Hereditary Hearing Impairments: What Causes Non-Syndromic Hearing Loss?
When a child is diagnosed with a hearing impairment, it often comes as a shock to parents. Discussions surrounding the causes of hearing loss often disregard the role of genetic factors.
Even though researchers have, to date, discovered over 400 genetic causes involving different forms of hearing loss, most people still know little about childhood hearing loss, its causes and treatment options.
With over 50 per cent of childhood hearing loss being inherited, it’s time we examine one of the most common forms of genetic hearing impairments – non-syndromic hearing loss.
|Note: As is common practice amongst hearing health care professionals, this article will use the terms ‘hearing impairment’ and ‘hearing loss’ interchangeably to describe hearing below the threshold levels for normal hearing.|
Hearing Loss Onset: Prelingual or Postlingual?
Before we jump in, you need to understand the difference between prelingual and postlingual hearing loss. The two terms will be referenced throughout this article.
Prelingual: Hearing loss present before the child develops speech
Prelingual hearing loss is present before a child begins to speak. This means congenital hearing loss – a hearing loss present before or at birth – is always prelingual.
|Congenital hearing loss causes include infections during pregnancy, brain or nervous system disorders, and syndromic and non-syndromic genetic causes.|
- Approximately 80% of congenital hearing loss is caused by genetics.
- The remaining 20% is attributed to environmental and acquired causes.
Note: Not all prelingual hearing loss is automatically congenital since hearing loss can also be acquired after birth.
After birth, hearing loss can be caused by illness, trauma, low birth weight and bacterial infections.
To learn about non-genetic risk factors for hearing loss in babies, click here.
Postlingual: Hearing loss sets in after the child has developed speech
Postlingual hearing loss occurs after the development of normal speech.
Causes may include syndromic and non-syndromic genetic causes, ear infections, meningitis, eardrum perforation, and trauma to the head.
Syndromic vs Non-Syndromic Hearing Loss
As mentioned before, both pre and postlingual hearing loss can be syndromic or non-syndromic. But what does that mean?
Syndromic Hearing Loss
As the name suggests, syndromic hearing loss is a hearing loss whose symptoms are visible, mainly because syndromic hearing loss is associated with malformations of the outer ear.
It often occurs with medical problems that also affect other parts of the body.
Of course, the overall contribution to all hearing loss is much smaller.
Syndromic hearing can be categorised by mode of inheritance.
Non-Syndromic Hearing Loss
Non-syndromic hearing loss, on the other hand, is a hearing impairment that isn’t associated with visible abnormalities of the outer ear. It causes partial or total loss of hearing in both ears.
Nonsyndromic hearing loss may, however, be associated with abnormalities of the middle ear or inner ear.
- Approximately 80% of prelingual hearing loss is non-syndromic.
Non-Syndromic Hearing Loss – Types of Inheritance
Different patterns of inheritance can classify non-syndromic hearing loss.
Causes of prelingual hearing loss, Source: NCBI
With certain exceptions, the onset and severity of each type follow a similar clinical pattern.
|75–80% of non-syndromic hearing loss|
|20% of non-syndromic hearing loss|
|2–5% of non-syndromic hearing loss|
|1% of non-syndromic hearing loss|
Genetic Heterogeneity of Non-Syndromic Hearing Loss
|Genetic heterogeneity occurs when several genes can cause a single clinical disorder. Non-syndromic hearing loss is the perfect example.|
Non-syndromic hereditary hearing loss is characterised by extreme genetic heterogeneity.
The to-date largest multigene panel study identified more than 40 causative genes among 440 individuals with a genetic non-syndromic hearing loss diagnosis.
A total of more than 6,000 causative variants have been identified across 110 genes.
This may explain why there’s such variability in non-syndromic hearing loss cases, even within families with dominant cases.
Researchers also suggest that environmental factors play a much more important role than previously assumed.
Wrapping up: What Causes Non-Syndromic Hearing Loss?
To choose the appropriate treatment path, it is necessary to understand genetic causes.
But whilst we can pinpoint it down to a few hundred out of 25,000 genes, only a comprehensive hearing assessment paired with genetic testing can give a definite answer to this question.
A newborn hearing screening (NBHS) is often the first step in the right direction. These tests are offered free of charge to all newborn babies in both public and private facilities in Australia.
For more information or to book an appointment with an accredited hearing healthcare provider, contact the friendly and knowledgeable team at Attune Hearing.