41. Congenital Abnormalities of the Ear (Microtia)
1. Essential Embryology:
a. The whole of the external ear, temporal bone, the auditory canal, the tympanic membrane and the ossicles which form the vibratory link for conduction of sound, develop from the mandibular and hyoid arches in the early weeks of gestation (Fig. 1). These structures constitute the conductive system of sound and, except for the external ear, are located within the temporal bone. The inner ear or the perceptive system beyond the ossicles (medially) develops from the neuro-ectoderm and is in fact an extension of the brain and its development precedes the development of the conduction system. These two systems get linked over a period of time but because they develop separately, the chance that they will both fail to develop in a single individual is a rare occurrence. In the event, abnormalities including a complete absence of the conducting system will still allow a child to hear because bone will allow sound to be transmitted to the inner ear above 35-40 decibels. Reconstruction of ossicles falls under the discipline of otology and has remained somewhat difficult. Therefore a hearing aid which will amplify bony conduction should be advised in all children with this abnormality so that speech develops optimally and the child does not develop a complex. This is particularly true when the condition is bilateral.
b. The fact that the development of the external ear as well as its conducting system occurs in the mandibular and the hyoid arches, means that several other structures which develop from these arches might be involved together with the abnormality of the ear when present. This abnormality might be of varying severity and goes by the term microsomia. When restricted to one side it is called hemi facial microsomia and strangely perhaps because it occurs independently on either side of the face and varies in severity, when both sides of the face are involved, the condition is called bilateral hemifacial microsomia (Fig. 2-4). Though the facial nerve is neuroectodermal in origin because of its passage through the temporal bone, it is sometimes affected in this syndrome as well as in isolated congenital ear abnormalities and a facial palsy might be present.
2. Anatomy: The external ear can perhaps be described as an exquisitely carved product of nature. It has several cartilaginous projecting parts as well as concavities over which is draped some very thin skin. It rises from the cranium at an angle called the auriculocephalic angle after developing from separate hillocks which together finally form a uniquely curved structure of the ear with a sulcus at the back. The ear is divided into a helix (spiral curve) with its crura (root) and the helix has a tubercle. Opposite the helix is the anti-helix with its two crura and between them is the scapha (from scaphoid, like a boat). Between the two crurae of the anti-helix lies the triangular fossa. Medial to the anti-helix is a shell like concavity called the concha which leads to the external auditory meatus. Anterior to the meatus is the tragus (a horn) opposite which lies the anti-tragus on the anti-helix. The soft lobule completes the ear (Fig. 5-7).
3. Classification of developmental anomalies: These are usually divided into five types with some overlap.
a. Anotia: Where there is no development of the external ear or the conducting system (Fig. 8).
b. Microtia: Where almost all the cartilaginous constituents are missing and what is usually seen is a soft tubular structure with the consistency of the ear lobule. Several variations of this are possible. Very often the external auditory canal as well as the rest of the conducting system also fails to develop (Fig. 9-11).
c. Lop ear: Here the cartilage of the upper part of the ear is abnormal in that the helicial cartilage does not develop properly. Deformities of the middle ear are less common (Fig. 12-14).
d. Cup ear: Here the cartilage of almost the whole of the ear is deficient together with a deficiency of the skin. The ear does not unfurl and looks like a cup. Here too as in a lop ear deficiencies of the middle ear may or may not be present (Fig. 15-16).
e. Cryptotia: Almost the whole of the cartilaginous frame of the ear is developed and present but is contained within a pocket of skin. The ear fails to separate and rise and an auriculo-cephalic sulcus is not produced (Fig. 17-19).
f. Bat ear: The ear is developed normally but the auriculo-cephalic sulcus is exaggerated and the normal helical architecture is less pronounced (Fig. 20).
Note: Each type of deformity except anotia presents a range of variations in terms of deficiency and distortion of cartilage as well as the skin. A brief historical review as well as the current standard technique for reconstruction will be discussed in the next chapter.
The compiler of these short notes wishes to express his thanks to Parag Telang, a plastic and reconstructive surgeon from Mumbai for the several illustrations and also help in writing this chapter.