The Hearing Problem

Anatomy and Function of the Ear

the ear is comprised of three parts: the outer, middle, and inner ear. Basically, the outer ear consists of the funnel-like auricle and its canal, whereas the inner ear contains the snail-shaped cochlea that acts as the "electrical system" that relays the message along the auditory nerve to the brain for interpretation; the middle ear is an air-filled space between the two that houses three linked, moveable little ear bones (ossicles), known as the malleus (hammer), incus (anvil), and stapes (stirrup) (see Fig. 2).

Fig. 2 - Anatomy of the External, Middle, and Inner Ear.

Figure 2 - Anatomy of the External, Middle and Inner Ear

The phenomenon of hearing is initiated when sound waves reach the auricle, enter the canal and strike the tympanum (eardrum). Attached to the reverberating drum itself, the hammer mechanically transmits the wave as vibrations to the anvil and stirrup, whose footplate pushes on the membrane which is stretched across the tiny oval window of the fluid-containing inner ear. There, sound is converted to fluid waves whose frequency and height determine the pitch and intensity; this loudness is measured in decibels (dB). Normal conversation is usually at 45-60 dB, whereas a whisper registers about 30 dB.

The Hearing Deficiency Defined

At consultation, parents of a microtic infant are usually most concerned with the hearing problem. They either think that the child is completely deaf on the affected side or that hearing can be restored by merely opening a hole in the skin. As these are misconceptions, the physician can do much to alleviate parents' anxieties by fundamentally explaining ear embryology.

Because the human ear's receptive (inner) portion is derived from different embryological tissue than the conductive (external and middle) portion, the inner ear is rarely involved in microtia, and these patients have at least some hearing in the affected ear. The problem is sound conduction, which is impeded by the malformed middle and external ear complex. Typically, these patients have a hearing threshold of 40-60 dB on the affected side (by comparison, normal function allows us to hear sounds between 0-20 dB).

The middle ear and external ear develop from a common block of tissue, chiefly from the first and second branchial arches. The auricle itself is formed from six "hillocks" of tissue which lie along these arches and can be first seen in the five-week embryo [3, 54, 94].

On the other hand, the inner ear first appears at three weeks and is derived from tissues of distinctly separate origin. Perhaps this explains why it is usually spared the developmental mishap that almost invariably involves the middle ear of microtic patients. Refinements in radiographic technique (various X-ray and related methods, such as CT or CAT scans) have occasionally demonstrated underdevelopment of the inner ear [79, 88].

However, in evaluating approximately 2,500 microtic cases over a 25-year period (nearly 1,500 of which I have repaired), I have seen only three patients who were totally deaf. Remarkably, these were unilateral microtias who had no family history of hearing problems or microtia.

Determining the Child's Hearing

One need not wait until a child is old enough to respond to conventional behavioral hearing tests: An infant's hearing may be objectively assessed shortly after birth by auditory brainstem response testing (ABR) to determine whether or not the cochlea reacts normally to sound stimulation and whether the middle ear is normal. This is worthwhile even in the unilateral cases, for even if the opposite pinna appears relatively normal, sometimes its underlying middle ear is malformed.

Because of their normal inner ear, even patients with bilateral microtia usually have serviceable hearing when using bone conductive hearing aids to overcome the transmission block. They usually develop normal speech. These patients should be referred to an audiologist so these aids can be used as soon as possible; there is no point waiting several months, but instead, they should be applied within weeks of birth.

Because these bone-conductive hearing aids are cumbersome and label the child as "different" from his/her peers, it is ideal to surgically correct the hearing deficit to eliminate the aids. However, correcting this conductive problem surgically is difficult as the middle ear beneath the closed skin is not normal.

Surgical Correction of Hearing Problem

Exploration involves cautiously avoiding the facial nerve while drilling a canal through solid bone. One must usually create the drum with tissue grafts; the distorted or fused ossicles may be irreparable. Skin graft "take" on the drilled bony canal can be problematic and result in chronic drainage or canal stenosis. However, in the hands of a competent otologist with large volume experience, restoring middle ear function surgically can be rewarding for the microtic patient -- even if only one ear is involved.

Fortunately, nine out of ten microtia patients have one normal ear and are "born adjusted" to the monaural condition and do quite well in life even without treatment for hearing. Their main problem is sound localization, although they may also have hearing problems in noisy crowds. This can cause difficulty in a school environment where following verbal instructions and paying attention are necessary. The teacher's knowledge of the problem and cooperation in placing the child in a strategic position in the classroom are helpful. Some microtia patients may require speech therapy. Also, it is important to protect the normal ear from unusually harmful noise.

Because of the technical surgical and healing problems described above, many surgeons presently feel that potential gains from middle ear surgery in microtia are outweighed by potential risks and complications, and that this surgery should be reserved for bilateral cases. It is my conviction that if the otologic surgeon is not comfortable in tackling a unilateral case, then he certainly shouldn't be operating on the bilateral cases either! In the hands of a competent otologist, middle ear functional restoration surgery can be very rewarding for the unilateral as well as the bilateral patient.

As with surgical construction of the outer ear (or anything else for that matter), the outcome of such surgery strongly depends upon the experience of the surgeon. Not only must the otologic surgeon have the expertise and experience to perform this delicate and sophisticated surgery, but he must also be well versed in evaluating the patient's middle ear development to determine his/her candidacy for surgical repair in the first place. Jahrsdoerfer uses a ten-point scoring system in evaluating the CT scan of each microtic patient's middle ear [59, 107]. He will only operate on unilateral cases if they score six or higher; but he will sometimes operate on a patient with bilateral microtia with a lesser score. Using these criteria, Jahrsdoerfer finds that only 50 percent of patients are suitable candidates for surgery. He finds that not only does this grading system help to predict the degree of success one may expect in an individual patient (e.g., a score of eight translates into an 80 percent chance of restoring hearing to normal or near-normal levels, i.e., 15-25 decibels speech threshold), but it also prevents one from tackling impossible cases.

When middle ear surgery is contemplated, a team approach must be planned with an extremely experienced, competent otologist. In these cases, the auricular construction should precede the middle ear surgery, as once an attempt is made to "open the ear," the virgin skin is scarred, which compromises a satisfactory auricular construction.