What are MYO7A-related Disorders?
MYO7A-related disorders are an inherited group of conditions associated with hearing loss with or without vision loss. This group of disorders does not affect intelligence or cause any other primary health problems. MYO7A-related disorders are caused by harmful changes, or mutations, in the MYO7A gene. The three main disorders that make up the MYO7A-related disorders include Usher Syndrome Type IB (USH1B), DFNB2, and DFNA11. These conditions are very similar, and the primary difference between them is the severity of the condition.
Usher Syndrome Type IB
USH1B is the most common form of MYO7A-related disorders. The symptoms of USH1B include hearing loss, balance problems, and progressive vision loss. Infants with USH1B are profoundly deaf in both ears at birth. Affected individuals usually have severe balance problems, caused by abnormalities of the inner ear (vestibular system) that can lead to delays in development. In general, children with USH1B sit and walk at later ages and have difficulties sensing changes in speed or direction. In childhood or by early adolescence, individuals with USH1B develop retinitis pigmentosa, an eye disease that causes night blindness and a gradual loss of vision on the sides of the head (peripheral vision). Eventually only the central vision remains, creating "tunnel vision." This central vision can also become impaired and can lead to total blindness in a small number of individuals with the disease. In some cases, individuals with USH1B develop clouding in the lens of the eye (cataracts), which can further impair vision.
DFNB2 is characterized by hearing loss but no vision loss. Individuals with DFNB2 develop profound hearing loss anywhere from birth to adolescence and may also develop balance problems. Progressive vision loss is not expected in this condition, but some individuals re-evaluated later in life have developed symptoms of retinitis pigmentosa, indicating variability or overlap in the conditions associated with this gene.
DFNA11 is an autosomal-dominant form of nonsyndromic hearing loss and deafness. An autosomal-dominant condition is one where symptoms can be present when an individual inherits only one mutation in the associated gene. Individuals with DFNA11 seem to develop moderate to severe progressive hearing loss after learning how to talk (i.e., in late childhood or adolescence). Individuals with DFNA11 may have repetitive, uncontrolled eye movements (nystagmus) or mild balance issues. However, affected individuals typically do not have vision loss. This condition seems to be the mildest form of MYO7A-related disorders. DFNA11 is very rare, and carriers of mutations in the MYO7A gene do not generally experience any symptoms.
How common are MYO7A-related Disorders?
The global incidence is unknown for all MYO7A-related disorders. The incidence of USH1B is approximately 1 in 90,000 individuals. There are regions where founder effects (high frequency of the disease because the group arose from a small, possibly isolated population) occur, such as in indigenous populations in South Africa, South Asia, and the Middle East.
DFNB2 and DFNA11 are extremely rare disorders. DFNB2 has been been reported in at least three families and DFNA11 in at least five families of various ethnicities. Other presentations or variability in these two disorders may not be recognized as of yet.
How are MYO7A-related Disorders treated?
There is no cure for MYO7A-related disorders, but early treatment is important to give children with the condition the best opportunity to develop communication skills. While a child is young, his or her brain is most receptive to learning language, either spoken or signed. It is also important to take advantage of the time when the child's vision is normal. Individuals with USH1B generally do not respond to hearing aids, but cochlear implants may help regain some form of hearing. Sign language is a good option for communication. Specialists can introduce other tools and methods of instruction available to individuals with hearing loss. It is often helpful if the whole family undergoes such instruction and, as a family unit, helps the child adapt.
For those individuals that develop vision loss, visual aids and specialized instruction (for example, in tactile signing) help children adapt to their limited vision. Affected individuals can be prone to accidental injury due to their vision loss and balance problems. Well-supervised participation in sports may help an individual with Usher syndrome type 1 compensate for balance issues, but swimming may be particularly difficult, and strategies to ensure safety are needed. Use of UV-A and UV-B blocking sunglasses is recommended, and other optical aids may increase eye comfort. Therapy with vitamin A palmitate may slow retinal degeneration for some.
What is the prognosis for an individual with an MYO7A-related Disorder?
USH1B results in severe hearing and vision impairment, and DFNB2/DFNA11 results in hearing impairment only. However, none of the conditions affect one's lifespan or intelligence.