The majority of deaf children are born to parents with normal hearing even though most deafness is genetic. Yet most deaf children are not brought to geneticists who could help parents to understand the nature of their child's illness and its causes. At least one doctor is working to change that, however.
Dr William Reardon, consultant clinical geneticist at Our Lady's Hospital for Sick Children at Crumlin, has been working for the past 12 years to identify the genetic causes and forms of various types of deafness. Inherited deafness occurs in one in 2,000 births.
"If parents knew what type of deafness their child had, they would know what other symptoms to expect, and that would lead to much better medical care," Dr Reardon said. "And if they had this information early on, they would have a better idea about their chances of having another deaf child."
The type of deafness Dr Reardon has been working on since 1996 is caused by Pendred syndrome - a syndrome first reported in 1896 that commands only a few lines in mainstream medical books. In addition to causing deafness, the syndrome creates a malformation in the inner ear - known as a dilated vestibular aqueduct.
The syndrome causes a severe swelling of the thyroid gland by the time the person reaches their teenage years. Most doctors, and therefore parents, do not however realise that the thyroid problems and the deafness are related. "When parents discover their child is deaf, they spend years going to ENTs (ear, nose and throat departments) looking for a solution," Dr Reardon explained. "But after a while, they realise there is no solution and they kind of fall away from the whole medical thing. Then, when the child develops a huge goiter, they go to a different doctor to have that worked on. They never connect the two symptoms and neither do their doctors."
For years it was thought that Pendred syndrome was incredibly rare, but this had more to do with faulty diagnosis rather than incidence. Radiologists would look at CAT scans of the inner ear of deaf patients and see the dilated vestibular aqueduct, but would never connect that with the other symptoms because they weren't working directly with the patient.
Now Dr Reardon has proved that more than 20 per cent of all patients with inherited deafness present with the conditions of the syndrome. "We are so over-specialised now that every symptom is looked upon as a separate illness," Dr Reardon said. "It's not the best for patient care because the bigger picture is often missed."
With gene mapping, Dr Reardon has found the source of Pendred syndrome. On chromosome seven there is a mutation of a gene that regulates the transport of iodine across the cell membrane, thus causing an abnormality in the thyroid gland and excess of fluid in the ear. The excess fluid causes the bone in the ear to bulge, in the process creating the dilated vestibular aqueduct.
Dr Reardon is working to create genetic tests that can be used to identify the type and cause of a child's deafness. He is creating a test for "Connexion 26", the most common form of deafness, which is caused by a mutation on chromosome 13. Like those suffering from Pendred syndrome those with Connexion 26 are born to normal parents. However, Connexion patients have normal thyroids and usually do not suffer any other problems.
Dr Reardon is also working on a test for "Coch", another common form of deafness associated with an abnormality on chromosome 14. Coch patients are usually born into families that have had deafness for several generations. The patients suffer from balance disorders and from tinnitus - a constant ringing in the ears.
Determining which test to give a deaf child is the tricky part. Normally, a geneticist would begin with a test of the most common form of deafness, and if that comes out negative, would move on to the next most common form. Few deaf children here are screened in this way however.
"When I was working in England I was seeing 10 children a week," Dr Reardon explained. "I've been here since August [1999] and I have only seen four. We have to make doctors and consultants more comfortable with the idea of genetics, and help them to get a clear idea about how it can contribute to the health and treatment of the deaf."