How We Hear
The ear can be divided up into three parts: the outer, middle and inner ear. All three of these sections play an important role in transferring information to the brain for processing. In each case, a breakdown results in a hearing loss.
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 oval window on the other. There are three bones in the middle ear that form a sort of bridge between the eardrum and the oval window. 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.
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 audiometry and/or otoacoustic emissions results.
Degrees of Hearing Loss
When we describe hearing loss in terms of how much loss a child has, we use terms such as slight, mild, moderate, moderate-to-severe, severe and profound. In each case, there are difficulties understanding speech information associated with that degree of hearing loss.
-10 – 20 dB HL
|Children have better hearing sensitivity than the accepted normal range for adults. A child with hearing sensitivity in the –10 to +20 dB range will detect the complete speech signal even at soft conversation levels. However, good hearing does 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 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 speech signal. The degree of difficulty experienced in school will depend upon the noise level in classroom, distance from the teacher and the configuration of the hearing loss. Without amplification, the child with 35-40 dB of hearing loss may miss at least 50% of 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 loss is present. Even a mild hearing loss can have serious effects on speech and language development.|
41 – 55 dB HL
|A child can understand conversational speech at a distance of 3-5 feet (face-to-face) only if structure and vocabulary are controlled. Without amplification, the amount of speech signal missed can be 80-100% with a 50 dB hearing loss.|
56 – 70 dB HL
|Without amplification, children will have difficulty understanding conversational speech. A 55 dB loss can cause a child to miss up to 100% of 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. Speech 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 ability of 90 dB or better should be able to identify environmental sounds and detect speech sounds presented at a conversational level.|
91 + dB HL
|Without amplification, a child with a profound hearing loss will not hear loud speech or environmental sounds. He/she will be aware of vibrations more than tonal pattern. Most children will rely on vision rather than hearing as 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
Problems with sound traveling via the outer or middle ear result in what we call a CONDUCTIVE HEARING LOSS. Most of these conditions can be medically treated. Some of the most common factors that result in a problem in the outer or middle ear that result in a hearing loss 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).
Issues with sounds traveling in the inner ear are called SENSORINEURAL HEARING LOSS. For the most part, we can treat the problem via a hearing aid or cochlear implant, but we can’t “fix” the problem medically. For sensorineural hearing losses in children, we often try to determine if the loss 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 disorder, the inner ear or cochlea seems to receive sounds normally, but signals leaving the cochlea may be disorganized or the hearing nerve itself may not process sound normally. With auditory neuropathy, hearing loss can range from mild to profound and often fluctuates.
For hearing losses in children, we often try to determine if the loss was congenital, meaning present at birth, or acquired at a later age. We also try to determine the etiology or cause of the hearing loss. We know that approximately 50% of all childhood hearing loss is due to hereditary or genetic factors. However, there is a good chance that the reason for the hearing loss 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 loss, we mean that the hearing loss is carried down through the family. There are two main forms of this type of hearing loss: syndromic and non-syndromic. A syndromic hearing loss means that there are other issues in addition to the hearing loss, and non-syndromic means that the hearing loss is the only issue for the child. The most common one is non-syndromic hearing loss, which includes 2/3 of all genetic hearing losses.
The following is a list of the most common forms of syndromic hearing loss and the other medical conditions that are commonly associated with them:
|Alport||Hearing loss 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 loss is 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 loss maybe be sensorineural or mixed and can range from mild to profound hearing loss. 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 loss 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 loss 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 loss may be mixed or sensorineural and may be progressive. Abnormal inner ear anatomy and thyroid enlargement can also be present.|
|Stickler||Hearing loss is usually conductive but can be mixed or sensorinural and can be progressive. Unusual facial features, cleft palate, eye problems and arthritis are common.|
|Treacher-Collins||Hearing loss 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 problems are common.|
|Usher||There are three types of Usher syndrome with different types of hearing loss but in all cases the losses are sensorineural and can have balance issues associated with them. All types have progressive vision loss.|
|Waardenburg||Hearing loss may be present in one or both ears and is a sensorineural type of loss. The striking features of Waardenburg syndrome can include premature graying hair, white forelock, fused eyebrows (synophrys), and skin pigment changes.|
Of the 50% childhood hearing loss that is 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 loss or about 1/3 of all the cases of genetic hearing loss. Because such a large percentage of hearing loss is due to a genetic component, it is often recommended that you have genetic testing. Genetic testing can help identify the cause of the hearing loss in your child and help predict if the hearing loss might worsen.
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