Congenital aural atresia is an ear disorder that can be of varying severity, affecting only the outer ear or middle ear. Ear malformation can be a single problem or be part of a syndromic picture. Altered development of the first and second branchial arches and first branchial clefts may be responsible for CCA. Four degrees of severity are classified based on the combination of Schuknecht high-resolution computed tomography (CT) scan and surgical findings.
Type A: narrowing of the fibrocartilaginous canal and presence of cholesteatoma distal to the stenotic region
• Type B: Narrowing and tortuosity of the fibrocartilaginous and bony part of the canal, usually associated with abnormal tympanic membrane and malleus.
• Type C: Complete atresia with different combination of fibrous tissue and bone. Typically, the malleus and incus fuse together, the manubrium and tympanic membrane are missing, and the stapes are mobile
• Type D: Complete atresia with reduced pneumatization of the mastoid. Ossicular abnormalities more severe than Type C and often facial nerve abnormal
Other researchers have also classified CAA, and one of the most famous is Weerda, which classifies these malformations into three types (A, B, and C). Altmann defined a histopathological classification that relates the severity of CAA by defining three categories: mild, moderate, and severely malformed species. Many authors have since changed this classification system, further subclassing type II based on surgical findings and functional outcome.
Embryology and Etiopathogenesis of CAA
According to the researchers, they contribute to auricular development and both may play a role in the etiopathogenesis of CAA. The auricle begins to develop between the third to sixth weeks of embryonic life, when mounds appear on the arches, and its formation is completed by the fourth month of pregnancy. The base of the tragus, the helix root and the upper part of the helix come from the anterior three crests derived from the first arch. The posterior hillock emerging from the second arc is responsible for the formation of the antihelix, antitragus and lobule.
The middle ear cavity arises from the first pharyngeal arch from the 4th week of pregnancy. Pinna develops around the outer flesh that is canalized in the 28th week of embryological life. The middle ear cleft is formed in the 8th week and the cavity is completed in the 30th week. The first arch cartilage forms malleus and incus at the 8th gestational week, which begins to ossify in the 4th month of pregnancy. From the second arch, cartilage emerges from the stapest except the medial lamina of the foot plate derived from the otic capsule. At the 9th week, ectodermal cells proliferate, filling the meatus lumen and forming a “meatal plug” (MP). Then at week 10, the MP follows a horizontal plane and extends in a disc-like fashion and the interior of the MP begins to thin to form the future tympanic membrane.
At the same time, the plug in the proximal part of the neck starts to be reabsorbed. At 13 weeks MP is in contact with the primordial malleus and this contact will contribute to the thinness of the interior of the MP that will form the tympanic membrane at week 15. At 16 weeks, the outer ear canal is completely open but still narrow and curved. The meatus is fully extended at the 18th week and begins its opening to be completed at the 28th week.
Any adverse event that occurs during the 4th and 25th weeks of gestation and interrupts one of these developments may be responsible for one of the different types of CAA. Adverse event may be due to genetic abnormalities, vascular accident (fetal hypoxia), teratogenic substances (aminoglycoside antibiotics, hydantoin, alcohol, nicotine, herbicides), maternal infection (rubella, cytomegalovirus, measles, hepatitis, toxoplasmosis, lues) and the mother. metabolic disease (thyroid hormone deficiency or diabetes). CAA can be a single malformation or be associated with other malformations, such as in the case of auto-facial dysostosis (Treacher-Collins syndrome, Goldenhar syndrome), craniofacial dysostosis (Crouzon syndrome, Apert syndrome), auto-cervical dysostosis (Klippel-Feil syndrome). Wildervanck syndrome), auto-skeletal dysostosis (Van der Hoeve-De Klein syndrome, Albers-Schonberg disease) and chromosomal syndromes (trisomy 13, 18, 21, and 18q syndrome).
Indications for Using BAHI in CAA Patients
CAA is predominantly unilateral (about 70–90%) and the malformation mostly affects the right ear, perhaps this side may suffer from hypoperfusion more often than the left side where the heart is located and usually has a pressure 10 mmHg higher to the right side. The incidence of ear malformations is approximately 1 in 3800 newborns. Some children may exhibit a bilateral CAA when the malformation is not an isolated disease, but a syndrome is contextualized, such as CHARGE syndrome, where up to 60% of children are affected by bilateral atresia.
Patients can be affected by different severity of external and middle ear malformations. Different forms of hearing loss can be identified because of the severity. While CAA typically causes conductive hearing loss (CHL) in 80-90% of cases, it is the component of sensorineural hearing loss (SNHL) in the remaining patients. CHL is typically in the moderate hearing loss range of 40-60 dB; this is the range in which BAHIs work better.
The image on the back shows the ideal condition for using BAHI, while the super impacted yellow banana shows the distribution of sound frequency. Offering an auditory threshold within 45 dB, CHL can benefit from a BAHI because the implant guarantees good recovery of auditory functions in the range of 500 to 4000 Hz. In the case of children suffering from unilateral CAA and sensorineural hearing loss (10-20% of children with CAA), BAHI can be used to restore hearing function if the contralateral normal hearing function is preserved.
Author: Ozlem Guvenc Agaoglu