The ear is made up of three main parts: the outer, middle, and inner ear. After sound passes through the inner ear the information is sent to the nervous system where the sound is interpreted and your brain can actually give it meaning. We will just be focusing on the three main parts, their functions, and where and how hearing loss can occur in each area.
The outer ear is made up of the pinna, or the part of the ear we can actually see and what we think of when we think of an “ear”, and the external auditory canal. Sound is funneled through the pinna into the ear canal and on to the tympanic membrane, or eardrum. The main purposes of the outer ear is to protect the delicate structures in the middle and inner ear, localize and funnel sound, and give a natural boost for high-frequency sounds.
Although there are multiple disorders and conditions of the outer ear that may affect the ability to hear, all of these issues are considered conductive losses and usually cause muffled hearing but rarely deafness. One of the most common outer ear issues is built up or impacted cerumen, or earwax. This can usually be removed by your hearing healthcare professional or a water wash and hearing will return to normal levels. Deformities such as a small pinna, absence of a pinna, or an underdeveloped or undeveloped ear canal are also abnormalities that can cause permanent conductive hearing loss.
After sound passes through the outer ear and tympanic membrane, it reaches the middle ear. The middle ear is a small cavity connecting the outer and inner ear and contains three bones which together are called the ossicles. The first bone which is attached to the tympanic membrane is the malleus, which then connects to the incus, and finally the stapes. These are the smallest bones in the human body and the stapes is the smallest of them all! The function of the ossicles is to transfer sound energy from the outer to the inner ear. Another important part of the middle ear is the eustachian tube (ET). The ET connects to the back of the throat and is responsible for ventilation and drainage to keep the middle ear at an ambient and comfortable pressure.
Damage or dysfunctions of the middle ear can also cause conductive hearing loss. The most common issues are usually resolvable and caused by sinus issues, allergies, or the common cold as the ET becomes blocked or swollen. Ear infections, such as otitis media, are also common causes of temporary mild conductive loss. Permanent loss in the middle ear can be caused by high-pressure trauma, otosclerosis – an abnormal bony growth on the stapes, cholesteatoma – abnormal skin cell growth, repeated ear infections, and normal aging.
Finally we arrive at the inner ear. The inner ear is made up of the cochlea which contains many smaller structures that contribute to creating the electromechanical event that causes neurons to fire. As sound travels from the outer ear through the ear canal it causes the tympanic membrane to vibrate and activate the ossicles. The inner ear is fluid filled so as the ossicles push against the oval window pressure waves are created in the fluid. These waves activate and bend tiny hair cells, when these are stimulated neurons begin to fire to the brain where the auditory nervous system takes over to interpret sound and give it meaning.
Most hearing loss occurs in the inner ear, we call this Sensori-Neural Hearing Loss (SNHL). This type of hearing loss can have a range of severity but is typically permanent and progressive. Aging and noise-induced hearing loss are the most common causes of SNHL. Other issues that can cause hearing loss in the inner ear are ototoxic drugs that poison and damage the cochlea, meniere’s disease which causes an increase in volume and pressure of fluid in the inner ear, and acoustic neuroma which is a tumor on the acoustic nerve. Unfortunately none of the causes of hearing loss in the inner ear are reversible or resolvable. The only preventable loss is noise-induced, when exposed to loud noises it is important to wear hearing protection to save your hair cells from that permanent damage!