Many people think that having hearing loss is like listening to a radio set to a low volume — the sound is simply not as loud. Although it is true that certain kinds of hearing loss can make sounds noticeably softer and more difficult to hear, there are in fact different types of hearing loss that can have vastly different effects on how sounds are heard and understood. The different types of hearing loss tend to have different causes, and it appears that having diabetes can contribute to the development of certain types of hearing loss.
The mechanics of hearing
Hearing is a process in which the ear is only the beginning of the story. The chain of events starts when sound enters the ear canal and causes the eardrum to vibrate. The vibrations set in motion the three tiny bones that form a chain in the middle ear space that connects the eardrum to the cochlea — a hollow structure that is coiled in the shape of a snail’s shell, containing three tubes filled with fluid. The last bone in the middle ear chain is connected to a membrane covering a small opening called the oval window at one end of the cochlea, and the vibrations of this membrane cause waves in the fluids inside the cochlea. This, in turn, causes movement of microscopic structures called hair cells, which are present in one of the tubes in the cochlea. The movement of these tiny hair cells creates an electrical signal that is sent to the hearing nerve, which connects the cochlea to the brain stem. The electrical signal travels up the brain stem and through a system of nerve pathways before arriving at specialized auditory centers of the brain where the message is finally processed. Amazingly, this entire chain of events takes only tiny fractions of a second. (See “A Look Inside the Ear” for more information about the mechanics of the ear.)
Types of hearing loss
Damage can occur anywhere along the hearing pathway. The location of the damage determines the type of hearing loss that occurs.
Conductive hearing loss (outer and middle ear). Trauma to the structures of the ear that physically transmit sound, such as the eardrum and the bones in the middle ear, can result in conductive hearing loss, which reduces the ear’s ability to physically conduct sound vibrations. The eardrum can be damaged by chronic infection, trauma resulting from pressure changes in the ear (such as those that occur in deep-sea diving), or blunt force to the ear or head. The tiny bones in the middle ear also can be damaged by blunt force. A condition called otosclerosis, which involves abnormal growth of bone in the middle ear, can reduce the strength of the sound vibrations that are transmitted into the cochlea, thereby reducing the volume at which sounds are heard.
Conductive hearing loss causes a reduction in the overall volume of sounds, but if speech can be made loud enough — by means of a hearing aid or the speaker talking louder, for instance — it can most often be understood. In many cases, areas of the ear involved in conductive hearing loss may be treated with medicines or repaired with surgery.
Sensorineural hearing loss and central processing disorders (inner ear and central hearing pathway). Damage to the inner ear or to structures along the nerve pathway is called sensorineural hearing loss because it involves either the delicate sensory hair cells in the cochlea or the hearing nerve, and sometimes both. When the nerve pathway from the ear to the brain is damaged, this is usually referred to as a central processing disorder. Unlike people with conductive hearing loss, those with sensorineural hearing loss or processing disorders may have difficulty understanding speech even when it is amplified. In fact, too high a volume can result in distortion of the speech, causing an unpleasant sound and making it even more difficult to understand.