DFN3 deafness, also known as X-Linked mixed deafness type 2, is the most common form of X-linked congenital deafness (present from birth). DFN3 deafness is mixed because the changes causing hearing loss are found in both the middle (conductive hearing loss) and inner ear (sensorineural hearing loss). The middle ear contains three tiny bones that help transfer sound from the eardrum to the inner ear. In DFN3, one of these tiny bones, the stapes, cannot move properly (stapes fixation). The inner ear consists of three parts: a snail-shaped structure called the cochlea that helps process sound, nerves that send information from the cochlea to the brain, and structures involved with balance. In DFN3, there are several specific abnormalities in the inner ear.
DFN3 results from a mutation or error in the POU3F4 and is inherited (runs in families) as an X-linked trait. Normally, we have two copies of every gene, one on a chromosome inherited from each of our birth parents. The sex chromosomes are different. A male has one X and one Y chromosome, whereas a female has two X chromosomes. So a male only has one copy of a POU3F4 gene and if this one copy has the error, he will have DFN3 deafness. A female has two copies of the POU3F4. If only one of her copies is changed, she will be a carrier and may have mild hearing loss. If a woman is a carrier of DFN3, her sons will have a 50% chance of having DFN3 deafness. DFN3 can be diagnosed through various hearing tests and genetic testing. There are a number of options available for individuals with mixed deafness or sensorineural hearing loss. Parents should seek information about the range of options open to their child before deciding which options may work best for their child and family. Specialists may include an otologist, an audiologist, a speech-language pathologist, a clinical geneticist, and a genetic counselor. Support groups are also good resources of support and information.