Unilateral hearing loss

Unilateral hearing loss (UHL) or single-sided deafness (SSD) is a type of hearing impairment where there is normal hearing in one ear and impaired hearing in the other ear.

Symptoms
Patients with unilateral hearing loss have difficulty in
 * hearing conversation on their impaired side
 * localizing sound
 * understanding speech in the presence of background noise.

In quiet conditions, speech discrimination is approximately the same for normal hearing and those with unilateral deafness ; however, in noisy environments speech discrimination varies individually and ranges from mild to severe.

Causes
Known causes include physical trauma, acoustic neuroma, meningitis, or mumps (Epidemic parotitis).

Prevalence
A 1998 study of school children by Lee, Gomez-Marin, and Lee found that per thousand school children, some 6-12 had some form of unilateral hearing loss and 0-5 had moderate to profound unilateral hearing loss. They estimated that in 1998 some 391,000 school-aged children in the United States had unilateral hearing loss.

Profound unilateral hearing loss
Profound unilateral hearing loss is a specific type of hearing impairment when one ear has no functional hearing ability (91dB or greater hearing loss). People with profound unilateral hearing loss can only hear in monaural (mono). It is known to cause:
 * Irritability
 * Frequent headaches, stress
 * Social isolation
 * Trouble figuring out where sounds are coming from.
 * Variable light dizziness
 * Trouble paying attention to what people are saying: evasive behaviour.
 * Lack of sound depth: any background noise (in the room, in the car) is flat and wrongly interpreted by the brain. The effect is similar to what happens when people with unimpaired hearing try to watch, on a  mono TV, someone speaking in a noisy crowd.  The effect is also similar to talking on the phone to someone who is in a noisy environment.
 * Inability to filter out background noise or selectively listen to only the important portion of the noise in the environment.
 * For sensorineural hearing loss, the lack of input coming from the damaged sensory apparatus can cause "ghost beeps" or ringing/tinnitus as the brain attempts to interpret the now missing sensory data. The frequency and the volume of the noise can increase according to one's physical condition (stress, fatigue, etc.). This can aggravate social problems and increase the difficulty of speech comprehension.

Treatment Options
Learning of the central nervous system by "plasticity" or biological maturation over time does not improve the performance of monaural listening.


 * CROS hearing aid: a hearing aid that takes sound from the ear with poorer hearing and transmits to the ear with better hearing. This kind of hearing aid can involve two behind-the-ear units connected either by wire or by wireless transmission.  There are also systems incorporated into eyeglasses.
 * Bone Anchored Hearing Aid: transfers sound through bone conduction and stimulates the cochlea of the normal hearing ear. One study of the system showed a benefit depending on the patient's transcranial attenuation .  Another study showed that sound localisation was not improved, but the effect of the head shadow was reduced.

A recent study compared the CROS hearing aid with a bone anchored CROS system and found that the latter yields greater benefit on the deaf ear.

Trivia
When wearing stereo headphones, people with unilateral hearing loss can hear only one channel, hence only half of the components of the music, e.g., bass or piano, but not both. The need for headsets for cellphones and VOIP communication has made monaural headphones, which often combine stereo to mono sound, readily available to solve the problem.