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additional eminences, the styloid and the chordal, and two other depressions, the posterior and lateral tympanic sinuses, are also probably identifiable on CT; however, their importance is limited as well. The styloid and chordal eminences are inferior and posterior to the pyramidal eminence, respectively. The posterior tympanum is derived virtually in its entirety from the second branchial arch.7,12 These recesses may be hidden from view during surgery and are often the site of residual collections of granula- tion tissue or CH. They are consistently well seen on axial CT section.4,49,50CT visualization of the sinus tympani is especially important preoperatively, as extensive involve- ment in this location may require a retrofacial (nerve) surgical approach (Fig. 3.14and Fig. 3.24). A highly posi- tioned jugular bulb and a contracted space between the facial nerve and the posterior semicircular canal preclude this type of exploration.47,48
These structures form the posterior portion of the tym- panic cavity proper. The lateral border is the TM, and the medial border is the labyrinth, particularly the promon- tory. The roof of the epitympanum, which is referred to as the tegmen tympani, separates the epitympanum from the middle cranial fossa. The inferior wall of the tympanic cavity (hypotympanum) is separated by plates of bone anteriorly from the carotid canal and posteriorly from the jugular bulb (Fig. 3.22).
A
C
B
Fig. 3.23 (A)Coronal anatomy (lsc, lateral semicircular canal; ow, oval window [posteriormost aspect]; v, vestibule; rw, round window; hypo, hypotympanum; sub, subiculum). (B)Coronal computed tomography image. Outlined black arrow, oval window (posterior portion); thin white arrow, ponticulus; outlined white arrow, round window (cephalad border); thin black arrow, facial nerve canal, second genu. (C)Vestibule (V). Black arrow, round window (leading to scala tympani of basilar cochlear turn); white arrow, round window niche; large white arrow, hypotympanum.
Fig. 3.24 Coronal anatomy (st, sinus tympani; f.mast, facial nerve, mastoid segment; smf, stylomastoid foramen).
theory suggests that mastoid size is independent of the status of the mesotympanum. The environmental theory suggests that the degree of childhood middle ear disease determines the size of the mastoid air cell system.52 There is no uniform agreement on the embryologic onset, which is typically a symmetric process unless compli- cated by inflammatory disease. The frequency of bacter- ial infection (eustachian tube function) therefore plays a critical role. The eustachian tube is responsible for mid- dle ear ventilation, protects it from pathogenic organ- isms, equilibrates pressure across the TM, and allows drainage of secretions. It is essential for maintenance of normal ventilation.1
Individuals with unilateral depressed pneumatization will often have a history of chronic otitis media (COM).
Interestingly, patients with cystic fibrosis typically have excellent pneumatization and a low incidence of infection.53 Pneumatization is described as pneumatic when complete, diploic when partial, and sclerotic when essentially absent.54 In the latter two categories aeration is limited to an antrum and central mastoid tract of variable size. The nonpneuma- tized portion consists primarily of bone marrow (diploic) or dense bone (sclerotic).24Five pneumatized regions are described: the middle ear, mastoid, perilabyrinthine, petrous apex, and accessory regions.55There is extensive intercom- munication (Table 3.4).
Chapter 3 The Middle Ear and Mastoid
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A
B C
Fig. 3.25 (A)Vascular supply of the middle ear, sagittal (oblique) drawing. (B,C)Insets (ec, external carotid; ic, internal carotid; pa, posterior auricular; ap, ascending pharyn- geal; o, occipital; im, internal maxillary; da, deep auricular; at, anterior tympanic; s, stylomastoid; it, inferior tympanic; st, superior tympanic; ct, caroticotympanic; pt, posterior tympanic; p, petrosal; et, eustachian tube). (Adapted from Hesselink JR, David KR, Taveras JM. Selective arteriography of glomus tympanicum and jugulare tumors:
techniques, normal and pathologic arterial anatomy. AJNR Am J Neuroradiol 1981;
2:289–297. Reprinted with permission.)
The middle ear is divided into three compartments in the coronal plane (epitympanum, mesotympanum, hypotympanum) and in the axial plane (protympanum, mesotympanum, posterior tympanum) (Fig. 3.26). The mesotympanum is located immediately opposite the pars tensa and may be separated from the more superi- orly placed epitympanum and more inferiorly placed hypotympanum by tangential lines drawn through the superior and inferior margins of the EAC, respectively.
Similarly, the mesotympanum may also be separated from the more anteriorly located protympanum and more posteriorly located posterior tympanum by tan- gential lines drawn through the anterior and posterior aspects of the EAC, respectively (Fig. 3.26). Thus, there are five definable areas of pneumatization within the middle ear.7
The epitympanum (attic) contains the malleus head and incus body and lies within the notch of Rivinus, a fan- shaped recess in the tympanic bone. The tympanosqua- mous suture line forms the anterior boundary and the tympanomastoid suture, its posterior boundary.
Two communications have been extensively described between the mesotympanum and epitympanum. These are referred to as the anterior and posterior tympanic isthmus.7,44,49,56,57These isthmi are vertical in orientation and pierce the tympanic diaphragm, a series of mucosal folds and ligaments that separate the mesotympanum from the epitympanum. The anterior isthmus lies between the tensor tympani tendon and the incudostapedial region, and the posterior tympanic isthmus between the short process of the incus and the stapedius tendon. They may be appreciated in cross section on axial CT.4,5,49They are Table 3.4 Pneumatization
Types Pneumatic Diploic Sclerotic Regions Middle ear
Epitympanum (attic) Mesotympanum Hypotympanum Posterior tympanum Protympanum Mastoid Antrum
Central mastoid tract Peripheral
Perilabyrinthine Supralabyrinthine Infralabyrinthine Petrous Apex Petrosal Apical Accessory Zygomatic Squamous Occipital
Source:From Schuknecht HF. Pathology of the ear. 2nd ed.
Philadelphia: Lea & Febiger; 1993. Reprinted with permission.
Fig. 3.26 Artist’s rendering of compartments of middle ear. (A)Coronal (Epi, epitympanum; Meso, mesotympanum; Hypo, hypotympanum).
(B)Axial (Pro, protympanum; Meso, mesotympanum; Post, posterior tympanum). (Adapted from Hesselink JR, David KR, Taveras JM. Selective
arteriography of glomus tympanicum and jugulare tumors: techniques, normal and pathologic arterial anatomy. AJNR Am J Neuroradiol 1981;2:289–297. Reprinted with permission.)
A B
prone to occlusion in the poorly pneumatized middle ear.
Recently, other authors have disputed the anatomic pres- ence of these isthmi and doubted that occlusive change predisposes to CH development.58,59
The mastoid region is subdivided into the mastoid antrum, which communicates with the epitympanum (attic) via the aditus, the central mastoid tract (direct inferior extension of the antrum), and the peripheral mas- toid area (additional cells that arise from the antrum).37 This latter region is further subdivided into tegmental cells (above the EAC), posterosuperior sinodural cells, posteroinferior sinal cells (closely related to the sigmoid sinus), facial cells (related to mastoid segment of facial nerve canal), and mastoid tip cells, which are divided into medial and lateral portions by the digastric groove.7,24The mastoid antrum is present at birth. Peripheral pneumati- zation continues through early childhood.
The tegmen is bone of variable thickness that forms the roof of the epitympanum/middle ear (tegmen tym- pani) and the roof of the mastoid (tegmen mastoideum) (Fig. 3.8). It is lined on the superior surface by dura and on the undersurface by mucosa.60 As such, the tegmen provides a crucial barrier preventing the spread of infec- tion and leakage of cerebrospinal fluid. In autopsy series, 1 to 5 focal defects are described in the tegmen in 15 to 34% of cases, most often in the well-pneumatized mas- toid. Rarely, multiple defects are described and may be the source of some morbidity including spontaneous CSF leaks and pneumocephalus.
The anterolateral portion of the mastoid is derived from the squamous portion of the temporal bone; the posteromedial portion including the mastoid tip arises from the petrous portion.61 The Koerner’s (petrosqua- mous) septum (KS) is a structure of interest to both the radiologist and otologic surgeon due to the surgical impli- cations when it is particularly thick, as it may be confused with the medial wall of the antrum (see Normal Varia- tions subsection below) (Fig. 3.19). This structure begins at the glenoid fossa and extends inferiorly lateral to the facial nerve canal toward the mastoid tip. The KS is vari- ably sized and predisposes the patient with chronic otitis to so-called attic block.46This septum may be divided into three portions: ventral (temporomandibular), middle (tympanic), and dorsal (mastoid). The ventral extremity can be used as a surgical landmark for the mastoid seg- ment of the facial nerve canal.62 The septum appears essentially absent in the well-pneumatized temporal bone; however, recent studies indicate that its size bears no relationship to the extent of pneumatization itself.62,63 The perilabyrinthine cells are located posterior to a vertical plane passing through the modiolus of the cochlea and are described as being either supra- labyrinthine or infralabyrinthine.7 The cochlea and the petrous apex region are anterior to this plane and consist
of peritubal cells (adjacent to the eustachian tube, antero- lateral to the carotid canal) and apical cells (anteromedial to the carotid canal).55,64Pneumatization is far more common in the peritubal region (65%). These peritubal cells often com- municate directly with the eustachian tube instead of the tympanic cavity. This anatomic fact may explain persistent CSF leaks in individuals who have undergone middle ear obliterative surgery.65 Eradication of CSF otorhinorrhea in this circumstance requires obliteration of several mil- limeters of the osseous eustachian tube orifice. This may be difficult due to the anatomic proximity of the carotid canal.
Accessory pneumatization may extend beyond the confines of the aforementioned regions into the zygomatic, squamous temporal bone, occipital bone, and styloid process.24,37
Developmental tracts of pneumatization may serve as pathways for disease within the temporal bone. These include the posterosuperior cell tract, the posteromedial cell tract (superior retrolabyrinthine), the subarcuate cell tract (translabyrinthine), the perilabyrinthine tract, and the peritubal tract. These tracts all intercommunicate and are named according to location. Of interest is that the petrous apex may be pneumatized by all of these tracts.
The petrous apex is a truncated pyramid medial to the inner ear, the carotid artery, and the eustachian tube, and is pneumatized in 30 to 35% of temporal bones.66–70This pneumatization occasionally consists only of a single giant air cell.71Although these giant air cells were per- haps a differential diagnostic problem with conventional radiographs and multidirectional tomography, their char- acteristic air density on CT presents little diagnostic diffi- culty. Interestingly, it is the nonpneumatized petrous apex that can cause difficulty on magnetic resonance imaging (MRI) (Fig. 3.27).67Bright signal from marrow fat can cause confusion because pathologic processes associ- ated with hemorrhage, specifically cholesterol granuloma, may be imitated by the rapid T1 relaxation times. Com- puted tomography will typically be diagnostic in identifying asymptomatic petrous apex penumatization. T2-weighted sequences should reveal less signal intensity and as such will easily document the presence of bone marrow (fat).
The os suprapetrosum of Meckel is a small bony structure located adjacent to the petrous apex that occurs as a normal variation.72
A mastoid pneumocele is a thin-walled generalized air cell enlargement presumably due to a one-way ball valve. This is differentiated from a pneumatocele, which is a collection of loculated air under tension within the soft tissues outside the abnormal cell. Compromise of the external auditory canal associated with conductive hearing loss has been reported in this context.73Defects within the tympanosquamous and tympanomastoid sutures are likely causative. Pneumocephalus has also been described.
Chapter 3 The Middle Ear and Mastoid