This section provides descriptions of different types of hearing loss and disorders briefly discussed in other sections of the Medical Home Portal. This section does not provide an exhaustive description of all types of hearing loss. Diagnosis and management is reviewed in the Hearing Loss and Deafness module. This section includes:

• Sensorineural Hearing Loss
• Conductive Hearing Loss
• Mixed Hearing Loss
• Auditory Neuropathy
• Central Hearing Loss and Central Auditory Processing Disorder
• Unilateral Hearing Loss
• Hereditary Nonsyndromic Hearing Loss

Sensorineural hearing loss (SNHL) results from damage to the inner ear (cochlea) or to the nerve pathways from the inner ear to the brain. SHNL is sometimes called “nerve deafness.” The “sensory” part of the name refers to damage to the inner ear. The cochlea is contained in the inner ear, and along it are located hair cells, which are nerve endings that respond to different pitches of sound. The “neural” part of the name refers to damage to the vestibulocochlear nerve, also known as the 8th cranial nerve, the auditory nerve, or the acoustic nerve. The vestibulocochlear nerve transmits messages to the brain regarding sound and balance. SNHL reduces the ability to hear faint sounds; speech that is loud enough to hear may sound muffled or unclear. SHNL is the most common type of hearing loss and usually cannot be medically or surgically corrected. Causes of SNHL in children include:

• Illnesses, including CMV, bacterial meningitis
• Drugs that are toxic to hearing
• Hearing loss that runs in the family (genetic or hereditary)
• Head trauma
• Malformation of the inner ear
• Exposure to loud noise
• Eighth nerve lesions
• Prematurity

Most traditional hearing aids are designed for SNHL. Children with profound sensorineural loss may be candidates for cochlear implants.

Diagram of the hearing apparatus:

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Conductive hearing loss (CHL) occurs when sound is not conducted efficiently through the outer and middle ear. The middle ear, which includes the tympanic membrane (eardrum) and ossicles (tiny bones) transmits sounds from the air environment of the outer ear to the fluid-filled environment of the inner ear. This transmission process uses the middle ear’s mechanical vibrations to adapt the impedance of air to the impedance of the fluids in the inner ear. Without a good impedance match between the two, sound energy is reflected or absorbed rather than conducted, and a hearing loss occurs. Conductive hearing loss results in inability to hear faint sounds.
CHL is the most common cause of hearing loss in childhood and is usually caused by otitis media with effusion (OME). OME refers to a collection of fluid in the middle ear, which impairs tympanic membrane function but does not cause acute symptoms of an ear infection. It is generally a self-limited disorder with most cases resolving spontaneously in in 3-6 months. In a minority of cases, persistent OME may lead to significant hearing loss and/or tympanic membrane abnormalities, which may ultimately affect permanent hearing and language development. Other causes of CHL are less common, including anatomic craniofacial abnormalities, ossicular malformations, foreign body or cerumen in the external auditory canal, cholesteatoma, and trauma to the tympanic membrane or middle ear. [Gifford: 2009]
In general, traditional hearing aids work well for conductive hearing loss. However, some causes of conductive loss can be corrected medically or surgically. Treatment is individualized, guided by the otolaryngologist/audiology team. For example, middle ear fluid or Eustachian tube dysfunction may be resolved with treatment of infection or placement of ventilating tubes. Other interventions may include foreign body removal, tympanoplasty, mastoidectomy, or amplification with bone-anchored hearing aids. [Doyle: 2003] In some cases, absence of an outer ear or ear canal can be surgically repairable, but may require hearing aids for some time. Absence of the middle ear usually requires permanent use of hearing aids.

Mixed hearing loss describes a condition where a conductive hearing loss occurs in combination with a sensorineural hearing loss (SNHL). This means there is impaired conduction of sound through the outer and middle ear, as well as damage to the cochlea or auditory nerve in the inner ear. Examples in children occur when a child has SNHL and also experiences conductive loss because of otitis media, causing hearing to be poorer than it was from the SNHL alone. Traditional hearing aids can be used with mixed hearing losses. Medical or surgical correction of impaired conduction is typically a focus of treatment in mixed hearing loss.

Hearing loss may also be related to a hearing disorder called auditory neuropathy, or impaired transmission of signals from the inner ear to the brain, which results in poor speech perception, or disorganized sounds reaching the brain. Auditory neuropathy typically implies a processing defect of the auditory (cochlear) portion of the 8th cranial nerve, but similar test results are found with defective inner hair cells. This type of hearing impairment is diagnosed when there is an abnormal ABR but a normal OAE result. Variability or absence of responses to behavioral testing is also an indication of auditory neuropathy. Because of the wide variability in auditory performance, the term Auditory Neuropathy Spectrum Disorder (ANSD) was adopted in 2008. This hearing disorder is complex, as evidenced by the many names, explanations, and classifications of the disorder, which has also been called Dysynchrony and Central Auditory Neuropathy. Due to the involvement of the cochlear portion of the 8th cranial nerve (and sometimes the vestibular portion as well), this disorder technically can be considered a subset of SNHL, but is often differentiated due to dissimilar function and test results.
Children presenting with this hearing loss may have sporadic windows of hearing or near normal hearing and at other times may not respond to sound at all. The children that do respond to speech will often will have difficulty distinguishing one sound from another and trouble understanding speech clearly. In some cases, ANSD causes only mild hearing difficulties and is only a problem in noisy situations. In the majority of cases, however, it leads to significant hearing loss. Due to the variability in auditory performance, habilitation recommendations are based on speech and language development and can range from audiological monitoring to hearing aid fitting and cochlear implantation.
The causes of ANSD are unknown, but children who are born prematurely or have a family history of the condition are at higher risk for it. Symptoms can develop at any age, but most kids with ANSD are born with it and diagnosed in the first months of life. As ANSD becomes better understood, it is diagnosed more frequently and now accounts for about 10% to 15% of cases of hearing loss. Case reports suggest that auditory neuropathy may be associated with other types of neuropathies, such as Charcot-Marie-Tooth, Friedrich’s Ataxia, and Refsum syndrome, as well as associated with AIDS infection. [Hain: 2017]

In central hearing loss, the central nervous system, specifically the brain, encounters difficulty processing speech. Even when sound is effectively transmitted through the outer, middle, and inner ear, the brain is unable to properly interpret these signals. Central hearing loss therefore can also be considered a central auditory processing disorder. Auditory processing disorder is type of learning disability that affects many different people, however the problem itself is not well understood. Affected people may have difficulty understanding speech in certain situations, for example when there is significant background noise or when they are already attending to other auditory stimuli. This is not the same as functional hearing loss, in which an individual with normal hearing mechanisms may not respond to auditory cues due to behavioral or psychological differences. The functional difference between a central auditory processing disorder and an auditory neuropathy (above) can be slim, although test results can help differentiate the disorders.

In this disorder, routine hearing tests may be normal, reflecting the normal function of the outer, middle, and inner ear and 8th cranial nerve, however the individual may still struggle to understand and interpret speech. Testing for this condition is difficult, so diagnosis typically requires a person who has experience recognizing it. In children, awareness of this condition can help increase needed supports, such as limiting background noise to enhance classroom performance and learning. Frequency modulation auditory trainers have been used for children with central auditory processing problems.

Children with unilateral (one-sided) hearing loss are at risk for academic failure, experience considerable difficulty in understanding speech in a background of noise, experience trouble with localization (with resulting safety issues), and appear to exhibit more behavioral problems in school. Despite these challenges, clinical practice in terms of intervention for this population is variable.[Fitzpatrick: 2010] Children at greatest risk for educational problems exhibit a specific profile that includes, early age of onset, perinatal and/or postnatal complications, severe-to-profound sensorineural impairment, and right ear impairment. Children with unilateral hearing loss can be helped in the academic setting through careful classroom management and amplification.

• Children with unilateral hearing loss should be given seating preference and, when a choice of classrooms is available, the child should be placed in the classroom with the least amount of ambient noise and reverberation. Proper illumination can help the child to augment his/her understanding through visual cues and speech reading. Teachers should be informed that the child might have difficulty with speech understanding and localization especially under noisy conditions. In-service training may help teachers to recognize potentially frustrating educational settings and provide appropriate solutions.
• Students with unilateral hearing loss may also benefit from using frequency modulation (FM) auditory trainers in the classroom setting. The main advantage of FM auditory trainers is their ability to transmit the speaker's voice without the attenuation caused by physical distance and background noise. Such a system may be particularly helpful for students who are experiencing difficulty in the academic setting or have special listening problems that cannot be solved by changes in the classroom environment.
• Fitting the affected ear for a hearing aid may enhance communication outside the classroom setting.
• Children with unilateral hearing loss should be monitored carefully for conductive hearing loss from otitis media with effusion in either ear and for progressive hearing loss. Several authors suggest that these children should receive annual audiologic evaluations and, particularly when the hearing loss is progressive, more periodic evaluations may be desirable.
• Parents and teachers should be counseled that these children might encounter specific dangers in common situations such as crossing busy streets or riding a bike in heavy traffic. They should also be encouraged to provide protection from high noise levels.

Over 100 genes are predicted to cause hereditary forms of nonsyndromic hearing loss (hereditary forms of hearing loss not associated with a broader genetic syndrome). As the chromosomal locations (loci) of the various genes responsible for nonsyndromic hearing loss are discovered, they are given specific designators based on their inheritance pattern and their order of discovery. Loci of genes causing X-linked forms of hearing loss are given the designation DFN and then numbered sequentially (DFN1, DNF2, DFN3...). Autosomal dominant loci have DFNA as their common designator (DFNA1, DFNA2, DFNA3...) and autosomal recessive loci have DFNB as their common designator (DFNB1, DFNB2, DFNB3...).

Different mutations within the same gene may cause different kinds of nonsyndromic or syndromic hearing loss. Different mutations in the Connexin 26 gene, for example, can lead to a dominantly inherited form of hearing loss associated with the DFNA3 locus on chromosome 13q12 or an autosomal recessive form of hearing loss associated with the DFNB1 locus at the same chromosomal location. Some specific mutations may cause both a syndromic form of hearing loss and a nonsyndromic form. Mutations in the Pendred syndrome gene (PDS), for example, cause both Pendred syndrome (sensorineural hearing loss and goiter) and an autosomal recessive form of nonsyndromic hearing loss associated with the DFNB4 locus on chromosome 7q31.