and are
often spared.
23 year old male with right sided hearing
loss. Axial CT through the right temporal
bone demonstrates a linear bone density
extending from the malleus to the superior
and anterior wall of the epitympanum
(arrow). This is consistent with a malleus
bar, which acts to dampen sound
transmission and causes conductive
hearing loss.
Middle Ear
5 year old male with a history of Treacher-Collins Syndrome presents
with conductive hearing loss. Coronal (left) and Axial (right) images
through the right temporal bone demonstrate a grossly abnormal
external and middle ear cavity. A bony plate is seen in the expected area
of the tympanic membrane (arrowhead). A single fused ossicle is seen in
the region of the malleolar head (short arrows) with a short osseous
process fusing with the temporal bone anteriorly.
Middle Ear-Facial Nerve
• The facial nerve is
derived from the second
branchial arch.
• It can take an
anomalous course through
the middle ear.
• Dehiscence of the
facial canal is most
common in the tympanic
segment.
15 year old female presented with congenital hearing
loss. Axial (left) and coronal (right) CT through the right
temporal bone show an aberrant course of the facial
nerve where it extends through the middle ear cavity (*),
• Preoperative
overlying the oval window. The stapes (arrowhead) does
knowledge of an abnormal not articulate with the oval window and may be in contact
facial nerve course could
with the facial nerve (arrow).
prevent nerve injury during
surgery (facial paralysis).
Middle Ear-Congenital Cholesteatoma
•Most commonly presents
in children as a soft tissue
mass in the anterosuperior
mesotympanum (versus
posteriosuperior in acquired
cholesteatomas).
•External auditory canal
and tympanic membrane
are normal and intact.
3 year old patient who presents with a suspected
congenital cholesteatoma found on physical exam.
•Congenital choleastoma
Axial and coronal CT show a soft tissue density
etiology is controversial but (arrow) in the mesotypanum extending into the
is thought to be due to
hypotympanum. This soft tissue is surrounding the
epidermoid formation
long process of the incus (*). The tympanic membrane
(squamous cell rests with was intact. This lesion is most consistent with a
congenital cholesteatoma.
unknown function).
Middle EarArterial Anomalies
• An aberrant course of the internal
carotid artery (ICA) through the middle
ear is rare.
• This variant is thought to be due to
embryologic regression of the petrous
ICA with collateralization of flow via the
inferior tympanic and caroticotympanic
arteries.
• The stapedial artery normally
regresses during development and is
thought to be involved in formation of
the crura of the stapes.
• A persistent stapedial artery (not
shown) is a rare variant that can
originate from the ICA. This enters the
middle ear cavity, enters the facial
canal and exits via the facial hiatus.
• Hemorrhage can occur following
myringotomy without presurgical
knowledge of these aberrant vessels.
Top: Axial CT demonstrates bilateral aberrant
ICAs coursing through the middle ear.
Bottom: Coronal CT of the right temporal bone
demonstrates an aberrant ICA abutting the
cochlear promontory.
FN=Facial Nerve
Middle Ear- Jugular Vein
11 year old female presented with a right middle ear mass. Axial CT on
the left reveals a dilated jugular vein which has eroded into the middle ear
cavity and is abutting the long process of the incus. A corresponding
jugular flow void was seen on the MRI on the right (*). The jugular vein can
vary in course in relation to the middle ear. A high jugular bulb with or
without wall dehiscence can be associated with tinnitus.
Inner Ear Development- Otic Vesicle
When the embryo is about 2 mm crown rump length, the neuroectoderm
begins to focally thicken, forming the otic placode. This thickened tissue
involutes to form the otic pit and subsequently partially fuses to become
the otic vesicle (otocyst). The otocyst migrates deep to approximate the
pharyngeal pouch/branchial groove.
Inner Ear Development
• The endolymphatic
apparatus forms from the
otocyst early in development.
The otocyst also forms an
utriculosaccular portion, which
will develop into the cochlea,
vestibule and semicircular
canals (Picture 2).
• The saccule and utricle
are formed by the 11th week
gestational age.
• The superior and posterior
semicircular canals develop
from a pouch on the dorsal
margin of the otocyst while
the lateral semicircular canal
originates from a separate
outpouching.
• The semicircular canals
begin forming at the 6th week
gestation and have completed
formation by the 22nd week.
Graphic representing progressive formation of the
membranous labyrinth of the inner ear from the otocyst
to the fully formed cochleovestibular apparatus. The
semicircular canals form sequentially, starting with the
superior SCC, then the posterior SCC and finally the
lateral SCC.
Inner Ear Malformation-By Stage of
Arrest (Gestational Age)
The cochlea is thought to develop in a progressive manner, with different
malformations occurring at different stages of arrest:
Malformation/Stage of Arrest (GA)
-Labyrinthine Aplasia (Michel’s Deformity)/3 weeks
-Cochlear Aplasia/5 weeks
-Common Cavity Deformity/5-6 weeks
-Cochleovestibular or cochlear hypoplasia/6 weeks
-Incomplete Partition Type 1/6-7 weeks
-Incomplete Partition Type 2/7 weeks
-The cochlea finishes development by the 8th week
A way to differentiate between labyrinthine or cochlear aplasia versus
labyrinthitis osfficans is to find the cochlear promontory, which will be present
in the latter case. In labyrinthine aplasia, the internal auditory canal is also
aplastic.
Common Cavity
2 year old male with a right sided cochlear implant. Axial (right)
and coronal (left) images through the temporal bone demonstrate a
single cystic structure (arrows) extending from the internal auditory
canal. The semicircular canals, vestibule and cochlea were not
formed. The patient had normal morphology of the external ear
structures. Findings are consistent with a common cavity
corresponding to a developmental arrest at 5-6 weeks gestation.
Cochleovestibular Hypoplasia
11 month old with CHARGE syndrome presents with right sided
hearing loss. Hypoplastic semicircular canals were seen originating from
an enlarged dysmorphic vestibule (red arrow). Axial image on the right
demonstrates an elongated channel connecting a small dysplastic
cochlea to the dysmorphic vestibule.
Incomplete Partition- Type 1
2 year old male with right sided hearing loss. Axial CT on the right
shows an enlarged dysplastic vestibule (yellow arrows). Well formed
but slightly enlarged semicircular canals were identified (black arrow).
The cochlea has a cystic appearance (*) and directly communicates
with the dysplastic vestibule. The IAC is also enlarged. These findings
are characteristic of an incomplete partition type 1.
Incomplete Partition-Type 2
Axial CT and T2 weighted MRI of the temporal bones in a 13 year old
male demonstrating a deficiency in the apical turns of the cochlea
bilaterally (arrows). The vestibular aqueducts were enlarged (EVA) and the
vestibules were also dysplastic, all findings that are seen in type 2
incomplete partition. Type 2 incomplete partition is the most common
congenital abnormality affecting the cochlea.
Inner Ear-Semicircular Canals
11 year old female with Apert’s syndrome. Axial cut through the temporal
bone shows bilateral fusion of the lateral semicircular canals and vestibule
(black arrows). The posterior (yellow arrows) and superior semicircular canals
as well as the cochlea was normal bilaterally. The lateral SCC is the last to
develop and therefore is the most commonly malformed canal. Isolated
malformation of the posterior SCC has been reported in Alagille and
Waardenburg Syndromes, whereas thalidomide toxicity has been reported to
cause an isolated malformation of the superior SCC.
Inner Ear-Semicircular Canals
15 year old female presents with congenital hearing loss
and dizziness. The roof of the superior semicircular canal was
found to be deficient on coronal imaging (yellow arrow).
Incidentally, the facial nerve (white arrow) was also abnormally
positioned in front of the oval window (black arrow).
Enlarged Vestibular Aqueduct (EVA)
•
•
•
Vestibular aqueduct
should not be wider than
the adjacent posterior
semicircular canal.
The vestibular aqueduct
is the last inner ear
structure to finish
development and is not
fully formed until early
childhood.
An EVA is less
commonly found in
isolation and, if present,
should raise concern for
Axial CT demonstrates an enlarged vestibular
additional inner ear
aqueduct (long arrow) which is dilated compared
malformations.
with the adjacent posterior semicircular canal.
This patient was also found to have a type 2
incomplete partition (not shown).
X-Linked Stapes Gusher Syndrome
Axial (right) and coronal (left) CT of a 56 year old male demonstrates a
deficient modiolus (black arrow), enlarged IAC and a dysplastic vestibule
(*). The base of the cochlea is deficient and is contiguous with the IAC
(yellow arrow). These findings are concerning for X-Linked Stapes Gusher
Syndrome. The stapes footplate is typically fixated, which is not readily
appreciable on CT. Classically, during surgical correction of the fixated
footplate, a large amount of CSF will pour into the middle ear from a
pressurized vestibular system.
Conclusion
• The middle and external ears are derived from the first and second
branchial arches and the first pharyngeal pouch.
• The inner ear structures derive from a common precursor, the
neuroectoderm, and develop into the cochleovestibular apparatus
through a complex series of steps.
• The vestibular aqueduct is the last embryologic structure to develop
and, when enlarged, a high suspicion for other inner ear
abnormalities should be maintained.
• Knowledge of the embryologic origins of the temporal bone provides
a better understanding of congenital malformations and attunes the
interpreting physician to concomitant abnormalities.
References
•
•
•
•
•
•
•
•
Curtin HD, Gupta R, Bergeron RT. Chapter 15: Embryology, Anatomy and Imaging of the Temporal Bone.
In: Som PM, Curtin HD, eds. Head and Neck Imaging. 5th ed. St. Louis, MO: Mosby-Elsevier;2011:10531096.
Sennaroglu L, Saatci I. A New Classification for Cochleovestibular Malformations. Laryngoscope
2002;112:2230-2241.
Mukerji SS,Parmar HA, Ibrahim M et al. Congenital Malformations of the Temporal Bone. Neuroimag Clin
N AM 2011;21:603-619.
Rodriguez K, Shah RK, Kenna M. Anomalies of the Middle and Inner Ear. Otolaryngol Clin N Am
2007;40:81-96.
Persaud R, Hajioff D, Trinidade A et al. Evidence-based review of aetiopathogenic theories of congenital
and acquired cholesteatoma. The Journal of Laryngology & Otology 2007;121:1013-1019.
Atmaca S, Muzaffer E, Kucuk H. High and dehiscent jugular bulb, a clear and present danger during
middle ear surgery. Surg Radiol Anat 2014;36:369-374.
Pyle MP. Embryologic Development and Large Vestibular Aqueduct Syndrome. Laryngoscope
2000;110:1837-1842.
Celebi I, Oz A, Yildirim H et al. A case of an aberrant internal carotid artery with a persistent stapedial
artery: association of hypoplasia of the A1 segment of the anterior cerebral artery. Surg Radiol Anat
2012;34:665-670.
*All graphics used were created digitally by Ryan Brandt