Congenital mixed hearing loss is that which is present at birth or develops during the first month of life and is comprised of both conductive and sensorineural components. Mixed hearing loss that occurs later in life is considered acquired mixed hearing loss and is discussed in another article (see: Acquired mixed hearing loss.) There is significant crossover between these two types of hearing loss; however, there are some entities that are specific to each. Conductive hearing loss occurs when the outer or middle ear fail to transmit sound wave energy to the inner ear. Sensorineural hearing loss is secondary to a problem with converting the mechanical energy into an electrical signal. The error may occur in the inner ear (cochlea) or in the pathway of transmission to the auditory cortex.
Congenital hearing loss (conductive, sensorineural, and mixed) occurs in approximately 1.4 per 1,000 neonates in America (CDC 2011). Of these, 50% are caused by environmental insults in utero and 50% are caused by inherited defects. Genetic hearing loss can be further broken down into non-syndromic and syndromic associations (70% and 30%, respectively). Furthermore, 80% of the non-syndromic congenital hearing loss is a result of autosomal recessive genetic mutations (Yildirim and Yilmaz 2006).
Congenital Mixed hearing comprises only a small subset of congenital hearing loss (see also: Genetic sensorineural hearing loss ). At the time of writing this article (2014), there are no published statistics, to our knowledge, on the prevalence of congenital mixed hearing loss. However, it can be inferred that it is less than 15% of all congenital hearing loss (<15 in 200,000 neonates), as it most commonly occurs within the category of congenital syndromic hearing loss.
A mixed hearing loss, by definition, must include impairment to both the conductive and sensorineural parts of the hearing pathway. The conductive component comes from pathology associated with the external or middle ear. This includes all components from the auricle to the oval window of the cochlea. The sensorineural component comes from pathology to the inner ear through the auditory pathway to the brain. Table 2 presents a list of congenital ear abnormalities, which of course is not all encompassing.
Any combination of a conductive and sensorineural congenital abnormality, in theory, would cause a mixed hearing loss. Additionally, there are distinct congenital syndromes associated with mixed hearing loss. These will be discussed later in the differential diagnosis portion of this article.
It is important to emphasize the embryology behind ear development and highlight some key steps to help better understand congenital malformations. The auricle develops from the fusion of six mesenchymal proliferations, called hillocks, at the dorsal ends of the first and second pharyngeal arches surrounding the first pharyngeal cleft (Sadler 2004). The first pharyngeal cleft forms the external auditory meatus and canal. Improper fusions of the hillocks result in abnormal auricles, microtia, or even anotia. The exact mechanisms behind these abnormalities are unknown; however, isoretinoin, thalidomide, and mycophenolate mofetil have an associative relationship with microtia (Murakami et al. 2010).
The tympanic membrane has three layers and is formed by the in-growth of the first pharyngeal cleft (ectodermal epithelial lining) and the out-growth of the first pharyngeal pouch (endodermal lining) with remaining mesenchyme in between (fibrous layer). Failure of the first cleft to recanalize at 6 months of fetal life results in canal atresia or stenosis. It is important to understand that in severe cases of atresia, the middle ear space and structures are often involved. For this reason, the Jahrsdoerfer classification is often helpful to evaluate the severity of middle ear abnormalities and help judge the likely success of atresia repair (Rodriguez et al. 2007).
The middle ear cavity itself is endoderm in origin. It is derived from the first pharyngeal pouch during week 3 of gestation from the tubotympanic recess. This recess develops from expansion of the first and possibly the second pharyngeal pouch. The ossicles are developed from the differentiation of neural crest mesenchyme of the middle ear, and lined by the epithelium of the first pharyngeal pouch. This development starts during the fourth to sixth week. Initially the neural crest mesenchyme forms cartilaginous models of the ossicles which slowly ossify by 30 weeks and the mesenchyme resorbs, allowing the ossicles to be free (Rodriguez et al. 2007). Any impairment during this maturation process may cause ossicular chain abnormalities (e.g., fixation, complete absence or shortening, ankylosis, or malformation).
Lastly, the inner ear development starts during the fourth week of gestation with the otic placodes, a pair of surface sensory placodes in the head. The placodes eventually invaginate to form otocysts which later give rise to a ventral component (forming the saccule and cochlear duct) and a dorsal component (forming the utricle, semicircular canals, and endolymphatic duct) (Sadler 2004). Together these epithelial structures form the membranous labyrinth. Although the exactmechanisms are complex, it is important to understand that disruptions during this very complicated developmental process may cause incomplete development of one or all parts of the inner ear. Systemic diseases such as cretinism or infections in utero tend to favor disruption of the Organ of Corti, causing hearing loss that is sensorineural. In cretinism, oftentimes the ossicles are also effected creating a mixed hearing loss picture (Pellitteri et al. 2010). Common infectious pathogens include maternal toxoplasmosis, rubella, CMV, and syphilis.
As implied in its name, congenital mixed hearing loss is present at birth or develops during the first month of life. Ideally, this is discovered during the first few days of life with a failed newborn hearing screen. In areas where universal screening is not mandated, patients may present later in life when they are of school age and having difficulty hearing in school. Often times, pediatricians will be the first to detect an abnormality and initiate the workup with an audiogram. At this point, it may be difficult to differentiate between a congenital and an acquired hearing loss.
The first consistent opportunity to detect hearing loss is after birth via a newborn hearing screen. In the past, hearing screening was only conducted when known risk factors were present (craniofacial anomalies, family history of hereditary hearing loss, NICU admission >2 days, intrauterine infection, or other syndromes associated with hearing loss). Over the past decades, there has been a greater push to mandate universal newborn hearing screening. Groups such as the Joint Committee on Infant Hearing (JCIH), Centers for Disease Control and Prevention (CDC), and American Academy of Pediatrics (AAP) recommend universal newborn screening (Wrightson 2007). In fact, the 2009 National Centers for Disease Control and Prevention’s Early Hearing Detection and Intervention (EHDI) program reported 97.4% screening in all documented US births, a considerable improvement from previous years (CDC 2011).
The two main screening modalities in infants are otoacoustic emissions (OAEs) and acoustic brainstem responses (ABRs). Specifically, transiently evoked otoacoustic emissions (TEOAEs) and automated acoustic brainstem responses (AABRs) are used. TEOAEs evaluate the function of the peripheral auditory system through a small probe placed in the ear canal which delivers a click stimulus. The outer hair cells of the cochlea transmit a physiologic response to these clicks which is detected by a microphone in the same ear probe. These responses are then compared to emission norms and a pass or fail report is generated. AABRs are able to measure the auditory function from the eighth nerve and auditory brainstem by using electrodes placed on the patient’s forehead and neck, with the ears being covered with earphones. A series of clicks are emitted through the earphones, and the brain wave responses are measured and compared to normal response templates.
There currently is no evidence supporting the use of AABRs over TEOAEs. At our institution, all newborns on the postpartum floor receive distortion product otoacoustic emissions (DPOAEs) and all neonates admitted to the NICU receive AABRs and DPOAEs. DPOAEs are very similar to TEOAEs, except in that two tones or frequencies are delivered instead of one. If a newborn fails their initial DPOAE testing, they receive a repeat exam at 1 month of age. Failure at the second test warrants an ABR. If any NICU patient fails either exam, they will be referred to an audiologist for a formal examination with an ABR (see the auditory/ vestibular testing entry for more details). Mixed hearing losses can be detected on formal ABRs. As the patients are followed through life, and binaural subjective data can be obtained, traditional audiograms should be pursued.
Once hearing loss is identified, a congenital hearing loss evaluation algorithm should be used. Currently there is no consensus on a standard workup, however many have been proposed. First, a thorough history and physical should be conducted. Emphasis on maternal infections or systemic illnesses during pregnancy as well as a family history of hearing loss should be delineated in the history. The physical exam should assess for syndromic facial characteristics (e.g., branchial clefts, auricle abnormalities, thyroid goiters, facial symmetry). Thorough ophthalmologic examination should be included due to the high prevalence of concurrent ophthalmologic problems. Additionally, hearing-impaired children are so heavily reliant on visual input. Experts generally agree that an EKG (abnormalities present in Jervell Lange-Nielsen Syndrome), Connexin 26 screening (most common cause of autosomal recessive non-syndromic hearing loss), and radiographic workup should also be considered (De Leenheer et al. 2010).
A high-resolution computed tomography (HRCT) scan of the temporal bones is the imaging study of choice for most congenital hearing loss because most anatomic causes are due to abnormalities of the middle or inner ear (St. Martin and Hirsch 2008). Abnormalities of the ossicles to include fused, malformed, or absent ossicles, lateral chain fixation, and facial nerve abnormalities are particularly important when looking for either a conductive or mixed hearing loss etiology. Inner ear malformations such as a mondini malformation, Michel’s aplasia, round or oval window atresia, and internal auditory canal stenosis can also be visualized on HRCT studies.
Even after a thorough evaluation, a child with congenital mixed hearing loss may still not have a firm diagnosis. Direct visualization of the middle ear space in the operating room may be necessary later in life. Additionally, the presence of a middle ear effusion in the setting of congenital sensorineural is not congenital mixed hearing loss, but rather acquired. Listed below are the more common conditions that have congenital mixed hearing loss associations. However, it is important to remember that the incidence of these conditions is very low.
Cretinism is a condition of physical and mental growth retardation due to untreated congenital hypothyroidism. Congenital hypothyroidism is most frequently due to iodine deficiency which has been largely eliminated in the developed world. Progressive mixed hearing loss is reported in one half to nearly all children with endemic cretinism (Meyerhoff 1979). Hearing loss with hypothyroidism may be conductive, sensorineural, or mixed. It tends to be more severe and mixed in congenital than in adult hypothyroidism. Oftentimes, there are ossicular chain abnormalities and edema of the eustachian tube mucosa causing a conductive hearing loss. The sensorineural hearing loss is a result of the myxedematous changes in the organ of corti and tectorial membrane. In adults, severe myxedema may cause bilateral symmetric and progressive sensorineural hearing loss.
CHARGE association refers to the presence of four of the six listed congenital anomalies that occur in random which include colobomas, heart defects, choanal atresia, retarded growth (neurological), genital hypoplasia, and ear anomalies (Edwards et al. 1995). It is associated with mutations in chromosome 8. Among the ear anomalies, CHARGE includes the external, middle, and inner ear malformations which can cause a conductive, sensorineural, or mixed hearing loss. In the external ear, there can be wide and short pinnas and cup deformities. In the middle ear, Wright et al. describe absence of stapedial muscles, absence of the oval window, aberrant course of the facial nerve, and dehiscence of the facial nerve canal. In regard to the inner ear, Wright also describes a Mondini-type malformation of the cochlea in addition to multiple anomalies of the vestibule (Wright et al. 1986). CHARGE can also be associated with lateral semicircular canal dysplasia which can cause an isolated conductive, sensorineural, or mixed hearing loss (Johnson and Lalwani 2000). A CT scan, in particular, is useful to help delineate these middle and inner ear abnormalities.
Branchio-Oto-Renal (BOR) Syndrome
BOR is an autosomal dominant syndrome with an estimated prevalence of 1:40,000 caused by a mutation in the EYA1 gene. BOR presents with a spectrum of clinical manifestations to include second branchial clefts; hearing loss; malformations of the outer, middle, and/or inner ear; and renal anomalies ranging from mild hypoplasia to complete agenesis (Kemperman et al. 2004). CT scans are also helpful to visualize cochlear hypoplasia or dysplasia, large vestibular aqueducts, ossicular chain abnormalities, and cochlear nerve hypoplasia which are commonly found with this syndrome (St. Martin and Hirsch 2008).
Treacher Collins Syndrome
Treacher Collins Syndrome, also known as Franceschetti syndrome or mandibulofacial dysostosis, is a rare autosomal dominant congenital disorder associated with a TCOF1 gene, and is characterized by craniofacial deformities such as absent zygomas, malformed small ears, auditory pits, down slanting palpebral fissures, and micrognathia. Hearing loss in these patients is conductive 55% of the time in concert with a high proportion of high-frequency sensorineural hearing loss.
Hemifacial microsomia is a craniofacial disorder that occurs in 1 in 5,600 births which is also commonly known as Goldenhar syndrome, oculoauriculovertebral sequence, oculoauriculovertebral dysplasia, and “first and second branchial arch” anomalies. The involvement of the ear can be highly variable. External and middle ear anomalies of this syndrome include a flattened helical rim of the pinna, preauricular skin tags, microtia, external canal atresia, ossicular chain abnormalities, and anotia (Carvalho et al. 1999). Although the inner ear is not classically affected, there seems to be an increasing association of sensorineural hearing loss.
Klippel-Feil syndrome (KFS) is a congenital abnormality of the vertebrae characterized by vertebral malformations of the atlas and axis resulting in a short neck, low posterior hair line, and limited neck mobility. KFS also has extraskeletal manifestations to include renal, cardiac, neural tubes, cleft palate, and ear abnormalities. Yildirm et al. described a spectrum of ear findings to include low-set ears, microtia, and numerous ossicular malformations (deformed caput mallei, rudimentary incus head, shortened incus, stapes footplate fixation) causing conductive hearing loss. Mondini deformities are also common, causing a sensorineural hearing loss (Yildirim et al. 2008).
Fifty percent of congenital hearing loss is caused by inherited genetic defects and are largely unpreventable; therefore, prophylaxis does not apply after birth. For the other 50% of cases caused by environmental issues, appropriate prenatal care cannot be overemphasized. Good prenatal care will serve to detect (or prevent via vaccinations) infectious etiologies such as TORCHES infections. Prenatal care will also serve to educate mothers on avoiding teratogens (e.g., thalidomides and retinoic acid). Early health screening can also detect maternal metabolic and endocrine issues.
The main therapy for congenital hearing loss is bilateral amplification. The push for early detection and intervention has become standard of care, and ensures normal or near-normal speech, language, and social skill development in children. The Joint Committee on Infant Hearing (JCIH), in their published position statement in 2007, emphasized early hearing screening and amplification within 1 month of detection. Depending on the severity of hearing loss, different types of hearing amplification (nonsurgical and surgical) are available. Middle ear explorations as a neonate are not recommended. Some patients may benefit from a middle ear exploration with concomitant ossicular chain reconstruction to help their conductive loss, but this should not be considered until later in life.
Congenital mixed hearing loss secondary to congenital cretinism may be one of the few disease processes that are amenable to medical therapy. Even in these populations, 10% of deafness still persist after T4 therapy (Pellitteri et al. 2010).
Hearing aids are small electroacoustic devices that simply amplify and modulate sound for the wearer. Due to the constant advances in technologies, hearing aids are discrete, powerful, and provide better quality of life for patients with hearing loss than ever before. For congenital hearing loss, in particular, any newborn child with hearing loss greater than 20 dB should be fitted for hearing amplification. Hearing aids come in various varieties. The behind the ear (BTE) hearing aids are most appropriate for children under the age of 7 as their ear canals are perpetually increasing in size. In the ear (ITE) and completely in the canal (CIC) aids tend to be less appropriate. In addition, these smaller hearing aids are more easily lost and are potential airway foreign bodies which are a safety issue for children.
Frequency Modulation (FM) systems should also be encouraged in hearing losses of 55 dB and above to improve the speech-to-noise ratio. A receiver attached to the child’s hearing aid and a microphone worn by the caregiver can improve feedback issues in infants who have poor head control and mobility (Norton et al. 2010).
In patients with auricular abnormalities, such as microtia or atresia, bone-anchored hearing aids (BAHA) should be considered because hearing aids may be poor fitting. BAHAs work by rerouting sound directly to bone via a sound processor and an osseointegrated implant that is located behind the ear. The sound processor detects sound waves, enhances the signal, and converts it into vibrations that are transmitted through the abutment implanted into the skull. This allows sound energy to bypass the outer and middle ear malformations and be transmitted directly to the inner ear for conversion to sensorineural input.
Disadvantages of BAHAs include the complications that occur to the skin surrounding the transducer and osseointegrated abutment. Meticulous skin care is required to prevent overgrowth or breakdown of skin around the implant. Poor osseointegration and need for revision implants can also be an issue. Lastly, BAHAs can have poor cosmesis in patients who wear their hair short. The clear benefit of BAHAs, however, is its reversibility and ability to bypass the conductive components of hearing.
Implantation is limited to the cortical thickness of the skull, so most patients are unable to implant until about 3–4 years of age (Snik et al. 2008). In the meantime, the BAHA can be attached to a special disk held in place by a steel headband of an elastic headband called the BAHA Softband. This can be used bilaterally if necessary with no impact on speech and language development. The BAHA Softband is also a great modality to trial the efficacy of a BAHA prior to implantation in older patients (Cochlear 2011).
Cochlear implantation (CI) is an excellent modality for profound sensorineural hearing loss when hearing aids and BAHAs are of little benefit (please refer to the entry on surgically implanted devices for more information). CIs can bypass the conductive components of hearing and can directly stimulate nerve hearing which is beneficial for congenitally mixed hearing loss patients. Major contraindications for CI are inner ear or eighth nerve abnormalities; so CT temporal bones and an MRI of the internal auditory canals are imperative for preoperative planning.
Middle Ear Implantable Devices
There are several middle ear implantable devices on the market. Most are indicated for sensorineural hearing loss, but work in Europe has shown significant benefits in patients with mixed hearing loss. Vibrant Sound Bridge (VSB) is an example of an active middle ear implant that utilizes a floating mass transducer on the round window, oval window, or medial ossicular chain to transmit sound wave energy into the inner ear. For this reason, it is also an excellent device for patients who have middle and/or external ear abnormalities. Sound is picked up from a microphone on the audio processor which is a removable device worn on the skull behind the ear. This device transforms the sound energy into an electrical signal and transmits it across the skin to an implanted receiver which then relays the signal down to the floating mass transducer resting in the middle ear. This is an excellent option for patients who have failed BAHAs or hearing aids.
Recently, an international committee convened in Frankfurt, Germany and deemed the Vibrant Sound bridge “appropriate for application in subjects under age 18, when the subjects satisfy the inclusion criteria, including having adequate anatomy to allow placement of the VSB and when weighed against and compared to other potential therapies” (Cremers et al. 2010). The exact lower age limit for implantation, however, is still in question. Please refer to the Hearing Aids article for more details. With any of the above modalities, close follow-up with audiologist for aid adjustments is vital to normal speech and language development.
Congenital mixed hearing loss, with the exception of hearing loss caused by congenital hypothyroidism, is a permanent condition and often progressive. Emphasis should be made on early detection and intervention to minimize the sequelae (speech and language development) of this disease process.
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