What is a CROS or biCROS Hearing Aid?

If you hear poorly in one ear, and cannot benefit from amplification in that ear, your audiologist may recommend a CROS or biCROS hearing aid.

So what are CROS and BiCROS hearing aids? CROS stands for Contralateral Routing of Signals. In this style of hearing aid, a transmitter on the poorer ear collects sound from that side and sends it to a receiver on the better ear. The receiver takes the sound and delivers it into the canal of the better ear. If someone has hearing loss as well in the better ear, a biCROS aid will collect and amplify sound on the better side as well.

Before you hand over your credit card, make sure that you understand the benefits, limitations, and alternatives to CROS technology. Use the information that follows in this post to ask your audiologist questions about their reasons for their recommendation.

Candidacy

During your hearing test, your audiologist determined the softest levels at which you hear tones at different pitches in each ear. Your audiologist also measured your speech recognition ability. This test provides an indication of how well each ear will perform with amplification. If one ear has a poor speech recognition score while the other ear understands speech well, the audiologist may suggest CROS or biCROS technology.

For CROS to work well, the better ear should have near-normal hearing. The technology will not amplify sound directed towards the side of your good ear. If you have some impairment in your good ear, biCROS may be recommended. In this configuration, microphones placed on both the receiver and transmitter collect sound which then is combined by the receiver for projection into the better ear.

Patients with cognitive impairment or dexterity problems may have difficulty managing CROS technology. A patient will need to understand how to adjust the hearing aids for different listening situations.

Causes and Effects of Unilateral Hearing Loss

Single-Sided deafness occurs in about three percent of school-aged children and in about seven percent of adults. Causes of asymmetrical hearing loss may include:

Congenital defects
Genetic or hereditary causes

Syndromes
Tumors
Severance of auditory nerve during surgery

Traumatic brain injury
Infections
Noise exposure directed to one side, such as a blast

Head injury

Individuals with single-sided deafness experience difficulty understanding speech in noise and localizing sounds. A person’s ability to follow speech in a noisy environment is measured by the noise level at which the person can still understand fifty percent of the words. The noise level is expressed in terms of decibels relative to the speech level. This is known as the signal-to-noise ratio (SNR).

Research has found that people with unilateral hearing loss need 13 dB greater SNR than people with normal hearing to achieve comparable performance.

Unilateral hearing loss in children requires management from audiologists to mitigate its effects. About a quarter of children with single-sided deafness fail a grade. School personnel may need to be educated about the effects of the impairment; uninformed teachers and administrators may assume that a child will not have difficulty if he or she hears normally in one ear.

Advantages and Disadvantages of CROS Hearing Aids

About half of the patients who try CROS hearing aids keep the technology after the trial period. This style of hearing aid may help with some situations, but it cannot fully compensate for all of the difficulties caused by single-sided deafness.

The major advantage of CROS is the reduction of the head shadow effect. If you cannot hear in one ear, sounds coming towards that side of your body will be reduced by the time they reach your better ear. This effect mostly impacts higher-pitched sounds above 2000 Hz. The pitches at and above 2000 Hz contribute a great deal to the clarity of speech.

With CROS, you will more easily understand a friend standing next to you on your poorer side. In addition, you will pick up more environmental signals, which could improve your safety.

However, CROS hearing aids cannot fully restore the ability to localize sounds and hear in background noise. The auditory system does complex processing of information as neurons relay signals from your ear to the brain.

Neural circuits responsible for sound localization and background noise suppression depend on comparing signals from both of your ears. In a CROS or biCROS fitting, you only receive sound in one ear.

Styles and Features of CROS Hearing Aids

Often, the receiver and transmitter in a CROS setup will look similar to conventional amplification. The difference from conventional amplification is that, in CROS, only the transmitter side has a microphone and only the receiver side has a speaker projecting sound into the ear canal.

In BiCROS, both sides have a microphone and the sound from both devices combine into the signal that the receiver sends into the good ear.

Depending on the hearing aid brand, you may choose a behind-the-ear (BTE) model or an in-the-ear (ITE) model. ITE models are custom shells manufactured to fit inside of your ear canal and the outer bowl of your ear (called the concha) perfectly. BTE models have a few different coupling options.

On the transmitter side, audiologists will generally use a thin tube with a dome on the end, or an open-style earmold, to keep the device secured on your ear. The receiver side can be fit with a thin tube, a receiver, or a standard earmold. Your audiologist will recommend the best choice for you depending on the hearing acuity in your better ear and your ear’s anatomy. Be sure to let your audiologist know about any vision, dexterity, or skin allergy problems that may impact the recommendation.

Special Considerations in Fitting CROS Aids

Sometimes patients complain that they hear hearing aid circuit noise in their near-normal better ear, or that amplification sounds artificial. Placing the receiver in your better ear may cause a feeling of fullness or change the sound of your own voice (called the occlusion effect), although your audiologist will strive to minimize this.

Your audiologist will work with you to achieve a good balance of sound from your poorer and better sides. In the ideal mix, the sound from the transmitter side will not interfere with your good ear’s ability to understand speech. Depending on the hearing aid brand, you may be able to have different programs that vary the mix.

Research has demonstrated that CROS systems help most when the speech that you want to hear is on your poorer side and the background noise comes from another direction. If the noise is on your poorer side and the speech is coming from another side, then the CROS transmitter will pick up and send the noise to your better ear and interfere with your ability to understand speech.

Therefore, make sure that your audiologist provides you with the ability to turn the transmitter on and off depending on the situation. If you have problems with dexterity, ask about a remote control or a smartphone app.

Alternatives to CROS Hearing Aids

There are three main alternatives to CROS hearing aids. Your audiologist can talk with you about all of your options.

Traditional hearing aid fitting: You may choose to wear standard amplification on the poorer ear, which may help with environmental sound awareness. During your trial period, you can determine whether you find this helpful.

Transcranial CROS hearing aid: This option involves placing a hearing aid in your poorer ear which produces enough output that the sound crosses over to your better ear via bone conduction. That is, your skull transmits the sound from your bad ear to your good ear. Audiologists rarely recommend this option.

Baha system: The Cochlear Baha system is a device implanted into your skull. A processor with a microphone is attached to the outside of your head. When the processor picks up sounds, it vibrates your skull and transmits sound to your ears via bone conduction.