How We Hear
The ear can be divided up into three parts: outer, middle, and inner ear. All three of these sections play an important role in transferring acoustic information to the brain for processing. In each case, a breakdown results in a hearing difference.
The OUTER EAR is made up of the ear canal and the eardrum. Sounds, in the form of sound waves, enter the ear canal, travel the length of the ear canal and reach eardrum. Sound waves bounce off the eardrum, causing a vibration.
The MIDDLE EAR is an air-filled space with the eardrum on one side and the cochlea on the other. There are three bones in the middle ear that form a sort of bridge between the eardrum and the oval window of the cochlea. When the eardrum vibrates, this vibration is transferred via these bones to the oval window. The vibration then is transferred via the oval window to the fluid in the inner ear, or cochlea.
Movement of the fluid in the INNER EAR causes the hair cells in the inner ear to send electrical signals up the auditory nerve to the brain. The brain processes these electrical signals and we “hear” the sound.
How to Read an Audiogram
An audiogram is a graphical representation of a person’s hearing. When looking at an audiogram, you will see frequencies/pitches, measured in Hertz (Hz), on the horizontal axis (running from left to right- low pitch to high pitch) and Intensity, measured in decibels (dB), on the vertical axis (from top to bottom- soft to loud). A designation is placed on the audiogram for the softest level at a given frequency that your child responds to 50 percent of the time. An audiogram obtained at Rady Children’s will also report the results of other types of testing such as immittance and/or otoacoustic emissions results.
Degrees of Hearing Loss
When we describe atypical hearing levels, we use terms such as slight, mild, moderate, moderately-severe, severe, and profound to represent the degree of hearing difference compared to typically hearing individuals. In each case, there are difficulties understanding spoken language information associated with that degree of hearing thresholds.
-10 – 20 dB HL
|Children have hearing thresholds in the typical hearing range. A child with hearing sensitivity in the –10 to +20 dB range will detect the complete spoken speech signal even at soft conversation levels. However, typical hearing thresholds do not guarantee good ability to discriminate speech in the presence of background noise.|
21-25 dB HL
|May have difficulty hearing faint or distant speech. Even with a threshold of 21dB, a student can miss up to 10% of the spoken speech signal when the teacher is at a distance greater than 3 feet and when the classroom is noisy, especially in the elementary grades when verbal instruction predominates.|
26 – 40 dB HL
|At 30 dB, a child can miss between 25-40% of the spoken speech signal. The degree of difficulty experienced with listening in school will depend upon the noise level in classroom, distance from the teacher, and configuration of the hearing thresholds. Without amplification, the child with 35-40 dB of hearing thresholds may miss at least 50% of spoken class discussions, especially when voices are faint or the speaker is not in line of vision. The child will miss consonants, especially when a high frequency hearing difference is present. Even a mild hearing difference can have serious effects on spoken speech and language development./td>|
41 – 55 dB HL
|A child may understand simple, close conversational speech when face-to-face, but will struggle as conversation gets more complex, distance increases, or visual cues are removed. Without amplification, the amount of spoken speech signal missed can be 80-100% with 50 dB hearing thresholds.|
56 – 70 dB HL
|Without amplification, children will have difficulty understanding spoken conversations. Atypical thresholds at 55 dB can cause a child to miss up to 100% of spoken speech information. There will be marked difficulty in school situations requiring verbal communication in both one-to-one and group situations.|
71 – 90 dB HL
|Without amplification, the child will have difficulty hearing in all situations. Spoken language will be detected only if the speaker is talking loudly and no more than one foot from the ear. When amplified optimally, children with hearing thresholds of 90 dB or less should be able to identify environmental sounds and detect speech sounds presented at a conversational level.|
91 + dB HL
|Without amplification, a child with profound hearing thresholds will not hear loud speech or environmental sounds. They will be aware of vibrations more than tonal pattern. Most children will rely on vision rather than hearing as their primary avenue for communication and learning. When amplified, it is possible that some children will be able to detect some speech sounds|
Types of Hearing Loss
Hearing differences due to inability of sound to easily travel through the outer or middle ear are called CONDUCTIVE HEARING DIFFERENCES. Most of these conditions can be medically treated. Some of the most common outer or middle ear concerns that may result in a conductive hearing difference include:
- Excessive amount of ear wax.
- Fluid or infection in the middle ear.
- Hole or perforation in the eardrum.
- Deformity of the outer or middle ear.
- Breakage or fixation of the bones in the middle ear.
- Foreign body in the ear canal.
- Growth in the ear canal (exostosis) or middle ear (cholesteatoma).
Hearing differences due to the inability of sound to travel in the innear ear or along the auditory nerve are called SENSORINEURAL HEARING DIFFERENCES. For the most part, we can assist sensorineural hearing differences with a hearing aid or cochlear implant, but we can’t medically “fix” it. For sensorineural hearing differences in children, we often try to determine if the difference was congenital, meaning present at birth, or acquired at a later age.
Hearing losses can also be MIXED, meaning that they are partially conductive and partially sensorineural.
Another less common type of hearing loss is auditory neuropathy spectrum disorder (ANSD). With this type of hearing difference, the inner ear or cochlea seems to receive sounds typically, but signals leaving the cochlea may be disorganized or the hearing nerve itself may not process sound typically. With auditory neuropathy, hearing differences can range from mild to profound and often fluctuates.
For hearing differences in children, we often try to determine if it was congenital, meaning present at birth, or acquired at a later age. We also try to determine the etiology or cause of the hearing difference. We know that approximately 50% of all childhood hearing differences are due to hereditary or genetic factors. However, there is a good chance that the reason for the hearing difference will not be known.
The most common reasons for a child having a CONGENITAL hearing loss include:
- Illness, infection or medicine taken by the mother during pregnancy.
- A condition or syndrome that has hearing loss associated with it.
- Physical abnormality of any part of the ear.
- Hereditary factors.
Some of the most common reasons for a child to have an ACQUIRED hearing loss include:
- Complications at birth.
- Illness such as meningitis, scarlet fever or mumps.
- High fever.
- Neurodegenerative disorders.
- Ototoxic medications.
- Head injury or trauma.
- Repeated or constant ear infections.
- Noise .
Some common causes for a child to have ANSD include:
- Lack of oxygen at birth.
- Hyperbilirubinemia requiring blood transfusion, associated with severe jaundice.
- Infectious disease such as mumps.
- Immune disorders..
- Neurological disorder.
- Malformed inner ear and/or small auditory nerve.
Genetic Hearing Loss
When we speak of genetic hearing differences, we mean that the hearing difference is carried down through the family. There are two main forms of this type of hearing difference: syndromic and non-syndromic. A syndromic hearing loss means that there are other concerns in addition to the hearing difference, and non-syndromic means that the hearing difference is the only concern for the child. The most common one is a non-syndromic hearing difference, which includes 2/3 of all genetic hearing differences.
The following is a list of the most common forms of syndromic hearing differences and the other medical conditions that are commonly associated with them:
|Alport||Hearing difference may be sensorineural, conductive, or mixed and may be progressive. Other features include kidney problems, near-sightedness or cataracts, and palate abnormalities.|
|Brachio-oto-renal||Hearing difference could be conductive, sensorineural, or mixed. Cysts (or pits) can be found on the neck or in front of the outer ear and the outer ear may be malformed. Kidney problems can be life threatening.|
|Charge||The hearing difference maybe be sensorineural or mixed and can range from mild to profound in degree. The letters in CHARGE stand for Coloboma, Heart, Atresia of the choanae, Retardation of growth and development, Genital and urinary abnormalities, and Ear abnormalities. There may be partial facial paralysis, cleft palate, cleft lip, kidney problems, and feeding problems.|
|Jervell and Lange-Nielsen||Hearing difference is sensorineural. The other major finding is an abnormal heart rhythm which could lead to fainting spells and possible sudden death. These abnormal heart rhythms are successfully treated with medication.|
|Neurofibromatosis type 2||Hearing difference is sensorineural and involves tumors of the hearing and balance nerve. Café-au-lait (coffee with cream-colored) spots may appear on the skin, with freckling and cataracts.|
|Pendred||Hearing difference may be mixed or sensorineural and may be progressive. Abnormal inner ear anatomy and thyroid enlargement can also be present.|
|Stickler||Hearing difference is usually conductive but can be mixed or sensorineural and can be progressive. Unusual facial features, cleft palate, eye problems, and arthritis are common.|
|Treacher-Collins||Hearing difference is conductive. Striking facial features include cleft palate, down-slanting eye-slit openings, unusual pupil openings, under-developed cheek bones, absent/malformed outer ears, teeth alignment problems, and possible balance concerns are common.|
|Usher||There are three types of Usher syndrome with different types of hearing differences but in all cases the hearing differences are sensorineural. There can be balance concerns associated. All types have progressive vision loss.|
|Waardenburg||Hearing difference may be present in one or both ears and is a sensorineural difference. The striking features of Waardenburg syndrome can include premature graying hair, white forelock, fused eyebrows (synophrys), and skin pigment change.|
Of the 50% of childhood hearing differences that are genetic, an estimated 70% are due to recessive causes, about 15% have a dominant cause, and the remaining 15% include all the other forms of inheritance. One gene, known as Connexin 26 (abbreviated CX26), is estimated to be responsible for half of all the recessive cases of hearing differences or about 1/3 of all the cases of genetic hearing differences. Because such a large percentage of hearing differences are due to a genetic component, it is often recommended that you have genetic testing. Genetic testing can help identify the cause of the hearing difference in your child and help predict if it might worsen.
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