By Jana Soeldner Danger
City & Shore Magazine
Hearing aids and cochlear implants can improve the quality of life for many hearing-impaired individuals, but they usually can’t help those with nonfunctioning auditory nerves. A new electronic device called an auditory brainstem implant may offer hope, says Dr. Christine Dinh, a neurotologist at the University of Miami Miller School of Medicine.
The ABI was developed for people with neurofibromatosis type 2, a genetic condition that causes tumors to grow on the auditory, balance and other nerves in the brain and spinal cord. The auditory nerve tumors block signals from hearing aids and cochlear implants before they can reach the brain, and affect balance, Dr. Dinh says. Unless the tumors are caught early, surgical removal or treatment with radiation or chemotherapy often permanently destroys auditory nerve function.
With normal hearing, sound enters the outer ear and strikes the eardrum. The eardrum vibrates, transmitting the sound waves to the middle ear, where tiny bones also vibrate and transfer the sound to the cochlea, a structure in the inner ear. A cochlear implant bypasses a nonfunctioning cochlea and transmits signals to the auditory nerve, and if the auditory nerve is functioning, the patient should be able to hear sounds. But the tumors associated with neurofibromatosis 2 are often between the cochlea and the brainstem, and sound signals can’t pass through them, Dr. Dinh says. The auditory implant, on the other hand, is placed directly on the brain, so signals bypass the inner ear and auditory nerve to stimulate hearing.
How it works
A hook-shaped microphone that picks up sound is placed on top of the ear. It sends signals to an external sound processor, which in turn sends signals to a receiver or decoder placed under the skin. The receiver then sends electrical signals to a paddle containing electrodes surgically placed directly onto the brainstem. These signals stimulate cells within the brainstem, allowing patients to hear sound.
The implant doesn’t restore normal hearing, and some people do not experience any benefit. But most patients can distinguish sounds like a ringing telephone or a car horn. Some people can recognize words and even sentences, while others hear only more general sounds.
A 30 percent increase is considered a success. Patients who can lip read may improve their understanding significantly when lip reading is combined with the implant, Dr. Dinh says.
Clinical trials are underway to see if the ABI can also benefit adults and children with auditory nerves that do not function for other reasons, such as an abnormally shaped inner ear, the absence or malformation of inner ear structures or injury to the auditory nerves from skull fractures on both sides of the head.
After the surgery, the patient recovers for four to eight weeks before the implant is turned on. The patient must then meet regularly for several months with an audiologist to adjust the device’s programming. Side effects may include dizziness and facial twitching or weakness, but serious side effects are rare. Results are best when the procedure is done as soon as possible after deafness occurs, because when a patient has been deaf for a prolonged period, cells start to die, Dr. Dinh says.
The University of Miami was the first institution in Florida to perform the procedure, Dr. Dinh says. Since March, UM has done three of the surgeries, which require the skills of a neurosurgeon, a neurotologist, an audiologist and an electrophysiologist working together. The first patient has done extremely well: Using a combination of lip reading and the implant, she has achieved nearly 100 percent of environmental sound perception, word recognition and sentence understanding, Dr. Dinh says. “It’s an amazing success.”