Костная проводимость: возможности и особенности BAHA-имплантации.

A bone anchored hearing aid (BAHA) is a type of hearing aid based on bone conduction. It is primarily suited for people who have conductive hearing losses, unilateral hearing loss, single sided deafness and people with mixed hearing losses who cannot otherwise wear 'in the ear' or 'behind the ear' hearing aids. They are more expensive than conventional hearing aids, and their placement involves invasive surgery which carries a risk of complications, The sound quality is also impaired as much of the sound energy is lost in the soft tissue over the skull bone, particularly for the higher sound frequencies important for speech understanding in noise.

Bone anchored hearing aids use a surgically implanted abutment to transmit sound by direct conduction through bone to the inner ear, bypassing the external auditory canal and middle ear. A titanium prosthesis is surgically embedded into the skull with a small abutment exposed outside the skin. A sound processor sits on this abutment and transmits sound vibrations to the titanium implant. The implant vibrates the skull and inner ear, which stimulate the nerve fibers of the inner ear, allowing hearing. The surgery is often performed under local anesthesia and as an outpatient procedure. An important piece of information for patients is that if they for whatever reason are not satisfied with the BAHA solution, removing the implant is easy. No other ear surgical procedure is reversible like this. By bypassing the outer or middle ear, BAHA can increase hearing in noisy situations and help localise sounds. In addition to improved speech understanding, it results in a natural sound with less distortion and feedback compared with conventional hearing aids. The ear canal is left open for comfort, and helps to reduce any problems caused by chronic ear infections or allergies. In patients with single sided sensorineural deafness, BAHA sends the sound by the skull bone from the deaf side to the inner ear of the hearing side. This transfer of sound gives a 360° sound awareness. BAHAs may facilitate normal speech development.

This fairly common condition is often associated with continuous or intermittent drainage from the ear canal. These patients may also have a hearing loss and need amplification. A conventional air conduction aid with a mold placed in the ear canal opening may not be appropriate due to the drainage, and may even provoke drainage. If the hearing loss is significant an air conduction aid may have difficulty overcoming the dysfunction of the eardrum and middle ear bones. Bone conduction hearing device bypassing the middle ear may be a more appropriate treatment for these patients. Good transmission of sound in the bone, with reduced attenuation and distortion may be possible.

A person with unilateral hearing loss may have functional difficulty hearing even when the other ear is normal, particularly in demanding situations such as noisy environments and when several people are speaking the same time. A complication in single sided deafness is hearing impairment in the hearing ear. Conventional ear surgery involves a risk of hearing loss due to the surgical procedure. Ear surgeons may be reluctant to perform surgery on an only hearing ear. The BAHA surgery avoids this risk and may be an appropriate treatment. An extended trial of a BAHA system with a headband prior to surgery led to more realistic expectations. In the trial, 50% of the candidates wished to proceed to surgery.

Irritation in the external ear canal due to inflammation or eczema may be a condition for which a conventional air conduction aid is not an appropriate treatment. Direct bone conduction may be an option.

Patients with malformations are not always suitable for reconstructive surgery. Treacher Collins syndrome patients may have significant malformations with ossicular defects and an abnormal route of the facial nerve. These structures, as well as the inner ear, could be in danger at surgery. Patients with Down syndrome may have a narrow ear canal and middle ear malformation leading to impaired hearing. Some part of the cognitive delay seen in these children may be partly due to their poor hearing. The surgery can only take place once the skull is at least 2.5 mm thick. Children with certain syndromes may have a slighter build, thinner bone, or unusual anatomy. Other children may have a thicker skull at a younger age, so it is difficult to give a specific age for surgery. In the U. S., the Food and Drug Administration only approves BAHA implantation of children aged five years or older. For infants and young children prior to surgery, the sound processor can be worn on a head band or soft band which the infant wears to hold it against the skull.

The bone behind the ear is exposed through a U shaped or straight incision or with the help of a specially designed BAHA dermatome. A hole, 3 or 4 mm deep depending on the thickness of the bone, is drilled. The hole is widened and the implant with the mounted coupling is inserted under generous cooling to minimize surgical trauma to the bone. In a single stage procedure, the fixture and abutment are placed simultaneously. For children or patients who may damage the abutment prior to osseointegration of the fixture, a two stage procedure may be performed in which the fixture is placed first, and after osseointegration roughly 6 months later, the abutment is attached. In children, a sleeper implant may be placed to serve as a backup in case the main implant is damaged. Some surgeons perform a reduction of the subcutaneous soft tissue. The rationale for this is to reduce the mobility between implant and skin to avoid inflammation at the penetration site. This reduction of the soft tissue has been questioned and some surgeons do not perform any or a minimum of it. The rationale for this is that any surgery will result in some scar tissue that could be the focus of infection. The infections seen early during the development of the surgical procedure could perhaps be explained by the lack of seal between implant and abutment allowing bacteria to enter the space. A new helium tight seal may be advantageous and prevent biofilm formation. This will also allow the surgeon to use longer abutments should a need exist. Three to six weeks later or even earlier, the audiologist will fit and adjust the hearing processor according to the patient's hearing level. The fitting will be made using a special program in a computer. The original surgical procedure has been described in detail by Tjellström et al. 2001.

An area where skin is penetrated requires care and cleaning because of the risk of inflammation around the abutment. Daily cleaning is required.

Hearing is of importance for a normal speech development. The skull bone in children is often very thin and softer than in the adult. Surgery is thus often delayed until the age of four to five years. In the meantime, the child with bilateral atresia can be fitted with a band around the head with a coupling for a BAHA. This may be done at the age of one month. Infants at this age may tolerate this well. There are also other types of bone conduction hearing aids, such as implants and non surgical devices.

In the US, the cost of the Baha device is about $4,000. In the Netherlands, the cost of the device is around €3000 (in 2008). The cost of the titanium implant, surgery, and aftercare from surgeon and audiologist must also be considered.