November 13, 2023

Non-syndromic hearing loss : What is it? Genetic probability, Examples, Common symptoms, Causes

This text provides comprehensive information about non-syndromic hearing loss, covering what it is, genetic probability, examples of syndromic hearing loss, common symptoms, and underlying causes.

The process of hearing is intricate and lengthy. Initially, sound enters the external ear, and the middle ear must effectively gather and amplify the sound. Once this process occurs correctly, the structures in the inner ear convert the sound signals into electrical signals and transmit them to the brain.

As the process of hearing involves numerous tissues, any issue with even one of them can lead to congenital hearing loss.

Syndromic vs Non Syndromic Hearing Loss : What is the Non Syndromic Hearing Loss?

• Syndromic hearing loss is hearing loss that accompanies other health issues, and Usher Syndrome, Waardenburg Syndrome, Pendred Syndrome, Down Syndrome are most common syndromic hearing loss.

• Non-syndromic hearing loss is hearing loss that does not accompany other health issues.

The ratio of genetic hearing loss to environmental hearing loss

 Some individuals may refer to environmental hearing loss as 'non-organic hearing loss,' but it is more appropriate to describe it as hearing loss caused by environmental factors.

 For example, if a fetus is exposed to toxic substances during pregnancy, it is classified as environmental hearing loss. When considering such examples, categorizing hearing loss as genetic or environmental hearing loss becomes more intuitive for distinguishing the types of hearing loss.

Hearing loss is divided into 50% genetic hearing loss and the remaining 50% as environmental hearing loss.

In genetic hearing loss, there are syndromic hearing loss that are accompanied by other health issues, and non-syndromic hearing loss where hearing loss is the primary concern without additional associated problems.

The ratio of syndromic hearing loss to non syndromic hearing loss.

Syndromic hearing loss constitutes approximately 30% of genetic hearing loss, In other words, it accounts for 15% of total hearing loss. 

While the remaining 70% is non-syndromic hearing loss, In other words, it constitutes 35% of total hearing loss.

Syndromic Hearing loss Examples vs. Non Syndromic Hearing Loss Examples

Examples of syndromic hearing loss include:

• Usher Syndrome: This is a genetic disorder characterized by hearing loss and retinitis pigmentosa, a degenerative eye condition leading to vision loss.

 

• Waardenburg Syndrome: A genetic condition that can cause hearing loss and changes in pigmentation, such as different-colored eyes or a white forelock of hair.

 

• Pendred Syndrome: This syndrome is associated with both hearing loss and an enlarged thyroid gland.

 

• Down Syndrome: Individuals with Down syndrome often experience various health issues, including hearing loss.

Examples of nonsyndromic hearing loss include:

• DFNA1 (Autosomal Dominant Nonsyndromic Hearing Loss 1): The first identified type of autosomal dominant nonsyndromic hearing loss.

 

• DFNB4 (Autosomal Recessive Nonsyndromic Hearing Loss 4): An example of autosomal recessive nonsyndromic hearing loss.

 

• DFNX1 (X-Linked Nonsyndromic Hearing Loss 1): An example of X-linked nonsyndromic hearing loss.

 

• Mitochondrial Nonsyndromic Hearing Loss: Hearing loss caused by mutations in mitochondrial DNA without associated syndromic features.

Common Symptoms for Syndromic Hearing loss

Syndromic hearing loss is characterized by not only hearing impairment but also additional health symptoms.

Below are common non-hearing related symptoms associated with representative syndromic hearing loss:

• Usher Syndrome:

Vision Loss: Gradual loss of vision due to retinitis pigmentosa.
Balance Issues: Problems with balance due to issues in the vestibular system.

• Waardenburg Syndrome:

Pigmentation Changes: Varied pigmentation changes in the eyes, skin, and hair, including different-colored irises.
Facial Features: Distinctive facial features such as a white forelock of hair.

• Pendred Syndrome:

Thyroid Enlargement: Abnormal enlargement of the thyroid gland.

• Down Syndrome:

Intellectual Disability: Typically associated with intellectual disability.
Muscle Tone Impairment: Reduced muscle tone and weakened strength.
Facial Features: Distinct facial features, including slanted eyes, small ears, and a flat facial profile.
Heart Defects: Heart defects may be present.

Causes for nonsyndromic hearing loss : Genetic Probability

Non-syndromic hearing loss is a subcategory of genetic hearing loss, and its occurrence is determined by the genetic mechanisms involved.

The composition of non-syndromic hearing loss is as follows:

DFNB4 (Autosomal Recessive Nonsyndromic Hearing Loss 4) accounts for approximately 77%,

DFNA1 (Autosomal Dominant Nonsyndromic Hearing Loss 1) for about 22%,

DFNX1 (X-Linked Nonsyndromic Hearing Loss 1) around 1%,

and Mitochondrial Nonsyndromic Hearing Loss about 1%.

• DFNA1 (Autosomal Dominant Nonsyndromic Hearing Loss 1):

• DFNA1 is a type of non-syndromic hearing loss where having the hearing loss gene on only one of the two autosomes guarantees the expression of hearing loss. Autosomes come in pairs from each parent, and if one parent possesses the hearing loss gene on one of their autosomes and passes that autosomal chromosome to the child, the child will inevitably have non-syndromic hearing loss. In other words, there is a 50% chance of passing non-syndromic hearing loss to the child.

• DFNB4 (Autosomal Recessive Nonsyndromic Hearing Loss 4):

• Autosomal recessive hearing loss requires both autosomes to carry the non-syndromic hearing loss gene for its expression. In this case, the child has a 25% chance of inheriting the gene because both parents need to contribute an autosomal chromosome containing the hearing loss gene. DFNB4 can manifest even if neither parent shows hearing loss because both parents are likely to have just one autosomal of hearing loss gene.

• DFNX1 (X-Linked Nonsyndromic Hearing Loss 1):

• X-linked hearing loss can be expressed in a dominant or recessive manner. If the mother carries the gene and passes it to her son, it results in X-linked hearing loss.
• If the mother passes the X chromosome with the gene to her daughter, the daughter may not experience hearing loss due to the presence of a normal X chromosome from the father. However, the daughter can become a carrier of the hearing loss gene and potentially pass it on to her offspring.

• Mitochondrial Nonsyndromic Hearing Loss:

• Mitochondrial hearing loss is exclusively maternally inherited. It occurs when a mother, possessing mitochondria with the hearing loss gene, passes these mitochondria to all her children. Since mitochondria are inherited through the maternal lineage regardless of gender, if the mother carries the mitochondrial hearing loss gene, there is  100% chance that her children will experience non-syndromic hearing loss.