In order to better understand the hearing complications associated with atresia and microtia, a quick explanation of how the ear works would be a helpful start. Essentially the outer ear and canal serve to capture and amplify the vibrational energy of sound in the environment and then send it to the ossicular bones of the middle ear to convert those sound waves into mechanical motion. The movement of those bones drive hairs in the cochlea of the inner ear, converting that mechanical motion into electrical signals. Those signals are what the brain processes into what we call hearing.
Traditionally, children with unilateral hearing loss did not receive any hearing aid, since it was considered unnecessary. More recent investigations have shown though that while children with unilateral hearing loss function well in quiet environments, they have a difficult time in high background noise environments. This is because with two fully functional ears the brain is capable of many tasks that become exceedingly difficult with only one ear.
With two ears, one will hear a sound split seconds before the other, allowing the brain to locate where the noise is coming from. Practically speaking, if a child with unilateral hearing was walking through a mall and someone shouted their name they would have no way of telling where the person was shouting from. In part due to this reason, the injury rate for children with unilateral hearing is twice that of children with two fully functional ears.
The brain is essentially a specialized computer that processes electrical signals sent from the ears into something useful. Two ears make more data inputs accessible to the brain than one, allowing it to pick out the most useful information in high noise environments. Even adults with perfect hearing can have trouble following a conversation in a noisy restaurant or party. If one ear is plugged up that task becomes tremendously more difficult, eating up a lot more brainpower to follow along. That cognitive load increases dramatically if a task is being completed at the same time. For example, if a child with unilateral hearing is attempting to take notes on what a teacher is saying in a noisy classroom they are going to have a harder time listening and writing than a child with two functional ears.
Because of the way neurons in the brain are wired, if the same sound is heard in both ears the receiving neuron in the brain will respond twice as intensely as if it were stimulated by only one ear. With both ears the effective response is as if a much louder sound were heard than if the same decibel level were heard with one ear alone. Two ears allow much more intelligible hearing of quiet sounds.
The majority of development of the hearing system in the brain occurs within the first 10 years of a child’s life. Any effort to aid or correct a unilateral hearing loss should take place earlier rather than later to make the most of the plasticity of the young brain to adapt to systemic changes. The precise hearing aid or surgical technique used to repair the hearing loss needs to be determined by an ear specialist with CT scans and other hearing tests.