Given the limitation of hearing aids, the success of cochlear implants, and increasingly sophisticated electronics, it is not surprising that a number of companies have embarked on implantable (or semi-implantable) hearing devices.  This update will focus on the BAHA, which has proven very effective for specific clinical scenarios.  First, though, I will briefly review the broader field of implantable middle ear devices. 

Implantable middle ear devices are designed to treat conductive hearing losses and moderate to moderate-severe SNHL.  They use either electromagnetic or piezoelectric technology to transduce acoustic signals to movements of the stapes footplate (or in some cases the round window membrane itself).  Purported advantages include absence of acoustic feedback and the occlusion effect, improved cosmesis, and greater comfort. 

1)    Med El Vibrant Sound Bridge

This was the first FDA approved semi-implantable middle ear hearing device for SNHL.  A floating mass transducer (induction coil and magnet) is attached to the long process of the incus.  The microphone and processor/transmitter are placed behind the ear and convey, via electromagnetic induction, the signal to the implanted receiver/simulator.  This arrangement is similar to that of a cochlear implant.

2)    Esteem Envoy

This device is fully implantable and uses piezoelectric (PE) technology.  One PE crystal, the sensor, is attached to the incus.  The tympanic membrane serves as the natural microphone and vibrations of the incus are sent via a lead to the receiver placed subcutaneously in the postauricular area.  Once processed, the signal is sent via a second lead to another PE crystal, the driver, attached to the stapes.  This device received FDA approval in 2010. 

3)     Otologics Carina

This is a totally implantable electromagnetic system currently in PHASE II FDA clinical trials.  A transducer is attached to the incus via a small hole drilled in its head or body.  The processor is fixed to the bone above the mastoid cavity, and the microphone is placed subcutaneously. 

In general, these middle ear hearing devices are designed for individuals who do not tolerate traditional hearing aids (e.g., secondary to a draining ear, cosmetic concerns, etc.).  Insurance companies usually have exclusions for them, and out-of-pocket expenses can approximate $30,000 - $50,000.

The Bone Anchored Hearing Aid (BAHA) is used to rehabilitate a monaural or binaural conductive hearing loss or a single sided severe - profound SNHL. 

Conductive hearing losses that may be difficult to rehabilitate with conventional hearing aids and are amenable to a BAHA include: aural atresia, acquired EAC stenosis (e.g., fibrodysplasia, trauma, etc.), a draining middle ear, or frequent otitis externa.  In these clinical conditions, the device directly stimulates the ipsilateral cochlea, which should have normal hearing or no more than a mild - moderate SNHL loss. 

In single sided deafness (assuming a normal or near normal contralateral cochlea) the signal is picked up on the deaf side and transmitted via bone conduction (the skull) to the opposite cochlea.  In this setting, the BAHA is extremely helpful eliminating the head shadow effect (i.e., soft sounds presented to the deaf ear are now readily heard), but it does not help with determining direction of sound.  Listening in background noise may be improved somewhat. Studies have shown patient satisfaction is higher with a BAHA than with a CROS hearing aid system. 

The BAHA surgical procedure involves placing a 3mm or 4mm Titanium post in the temporal bone under general anesthesia.  A 3mm post is typically used for children, and a 4mm one for adults, assuming appropriate skull thickness before encountering dura.  After a 3-6 month period of osteointegration, the external sound processor can be attached to the post.  Daily cleaning around the post with a soft "toothbrush" is necessary.  An occasional reaction (erythema, granulation tissue) around the post can occur, but usually responds to topical antibiotics.  Most insurance carriers, including Medicare, cover this device.

Paul R. Lambert, MD

Professor and Chair