that is much more anteriorly and laterally located than normal (Fig. 3.29). This is much more common on the right presumably because the superior sagittal sinus often drains preferentially into the right transverse sinus.78The size of the mastoid antrum may be compromised, and this variation could make surgery more complicated, depending on the approach preferred by the surgeon.
Specifically, a postauricular approach could be quite haz- ardous in the presence of this type of sigmoid sinus anomaly. The distance between the sinus and the EAC varies directly with the degree of pneumatization of the mastoid.79
On occasion, we will encounter an extremely deep sinus tympani (Fig. 3.30).4Surgical exoneration of associ- ated debris within this recess might then be difficult, and the surgeon should be cautioned. The difficulties that may be encountered by the surgeon in the tiny middle ear cavity are self-explanatory.47A thick Koerner’s sep- tum has been described in the past to be a source of sur- gical concern. In this situation the surgeon may believe that the entire antrum has been explored, when indeed the more medial aspect has not.60,61,77The low-lying middle cranial fossa dura represents an obvious surgical hazard (Fig. 3.31). This occurs due to an absence of tegmental pneumatization (above the EAC) in those with congeni- tally thin superior EAC margins. This phenomenon is further discussed in Chapter 2. Other variations and anomalies will be considered under their appropriate sections and subsections.
Embryology
In this section, I provide a brief overview of the embry- ological development of the middle ear. For more detail, the reader is referred to Anson and Donaldson.33
The eustachian tube and tympanic cavity are formed from the first pharyngeal pouch (endoderm of tubotym- panic recess), which is a foregut outpocketing.30,80,81 The dorsal end of the pouch develops initially into the eustachian tube and subsequently forms tympanic cavity.12The extensions of the tympanic cavity, the attic (epitympanum), antrum, and mastoid air cells form after the tympanic cavity is developed. As these structures develop, the mesenchyme is replaced by endodermal epithelium. The tympanic cavity reaches adult size by 37 weeks of gestation.
Four endothelial primary sacs develop from the first pharyngeal pouch between the 10th and 30th weeks of gestation and form the tympanic cavity. These include the saccus anticus, saccus posticus, saccus superior (squamous), and saccus medius.12 Mucosal folds form where these sacs contact each other. The saccus medius is particularly important because it is responsible for the development of most of the epitympanum, antrum, and mastoid air cell system.
The saccus medius consists of three smaller saccules, the most medial of which forms Prussak’s space. The anterior epitympanum may be formed by the saccus anti- cus in some circumstances. When this occurs, the anterior and posterior epitympanum do not communicate.45The
Chapter 3 The Middle Ear and Mastoid
77
Fig. 3.28 Dehiscent (protruding) facial nerve. Coronal computed tomography image, right ear. There is a prominent inferior convexity to the tympanic segment of the facial nerve (arrow). Such a finding should always be called to the surgeon’s attention.
Fig. 3.29 Normal variant. Anteriorly and laterally placed sigmoid sinus (SIG).
saccus superior lies between the malleus handle and incus long process and is responsible for pneumatization of the squamous temporal bone. Areas pneumatized by the saccus medius and saccus superior subsequently become separated by a variable petrosquamous lamina (Koerner’s septum). The saccus posticus forms the recesses and ridges of the posterior mesotympanum.
Pneumatization of the tympanic cavity and epitympa- num is complete by week 34 of gestation in most cases;
however, further pneumatization of the temporal bone
and the rest of the mastoid air cell system continues for a variable time into childhood.7
The first and second branchial arches (mesoderm) dif- ferentiate into the ossicular chain and its supporting liga- ments, muscles, and tendons (Fig. 3.13F). The first branchial arch (Meckel’s cartilage) develops into the head of the malleus, the tensor tympani muscle and tendon, and the body and short process of the incus.82The second branchial arch (Reichert’s cartilage) develops into most of the rest of the ossicular chain and also into the stapedius A
C
B
Fig. 3.30 Usually deep sinus tympani (arrow). This represents a site where cholesteatomatous debris may be out of the direct vision of the operating surgeon. (A)Right ear. (B)Left ear. (C)Different patient, sinus tympani of extraordinary size. (C, Courtesy of Curtis Wushensky, MD.)
muscle and tendon. Other structures arising in whole, or in part, from the second arch include the mandibular condyle, styloid process, and facial nerve canal.83Ossicular development occurs simultaneously with the formation and differentiation of the middle ear cavity and its out- pouchings. The second half of this interval is primarily concerned with ossification, the ossicles having achieved adult size by the 15th week.84Formation of the stapes is not complete until week 38.85 Early in gestation the stapes primordium is pierced by the stapedial artery and is separated from the developing facial nerve and pyramidal eminence (laterohyale) by the interohyale, which becomes the stapedius tendon.86The stapes foot- plate has two layers: the tympanic portion, which is derived from the second brachial arch, and the vestibular portion (with its annular ligament), which develops from the otic capsule.81,84,85 The ossicles change little during life and, similar to the otic capsule, demonstrate a limited capacity for repair.
The TM and the supportive tympanic ring are formed by the 18th week of gestation. Portions of the TM are derived from all three germ layers. The outer epithelial layer is derived from the ectoderm of the first branchial groove (external auditory meatus). The middle fibrous layer is derived from the mesoderm, which insinuates itself between the tympanic cavity and the first branchial
groove. The inner mucosal layer is derived from the endo- derm of the first pharyngeal pouch.30
The tympanic ring (tympanic bone) is formed in mem- branous bone from four ossification centers. It is virtually completely developed by the 15th week of gestation.
There is a defect in the ring superiorly, which is known as the notch of Rivinus. The TM inserts in this location. The tympanic ring provides the scaffolding for the TM. The TM is relatively horizontal at birth and does not assume the adult vertical orientation until 3 years of age.81The tym- panic ring also contributes to the development of the styloid process. It is this tympanic bone that forms the sides and floor of the bony EAC as it elongates.