What is single-sided deafness?
Single-sided deafness (SSD) refers to the condition where a patient has normal hearing in one ear and non-aidable hearing loss or deafness in the other ear. The severity of the hearing loss is of such a nature that the patient cannot benefit from a conventional hearing aid in that ear.
What is the prevalence of single-sided deafness?
It is estimated that in the US, 5 out of 1000 children suffer from SSD and approximately 60 000 patients are diagnosed with this condition every year. The exact numbers are not known for the South African population.
How does it affect a patient?
Single-sided deafness causes numerous physiological, social, and psychological problems. Patients find it difficult to understand speech in background noise especially when the background noise is on the side of the normal ear and the speech on the SSD side.
Patients with single-sided deafness have trouble in localizing sound. They also have impaired spatial orientation. Patients may experience dizziness and disequilibrium.
Psychologically patients tend to withdraw from conversations in noisy environments and many have the exaggerated fears of losing their hearing in the only hearing ear.
Although SSD may go unnoticed in smaller children the effect on their development, spatial orientation, and 3-dimensional concept of space Is impaired. Their learning ability is negatively affected with up to 35% of children needing to repeat a year at school!
The causes of single-sided deafness?
Patients may be born with single-sided deafness (congenital) or develop it later in life (acquired). The most common cause of congenital single-sided deafness is an absent or abnormal cochlear nerve. The second most common cause is congenital cytomegalovirus (CMV) infection. Genetic abnormalities may be a reason, but in many cases, the cause remains unknown. In childhood, virus infections especially mumps are often the culprit.
Adult acquired single-sided deafness is most caused by idiopathic sudden-onset sensorineural hearing loss (SNHL). In idiopathic disease, the triggering factor is not known. Other common causes include virus infections including HIV, ear diseases such as cholesteatoma, circulatory problems, Meniere’s disease, labyrinthitis, meningitis, and vestibular schwannoma.
Trauma may lead to single-sided deafness and is often seen after head injuries. Even mild injuries may cause hearing loss.
Investigating single-sided deafness.
When neonatal hearing screening fails in a baby diagnostic testing may be required to confirm the hearing loss. This should be done as a matter of urgency aiming for confirmation not later than 6 months. In babies the hearing ability is investigated utilizing subjective electrophysiological testing such as brainstem evoked response audiometry (BERA), also referred to as auditory brainstem response (ABR) testing. Oto acoustic emissions are also utilized to measure the cochlear hair cell function.
Depending on the situation blood tests, metabolic tests, and imaging, including magnetic resonance imaging (MRI) and computed tomography (CT) scanning may be needed. Cone beam CT scanning is nowadays more often used and has the advantage of far less potential radiation than conventional screening.
Sometimes parents may be advised to consider genetic testing, specifically in cases where additional children are planned for the family
How is single-sided deafness treated?
Depending on the age of the patient and other factors, such as malformations or absence of the nerve, there may be a few viable options that can be offered.
The hearing loss per se can be managed with many different solutions, including frequency modulation (FM) systems, soft band bone conduction systems, contralateral routing of signal (CROS) hearing aid systems, and bone-conduction transcranial CROS devices.
Surgical devices include bone-anchored hearing systems (Ponto, BAHA, or BONEBRIDGE and even cochlear implantation (CI). The transcutaneous bone conduction systems have a titanium abutment that pierces the skin and although the auditory gain is satisfactory, these devices are falling out of favor due to skin problems and cosmesis, especially in children.
BONEBRIDGE is a active bone conduction implant with an external microphone and processor that connects with a magnet over intact skin. The latest and smaller BONEBRIDGE BCI 602 on the market has direct streaming abilities and is ideal for patients who cannot benefit from cochlear implantation.
The best option to achieve true binaural hearing, if applicable and with an intact nerve is a cochlear implant.
Cochlear implantation for patients with single-sided deafness.
Cochlear implantation is still the most effective way of restoring SSD as it is the only modality that utilizes the nerve on the SSD side. For it to work an intact cochlear nerve is a necessity. Other factors such as duration of deafness and age are also important as long-standing deafness and early age onset deafness may negatively affect the cortical brain structures of hearing. This may lead to sub-optimal outcomes after cochlear implantation. However, more and more publications have recently demonstrated the benefit and favorable outcome of cochlear implantation in SSD of early age onset as well as cases in adults of longstanding SSD of more than 35 years duration. Another benefit of cochlear implantation is that it can improve sound localization and even suppress tinnitus in some cases.