31 Otology Anatomy and Embryology with Radiology Correlates Flashcards
Which structures comprise the outer ear?
Which structures comprise the outer ear?
The external ear is composed of the auricle and the external ear canal, terminating at the tympanic membrane (Figure 31-1). The lateral third of the canal is cartilaginous and has hair follicles, along with ceruminous and sebaceous glands. The medial two thirds of the canal is osseous and free of hairs and adnexal structures. The length of the external canal, about 2.5 cm in adults, gives it a resonance frequency of 3 to 4 kHz.
What are the hillocks of His? What structure do they ultimately form?
What are the hillocks of His? What structure do they ultimately form?
The hillocks are six small buds of mesenchyme surrounding the dorsal end of the first branchial cleft.
- Hillocks 1, 2, and 3 arise from the mandibular (or first) branchial arch
- Hillocks 4, 5, and 6 develop from the hyoid (or second) arch.
These mesenchymal structures ultimately rearrange to form the auricle. Though the exact embryology is controversial, it is classically taught that the first hillock forms the tragus, the second and third form the helix, the fourth and fifth develop into the antihelix, and the antitragus is formed from the sixth (Figure 31-2).
What is the function of the auricle?
How does its unique structure contribute to auditory function?
What is the function of the auricle? How does its unique structure contribute to auditory function?
The cone-shaped auricle serves to collect and direct the sound down the ear canal toward the tympanic membrane. The shape of the auricle also creates small, unique, _high-pitched frequency resonances that contribute to the ability to localize sound in vertical space._
From which branchial structure does the external auditory canal develop?
From which branchial structure does the external auditory canal develop?
The external auditory canal develops from the first (or mandibular) branchial groove.
What are preauricular pits and tags? What is their clinical significance?
What are preauricular pits and tags? What is their clinical significance?
Preauricular pits and tags are benign malformations of the preauricular soft tissues. Pits are depressions in the skin located anterior to the ear canal. Epithelial mounds or pedunculated skin are known as preauricular tags. Structural abnormalities, including preauricular pits and tags, malformed pinnae, and stenotic or atretic ear canals, may indicate hearing loss and can be associated with congenital syndromes. Presence of these findings suggests the need for a thorough clinical exam for other congenital anomalies, audiometric evaluation, and possible genetic testing.
Which congenital syndromes are associated with external ear abnormalities?
Which congenital syndromes are associated with external ear abnormalities?
- Treacher-Collins syndrome (mandibulofacial dysostosis): Rare autosomal dominant condition with complete penetrance and variable expression consisting of downward-slanting palpebral fissures, auricular malformations with or without tags and preauricular blind fistulas, stenosis or atresia of the external ear canals, ossicular abnormalities, malar hypoplasia, flat nasal bridge, mandibular hypoplasia, cleft palate, and dental abnormalities.
- Goldenhar syndrome (Oculo-Auriculo-Vertebral syndrome): Rare disorder of unknown inheritance pattern characterized by anomalous development of the first and second branchial arch, which often results in unilateral craniofacial malformations, including hemifacial microsomia, eye anomalies, strabismus, anotia, preauricular skin tags, and stenotic or atretic ear canals. It is also associated with severe scoliosis.
- Branchio-oto-renal syndrome: Rare autosomal dominant disorder that is characterized by hypoplastic or absent kidneys, preauricular pits or tags, middle ear malformation or absence, and branchial cleft cysts or fistulae.
- CHARGE syndrome: Rare syndrome with a cluster of associated malformations, including coloboma of the eye, heart defects, atresia of the choanae, retardation of growth/development, genital defects (hypogonadism), and ear anomalies (asymmetric pinnae with low-set, lop ears).
- DiGeorge sequence: 22q11 chromosomal deletion resulting in absence or hypoplasia of thymus and/or parathyroid glands with cardiovascular and craniofacial anomalies, including low-set ears, micrognathia, hypertelorism, short philtrum, cleft palate, and choanal atresia.
- Crouzon syndrome: Rare autosomal dominant syndrome characterized by premature skull bone fusion (craniosynostosis). Other physical features include exophthalmos, hypotelorism, strabismus, beak-shaped nose, hypoplastic maxilla, low-set ears, and ear canal stenosis or atresia.
Describe the middle ear. What structures can be found within the middle ear?
Describe the middle ear. What structures can be found within the middle ear?
The middle ear is a 1 to 2 cm3 air-filled cavity that houses the ossicles, the stapedius and tensor tympani muscles, and the chorda tympani nerve (containing taste fibers from the anterior two-thirds of the tongue and parasympathetic fibers to the submandibular and sublingual glands). The middle ear is bounded laterally by the tympanic membrane and medially by the lateral wall of the inner ear (otic capsule). It is continuous with the mastoid air cells via the antrum and the nasopharynx via the eustachian tube (see Figure 31-1).
What are the ossicles? What is their embryologic origin? What is their function?
What are the ossicles? What is their embryologic origin? What is their function?
The ossicular chain is composed of the malleus, incus, and stapes. The malleus attaches laterally to the tympanic membrane, the stapes couples medially to the inner ear via the oval window, and the incus bridges these two bones.
- 1st branchial arch gives rise to the head and neck of the malleus and the body of the incus.
- 2nd branchial arch gives rise to the long process of the malleus, the long process of the incus, and the stapes suprastructure.
- The stapes footplate derives from both the 2nd branchial arch and the otic capsule.
The ossicles function to transform acoustic energy to overcome the impedance mismatch between the aerated external ear canal and the fluid-filled cochlea.
What is unique about the embryologic derivatives of the tympanic membrane?
What is unique about the embryologic derivatives of the tympanic membrane?
The tympanic membrane consists of three layers, each of which is derived from a different germ layer. The outer epithelial layer derives from ectoderm, the middle fibrous layer derives from mesoderm, and the inner epithelial layer derives from endoderm. Neural crest–derived mesenchyme around the lateral margin of the membrane forms the tympanic annulus, which begins to ossify in the third month of gestation.
Which muscles reside within the middle ear?
Which cranial nerves innervate these muscles?
Which muscles reside within the middle ear? Which cranial nerves innervate these muscles?
The stapedius and the tensor tympani muscles can be found within the middle ear. The stapedius is innervated by the facial nerve (CN VII), and a branch of the mandibular division of the trigeminal nerve (CN V3) innervates the tensor tympani.
What is the function of the stapedius and tensor tympani muscles?
What is the function of the stapedius and tensor tympani muscles?
Contraction of both muscles can be induced with high-intensity acoustic stimuli, with a more pronounced effect at lower frequencies*. When contracted, these muscles stiffen the ossicular chain, resulting in increased middle ear impedance and decreasing sound transmission to the inner ear. The exact function of this musculature *remains somewhat controversial, though it has been proposed that these reflexes serve either as:
- A mechanism for protection of the cochlea from intense sounds
- Reduce intensity of low-frequency background noise to preserve higher-frequency speech information.
Which structure provides aeration of the middle ear?
Which structure provides aeration of the middle ear?
The eustachian tube, by its connection to the nasopharynx, aerates and drains the middle ear. Its dysfunction can cause a plugged feeling or popping of the ear and is implicated in the pathophysiology of otitis media. The immature anatomy of the eustachian tube in children predisposes them to ear infections.
Describe the temporal bone. Which important structures does it contain?
Describe the temporal bone. Which important structures does it contain?
The temporal bone is a pyramidal structure (apex pointing medially) that forms part of the base and lateral side of the skull. Its major divisions are the squamous, petrous, tympanic, and mastoid bone segments. It houses the hearing and vestibular organs. Parts of the carotid, jugular, and facial nerve course through it. It also includes the middle ear cavity and the mastoid air cells.
Describe the tortuous path of the facial nerve through the temporal bone.
Describe the tortuous path of the facial nerve through the temporal bone.
After exiting the internal auditory canal, the facial nerve courses through the temporal bone via a z-shaped course in three divisions: the labyrinthine, the tympanic, and the mastoid segments (Figure 31-3).
- The labyrinthine segment begins as the nerve exits the internal auditory canal, traveling superior to the cochlea. Just lateral and superior to the cochlea, it angles sharply forward to reach the geniculate ganglion and then makes an acute posterior and slightly inferior turn. This “hairpin” bend is the first genu of the facial nerve.
- The tympanic segment extends from this point posteriorly and laterally along the medial wall of the tympanic cavity, above the oval window and below the bulge of the lateral semicircular canal, until reaching the pyramidal eminence. At this point, the nerve drops sharply inferiorly to form the second genu.
- The mastoid segment passes downward in the posterior wall of the tympanic cavity and the anterior wall of the mastoid to exit the base of the skull at the stylomastoid foramen.
Why does complete radiologic evaluation of the facial nerve involve both CT and MRI studies?
Why does complete radiologic evaluation of the facial nerve involve both CT and MRI studies?
When evaluating the facial nerve for possible lesion, both a dedicated CT scan and an MRI are useful. A CT scan can demonstrate the integrity of the osseous facial nerve canal while an MRI can reveal enhancement of the facial nerve itself.
What are the utricle and saccule?
What are the semicircular canals?
What are the utricle and saccule? What are the semicircular canals?
The utricle (oVEMP) and saccule (cVEMP) are vestibular organs that are responsible for detecting acceleration in a linear plane. The utricle detects horizontal accelerations and the saccule detects vertical accelerations, including gravitational force.
The semicircular canals detect rotational or angular acceleration. There are three canals (lateral/horizontal, superior/anterior, and posterior), which are oriented in different planes. The vertical canals (superior and posterior) are oriented roughly at 45 degrees in relation to the sagittal plane, and the horizontal canal is tilted upward about 30 degrees anteriorly from the horizontal plane.
From which embryonic structure does the labyrinthine membrane of the inner ear develop?
From which germ layer is this structure derived?
From which embryonic structure does the labyrinthine membrane of the inner ear develop? From which germ layer is this structure derived?
The structures of the inner ear and corresponding sensory innervation develop from bilateral otic placodes, which are ectodermal thickenings lateral to the rim of the neural tube. These placodes invaginate to become the otic pits and subsequently become enveloped by mesenchyme as vesicular structures known as otocysts. These bilateral otocysts will eventually differentiate into the membranous structures of the labyrinth. The otic capsule ossifies around the labyrinthine membrane between weeks 16 and 24 of gestation to form the bony labyrinth. _Fetal hearing is possible about 2 to 3 months before birth because hair cell and auditory neural development are essentially complete by 26 to 28 weeks gestation_.
What types of defects can result from abnormal cochlear development?
Which imaging modality is preferred to diagnose these defects?
What types of defects can result from abnormal cochlear development? Which imaging modality is preferred to diagnose these defects?
Inner ear malformations are described as being limited to the membranous labyrinth or involving both the osseous and membranous labyrinth. Dysplasia of the membranous labyrinth may be complete, limited to the cochlea and saccule, or involving only the basal turn of the cochlea. Membranous dysplasia is assumed to account for more than 90% of congenital deafness but can only be identified histopathologically. Only about 5% to 15% of congenitally deaf individuals have involvement of the otic capsule and thus show abnormality on imaging. These disorders include cochlear aplasia, cochlear hypoplasia, incomplete cochlear partition, and common cavity. The most severe agenesis is a complete aplasia of the entire osseous labyrinth (both cochlear and vestibular). Table 31-1 describes the osseous malformations in detail. High-resolution CT scan is the preferred imaging modality for diagnosing combined malformations of the osseous and membranous labyrinth.
Which rare disorder of the semicircular canals is associated with an anomaly of the temporal bone?
Which imaging modality is preferred for diagnosis?
Which rare disorder of the semicircular canals is associated with an anomaly of the temporal bone? Which imaging modality is preferred for diagnosis?
Superior canal dehiscence syndrome (SCDS) is characterized by conductive hearing loss, sound- or pressure-induced vertigo, and autophony resulting from absence of bone over the superior semicircular canal. The exact cause remains elusive, but dehiscent bone in SCDS has been proposed to be related to incomplete ossification of the otic capsule, leading to either absent or thinned bone susceptible to trauma-related injury. High-resolution CT scan is the imaging study of choice in diagnosing SCDS (Figure 31-4).
