Doctors at the University of Navarra have implanted an auditory brainstem implant (ABI) in the youngest patient ever attempted -- a 13 month-old girl:

A team of ear, nose and throat specialists and neurosurgeons at the University Hospital of Navarra, led by doctors Manuel Manrique Rodriguez, specialist in ear, nose and throat surgery and Bartolome Bejarano Herruzo, specialist in paediatric neurosurgery, have successfully operated on a 13 month-old girl from Murcia, who had been born deaf due to the lack of auditory nerves. She is the youngest patient in the world who has received an auditory implant in the brain stem. As a result of the operation, the child has begun to hear and started language development.

Previously, the medical centre had carried out, also successfully, a similar procedure on girl of eight years. Throughout the world there have only been 38 brain stem implants in children under the age of 12.

An ABI is an electrode that connects a microphone outside the head and stimulates the cochlear nucleus in the brainstem. The cochlear nucleus is the first stopping point of information coming from the ears into the brain carried by the cochlear nerve. The procedure is intended to bridge the gap in patients who for whatever reason do not have a functioning cochlear nerve.

The cochlear nerve runs from the cochlea -- the organ responsible for transducing noise into neural responses -- to the base of the brain called the brainstem. In doing so it runs through a bone called the petrous bone. When it enters the inside of the skull it joins the brainstem on the back right below the cerebellum. Here is a picture of the brainstem -- from Gray's. The picture labels the nerve as the "acoustic nerve":

There are a couple reasons that you might not have functioning cochlear nerves. (Remember that in this case both of them have to be non-functional because if you had one functioning you could still hear out of the unaffected ear.)

The two most common are the following:

1) Neurofibromatosis 2 resulting in bilateral acoustic neuromas -- NF2 is an dominant inherited genetic disease that results in multiple neurofibromas in the brain and spine. Actually this is kind of a misnomer because neurofibromas -- neural sheath tumors that incorporate Schwann cells and other cells -- are not found in NF2. They are associated with NF1 (von Recklinghausen disease). What you seen in NF2 are a lot of pure Schwann cell tumors called schwannomas as well as meningiomas and ependymomas. (Schwann cells form the myelin sheath of peripheral nerves and facilitate neuron conduction.)

These Schwannomas are relevant to hearing because they often form bilaterally on the cochlear nerve. (Here they are called acoustic neuromas. Sorry for the nomenclature, but there is kind of no way around it for this subject.) Removal of these tumors from the cochlear nerves often requires that they be severed, leading to hearing loss in these patients.)

(As an aside, lifespan with NF2 can be variable. Usually the tumors that develop are benign, so many patients do live a normal life. Trouble can happen when the tumors are located in unfortunate places and begin to press on the surrounding brain tissue.)

2) Congenital Cochlear Nerve Deficiency -- Congenital cochlear nerve deficiency is a (fortunately rare) disease in which the cochlear nerves fail to develop. Under normal circumstances when there is a developmental disorder in the hearing system, the patient is a candidate for a cochlear implant -- an implant that stimulates the cochlear nerve. However, these patients don't have one, so you have to go directly to the cochlear nucleus in the brain. I actually couldn't find information related to the frequency of this disorder, so if someone could recommend a review I would appreciate it.

ABIs (reviewed here) are intended to bridge the gap between the outer auditory world and the cochlear nucleus. To put in an ABI you take a plate of electrodes -- I think they have 21 at the moment -- and insert it into the cochlear nucleus at the base of the brain. A wire attached to these electrodes is then fed out of the brain to communicate with a microphone worn on the ear. The microphone can stimulate various electrodes to simulate stimulation by the ear. Pretty cool, right?

Well, there are some important caveats. First, we are dealing with a highly invasive surgery. The brainstem is a place where very important stuff happens in your brain -- like controlling breathing and consciousness -- so messing with it is a tricky business. From what I understand from the review in many cases the ABI is put in during the same surgery that they remove the tumors in NF2.

Second the device has to be tuned so that you know you are stimulating the right electrodes. During this process which happens repeatedly over several months, you activate different electrodes and ask the patient what kind of sounds they are hearing. They use this to calibrate the machine to provide usable information to the patient. They also do this because there are a lot of other things in this area of the brain that they might accidentally be stimulating. Occasionally one of the electrodes might not be in the cochlear nucleus and turning it on might trigger something else. According to the review this something is usually a sensation of tingling in the body. However, it could also be something like loss of consciousness or changes in homeostatic responses, so when you are rejiggering this implant you need to have emergency care standing by just in case. In many cases, some of the electrodes need to be inactivated because they produce anomalous or annoying results.

The improvements of as a consequence of this procedure are modest, and often less than those associated with cochlear implants. You have to have a lot of practice in order to interpret the sounds coming from this implant. This can be exceedingly frustrating at first. The authors of the review frame the use of the implant as a way to assist patients in lip reading, not a way to supplant lip reading.

On the other hand, the doctors in the case of the 13 month-old girl are bullish because the patient -- by being so young -- could adapt her hearing system to better interpret the information from the implant as she develops. The justification for putting the implant in at an early age is that hearing development requires...well...hearing. The doctors report that the patient has begun to respond to and produce sounds:

During the post-operational monitoring of this patient, it was observed that the child has begun to receive sounds and has even started to produce them. This is highly encouraging. The specialists have emphasised the importance of carrying out these operations at an early age when the capacity for learning is greater and the functional structure of the auditory centres is better prepared for receiving acoustic information.

Despite those good results, I am a little concerned about putting this device in a patient so young. How do the doctors intend to calibrate this implant without her input? If they can get a functional implant in early, the patient is much more likely to develop hearing and speech. I am just a little concerned that they won't be able to test whether all the electrodes are in the right place. It is certainly possible that they could evaluate responses to the electrode by looking at which direction the baby looks; they certainly test hearing in infants that way. But I don't know how you would know whether electrodes need to be inactivated in this patient.

I am 100% certain that this patient is going to go through a lot of follow-up. It will be interesting to see whether getting the implant early enough can allow her to fully compensate for her hearing deficit.