Ears

Introduction

The ear consists of three anatomical parts-external, middle, and internal. The external and middle parts are concerned mainly with transferring sound waves from the exterior to the internal ear. The internal ear contains the vestibulocochlear organ, which is concerned with equilibrium and hearing.

The internal ear is the first of the three anatomical divisions of the ear to appear, early in the fourth week of gestation. At that time, surface ectoderm gives rise to the otic vesicle which becomes the membranous labyrinth of the internal ear. The otic vesicle divides into

1. a dorsal utricular portion that gives rise to the utricle, the semicircular ducts, and the endolymphatic duct;

2. a ventral, saccular portion that gives rise to the saccule and cochlear duct.

The organ of Corti develops from the cochlear duct. The bony labyrinth develops from the surrounding mesenchyme.

The epithelium lining the tympanic cavity, the mastoid antrum, the mastoid air cells, and the auditory tube are derived from the endoderm of the tubotympanic recess of the first pharyngeal pouch. The auditory ossicles (malleus, incus, and stapes) develop from the cartilages of the first two branchial arches.

The epithelium of the external acoustic meatus develops from ectoderm of the first branchial groove. The tympanic membrane is derived from

1. the endoderm of the first pharyngeal pouch;

2. the ectoderm of the first branchial groove;

3. the mesenchyme between these layers.

The external auricle develops from six swellings called auricular hillocks (Fig. 7.52), which arise around the margins of the first branchial groove. The swellings are produced by proliferation of mesenchyme from the first and second branchial arches. As the auricle grows, the contribution of the first branchial arch becomes relatively reduced. The lobule is the last part of the auricle to develop. The external ears begin to develop in the upper part of the future neck region but, as the mandible develops, the auricles move to the side of the head and ascend to the level of the eyes.

Figure 7.52 Embryonic development of the ear Auricular hillocks derived from the first and second branchial arches

Figure 7.52 Embryonic development of the ear Auricular hillocks derived from the first and second branchial arches

First And Second Branchial Arches

The human usually has three external auricular muscles and six intrinsic muscles. A protruding or cupped auricle is usually the consequence of a defect in the posterior auricular muscle. A "lop ear" refers to a defect of the superior auricular muscle. Abnormalities of the intrinsic ear muscles may lead to abnormal ear creases and anatomy—for example, a prominent antihelix, or the crumpled ear of Beal syndrome. Absence of the superior crus is correlated with an increased risk for deafness.

Ears are almost as distinctive as fingerprints, and much has been written about their variation in form, size, and position. The main anatomical landmarks are noted in Fig. 7.53.

Evaluation of the ears should include an assessment of

1. both preauricular regions looking for skin appendages, fistu-lae, and pits;

2. the tragus, looking at the size in proportion to the size of the ear;

3. the external auditory meatus, whether normal, narrow, or atretic;

4. the shape of each ear and its symmetry, whether it has a free or attached ear lobe;

5. the anterior and posterior surface of each ear or helix;

6. the position and rotation of the ear;

7. the relative positions of the tragi, to see whether they are on the same level.

Figure 7.53 Landmarks of the external ear.

Helix curve

Crus superius anthelicis

Concha

Figure 7.53 Landmarks of the external ear.

Helix curve

Crus superius anthelicis

Concha

Incisura Anterior

Incisura anterior

The right auricle is usually slightly larger than the left in both height and width. Discrepant growth can also occur secondary to pressure phenomena. In severe torticollis, compression of one ear against the ipsilateral shoulder will cause increased growth of the ear. The same situation can pertain in utero. The frequency of missing or attached ear lobes varies with the ethnic group studied but in Caucasians it averages about 20-25 percent. Clinically, any patient with a malformed external ear should be evaluated for hearing loss and abnormalities of the urinary tract.

Complete absence of the auricle, or anotia, is extremely rare. In microtia, vestiges of the external ear are present, and a graded classification is applied. Type I microtia (Fig. 7.54a) consists of a small external ear retaining the typical overall structure. Type II (Fig. 7.54b) describes a more severe anomaly with a longitudinal cartilaginous mass. In type III microtia (Fig. 7.54c) the rudimentary cartilaginous or soft tissue mass does not resemble a pinna. In type IV, or anotia (Fig. 7.54d), no soft tissue mass is present. In cryptotia, there is abnormal adherence of the upper part of the auricle to the head, as the skin of the postauricular region directly joins the skin of the upper portion of the auricle. Cryptotia (Fig. 7.54e) is attributed to the persistence of fetal attachment of the auricle to the underlying skin.

The Darwinian tubercle is a small projection arising from the descending part of the helix. Many variations in shape can be observed in this area. Anterior ear lobe creases are seen in Beckwith-Wiedemann

Figure 7.54 Microtia classification type I (a), II (b), III (c), and IV (d), and cryptotia (e).

Figure 7.54 Microtia classification type I (a), II (b), III (c), and IV (d), and cryptotia (e).

syndrome. In this condition, there may also be scored grooves or notches on the posterior aspect of the superior helix or lobe.

Variations in ear position and rotation are discussed in detail in the measurement section. Subjective assessment of ear position and rotation is extremely imprecise and is influenced by the position of the patient's head, the size of the cranial vault, and the size of the neck and mandible. There are several different methods for objective assessment of ear position and rotation, which are detailed below.

Auricular appendages or tags are relatively common and result from the development of accessory auricular hillocks. They are usually anterior to the auricle and more often unilateral than bilateral. The appendages consist of skin but may also contain cartilage. They vary greatly in size and may be sessile or pedunculated. The most frequent location is the line of junction of the mandibular and hyoid arches. Less frequently, they occur in the line of junction of the mandibular and maxillary processes, on the cheek, between the auricle and the angle of the mouth.Appendages in this area are more often associated with microtia or oblique facial fissures.

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