The head and neckTemporal bone
Technique
High resolution computed tomography using thin 1-2 mm sections in both the axial and coronal projections is the preferred imaging method to examine the temporal bone.
Anatomy
The hearing and balance organs are imbedded in the pyramid of the temporal bone. These complex organ systems develop from all embryological layers. The inner ear is derived from the otocyst of the ectodermal plate, and is fully developed at birth. The labyrinth can therefore easily be seen on plain films of the newborn skull. The external ear canal and the middle car are derived from the first and second branchial arch apparatus.
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Figure 1.
Normal transverse cross sectional anatomy of the temporal bone at consecutive levels from bottom to top.
A) Junction between the hypotympanun and the mesotympanun at the level of the round window niche (2). The tensor tympani muscle can be seen extending towards the handle of the malleus (6) and the tympanic membrane.
B) At the level of the basal turn of the cochlea (3) forming the promontory which is part of the medial wall of the mesotympanum.
C) At the level of the internal auditory meatus (17) and the oval window (15). The suprastructures of the stapes can be seen (8).
D) Epitympanon (20) housing the head of the malleus (6) and the incus (7). The horizontal portion of the facial nerve canal is passing underneath the lateral semicircular canal.
E) Epitympanon (20) at the level of the lateral semicircular canal (18) and the aditus ad antrum (22).
F) Cross section of the two limbs of the superior semicircular canal (18) at the level of the tegmen tympani.
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1. Facial nerve canal
2. Round window
3. Cochlea
4. Cochlear aqueduct
5. Hypotympanon
6. Malleus
7. Incus
8. Stapes
9. Pyramidal eminence
10. Mesotympanon
11. Jugular bulb
12. Sinus tympani
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13. Posterior semicircular canal
14. Vestibular aqueduct
15. Oval window
16. Vestibule
17. Internal auditory canal
18. Lateral semicircular canal
19. Antrum
20. Epitympanon
21. Geniculate ganglion
22. Aditus
23. Superior semicircular canal
24. Subarcuate canal
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The normal anatomy of the middle and inner ear is depicted in Fig.1. The vestibule is the central part of the labyrinth and communicates with the basal turn of the cochlea and is in contact with the middle ear through the oval window. Posteriorly, the vestibule is in contact with the three semicircular canals which are situated in perpendicular planes to each other. The posterior semicircular canal runs parallel to the posterior surface of the pyramid. The superior semicircular canal is perpendicular to the long axis of the pyramid and its most superior portion creates the arcuate eminence seen on the upper surface of the pyramid. The lateral or horizontal semicircular canal indents the medial portion of the middle ear cleft above the oval window niche. The bony coverings of the cochlea have two and half turns and its central axis has an oblique anterolateral orientation. The cochlear aqueduct originating from the basal turn of the cochlea connects the perilymphatic space with the subarachnoid space in the posterior fossa. The endolymphatic duct courses through the vestibular aqueduct into a sac which lies along the dura on the posterior surface of the pyramid. The lamina cribrosa separates the vestibule from the internal auditory canal. The facial nerve runs in the anterior cranial portion of the internal auditory canal above the cochlear nerve, while the two vestibular nerves occupy the posterior half separated by the crista falciformis along the posterior wall of the canal.
The facial nerve enters the otic capsule and traverses the middle ear on its way to the stylomastoid foramen. The first part being the labyrinthine portion runs anteriorly lateral to the vestibule and just above the cochlea to the geniculate ganglion. Here the nerve swings back at the first genu and the tympanic portion runs underneath the lateral semicircular canal having a thin bony covering above the oval window. Reaching the posterior wall of the middle ear the nerve makes a ninety degree turn downward at the posterior (second) genu and the mastoid portion of the nerve runs down to the stylomastoid foramen before branching out in the parotid gland. The ossicular chain in the middle ear cleft connects the tympanic membrane with the oval window. The handle of the malleus is secured up against the tympanic membrane and the head articulates posteriorly with the body of the incus. The long process of the incus connects with the stapes through the incudostapedial joint. Many small ligaments suspend the ossicular chain along with the tensor tympani and the stapedius muscles which attaches to the handle of the malleus and the stapes respectively.
The middle ear forms together with the antrum and the mastoid cells a complex aircell system. The mesotympanum is the part of the middle ear that can be visualized through the tympanic membrane. Its medial border is the promontory which is the lateral bony covering of the basal turn of the cochlea. Hidden from direct inspection is the attic or epitympanum
 | Figure 2.
Congenital inner ear malformation computed tomography
Coronal section through the middle ear at the level of the oval window niche. Isolated malformation with a dilated lateral semicircular canal (arrow). The second portion of the facial nerve canal runs in a normal fashion underneath the lateral semicircular canal (crossed arrow). |
in which the head of the malleus and the body of the incus can be found. The roof of the epitympanum is the tegmen tympani which separates the middle ear from the middle cranial fossa. The epitympanum communicates with antrum and the mastoid aircells through the aditus ad antrum. The hypotympanum is located below the lower limbic margin of the tympanic membrane and the promontory. It connects with the eustachian tube through the prototympanum.
Pathology
Congenual malformations
The inner ear is fully developed in the 23rd week of pregnancy. Inner ear malformations are often bilateral when they occur. A blunt and dilated horizontal semicircular canal is the most common malformation (Fig. 2). If it is isolated it will often not be associated with any symptoms. The cochlea which normally has two and a half turns can have arrested its development and will then have fewer turns. Depending on the severity it will have a sac like appearance being the so called Mundane malformation. The Neutrogena hearing loss will depend on its severity. Computed tomography can depict the inner ear in detail and it is possible to assess the number of turns the cochlea as well as the semicircular canals along with the other canals of the inner ear.
The external and middle ear are derivatives of the branchial apparatus. This often leads to unilateral but combined malformations. Artesia of the external ear canal is often associated with malformed icicles in
 | Figure 3.
Artesia of the external ear canal- computed tomography.
Transverse section through the epitympanun. Normally developed inner ear structures and mastoid air cells white malformed ossicles are fused with the wall of the middle ear cleft (arrow). |
a small middle earcleft (Fig. 3).
Dysostosis mandibulo- facialis (Treacher-Collins syndrome), which is autosomal dominant inherited malformation, is the most common congenital ear malformation. The malformation is characterized by a hypoplastic mandible and maxilla leading to a low sitting outer ear and an atretic canal with malformed ossicles in a malformed middle ear.
Computed tomography is the primary method for evaluating external ear atresia in order to assess if the atretic plate is bony or cartilaginous and if there is a normal middle ear cleft with normal ossicles.
Inflammatory lesions
Acute otitis media
Acute infections of the middle ear most commonly occur in children and often in connection with an infection of the upper respiratory tract when bacteria can reach the middle ear through the eustachian tube. Imaging studies are of little value in uncomplicated acute cases. If the suspicion arises that the infection is progressing to a serious mastoiditis with threatening brain abscess computed tomography should be undertaken.
Chronic otitis media
Recurring acute middle ear infections or an inadequately treated acute infection can lead to chronic otitis media. Adults having chronic changes often had recurring ear infections as children. Computed tomography shows poorly developed opacified mastoid aircells having thick and sclerotic walls. The middle ear cleft is also often atelectatic and opacified.
 | Figure 4.
Chronic otitis - computed tomography
Transverse section through the epitympanun. The mastoid aircells are opacified having thickened sclerotic septae. A suspicious cholesteatoma is seen in the aditus with erosion of the short process of the incus (arrow). |
 | Figure 5.
Cholesteatoma - computed tomography
Coronal section through the anterior portion of the middle ear cleft. A small cholesteatoma interposed between the head of the malleus (white arrow) and the eroded scutum (black arrow). |
Part of the ossicular chain can also be demineralised or missing. The inner ear structures including the bony coverings of the lateral semicircular canal are intact (Fig. 4).
Cholesteatoma
Keratin producing squamous epithelium that becomes enclosed in a cavity like the middle ear cleft will produce a cholestetoma. Primary cholestetoma arise from aberrant epithelial rests, most commonly occurring in the epitympanun or the petrous apex. An acquired, or secondary, cholesteatoma arises in a retraction pocket of the tympanic membrane extending up into the attic and epitympanun or through a marginal perforation. Cholesteatoma expands in all directions like a ball of tissue. An acquired cholesteatoma can be seen on computed tomography as a rounded soft tissue collection in the epitympanun in between the lateral wall and the ossicles. The scutum being the upper margin of the
 | Figure 6.
Labyrinth fistula - computed tomography
Coronal section through middle ear at the level of the oval window (black arrow). A large cholesteatoma fills in the middle ear and has eroded the bone covering the lateral semicircular canal (white arrow). The ossicles have been eroded. |
tympanic ring will become eroded and blunted as will the medially displaced malleus and incus (Fig. 5). Continued expansion can lead to a labyrinth fistula with vertigo due to erosion of the bony covering of the lateral semicircular canal (Fig. 6). Acquired cholesteatoma is often found together with other chronic ear changes and diagnosis can therefore be difficult at times even by computed tomography (Fig. 4). A cholesteatoma should always be suspected if signs of expansion and erosion are found in patients with chronic middle ear changes.
Fractures
The ear is the most commonly affected sensory organ in severe head trauma. The fracture extension will depend on the type and direction of the trauma force as well of the development and pneumatisation of the temporal bone. The fractures can be classified according to their orientation along the long axis of the pyramid in longitudinal and transverse fractures. Longitudinal fractures are the most common and can be seen with blunt trauma to the forehead. These fractures run along the long axis of the pyramid through the middle ear out into the roof of the external auditory canal often leading to hematotympanun. Transverse fractures are perpendicular to the long axis can occur in trauma to the back of the head, as well as, the forehead. These fractures often start at the rim of the foramen magnum crossing the internal auditory canal or the labyrinth. Damage to the seventh and eight cranial nerves can therefore occur while the middle ear may not be affected.
Computed tomography makes it possible to immediately detect these fractures if thin section high resolution tomograms are added to the
 | Figure 7.
Facial schwannoma - computed tomography
Transverse section through the upper portion of the epitympanun. The labyrinthine portion of the facial nerve canal is expanded (black arrow) and an expansile lesion is present at the level of the geniculate ganglion (open arrow). |
regular head exam in the emergent phase. If a longitudinal fracture is found the patency of the ossicular chain should be evaluated along with any potential communications between the subarachnoid space and the middle ear that can lead to a cerebrospinal fluid leak. In transverse fractures it is the relationship of the labyrinthine portion of the facial nerve to the fracture that is important.
Tumors
Facial nerve schwannoma can occur anywhere along the nerve from the posterior fossa down into the parotid gland. The symptoms depend on the location of the tumor and paralysis can be a late symptom. The schwannoma grows in a spindle fashion along the nerve enlarging the facial nerve canal if situated within the temporal bone. Computed tomography will therefore display an expanded facial nerve canal (Fig. 7). Magnetic resonance imaging using Gadolinium enhancement will display the schwannoma as a high signal lesion along the enlarged portion of the nerve. Facial schwannomas within the internal auditory canal or the cerebellopontine angle cannot be differentiated from those originating from the acoustic nerve.
Paraganglioma or so called glomus tumors can occur anywhere where paraganglionic tissue is present including the temporal bone. These most often benign, but locally expansile tumors are named after their origin. Glomus jugulare tumors are found in adjacent to the jugular bulb, glomus tympanicum over the promontory in the middle ear (Fig. 8 a) and
 a | Figure 8.Glomus tympanicum - computer tomography
a) Coronal section through the middle ear at the level of the oval window. A soft tissue mass fills in the middle ear at the level of the promontory (arrow).
b) Selective external carotid angiogram shows a contrast blush corresponding to the soft tissue mass in the middle ear (arrow). |
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glomus vagale along the auricular branch of the vagus nerve. The origin of larger tumors can be difficult to assess. Tumors growing in the middle ear can be seen as a bluish mass behind the ear drum and will cause tinnitus as well as conductive hearing loss. High resolution computed tomography of the temporal bone and contrast enhanced computed tomography of the posterior fossa, alternatively magnetic resonance imaging with Gadolinium enhancement can demonstrate the tumors. Paraganliomas are highly vascularized and carotid angiography can help in making the diagnosis (Fig. 8). The vascular supply is often from the ascending pharyngeal and occipital arteries, both being branches of the external carotid artery.
Malignant tumors affecting the external ear canal and the middle ear are relatively rare. Squamous cell carcinoma is the most common malignancy in this area and often hide in changes of chronic otitis which can delay the discovery. Like squamous cell carcinoma in other areas of the facial skeleton these tumors will also cause destruction of the bordering skeleton early in their course. The tumors may grow down into the temporomandibular joint and the parotid bed leading to facial paralysis. Malignant (necrotising) external otitis, which is an infectious process often caused by psuedomonas aueriginosa and seen in elderly diabetics, can sometimes be difficult to differentiate from a true malignancy. Radionuclide studies, the pattern of disease as seen on CT and the clinical setting usually make the differential diagnosis straight forward.
Sven G. Larsson and Anthony A. Mancuso