Usher Syndrome – Symptoms and Causes of Usher Syndrome
Usher syndrome or Usher’s syndrome is a relatively rare disorder. It leads to deafblindness. A mutation in any one out of 10 genes makes it a genetic disorder of rare occurrence. The other names are –
- Hallgren syndrome
- Usher-Hallgren syndrome
- Retinitis pigmentosa-deafness syndrome
- Retinitis pigmentosa-dysacusis syndrome
- Dystrophia retinae dysacusis syndrome
- Deafness-retinitis pigmentosa syndrome
- Dystrophia retinae pigmentosa-dysostosis syndrome
- Graefe-Usher syndrome
Hearing loss and gradual loss in vision are the two typical signs of the Usher syndrome. An eye disease by the name, retinitis pigmentosa (RP) causes the loss of vision. This disease affects the light-sensitive layer of the tissue in the eye. This layer is present at the retina or the back of the eye. When these cells deteriorate, the vision is lost gradually. The hearing loss is associated with a defect in the inner ear.
Initially the effect starts with the loss of night vision, followed by development of a blind spot in the side vision area or peripheral vision. These blinds spots enlarge over time making the vision resembling tunnel. In a few cases, the vision may be impaired further by cataracts. Cataracts are characterized by the clouding of the front surface of the eyes. However, people with retinitis pigmentosa have also been observed to retain some central vision throughout their lives.
Classification of Ushers Syndrome
There are three major types of the Usher’s Syndrome that have been identified by the researchers till now. These include –
- type I
- type II
- type III
The classification is made according to the decreasing order of severity and age of the signs and the symptoms. The type I is further divided into 7 other subtypes. They are –
- types IA
- types IB
- types IC
- types ID
- types IE
- types IF
- types IG
The type II Sher’s Syndrome has three distinct subtypes. They are –
- type IIA
- type IIB
- type IIC
Hallgren syndrome type I
People, who are born profoundly deaf and gradually begin to lose their eyesight during the first decade of their life, are classified under the Hallgren syndrome type I. These people also show difficulty in balancing and appear slower in learning compared to other children. Such signs occur due to defective vestibular system. The vestibular system is responsible for providing the balance and senses for spatial orientation.
Hallgren syndrome type II
people placed under this category are also born deaf, but they do not show significant problems with balance. But they tend to lose their vision over time later in their second decade of life. They might also preserve some vision till their middle-age.
Hallgren syndrome type III
People who are not born deaf, but lose their hearing ability and eyesight gradually over time, are placed under this category. These people may or may not show balance difficulties. Most patients under this category get profoundly deaf when they are in their middle-age.
Symptoms of Usher Syndrome
The majority cases of deafblindness is because of the Hallgren syndrome. It is an inherited, autosomal recessive disorder.
- Inherited denotes it is carried in families and generations. A child is more likely to get it from his parents.
- Autosomal denotes that the sex of an individual has nothing to do with the occurrence of the disorder. Both male and female can be carriers and the victims. It is not carried by any of the sex chromosomes but by the other 22 chromosomes.
- Recessive refers to the the extent of expression of a particular allele. The appearance of the recessive allele is always dominated by the dominant allele making the chances of occurrence rare. In common terms, recessive indicates that both parents have to contribute for an appropriate gene for the syndrome to occur.
Symptoms in Usher syndrome type I
- Deaf by birth
- Difficulties in maintaining balance (problems in the vestibular system)
- Development of motor skills (such as walking) is often slow
Symptoms of Usher syndrome type II
- Severity of deafness varies (from mild to sever)
- Sufferers are mostly hard-of hearing compared to complete deafness
- Hearing does not get lost over time
- Vestibular system is mostly normal (no significant problem with balance)
Symptoms of Usher syndrome type III
- Gradual loss of hearing
- May or may not have defects with their vestibular system
- Gradual loss of eyesight over time
Causes of Usher’s Syndrome
About 3-6% of all cases of childhood deafness is the result of Usher-Hallgren syndrome. Usher-Hallgren syndrome accounts for about 50% of all cases of deafblindness in adults.
Causes of Usher syndrome type I
Mutations in any one of the several genes – CDH23, MYO7A, PCDH15, USH1C, and USH1G is the cause of the type I Usher syndrome.
These genes are responsible in development and maintenance of the structures in the inner ear. These structures aim at transmitting the sound and send signals to the brain. When any alterations are caused in these genes, the ability of balance is lost along with loss in hearing ability.
The stability of retina is dependent on the functioning of the photoreceptor cells in the eye. When the genes mentioned above are altered, they can influencing the structure and function of all the photoreceptor cells and the supporting cells causing retinitis pigmentosa and vision loss.
It is estimated that about 3-6 people out of 100,00 of the general population are affected by Usher-Hallgren syndrome type I. This disorder is prevalent in the people of central and eastern Europe and French-Acadian populations.
Causes of Usher syndrome type II
A mutation in any of the three different genes (like – GPR98, DFNB31 and USH2A) can cause Dystrophia retinae dysacusis syndrome. Usherin is the protein encoded by the gene USH2A.
This is located in the supportive tissues of the retina and the inner eye. Usherin is necessary for the development and maintenance of the supportive tissues and structures in the retina and the inner ear. So mutation or alteration in the gene can affect the functioning of the protein Usherin causing loss of hearing and vision.
But the role of other two proteins are yet to be known. The frequency of Dystrophia retinae dysacusis syndrome type II is same as the type I.
Causes of usher Syndrome type III
Mutation in the gene CLRN1 causes Usher syndrome type III. This gene is responsible in encodes a protein known as Clarin-1.
This protein is responsible for the development and maintenance of the retina and the inner ear.
But a detail understanding is not yet found out. The frequency of occurrence is highest in the Finnish population.