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Temporal Bone Malformations


INTRODUCTION

Background

  • Within a space of less than a few centimeters, the temporal bone contains highly specialized small anatomic structures that allow for hearing and balance.

  • The temporal bone is composed of five parts (petrous, tympanic, squamosal, mastoid, and zygomatic) and is subdivided as the inner ear, middle ear, and external ear.

  • The inner ear converts mechanical pressure into electrical signal to provide hearing and balance function. The inner ear contains the cochlea, vestibule, semicircular canals, cochlear aqueduct, vestibular aqueduct, oval window, and round window. The cochlea has 2½ turns, and the cochlear nerve enters the cochlea through the cochlear aperture to terminate on an osseous structure called the modiolus. The modiolus is attached to an osseous structure called the spiral lamina, forming a structure similar to a threaded screw. The spiral lamina has two membranes that separate each turn of the cochlea into three compartments. From lateral to medial the layers are the scala vestibuli → vestibular membrane (also known as Reissner membrane) → scala media (also known as cochlear duct) → basilar membrane → scala tympani. The scala vestibule terminates at the oval window, and the scala tympani terminates at the round window. The scala media contains the organ of Corti, which has the sensory receptors of the cochlear nerve and contains endolymph, whereas the scala tympani and scala vestibuli contain perilymph. The vestibule contains the utricle and saccule and with the three semicircular canals (lateral, superior, and posterior) provides balance information.

  • The middle ear is subdivided into the epitympanum, mesotympanum, and hypotympanum and contains the ossicles (malleus, incus, and stapes) ( Figure 13.2 ). The first branchial apparatus gives rise to the malleus, incus, and tensor tympani, while the second branchial apparatus gives rise to the stapes and facial nerve. The epitympanum communicates with the mastoid through the aditus ad antrum and the nasopharynx through the eustachian tube.

    Fig. 13.1, The Inner Ear. (A) The bony labyrinth (tan) is the hard outer wall of the entire inner ear and includes the semicircular canals, vestibule, and cochlea. Within the bony labyrinth is the membranous labyrinth (purple), which is surrounded by perilymph and filled with endolymph. Each ampulla in the vestibule contains a crista ampullaris that detects changes in head position and sends sensory impulses through the vestibular nerve to the brain. (B) Section of the membranous cochlea. Hair cells in the organ of Corti detect sound and send the information through the cochlear nerve. The vestibular and cochlear nerves join to form the eighth cranial nerve. (From Applegate E: The anatomy and physiology learning system, 4th ed, St Louis, 2010, Saunders. In Rogers JL, Brashers VL: McCance & Huether’s pathophysiology: the biologic basis for disease in adults and children, 9th ed, St. Louis, 2023, Elsevier.)

    Fig. 13.2, Compartments of the middle ear.

  • The external ear includes the auricle and external auditory canal and arises from the first branchial apparatus.

  • The facial nerve in the temporal bone consists of the canalicular, labyrinthine, genu, tympanic, and mastoid segments and exits the temporal bone at the stylomastoid foramen.

  • Malformations of the temporal bone can result in sensorineural and/or conductive hearing depending on the anatomic structure involved.

Imaging

  • An imaging approach to malformations of the temporal bone should include a systematic assessment of the inner, middle, and external ear. Conductive hearing loss is typically a problem of the external or middle ear, while sensorineural hearing loss is a problem of the inner ear.

  • MRI should be performed with 3D T2W sequences with 1-mm slice thickness. The advantage of MRI over CT is that MRI allows for visualization of the cochlear nerve, whereas CT may only infer the small size of the cochlear nerve based on a small internal auditory canal or cochlear aperture.

  • CT allows for thin section imaging with fast acquisition, which may eliminate need for sedation in some patients. CT imaging of the temporal bone should be acquired with bone algorithm using 1 mm or less axial slice thickness and reformatted into coronal plane. Sagittal and oblique reformatted images can be helpful for evaluating ossicular erosions or malformations and for evaluating dehiscence of semicircular canals. Soft tissue algorithms should be reviewed for intracranial or periauricular abnormalities.

  • MRI of the temporal bone should include thin-section 3D T2W sequences with 1 mm or less slice thickness and precontrast thin section (3 mm or less) T1W imaging. Precontrast T1W imaging is useful for evaluation of T1W hyperintense signal in the inner ear, which signifies proteinaceous or hemorrhagic fluid from a labyrinthitis, as well as for masses such as lipomas or dermoid cysts which have lipid. Thin-section postcontrast T1W imaging can be added if there is need for assessing for inner ear enhancement from labyrinthitis and for a vestibular schwannoma. The advantage of MRI over CT is that MRI allows for visualization of the cochlear nerve, whereas CT may only infer small size of the cochlear nerve based on a small internal auditory canal or cochlear aperture.

  • The following cases illustrate common and uncommon malformations in children affecting the inner, middle, and external ear.

INNER EAR MALFORMATIONS

Fig 13.1

Fig. 13.3, The spectrum of inner ear malformation imaging findings reflects the timing of insult during gestation. Earlier insults lead to less formation of inner ear structures. (Illustrations from StatDX, Copyright © 2022 Elsevier.)

INCOMPLETE PARTITION ANOMALY TYPE 1 (IP-1)

Key Points

Background

  • Cystic cochleovestibular anomaly

  • Developmental arrest in the fifth gestational week

Imaging

  • Snowman shape/figure of 8 shape to the cochlea and vestibule

  • Cystic cochlear malformation, which is separate from the vestibule

  • Absent interscalar septum

  • Absent modiolus

  • Wide internal auditory canal (IAC)

  • Vestibular aqueduct rarely large

  • Cochlear nerve often hypoplastic

REFERENCE

  • 1. Joshi V.M., Navlekar S.K., Kishore G.R., et. al.: CT and MR imaging of the inner ear and brain in children with congenital hearing loss. Radiographics 2012 May-Jun; 32: pp. 683-698.

INCOMPLETE PARTITION ANOMALY TYPE 2

Key Points

Background

  • Developmental arrest in the seventh week of gestation. More common than IP-1

Imaging

  • Malformation of the apical and middle turns of the cochlea, and normal basal turn

  • Absent interscalar septum between apical and middle turn

  • Large vestibular aqueduct

  • May have absent or deficient modiolus

  • Cochlear nerve and semicircular canals usually normal. Greater success with cochlear implant

REFERENCE

  • 1. Joshi V.M., Navlekar S.K., Kishore G.R., et. al.: CT and MR imaging of the inner ear and brain in children with congenital hearing loss. Radiographics 2012 May-Jun; 32: pp. 683-698.

LATERAL SEMICIRCULAR CANAL DYSPLASIA

Key Points

Background

  • Semicircular canals begin development between the sixth and eighth week of gestation and are complete at the 19th to 22nd weeks.

  • Superior canal develops first, then the posterior canal, then the lateral canal.

  • Earlier gestational insults will involve more of the semicircular canals. Consequently, the lateral semicircular canal is the most common canal to be malformed as well as malformed in isolation.

Imaging

  • Lateral semicircular canal appears as a short and wide canal, often unseparated from the vestibule.

  • Cochlea can be normal in appearance or malformed.

REFERENCE

  • 1. Joshi V.M., Navlekar S.K., Kishore G.R., et. al.: CT and MR imaging of the inner ear and brain in children with congenital hearing loss. Radiographics 2012 May-Jun; 32: pp. 683-698.

TIMELINE OF INNER EAR MALFORMATIONS

Fig 13.2

Fig. 13.7, Timeline of Inner Ear Malformations. The severity of the inner ear malformation reflects the timing of insult during gestation.

Fig. 13.4, Incomplete Partition Anomaly Type 1. (A) Axial T2W and (B) axial CT images demonstrate a cystic cochleovestibular anomaly with a “snowman” or “figure-8” morphology, absent interscalar septum, and absent modiolus. (B, inset image of snowman © istock.com/Vectorcreator.)

Fig. 13.5, Incomplete Partition Anomaly Type 2. (A and B) Axial and coronal 3DT2W CT and (C and D) axial images demonstrate malformation of the apical and middle turns of the cochlea and an enlarged vestibular aqueduct ( arrows ). A normal cochlear nerve is visible.

Fig. 13.6, Lateral Semicircular Canal Dysplasia. (A) Axial T2W and (B) axial CT images demonstrate malformation of the lateral semicircular canal ( arrow ) seen as lack of separation of the canal from the vestibule.

Fig. 13.8, Cochlear Nerve Hypoplasia. (A) Axial T2W, (B) right sagittal oblique T2W, and (C) left sagittal oblique T2W images demonstrate the lack of a right cochlear nerve beneath the facial nerve. Popular mnemonic “7 up, Coke down” refers to the normal appearance, which is cranial nerve 7 above and cranial nerve 8 below in the anterior aspect of the internal auditory canal.

Fig. 13.9, Branchio-oto-renal Syndrome. (A, C, D) Multiplanar CT images and (B) axial T2W images demonstrate characteristic unwound cochlea ( blue arrow ), enlarged vestibular aqueduct ( red arrow ), and prominent eustachian tube ( yellow arrow ). (A, inset image of yo-yo © istock.com/Michael Burrell.)

Fig. 13.10, X-Linked Deafness. (A) Axial T2W and (B) axial CT images demonstrate corkscrew malformation of the cochlea and absent modiolus. (A, inset image of corkscrew © istock.com/UASUMY)

Fig. 13.11, Oval Window Atresia. (A) Coronal reformat CT demonstrating thickened oval window/absence of normal thinning of the bone at the oval window ( arrow ) as seen on a normal patient coronal CT image. In addition, the tympanic segment of the facial nerve is not seen below the lateral semicircular canal as is seen on this (B) coronal reformat CT from a normal patient.

Fig. 13.12, Ossicular Anomalies. Malleus fixation: (A to C) Multiplanar CT images demonstrate an osseous bar fixating the head of the malleus ( arrows ). Monopod stapes: (D and E) Multiplanar CT images demonstrate a single thickened crus of the stapes ( arrows ). Incudo-stapedial dislocation: (F) Axial CT image demonstrates a gap between the incus and stapes ( arrow ).

Fig. 13.13, Aberrant Internal Carotid Artery. (A and B) Axial CT angiogram images demonstrate a small inferior tympanic artery ( arrows ), which traverses through the middle ear cavity to join the horizontal petrous segment of the internal carotid artery (ICA). (C) Illustration depicts the failure of the cervical ICA to develop ( dotted lines ) with the ascending pharyngeal ( white solid arrow ), inferior tympanic ( white open arrow ), and caroticotympanic ( white curved arrow ) arteries providing an alternative collateral arterial channel, resulting in an aberrant ICA. (D) Axial illustration of the left temporal bone illustrates a classic aberrant internal carotid artery ( white solid arrow ) rising along the posterior cochlear promontory and crossing along the medial middle ear wall to rejoin the horizontal petrous ICA ( black solid arrow ). At the point of reconnection to the horizontal petrous ICA, stenosis ( white open arrow ) is often present. (C and D illustrations from StatDX, Copyright © 2022 Elsevier.)

Fig. 13.14, Persistent Stapedial Artery. (A and B) Axial and coronal reformat CT images demonstrate linear density traversing the cochlear promontory consistent with a persistent stapedial artery ( arrows ). (C) Illustration shows the persistent stapedial artery (PSA) arising from the vertical segment of the petrous internal carotid artery ( white solid arrow ), passing through the stapes, and traveling along the tympanic segment of the facial nerve ( white open arrow ) to become the middle meningeal artery ( white curved arrow ). (C Illustration from StatDX, Copyright © 2022 Elsevier.)

Fig. 13.15, External Auditory Canal Atresia. (A) A 3D volumetric CT image demonstrating malformation of the auricle. (B) Axial CT and (C) coronal reformat CT images demonstrate absent external auditory canal, atretic middle ear cavity, dysplastic incus ( red arrow ), absent stapes, and oval window atresia ( yellow arrow ).

Fig. 13.16, Cochlear Cleft. (A) Axial CT and (B) coronal reformat CT images demonstrate the characteristic linear hypodensity lateral to the cochlear compatible with a cochlear cleft ( arrows ).

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