Genetic Hearing Loss: The Causes of Non-Syndromic Hearing Loss
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Hereditary Hearing Impairments: What Causes Non-Syndromic Hearing Loss?

Hereditary Hearing Impairments

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. 

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

Known causes include the Waardenburg syndrome (WS), Neurofibromatosis 2 (NF2), Stickler syndrome and Usher syndrome type I, type II and III.

  • 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.

Non-Syndromic Hearing Loss – Types of Inheritance 

Different patterns of inheritance can classify non-syndromic hearing loss.

Causes of prelingual hearing loss

Causes of prelingual hearing loss, Source: NCBI

With certain exceptions, the onset and severity of each type follow a similar clinical pattern.

Autosomal Recessive

75–80% of non-syndromic hearing loss
  • Generally prelingual with severe to profound hearing loss 
  • 70% of cases are non-progressive; they do not worsen over time 
  • Hearing loss is always bilateral – it affects both ears 
  • The degree of hearing loss may vary among siblings

Autosomal Dominant

20% of non-syndromic hearing loss
  • Primarily characterised as post-lingual, with an onset often between the second through to the fifth decades of life
  • Often progressive in nature
  • The rate of progression and the degree of hearing loss vary among family members​

X-Linked (Sex-Linked)

2–5% of non-syndromic hearing loss
  • Several genes are responsible for this type of hearing loss
  • The type and degree of hearing loss varies depending on the affected gene 
  • Due to the sex-linked pattern of inheritance, males may present with a severe hearing loss whilst their female siblings present with normal to mild hearing loss

Mitochondrial

1% of non-syndromic hearing loss 
  • Always transmitted through maternal inheritance
  • Mother can be a carrier but not suffer from hearing loss herself
  • Usually of progressive nature
  • May lead to sudden hearing loss when exposed to certain antibiotics
  • The mitochondrial mutation A1555G has been identified as one of the most common genetic causes  of deafness

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

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