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

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.

PSCC= PosteriorSemicircular Canal

IAC= Internal Auditory Canal

FN= Facial Nerve

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 (Klippel-Feil 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.

•Preoperative knowledge of an abnormal facial nerve course could prevent nerve injury during surgery (facial paralysis).

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 (*), overlying the oval window. The stapes (arrowhead) does not articulate with the oval window and may be in contact with the facial nerve (arrow).

Middle Ear-Congenital Cholesteatoma

3 year old patient who presents with a suspected congenital cholesteatoma found on physical exam. Axial and coronal CT show a soft tissue density (arrow) in the mesotypanum extending into the hypotympanum. This soft tissue is surrounding the long process of the incus (*). The tympanic membrane was intact. This lesion is most consistent with a 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.

•Congenital choleastoma etiology is controversial but is thought to be due to epidermoid formation (squamous cell rests with unknown function).

Middle Ear- Arterial 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 additional inner ear malformations. Axial CT demonstrates an enlarged vestibular

aqueduct (long arrow) which is dilated compared 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.

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*All graphics used were created digitally by Ryan Brandt