Causes of Hearing Impairment: Key Factors and Their Percentages - Otolaryngology

Dr. Ye Dawei reply Otolaryngology

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Dear Sir/Madam,
I received a sudden phone call from a senior colleague in the medical field whom I have always admired.
During the call, he mentioned that a friend of his complained about experiencing a subjective decline in hearing on one side while listening to music.
I casually inquired whether it was in one ear or both, whether it was high frequency or low frequency, and whether it occurred suddenly or gradually.
Perhaps it’s a professional habit, but whenever I hear someone complain about unilateral hearing loss, as a frontline otolaryngologist, I always hope to detect at least one case of an "acoustic neuroma" in my practice—just one case would suffice.
Hearing loss can be categorized based on the location of the pathology into "conductive hearing loss" and "sensorineural hearing loss." Sensorineural hearing loss can further be divided into "cochlear pathology" and "retrocochlear pathology," as explained below:
1) Conductive Hearing Loss: The pathology can occur in the ear, ear canal, eardrum, or middle ear cavity, including the ossicular chain, which prevents sound from being effectively transmitted.
However, since the auditory nerve remains intact, there is no distortion of sound; it is merely a reduction in volume.
For these patients, simply increasing the volume can compensate for their hearing loss, and communication is generally not problematic.
2) Sensorineural Hearing Loss (Cochlear Pathology): The pathology is located in the auditory cells (hair cells) of the cochlea.
Its characteristic is that it produces "distortion" and "reverberation" of sound.
"Distortion" means that when the volume is increased while speaking to the patient, they may complain of hearing distorted sounds; "reverberation" means that the patient may feel overwhelmed by the volume, even though others perceive it as normal.
A common saying applies here: "You can't hear at low volume, but loud sounds can be startling." This type of hearing loss often leads to communication difficulties.
3) Sensorineural Hearing Loss (Retrocochlear Pathology): The pathology is located in the auditory nerve.
Patients with this condition may experience "distortion" and "decline" in sound perception, meaning that the same sound may seem to diminish over time.
Clinically, otolaryngologists often use these characteristics to make preliminary judgments about the potential location of the patient's pathology, and audiometric tests utilize these principles to assist in diagnosis.
In audiology, there is a concept known as the "speech frequency range," which refers to high frequencies, as consonants are predominantly found in this range while vowels are in the mid to low frequency range.
When we speak, consonants are often quieter.
Therefore, if a patient has a hearing loss in the high frequency range, they may frequently ask others to repeat themselves or complain that they can hear sounds but cannot discern what is being said.
Unfortunately, the two most common types of hearing loss I encounter in the clinic—"presbycusis" and "noise-induced hearing loss"—both exhibit significant damage in the high frequency range.
For these patients, it is crucial not to simply provide standard hearing aids.
If high frequencies are amplified, it will also proportionately amplify unwanted low-frequency background noise.
If the design of the hearing aid is inadequate and does not sufficiently amplify high-frequency sounds, patients may complain that they cannot hear what they want to hear while being overwhelmed by unwanted noise.
The correct approach is to select hearing aids that amplify high frequencies more and low frequencies less.
Currently, more advanced compression or digital hearing aids can significantly improve upon the shortcomings of older models.
How can outpatient physicians easily screen and diagnose the type of hearing loss a patient has? When faced with a patient complaining of hearing loss, I first examine their eardrum.
If it appears normal, I then perform a tuning fork test to differentiate between "conductive" and "sensorineural" hearing loss.
If sensorineural hearing loss is suspected, it is essential to further distinguish whether it is cochlear or retrocochlear pathology.
This distinction is crucial because cochlear pathology typically requires conservative medical treatment, whereas retrocochlear pathology, such as an "acoustic neuroma," may necessitate surgical intervention.
Being able to diagnose an acoustic neuroma in the outpatient setting without the aid of technological instruments is a source of immense pride for an otolaryngologist.
If a patient clearly exhibits unilateral sensorineural hearing loss, I would recommend further audiometric evaluation at a hospital.
Generally, the "pure tone audiometry" arranged in a hospital's audiology department can quantify the degree of hearing loss at specific frequencies.
If there is a 30 dB difference between the hearing in the healthy ear and the affected ear, it certainly piques my interest.
Tympanometry can assess issues in the middle ear and eardrum (classified into five types: Type A is normal; Type As indicates otosclerosis, ossicular fixation, or thickened eardrum; Type Ad indicates ossicular discontinuity or a flaccid eardrum; Type B indicates middle ear effusion, eardrum perforation, cerumen impaction, or chronic otitis media; Type C indicates negative pressure due to Eustachian tube dysfunction).
An experienced physician can visually inspect the eardrum and correlate it with the instrument report.
The "stapedius reflex test" can simply diagnose cochlear pathology through the reverberation phenomenon and can also help determine the site of facial nerve paralysis.
The "stapedius reflex decay test" operates on the principle that in normal ears or cochlear pathology, the contraction of the stapedius muscle can last for 10 seconds; if the reflex contraction lasts less than half that time (i.e., 5 seconds), retrocochlear pathology must be strongly considered.
All of the aforementioned tests provide immediate reports.
If any abnormal patterns suggestive of retrocochlear pathology are identified, it often causes a stir among the residents, as it indicates the need for further testing.
At this point, I would arrange for an "auditory brainstem response (ABR)" test, which measures the electrical potential changes in response to sound stimuli above the auditory threshold of 60 dB, as abnormal waveforms in ABR can indicate the presence of an acoustic neuroma.
If, after navigating through these evaluations, retrocochlear pathology is still suspected, it is then time to arrange for a CT scan or MRI.
Three years into my practice, it is regrettable—or perhaps fortunate—that I have not encountered any patients requiring a CT scan based on this protocol, and thus, I have not detected any cases of acoustic neuroma.

Sincerely,
Dr.
Da-Wei Yeh
Department of Otolaryngology, Hsinchu Hospital

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Reply Date: 2009/10/20