Hearing Loss: Types and Causes
There are three main types of hearing loss in children: conductive; sensorineural; and mixed. These may occur bilaterally (in both ears) or unilaterally (in one ear).
Conductive: Hearing loss occurs in the outer or middle ear. Hearing loss happens when sound waves are disrupted or blocked before they reach the inner ear. Usually, this type of hearing loss can be corrected and is considered reversible.
- Blockage of the ear canal
- Punctured ear drum
- Fluid in the middle ear (otitis media)
- Bones in the middle ear (ossicles) that have been changed by infection or tumor
- Misshapen outer or middle ear and ossicles
- In most cases, it can be corrected either medically or surgically
- In those rare cases where this is not possible, hearing aids are very effective in correcting conductive loss
Sensorineural: Hearing loss occurs in the inner ear (cochlea), the auditory (hearing) nerve or the brain. This type of hearing loss is generally permanent, since nerve cells do not regenerate (grow back). This can be present at birth or get worse over time.
- Can be environmental or genetic (See section on Causes/Etiology of Hearing Loss)
- Use of amplification devices, such as hearing aids
- Osseointegrated “bone” hearing device
- Assistive Listening Devices and Systems (ALDS) such as FM and sound systems
- Sometimes cochlear implants
Mixed: Both conductive (middle ear) and sensorineural (cochlear or auditory nerve) hearing losses are present together in the same ear. In this case, when the conductive loss is corrected, there is still some hearing loss. It is important that after a child has his or her conductive loss corrected (for example, by insertion of tubes), a hearing test is done.
- Auditory Neuropathy Spectrum Disorder (ANSD), also known as Auditory Neuropathy/Auditory Dysyncrony (ANAD): Part of the cochlea is functioning normally, but either some hair cells or the auditory nerve is not working right. It results in a confused nerve impulse signal being sent to the hearing centers of the brain. This disorder is not fully understood. The treatment varies depending on the degree of functional hearing.
- Central Auditory Processing Disorder (CAPD): This is not a hearing loss, as the inner ear works normally. Rather, the brain seems to have a hard time interpreting and processing sounds. A child with CAPD has problems with where sounds come from and what they mean, and has difficulty understanding speech and following spoken instructions.
Causes or Etiology of Hearing Loss
Causes of hearing loss are divided into two general categories: environmental and genetic. In some cases, the exact cause is not known.
- Viral infections, like Rubella measles or cytomegalovirus (CMV)
- Certain medicines that can be toxic to the ear
- Injuries caused by lack of oxygen or inadequate blood flow
- Syndromic – those associated with other organ differences
- Nonsyndromic – not associated with other organic differences. Connexin 26 is the most common genetic cause. It is a nonsyndromic autosomal recessive genetic abnormality (see Helping Hand HH-III-125, Connexin Genetic Testing)
We can only test for a handful of the genes that are known to play a role in hearing loss. Connexin 26 testing is one of these available tests. If this testing is negative, it may be helpful for you to meet with our genetic counselors or geneticists to discuss further workup for causes of hearing loss.
There can also be structural differences in the inner ear that can be the cause of the hearing loss. We can look for this through imaging (CT or MRI). One example of a common structural difference is an Enlarged Vestibular Aqueduct (EVA) (see Helping Hand HH-I-329, EVA).
Your Hearing Clinic Visit
There are two main reasons for your visit:
- We want to find out the cause of your child’s hearing loss.
- We want to give you information about hearing loss to help your child succeed. We will work especially on your child’s speech and language development and success in school.
There are different tests that the doctor might order depending on your child’s type and degree of hearing loss. We will usually know after a complete patient and family history and physical examination if the hearing loss is due to genetic or environmental causes. We can find out about environmental causes by asking about the history of prenatal infections, birth history, injury or other illnesses.
We can sometimes find a genetic cause of hearing loss by blood tests or imaging tests.
- Connexin 26 and 30 testing: This is a blood test for two of the most common known gene problems that cause hearing loss. The blood test is easily available. There are hundreds of gene mutations that cause hearing loss, but only a few tests are available to check for them. We believe that the Connexin gene causes almost half of hearing loss we find at birth.
- Genetics Referral: We offer more detailed genetic testing if your baby’s first blood tests are normal. These are also blood tests. We recommend you meet with our geneticist or genetic counselors to talk about this testing.
- Imaging: With a certain type of hearing loss, CT or MRI scans can help us find out if there is a problem in the structure of the inner ear that could cause the hearing loss.
- Ophthalmology: It is important that your child has a complete eye exam. Many children with hearing loss also have vision problems. Fortunately, most of these eye problems are easy to fix. We recommend your child see an eye doctor by age three unless you are told to go before then.
- Hearing tests: Your child has seen an audiologist. Be sure to follow the instructions for follow up. At first, infants and young children may need more than one hearing test to be sure of the hearing loss. Another purpose of repeat testing is to make sure that your child’s hearing loss is not changing. Our Hearing Team audiologist is in Hearing Clinic to answer your questions about hearing loss and treatment options.
- Speech and Language: Our Speech-Language Pathologist, who specializes in working with children who have hearing loss, will discuss your child’s speech and language development. For children who have not yet obtained a baseline speech and language evaluation, a referral will be placed for this separate appointment. You will learn about expected speech and language milestones for your child.
- Electrocardiogram (EKG): This is a test for patients with severe to profound hearing loss in both ears. We want to check for a rare heart problem related to hearing loss.