Embryology, Anatomy, Normal Findings, and Imaging Techniques

Chapter 9


Embryology, Anatomy, Normal Findings, and Imaging Techniques




Embryology


A detailed study of the development of the ear is beyond the scope of this chapter; thus only the key embryologic events and their timing are discussed. It is important to appreciate that the inner ear is derived from neuroectoderm, whereas the middle ear and the external ear develop from the branchial apparatus, which explains why the abnormalities of the inner ear generally are isolated from the abnormalities of the middle and external ear and vice versa. It also is important to remember that the pharyngeal clefts give rise to the ectodermal structures, pouches give rise to the endodermal structures, and the intervening arches give rise to the mesodermal structures.



External Ear


The external auditory meatus (canal) is derived from the first pharyngeal cleft. At the beginning of the third month, the epithelial cells at the bottom of the meatus proliferate and form the meatal plug. The meatal plug dissolves in the seventh month, and the epithelial lining of the floor of the meatus participates in the formation of the definitive tympanic membrane. The lateral part of the tympanic membrane arises from ectoderm, and the medial part of it arises from endoderm. Located in between is an intermediate layer of connective tissue, the fibrous stratum, which is derived from mesoderm.


The pinna (auricle) develops from six mesenchymal proliferations, called hillocks, which are located at the dorsal ends of the first and second pharyngeal arches. They surround the first pharyngeal cleft (the future external auditory meatus). These hillocks gradually fuse and form the definitive auricle.1




Inner Ear


The otic placodes become visible on each side of rhombencephalon on day 22 of gestation; they are the first indications of developing ears. The otic placodes invaginate rapidly and form the otic (auditory) vesicles. From the ventral component of the otic placode, the cochlea and the saccule develop. From the dorsal components, the utricle, the semicircular canals, and the endolymphatic duct arise. These epithelial structures are collectively named the membranous labyrinth.


Between the sixth and eighth weeks of gestation, the cochlear duct, arising from the saccule, penetrates the surrounding mesenchyme and completes the two and a half turns. The thin residual connection between the cochlea and saccule is the ductus reuniens. At the tenth week, the cartilage that differentiated from the mesenchyme surrounding the cochlear duct undergoes vacuolization and two perilymphatic spaces are formed: the scala tympani and the scala vestibuli. The cochlear duct (scala media) is separated from the scala vestibuli by the vestibular membrane and from the scala tympani by the basilar membrane. The lateral wall of the cochlear duct remains attached to the surrounding cartilage by the spiral ligament. Medially, the cochlear duct is attached to the modiolus, which is the axis of the bony cochlea. At 6 weeks gestation, semicircular canals appear as outpouchings of the utricular (posterior) component of the otic vesicle.1



Anatomy and Normal Findings


The normal imaging anatomy of the temporal bone is exquisitely displayed on computed tomography (CT) and magnetic resonance imaging (MRI). An exhaustive review of the temporal bone anatomy is beyond the scope of this chapter.


The temporal bone consists of tympanic, squamous, petrous, and mastoid parts and a styloid process.2 The temporal bone may be divided into compartments when studying its anatomy or reviewing imaging studies. These compartments are described from lateral to medial.



Pinna and External Auditory Canal


Detecting subtle abnormalities of the pinna with imaging may be difficult, but in general it is possible to identify gross malformations or absence of pinna on CT. Ossicular abnormalities are present in 98% of the temporal bones with microtia and external auditory canal (EAC) abnormality.3


In cross section, the EAC is oval in shape. Its walls (anterior and posterior; superior and inferior) run parallel to each other throughout the course of the EAC. The EAC consists of two segments: a membranous (fibrocartilaginous) segment that makes up the lateral third of the EAC, and a medial bony segment that forms the remainder of it.4


The tympanic membrane separates the EAC from the middle ear cavity and is attached to the tympanic annulus, which is a circumferential bony prominence at the medial-most aspect of the bony EAC. Superiorly, the tympanic membrane attaches to the scutum (e-Fig. 9-1), which is an important bony landmark in the diagnosis and evaluation of cholesteatoma. If the tympanic membrane is not thickened, wide window settings on CT are required to visualize it.





Middle Ear Cavity (Cleft)


The tympanic cavity is an aerated space medial to the tympanic membrane. Air reaches the middle ear cavity from the nasopharynx via the eustachian tube. The middle ear cavity is arbitrarily divided into three compartments: epitympanum (attic), mesotympanum, and hypotympanum. With lines drawn parallel to the superior and inferior walls of the bony segment of the EAC on the coronal plane, the part of the middle ear cavity above the upper line is the epitympanum (attic), which is the largest portion and houses the bulk of the ossicular chain. The mesotympanum lies between the two lines. The hypotympanum is below the lower line and normally is nonexistent or very small in children (Fig. 9-2).




Ossicles


The malleus, incus, and stapes form the ossicular chain, which is responsible for amplification (~30%) and transmission of sound waves from the tympanic membrane to the oval window on the vestibule.5


The manubrium (handle) and the lateral process of the malleus are embedded in the tympanic membrane. The tympanic membrane has two parts, divided by the lateral process of the malleus: a smaller pars flaccida superiorly, and a larger pars tensa inferiorly.6 Cranial to the manubrium is the neck of the malleus, to which the tendon of the tensor tympani muscle attaches. The tensor tympani muscle is derived from the first branchial arch and is innervated by the only branch of the trigeminal nerve that has motor fibers: V3 (mandibular branch). The head of the malleus forms the “ice cream” of the familiar “ice cream and cone” appearance and articulates with the body of the incus (Fig. 9-3). The malleoincudal joint is a diarthrodial (synovial) articulation, as is the incudostapedial joint (Figs. 9-3 and 9-4).


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Dec 20, 2015 | Posted by in PEDIATRIC IMAGING | Comments Off on Embryology, Anatomy, Normal Findings, and Imaging Techniques

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