Deafness is a general term for various degrees of hearing loss. Deafness can be caused by lesions in or near the outer and middle ear sound-transmitting structures, the inner ear sound-sensing organs, and any part of the auditory nerve pathway. The prevalence of deafness is high, with the World Health Organization (WHO) estimating that the number of people with hearing disabilities worldwide was 42 million in 1985, 120 million in 1995, and increased to 250 million in 2001. In China, it is estimated that about 27 million people with hearing impairment are disabled, ranking first in the total number of disabled people. Proper and early diagnosis as well as treatment of hearing impairment is very important to improve the quality of life of patients.
Deafness can be divided into organic deafness and functional deafness, and organic deafness can be divided into conductive deafness, sensorineural deafness, and mixed deafness. Conductive hearing loss refers to hearing loss caused by the inability to transmit sound waves to the inner ear due to lesions in the outer ear, middle ear or eustachian tube, and otosclerosis of the bony capsule of the vagus. Sensorineural deafness is a general term for hearing loss caused by lesions of the inner ear, cochlear nerve, brainstem auditory pathway and auditory center; among them, sensorineural deafness caused by lesions of the cochlear auditory receptors is called sensorineural deafness, also known as cochlear deafness.
deafness); hearing loss due to lesions in the auditory nerve and central auditory pathways is called retrocochleardeafness. Functional deafness refers to the absence of organic lesions of the auditory system, and the patient complains of not being able to hear the sound while the objectively observed hearing is normal. The following is a brief description of how to make a general determination of the site of deafness using audiometric methods.
The methods of hearing function testing are divided into subjective and objective testing methods. Subjective testing methods include: table test, tuning fork test, pure tone hearing threshold with suprathreshold function test, speech audiometry, etc., which are recorded based on subjective judgments made by the subject to stimulus signals, also known as behavioral audiometry. In some cases (mental retardation, pseudo-deafness, etc.), the results do not fully reflect the actual level of hearing function of the subject. Objective testing methods include: acoustic conductance testing, electrical response audiometry, and otoacoustic emission testing, etc. The results are relatively objective and reliable, but their frequency characteristics are poor. Domestic judicial, labor and disability appraisals mostly take subjective observation of hearing.
Tuning fork test is a widely used and simple hearing examination method in otology. It is relatively easy and fast to diagnose the nature of deafness, and it is currently one of the oldest methods in the hearing examination methods. The tuning fork is placed next to the examined ear, in the mastoid region or in the forehead, and the air conduction and bone conduction hearing are measured respectively. The time between the two ears, between air conduction and bone conduction, between the normal ear and the diseased ear when the sound of the tuning fork can be heard is compared to estimate the degree of hearing loss in the diseased ear and to identify the nature of deafness initially.
The pure tone hearing threshold test, often called electroaudiometry, is a subjective test in which a pure tone audiometer emits pure tones of different frequencies and intensities, and the subjective judgment is made by the subject to understand the pure tone hearing gap in both ears. It is a subjective method to understand the hearing sensitivity of pure tones in both ears by testing the air-conduction hearing and the bone-conduction hearing through air-conduction headphones and bone-conduction headphones respectively. However, because pure-tone hearing test is a subjective test method, it requires a high degree of subjective cooperation from the subject, and the hearing condition should be judged by the subject’s response, so it has the disadvantage of poor objectivity, especially for children, its accuracy is poor, and it cannot be used for testing infants and children. General result analysis: 1) Normal: air-bone conduction hearing threshold curve are within 25db, there is no significant difference between them. 2) Conductive deafness: bone conduction is normal or close to normal, air-bone conduction hearing threshold is increased, air-bone conduction spacing is greater than 10db, generally not greater than 40db, maximum not more than 60db, conductive deafness air-bone conduction hearing threshold is increased mainly at low frequencies with an ascending curve, air-bone conduction difference is obvious at low frequency area. 3) In sensorineural deafness, the hearing curve of air-bone conduction is decreasing in a consistent manner. There are also cases in which the air-bone curve decreases at all frequencies, but a certain air-bone conduction spacing exists. (In the case of fixed or sclerotic auditory chain, the resonance frequency of the auditory chain is 2000HZ and the bone conduction threshold is increased by about 15, which is not a mixed deafness but still a conductive deafness curve.)
The purpose of audiometry is not only to clarify the nature of deafness, but more importantly, to clarify the nature of the lesion as much as possible, so that it can provide greater help for treatment. Conductive deafness is generally considered to be caused by lesions in the middle and outer ear, but it is gradually being recognized that sensorineural deafness caused by otitis media and mixed deafness account for a significant proportion of patients with otitis media. The reason for this is that as the course of middle ear inflammation increases, the thickness of the round window membrane gradually increases, and oxygen in the inner ear is diffused through the round window membrane, thus causing hypoxic damage to the inner ear; there is also inflammatory material diffused into the inner ear through the round window, and as the course of the disease increases, it first involves the basal gyrus causing temporary or permanent threshold shift, and then gradually involves As the disease progresses, it first involves the basal gyrus, causing a temporary or permanent threshold shift, and then gradually involves speech frequencies, so there is a bone conduction hearing loss from high to low frequency.
Chronic secretory otitis media can also cause bone conduction hearing loss. The mechanism may be 1) middle ear effusion affecting the phase difference between the two windows and affecting bone conduction hearing. 2) endotoxin entering the inner ear affecting inner ear function. 3) bone conduction afferents are currently thought to have three pathways, one is sound radiating through the mastoid to the external auditory canal and then through the middle ear to the inner ear, two is cranial vibration directly vibrating the auditory chain to the inner ear, and three is cranial oscillation directly sensitizing the inner ear. In middle ear lesions, the first two types of bone conduction afferents are affected and therefore affect bone conduction hearing.4) Inner ear immune mechanisms are involved and the immune process in secretory otitis media may affect inner ear function. Similar to otosclerosis, bone conduction hearing loss is more pronounced at 2 kHz, but some authors have suggested that hearing loss is most pronounced at 4 kHz. Some bone conduction hearing thresholds recover with treatment, but some do not, and may be related to tympanosclerosis. When analyzing some mixed deafness or conductive deafness predominantly with several frequencies of bone conduction hearing loss, it is important to note whether it is caused by inner ear pathology. Currently there are four inner ear pathologies that can cause conductive deafness as follows
Upper semicircular canal fracture syndrome: The main manifestation is low-frequency conductive deafness. The third window of the superior semicircular canal fissure, which can move reciprocally, may be the cause of conductive deafness: when the stapes foot plate vibrates and causes the inner ear to vibrate, the membranous closure of the superior semicircular canal fissure moves reciprocally, which affects the conduction of sound to the cochlea and causes a decrease in air conduction hearing. The bone conduction hearing is increased. Vestibular evoked potential thresholds are significantly lower than normal. Large vestibular canal syndrome: low frequency conductive deafness. It is now thought that this is also due to the effect of the third window on the air conduction hearing. Meniere’s disease: some of these can manifest as poor low-frequency bone air conduction, possibly due to fluid accumulation in the inner ear as well as increased ectolymphatic pressure that restricts the inward motion of the stapes floor. x-linked stapes wellingtons syndrome: these patients have a larger than normal third window, resulting in enhanced bone conduction hearing and therefore poor bone air conduction. When these low-frequency conductive hearing losses are seen and other tests show normal outer and middle ears, the possibility of inner ear pathology is considered, at which point further tests are performed, along with imaging.
Suprathreshold functional testing: Pure tone audiometry can only measure the air-bone conduction hearing threshold, but in practice, some people can have better hearing thresholds and some deficiencies = but can behave very deaf. Suprathreshold audiometry can provide a reliable diagnosis of the site of auditory damage.
The alternating binaural loudness balance test, monaural loudness balance test, tone intensity difference threshold test, and short increment sensitivity index test, all of which test the relationship between sound intensity and the patient’s subjective loudness, are positive to indicate cochlear deafness.
The threshold tone attenuation test first detects the patient’s hearing threshold, and then stimulates with that threshold. If the patient can still hear after 1 minute, it is negative, if the sound disappears within 1 minute, it is 5 dB increment, if it is less than 10 dB, it is negative, and more than 15 dB is positive, which is mostly seen in postcochlear lesions. The suprathreshold adaptation test uses 500, 1000, and 2000 Hz frequencies, and 110 Db SPL is used for continuous vocalization within one minute, which is positive if there is an answer within one minute, otherwise it is negative, indicating a posterior cochlear lesion.
Human speech is the most exposed sound in daily life, with a wide frequency spectrum, fast transients, and variable sound intensity, and the hearing threshold cannot be directly determined. At present, in audiological examination, speech intelligibility test can be used to determine, which is commonly referred to as speech audiometry.
The speech intelligibility test can be used to determine the speech intelligibility of a person’s ear by means of a voice recorder, a jukebox or a direct oral articulation, which is delivered to the examined ear through a speech audiometer. This curve represents how well the human ear hears and understands language at various sound intensities. Therefore, speech audiometry is a broad-based audiometric method that is consistent with the actual hearing situation. The instrumentation of speech audiometry is not complicated, pure tone audiometers with talking devices can be used to carry out audiometry, tape recordings are more convenient and accurate, and oral speech is also available.
Speech audiometry is commonly used in clinical practice to: (1) understand the match between intelligibility threshold and pure-tone practical hearing apparatus. (2) To identify the presence or absence of sensorineural lesions by speech recognition rate. (3) Identify the phenomenon of reverberation. (4) Matching hearing aids.
(5) To compare and observe the hearing progress before and after treatment or training, etc.
Acoustic conductance test is one of the methods for observing hearing. It is the use of a certain sound pressure level of low-frequency pure sound conducted into the external ear canal, causing vibrations or changes in structures such as the eardrum, auditory chain, oval window, tympanic cavity, eustachian tube and middle ear muscles. Due to the difference in elasticity, quality and friction of these organs and tissues, the magnitude of the displayed sound level changes differently. It does not measure the hearing valve of the human ear but rather the changes in the acoustic impedance of the human middle ear. This change is recorded to provide an objective basis for the analysis of middle ear pathology. The impedance test results are divided into As, Ad, B and C curves. The A curve indicates that the tympanic membrane is mobile and the middle ear structure is basically normal; the B curve indicates that fluid in the middle ear or a middle ear tumor affects the auditory chain and the movement of the tympanic membrane; and the C curve indicates negative pressure in the middle ear, which is usually caused by poor function of the eustachian tube. The acoustic reflex has diagnostic value in the degree of hearing loss, qualitative localization.
Acoustic reflex threshold: The difference between the acoustic reflex threshold and pure tone audiometry is below 60 dB indicating reverberation, which is a cochlear lesion. If the acoustic reflex threshold is 15 dB above normal, the impedance is normal or the pure tone threshold is less than 65 dB and the acoustic reflex is not induced, then a postcochlear lesion should be excluded.
Amplitude of acoustic reflex: Generally, the amplitude of non-crossed acoustic reflex is greater than the amplitude of poorer acoustic reflex, and the ratio of the two amplitudes is between 1.2 and 1.5 under normal circumstances.
Acoustic reflex attenuation: A decrease of more than 50% in the amplitude of otoacoustic emissions within 5 seconds of continuous acoustic stimulation indicates the presence of auditory fatigue, which is a sign of postcochlear pathology.
Acoustic reflex latency: The latency is shortened in cochlear lesions and lengthened in postcochlear lesions.
Otoacoustic emissions are another objective method that has been used clinically in recent years for hearing acuity testing. The mechanism of otoacoustic emissions is a positive feedback acoustic energy that may be present in the cochlea that enhances the vibration of the basilar membrane and may also result from the vibration of the spiral apparatus, particularly the telescopic activity of the outer hair cells and the forward fluctuating acoustic energy in the cochlea. Evoked otoacoustic emissions occur in 100% of able-bodied individuals and are mostly used clinically for hearing screening of infants and children and for differential diagnosis of cochlear deafness and postcochlear deafness.
The results of clinical tests in recent years have shown that the energetic nature of the outer hair cells is responsible for the appearance of otoacoustic emissions. The evoked otoacoustic reflex can only be elicited when the outer hair cells are normal. If the outer hair cells are dysfunctional in cochlear lesions, evoked otoacoustic emissions may not be elicited. If the postcochlear lesion does not affect the outer hair cells of the cochlea, evoked otoacoustic emissions can be elicited. Therefore, a postcochlear lesion that can cause evoked otoacoustic emissions without evoking brainstem evoked potentials is a postcochlear lesion, and an ear that does not evoke evoked otoacoustic emissions can be considered to be cochlear outer hair cell dysfunction after excluding conductive deafness. In addition to the abnormalities of the outer hair cells, there may be an underlying lesion in the middle ear. It is usually considered that TE is not easily elicited at hearing thresholds less than 30 dBHL. The functional status of the middle ear has a greater impact on TE than on pure tone hearing, as it affects both the incoming and outgoing sound transmission. Middle ear fluid accumulation mainly affects the low and middle frequency regions of the DP, with little effect on high frequencies. It is also related to the amount and viscosity of the fluid, and when the amount of fluid in the middle ear is less than 1/2, there is no significant effect on DP. When the periosteal perforation is small (1%), it affects the low frequency DP, and gradually progresses to the high frequency as the perforation increases.
Another method of objectively observed audiometry is electrical response audiometry. We already know that when the ear is stimulated by sound, the auditory system induces a series of potential changes in the channel from the peripheral nerve to the center, and the method of recording these potential changes is called electrical response audiometry. The potential evoked by hearing is very weak compared to other potentials in the body, and the size is only a few microvolts, so it is difficult to extract. It was not until the advent of electronic computers that it became possible to extract and record these evoked potentials from the background noise of electrical interference through the “superposition” technique, and thus use them in clinical practice. Electrical response audiometry records the potentials at the end of the auditory system and is called cochlear electrogram, while the central part is called brainstem electroresponse and cortical electroresponse audiometry.
Cochlear electrograms are generated in the cochlea and include cochlear microphonic potentials (CM), action potentials (AP) and summation potentials (SP). Cochlear lesions, such as Ménière’s disease, can have abnormal waveforms, but middle ear lesions can also affect cochlear electrograms, which can result in increased response thresholds but normal waveforms.
The brainstem evoked potential is a waveform map with five waves. Wave I indicates the proximal cochlear nerve, wave II indicates the proximal cranial end of the cochlear nerve, wave III indicates the cochlear nucleus, wave IV indicates the superior olivary nucleus, and wave V indicates the lateral thalamus. If the V/I amplitude ratio is less than 1/2, it is a sign of posterior cochlear lesion, and the difference between the I-V wave intervals in both ears is greater than 0.4 ms. In patients with conductive deafness, the latency of each wave of ABR is prolonged, the interwave interval remains unchanged, and the Ⅰ wave is often not induced. However, it is important to note that ABR only responds to high frequency hearing, not low frequency, and only responds to peripheral hearing acuity and nerve conduction function in the brainstem pathway, not true hearing.
They can be used to objectively determine the true hearing of a deaf patient, and faithfully reflect the function of the auditory conduction pathway (including the function of the hair cells, auditory nerve and auditory center), and are particularly suitable for infants and children, pseudo-deafness and psychiatric patients. However, attention should be paid to lesions in the higher auditory centers above the brainstem.
Medium latency potentials as well as 40 Hz correlation potentials, and slow responses in higher cortices can then be used to identify central deafness, functional deafness, and pseudo-deafness.