Neurotology Overview

We are born with a relatively small number of auditory hair cells (about 15,000 per cochlea) and these have to last a lifetime. The very low number of auditory hair cells present in the cochlea is one of the major reasons hearing loss is so common.

Once the cells in a particular area of the cochlea are lost, hearing at that frequency is also lost. There is little in the way of compensatory hearing offered by neighboring hair cells as they are tuned to respond to a particular frequency. It is the irreversible injury or loss of auditory hair cells, supporting cells, and neurons that cause the majority of hearing loss.

50
People in the US with some form of hearing loss
360
People worldwide with some form of hearing loss

When is the inner ear at risk?

The following indicates maximal Sound Pressure Levels (in deciBels) and durations of sound before the inner ear is at risk of permanent hearing loss:

140
No Exposure
130
< 2 minutes
120
> 7.5 minutes
110
> 30 minutes
100
> 2 hours
90
> 8 hours

A global problem

Approximately 50 million Americans and 360 million worldwide have some form of hearing loss, a number that is up 44% globally in the past 15 years. Hearing loss constitutes the most prevalent neurologic health issue globally, more than those living with Parkinson’s, epilepsy, Alzheimer’s and stroke, combined.

Noise Induced Hearing Loss

Exposure to loud noises can result in temporary and permanent hearing loss. Temporary hearing loss is now understood to be additive, similar to concussions, and accelerate rates of permanent hearing loss and age-related hearing loss. Permanent hearing loss is usually due to a rapid inner ear cell death due to reactive oxygen species. The NIH reports that about 15 percent of Americans aged 20 to 69 have high frequency hearing loss related to occupational or leisure activities. Further, the DoD identifies noise-induced hearing loss among the top disabilities associated with current conflicts.

Chemotherapy Induced Ototoxicity

Chemotherapy with cisplatin and other platinum based therapeutics saves many lives, but can have the unwanted side effect of ototoxicity (toxicity to the inner ear). Ototoxicity often causes permanent sensorineural hearing loss, tinnitus, and disequilibrium or dizziness. High frequency hearing is generally affected first and extends into speech frequencies as chemotherapy treatment continues leading to permanent hearing loss that may require amplification with hearing aids. While treating the cancer is of primary importance, it is also important to preserve hearing, prevent tinnitus, and balance problems throughout treatment. In children, this can be particularly disabling, compromising language and cognitive development, learning ability and quality of life.

Tinnitus

Tinnitus is the abnormal perception of sound in the absence of sound. This can take the form of ringing, buzzing, hissing, crickets, screeching, sirens, whistling, whooshing, roaring, pulsing, ocean waves, clicking, dial tones, and even music or fragments of songs. Tinnitus can occur in one or both ears and often accompanies noise induced hearing loss, age related hearing loss and ototoxicity. However, many people report tinnitus without noticeable hearing loss. The US Center for Disease Control estimates that over 50 million Americans experience some form of tinnitus, 16 million of which seek medical attention and approximately 2 million describe their symptoms as extreme and life altering. Tinnitus is also a common disability in veterans, especially in those who have had exposures to loud noise during their time in military service.

Meniere’s Disease

Meniere’s disease describes a triad of episodic symptoms including vertigo, tinnitus and hearing loss. A fullness sensation of the affected ear has also been described. Episodes usually last from 20 minutes up to many hours. The underlying cause of Meniere’s Disease is unknown and typically affects individuals between 20 and 50 years old. In the US, approximately 600,000 individuals have been diagnosed with Meniere’s Disease. There is no cure for Meniere’s Disease and current treatments rely on symptom management. Affected individuals are usually on low-salt diets and diuretics. Surgery is an option for the most severe cases.

Antibiotic Induced
Ototoxicity

Aminoglycoside antibiotics can cause ototoxicity in Cystic Fibrosis patients undergoing frequent treatments as part of maintenance and therapeutic strategies. Aminoglycosides have well-documented side effects consisting of both nephrotoxicity and ototoxicity. While these antibiotics are of critical importance in keeping Cystic Fibrosis patients from succumbing to otherwise potential life threatening infections, frequent and intense use can lead to permanent hearing loss, tinnitus, and vertigo or dizziness. Often, the ototoxicity side effect will limit be the limiting factor in the dose of aminoglycoside a physician can prescribe.

Age-Related Hearing Loss (Presbycusis)

Age-Related Hearing Loss is progressive and most often occurs in both ears, although not necessarily at the same time or rate. As presbycusis advances, sounds seem less clear and lower in volume which makes hearing and understanding speech more difficult especially in noisy environments. It is a common problem that affects a third of people between 65-75 years of age and up to half of people over 75 years old. Presbycusis has a negative impact on the quality of life, and is significantly associated with decreased function in both mental and physical evaluations. While presbycusis can often be managed with a hearing aid, it is estimated that less than 1 in 5 persons who could benefit from hearing aids actually use them regularly.