By Daniela Carvalho, MD, MMM
Most of my patients know me as Dr. C (my sign language name is Dr. with the C by the heart). I am a pediatric otolaryngologist who has been caring for children with hearing loss for the past 20 years. Often when I meet a family in my office, their first question is, “why does my child have hearing loss?” That question can easily lead to an hour-long conversation, as the assessment of the cause of hearing loss will depend on so many factors, including the type, degree, progression, and age of onset. I will try to summarize this here, and to do so, I thought it would be easier to “start from the beginning.”
Hearing loss (HL) can be divided into:
- conductive (CHL),
- sensorineural (SNHL),
- and mixed (which has a conductive and sensorineural component).
There is also auditory neuropathy, another type of HL that deserves an entirely separate discussion due to its singularities.
Conductive hearing loss (CHL)
Conductive hearing loss (CHL) can be caused by any obstruction of the sound on its way to the inner ear. That means that wax blocking the ear canal, fluid in the middle ear, and abnormalities of the middle ear bones can cause CHL. Children that have CHL have a normal inner ear (cochlea and hearing nerve). There are children born without the ear canal, and most of them have microtia (malformed outer ear). Most are unilateral, and some can be associated with syndromes, such as facio-auricular-vertebral spectrum. For patients with absent ear canals, we usually order a CT scan around five years of age to assess their ear anatomy.
Sensorineural hearing loss (SNHL)
Sensorineural hearing loss (SNHL) can be:
- in one or both ears,
- prelingual (hearing loss occurs before a child develops speech and language, which includes children born with hearing loss),
- postlingual (hearing loss develops after the child develops speech and language),
- stable or progressive,
- symmetric or asymmetrical.
The causes of hearing loss will be different for each of those, so we will focus on some of them.
Most of the time, children born with unilateral SNHL have abnormal inner ear anatomy or hearing nerve. Our choice of assessment for unilateral SNHL is usually an MRI. Still, in some cases, a CT scan might be best or necessary (as it gives better information of the ear’s bony structures).
The most common abnormality seen in imaging (CT or MRI) of children with congenital SNHL (unilateral or bilateral) is something called enlarged vestibular aqueduct (EVA). EVA can be related to a genetic mutation, so we recommend genetic assessment for Pendred syndrome (a syndrome that causes hearing loss, EVA and an enlarged thyroid gland).
A little about genetics and other causes
The causes of prelingual bilateral SNHL can be divided into preventable and non-preventable. Preventable causes include infectious causes (such as cytomegalovirus, or CMV), significant pregnancy complications, prematurity, low-birth weight, and ototoxic medications. Non-preventable causes are due to genetic abnormalities. Recessive non-syndromic hearing loss (that’s a mouth full!) accounts for almost 70% of those.
Before we discuss this, we need to explain how we inherit our parents’ genes. We have 23 pairs of genes, half from our mother and the other half from our father. In autosomal recessive gene inheritance, both copies of the gene have mutations. In this case, both parents carry one copy of the mutated gene, but they do not show any signs and symptoms (they are called “carriers”). Autosomal recessive disorders are usually not seen in every generation of the family. In autosomal dominant inheritance, just one mutated copy of the gene is sufficient for a person to have the condition. In this case, several family members can have the same disorder, unless a new mutation of the gene caused it.
For bilateral sensorineural hearing loss, autosomal recessive non-syndromic hearing loss (AR) is by far the most common genetic cause. Non-syndromic means that the child only has hearing loss and that no other organs are affected by the genetic mutation. Among AR, about 50% are due to mutations in the gene GJB2, which codes the production of the protein Connexin 26. A simple blood test can evaluate if the child has HL due to GJB2.
Causes of autosomal recessive syndromic hearing loss include Usher syndrome (that can cause progressive vision loss) and Pendred syndrome. Certain genetic panels (blood tests) can evaluate more than 150 genetic mutations related to hearing loss. If the otolaryngologist caring for your child believes that he/she might have hearing loss due to genetic causes, they should refer your child to a genetic counselor/geneticist to do a thorough evaluation and decide if the genetic panel is the best option.
Autosomal dominant syndromic hearing loss includes Waardenburg syndrome and Treacher Collins syndrome. These syndromes have very characteristic features, so they often can be suspected on physical examination. A genetic test can then confirm the diagnosis.
What about noise?
Another cause for hearing loss (that, unfortunately, is a lot more common than we think) is excessive and prolonged exposure to noise (noise-induced hearing loss). It can be easily prevented by turning down the volume of headphones and ambient music and decreasing the time exposed to the sound source. I like to use the 60/60 rule: less than 60% of the maximum volume for less than 60 minutes (then take a short “sound” break). It is also essential to protect your and your child’s ears with earplugs if in really noisy environments.
To sum it up
Regardless of the cause, as children with hearing loss will also use other means of information acquisition, such as their vision, your child’s doctor should refer him/her to a pediatric ophthalmologist for a thorough exam and follow-up.
As we can see, the necessary exams to assess a child’s hearing loss will depend on multiple factors, and there isn’t a “one size fits all” test. The causes of hearing loss are many, and the physician needs to do a thorough exam, as well as assess the family and the child’s history carefully. An otolaryngologist with expertise in pediatric hearing loss can help you through this journey. Your family and your doctor should become a part of a great team, whose goal is to help your child achieve his/her maximal potential (independent of language modality), stay healthy, and to be happy!
Editor’s note: Dr. Carvalho is a Pediatric Otolaryngologist at Rady Children’s Hospital of San Diego (RCHSD), Director of the Hearing Program at RCHSD, and Professor of the University of California, San Diego (UCSD) and serves on the Advisory Board for California Hands & Voices.