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Thermodynamics and an Introduction to Themostatistics , 2nd edition, New York: Theoretical Phenomenology Computational Experimental Applied. Electrostatics Magnetostatics Plasma physics. Due to the complexity of treatments, communication between the reconstructive plastic surgeon and the otologist is necessary to detect hearing loss and determine the best method of restoring hearing in conjunction with microtia repair. Children born with these problems face difficulties in hearing and communication, as well as the social stigma of auricular deformity.

Here we focus on otologic and audiologic considerations for these patients because they are often encountered together. Canal atresia is a broad term referring to a lack of the external ear canal. It describes a spectrum that also includes a stenotic ear canal. Associated with this malformation is improper development of the middle and inner ear. Therefore, a discussion of embryologic development is important to understand the need for a thorough diagnostic workup.

Otologic structures are largely derived from the first and second branchial apparatus and the otic placode. Each branchial apparatus is made up of the branchial arches, pharyngeal pouches, branchial clefts, and branchial membranes. The placode is first visible as an ectodermal thickening on the lateral aspect of the head. The placode invaginates, forms a pit, and then detaches from the surface as a vesicle.

This otocyst goes on to form the inner ear membranous structures. These structures are subsequently innervated by the cochlear and vestibular nerves. As they enter the otocyst, they exert an inductive influence to produce neuroepithelium. The neuroepithelium becomes the sensory cells of the semicircular canals, saccule, utricle, and cochlea of the inner ear.

These structures are enveloped by a cartilaginous capsule that develops into the petrous portion of the temporal bone. In the fourth week, the first pharyngeal pouch forms the tubotympanic recess. This develops into the tympanic cavity, mastoid air cells, and the Eustachian tube one week later. The Eustachian tube is critical in ventilation of the middle ear space by connecting with the nasopharynx.

The first branchial cleft is the only cleft to remain as a postnatal structure. It develops into the external auditory canal and its epithelium in the fourth week. This tissue becomes the tympanic membrane and its middle fibrous layer is derived from the mesodermal layer. The ossicles span from the tympanic membrane to the oval window of the inner ear.

The first arch cartilage ossifies to form the upper portions of the malleus and incus in the sixth week. The second arch cartilage ossifies to form the manubrium of the malleus, incus long process, and the stapes suprastructure head, neck, and crura. It also forms the styloid process. The stapes footplate is derived from the otic capsule. Subsequently, at 8 weeks, the primordial external canal becomes occluded by an ectodermal plug.

This plug gradually resorbs as its cells degenerate. By the 28th week, the canal is fully canalized. Any perturbations to this process can lead to canal tortuosity, stenosis, or atresia.

In cases of atresia, an atretic plate remains. The external pinna is developed from a cluster of mesodermal thickenings referred to as the hillocks of His. The first branchial arch contributes to the anterior cluster and the second branchial arch contributes to the posterior cluster.

The first branchial cleft separates them. Eventually, they develop into distinct parts of the auricle. The structures of the inner ear, middle ear, external auditory canal, and auricle derive from different embryologic structures at various times of development.

Therefore, it is accepted that one part may develop abnormally while adjacent structures are normal. However, it easy to see that development of one structure can have a direct impact on the development of its neighbor. This is why a patient with an obvious microtia deformity must be thoroughly evaluated for deformities of the remainder of the external, middle, and inner ear. It has been associated with many congenital syndromes, but is more often seen in isolation.

It occurs more commonly in males and when unilateral, in the right ear. At initial evaluation, patients can be easily divided into unilateral or bilateral atresia. A thorough history and physical is necessary to determine contributing factors and to determine if the atresia is part of an associated syndrome or concomitant abnormalities. The external auditory canal can be graded as normal, stenotic, blindly ending, or atretic. A stenotic or blindly ending canal may elude diagnosis as it may only be appreciated on otoscopy.

An important pathologic finding in stenosis and atresia is canal cholesteatoma. Cholesteatoma is a term for trapped keratinizing squamous epithelium occurring in the temporal bone.

Cholesteatoma can be recurrently infected and cause local bone destruction. Patients with unilateral atresia with normal hearing on the contralateral side typically go on to develop normal speech. Because it is not regarded as a significant functional deficit, some authors advocate proposing surgery once the patient becomes an adult. Others have demonstrated consistent and successful results to restore hearing and have advocated the surgery to restore binaural hearing.

Bilateral canal atresia is approached differently secondary to the inherent deficit of hearing of the child. Hearing is vital within the first few years of life for development of normal language.

Drawing correlates from patients with congenital deafness, aural rehabilitation is initiated within the first few years of life, with many otologists performing cochlear implantation before one year of age.

Delay of treatment beyond 5 years of life results in poor development of speech and language. Additionally, cognitive and social—emotional development can be affected. Ability to hear sound is divided into a conductive component and a sensory neural component. The conductive component relies on the external and middle ear; the sensory neural component is dependent on the inner ear cochlea and the cochlear nerve.

Together, the mechanical energy of sound is converted to neural signals that are delivered to higher-level auditory processing centers of the brain. Atresia or stenosis of the canal can present with a 45 to 60 dB hearing loss secondary to conductive impairment. Audiologic testing is therefore important to determine the presence of sensorineural hearing.

The goals of a surgical intervention would be to either bypass or correct the conductive deficit. Therefore, any intervention is contraindicated if there is any sensorineural hearing loss.

Alternatively, aural rehabilitation with nonsurgical methods could be initiated quickly if a hearing loss is detected. Audiologic testing in pediatric patients is highly dependent on the patient's ability to participate in responding to stimuli. A well-defined series of electric potentials can be recorded from the scalp when acoustic stimuli are presented to the ear. Waves can be observed that correlate with neurologic function beginning at the cochlear nerve and through auditory processing centers of the brainstem.

These responses are not affected by sleep or sedation. Alternatively, poor patient cooperation can impede accurate results. Therefore, the testing is usually done while the patient is sedated.

Multichannel air and bone conduction ABR is used in canal atresia, especially when it is bilateral. It is particularly useful when conditions of hearing loss may make it difficult to determine the side of origin of a response. One scenario is when the patient has a large conductive deficit in one ear and a large sensorineural deficit in the contralateral ear.

When measuring bone conduction, an oscillator is placed on the temporal bone and vibratory stimuli are passed through the bone. In adults, an intensity of less than 10 dB can be transmitted to the contralateral ear, eliciting a response. Fortunately, the levels are as high as 15 to 25 dB in 1-year-old infants and as high as 25 to 35 dB in neonates, making it easier to determine the side of a response.

Visual reinforcement audiometry VRA is a mode of audiologic testing used on patients between 5 to 6 months of age. At this age, patients begin to respond to sounds by turning their head laterally. VRA relies on operant conditioning techniques where head turning toward sound stimuli is reinforced with a pleasant visual event, such as seeing a toy.

The conditioning delays habituation to the sounds and enough stimuli can be presented to determine a threshold of sound intensity. This technique can determine ear-specific responses if earphones are used and can also determine bone conduction. Infants who are 12 months old generally have more accurate and reliable results. As children get older, conditioned play audiometry CPA becomes the preferred testing modality. With CPA, the child is taught a motor skill in response to detecting a sound stimulus.

Tasks include dropping a building block into a bucket or placing a peg into a board. By varying the motor tasks, audiologists can present enough stimuli to determine both air and bone thresholds for a child beginning between 2 to 3 years of age.

Conventional measures of hearing involve multiple tests, but the most commonly used is the pure tone average. This test involves presenting sound stimuli to each ear via air conduction or bone conduction at various frequencies and intensities. The patient then responds to a detected stimulus by pressing a button or raising their hand.

The intensity level where half of the sounds are heard determines a threshold. Generally, patients as young as 5 years old can cooperate to successfully undergo this testing. Other aspects of conventional testing that are useful are tympanometry and acoustic reflexes. Imaging studies are important for all children with canal atresia.

It provides valuable information regarding the development of the middle and inner ear. Noncontrast temporal bone computed tomography CT scanning is the preferred study. The most important prognostic factors for a good outcome are the size of the tympanic cavity, pneumatization of the mastoid, the status of the ossicles, and the course of the facial nerve Fig. Generally, a CT is performed around age 4, once the middle ear and mastoid are fairly aerated and the patient is close to potential surgery.

It could be performed earlier to rule out a canal cholesteatoma. Coronal computed tomography scan of the temporal bone in a patient with canal atresia. Note the thick atretic plate where the external auditory canal is usually found.

Various grading systems have been developed to categorize the severity of atresia, but the system proposed by Jahrsdoerfer has given surgeons the ability to determine the likely success of performing an atresia reconstruction Table 1. The threshold for offering a surgical correction is much lower for patients with bilateral canal atresia.

It is important to understand that not all treatment for congenital canal atresia involves a surgical intervention. Additionally, when surgeries are considered, there are usually multiple stages required to restore hearing successfully, which is in conjunction with the multiple stages usually required for microtia repair.

Hearing aids should be considered for any child who demonstrates a permanent bilateral hearing loss exceeding 20 dB Hz between and Hz. Bone conduction hearing aids are used since the canal atresia precludes the use of standard hearing aids. Ideally, they should be initiated within the first few months of life. Bone conduction hearing aids are similar to conventional air hearing aids, except they have an oscillator that replaces the speaker.

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