Hearing Loss and Auditory System Function

Hearing Overview Hearing Overview Prof. Ehab Taha Yaseen FICMS, FRCS Head of Al-Yarmouk Center for Postgraduate Study Consultant Otolaryngologist

Agenda Agenda I will discuss briefly the following points: Epidemiology Hearing Facts. Terms. Definitions. Auditory system. How hearing occur. Sound detection. Sound discriminations. Hearing loss categories.

Epidemiology Epidemiology Millions of peoples experience some degree of hearing loss. - The prevalence of hearing loss increases with age: - 1.26% in Children (based on family report) - 5% children (under 18 years) suggested by others - 17 % of adults in the United States. - 20.8 % of adult men vs. 14.1% of adult women.

- 8.4 % of the population ages 18-44 - 20.6 % of the population ages 45-64 - 34.1 % of the population ages 65-74 - 50.4 % of the population age 75 and older - These estimates include persons with conductive and sensorineural hearing loss.

Hearing Hearing Basically, the function of the ear is: A sensory function, in which sound waves in the air are converted to bioelectric signals, which are sent as nerve impulses to the brain, where they are interpreted Hearing allows one to: Identify and recognize objects in the world based on the sound they produce. Hearing makes communication possible by using sound.

How Hearing Occur How Hearing Occur 1) Sound is derived from objects that vibrate producing pressure variations. 2) A pressure wave is propagated outward in a sound-transmitting medium, such as air. 3) When the pressure wave encounters another object, the vibration can be conveyed to that object and the pressure wave will propagate in the medium of the object.

4) The sound wave may also be reflected from the object, or it may deflect around the object. 5) Thus, a sound wave propagating reach the eardrum of a listener causing the eardrum to vibrate and initiate the process of hearing.

A sound waveform has three basic physical attributes: 1)Frequency: refers to the number of times per second that the vibratory pattern (in the time domain) oscillates. Measured in units of hertz (Hz), cycles per second. 2) Amplitude: refers to sound pressure which proportional to sound intensity (power) 3) Temporal variation, there are many aspects related to the temporal variation of sound, such as sound duration, intensity and frequency.

Some definitions of terms and measures used to describe sound: Sound pressure (p) = Force (F) produced by the vibrating object divided by Area (Ar) over which that force is being applied: p = F/Ar. Sound intensity (I) is a measure of power carried by sound waves per unit area in a direction perpendicular to that area. Intensity = p2divided by the density (po) of the sound-transmitting medium times the speed of sound (c): I = p2/poc.

Intensity: is the logarithmic ratio of the measured sound intensity to the reference sound intensity in decibels. Decibel: unit for expressing the ratio between two physical quantities, usually amounts of acoustic or electric power, or for measuring the relative loudness of sounds Equal to one tenth of a bel (B). Decibel (dB): dB = 10 log10(I/Iref), ref is a referent intensity The reference intensity I0, corresponding to a level of 0 decibels, is approximately the intensity of a wave of 1,000 hertz frequency at the threshold of hearing Hertz (Hz): hertz is the measure of vibratory frequency cycles per second

Tone (a simple sound): a tone is a sound whose amplitude changes as a sinusoidal function of time. Complex sound: any sound that contains more than one frequency component. Noise: a complex sound that contains all frequency components, and whose instantaneous amplitude varies randomly. White noise: a noise in which all of the frequency components have the same average level. Narrowband noise: is concentrated within a narrow range of frequencies

Auditory System Auditory System The ear is a very efficient transducer. Transducer is a device that changes energy from one form to another The external ear, middle ear, inner ear, brainstem, and brain each have a specific role in this transformation process, i.e., changing sound pressure in the air into a neural-electrical signal that is translated by the brain as speech, music, noise, etc.

How hearing occur How hearing occur The external ear includes the pinna, capture sound in the environment. The EAC will direct the sound to the tympanic membrane. The TM and ossicles, assist in the transfer of sound pressure in air into the fluid- and tissue-filled inner ear. When pressure is transferred from air to a denser medium, (inner ear environment), most of the it is reflected away. Thus, the inner ear offers impedance to conducting sound pressure to the fluid and tissue of the inner ear. The transfer of pressure in this case is referred to as admittance.

Impedance is the restriction of the transfer of pressure. The term acoustic immittance is used to describe the transfer process within the middle ear: the word immittance combines the words impedance and admittance (im + mittance) As a result of this impedance, there is nearly a 35 dB loss in the transmission of sound pressure to the inner ear. The outer ear, tympanic membrane, and ossicles will overcome the 35 dB loss. Thus, the fluids and tissues of the inner ear vibrate in response to sound in a very efficient manner.

Sound waves are transmitted through the ossicles to the stapes footplate. The footplate rocks in the oval window, setting the fluids of the inner ear in motion, with The parameters of that motion being dependent on the intensity, frequency, and temporal properties of the signal. The inner ear contains both the vestibular system and the cochlea. The cochlea has three separate fluid compartments; - Scala tympani and vestibuli, both contain perilymph, (= extracellular fluid), is rich in sodium and low in potassium and calcium. - Scala media, contains endolymph, (= intracellular fluids). Also contain the hair cells (are sensorineural cells, stimulated by fluid and tissue vibration). Endolymph is rich in potassium and low in sodium and calcium.

There are two types of hair cells: 1)Inner hair cells are the auditory bio-transducers translating sound vibration into neural discharges. The shearing (a type of bending) of the hairs (stereocilia) of the inner hair cells induces a neural-electrical potential that activates a neural response in auditory nerve fibers of the eighth cranial nerve that neurally connect the hair cells to the brainstem. 2)Outer hair cells serve a different purpose. When their stereocilia are sheared, the size of the outer hair cells changes due to a biomechanical alteration. The rapid change in outer hair cell size (especially its length) will intensify sounds that are low-level, mechanically entering into fluids of the cochlea.

The structures of the cochlea vibrate in response to sound. This vibratory pattern (the traveling wave) of cochlea serve 2 benefits: 1) Sort out the frequency. 2) Allows the inner hair cells to send signals to the brainstem and brain about the sound's vibration and its frequency content.

Theory of frequency processing: different frequencies of sound are coded by different auditory nerve fiber. The auditory nerve is said to be organized in that each nerve fiber carries information to the brainstem and brain about a narrow range of frequencies. In addition, the temporal pattern of neural responses of the auditory nerve fibers responds to the temporal pattern of oscillations of the incoming sound as long as the temporal variations are less than about 5000 Hz.

In general, the more intense the sound is, the greater the number of neural discharges that are being sent by the auditory nerve to the brainstem and brain. Thus, the cochlea sends neural information to the brainstem and brain via the auditory nerve about the three physical properties of sound: frequency, temporal variation, and level. The biomechanical response of the cochlea is very sensitive to sound, is highly frequency selective.

At 60 dB SPL the bones of the skull begin to vibrate, bypassing the middle ear system. This direct vibration of the skull can cause the cochlea to vibrate and the hair cells to shear and to start the process of hearing. This is a very inefficient way of hearing, in that this way of exciting the auditory nervous system represents at least a 60 dB hearing loss.

There are many neural centers in the brainstem and in the brain that process the information provided by the auditory nerve. The primary centers in the auditory brainstem in order of their anatomical location from the cochlea to the cortex are: cochlear nucleus, olivary complex, lateral lemniscus, inferior colliculus, and medial geniculate. Note: The peripheral auditory system: outer, middle, and inner ears and the auditory nerve The central auditory nervous system: brainstem and brain.

Sound Detection Sound Detection The healthy, young auditory system can detect tones in quiet with frequencies ranging from approximately 20 to 20000 Hz. The detection of tones is the basis for the primary measure of hearing loss. The audiogram is a plot of the thresholds of hearing. Thus, a person with no hearing loss at all will have a flat audiogram at zero dB HL. A person with a 40 dB hearing loss would be said to have a threshold of 40 dB HL.

Sound Discrimination Sound Discrimination The ability to recognize, compare and distinguish between distinct and separate sounds or similarities and differences between sounds. Humans are most sensitive over a range 500 to 4000 Hz and 35 to 80 dB SPL, in which listeners can discriminate a change of about one decibel in sound level and about a 0.5% change in tonal frequency. For instance: o50 dB SPL sound can be just discriminated from a 51 dB SPL sound o2000 Hz tone can be just discriminated from a 2010 Hz tone. This allows people to distinguish between phonemes in words. and allows someone to tell the difference between words and sounds that are similar and words and sounds that are different.

Sound Discrimination Sound Discrimination It can help children tell the difference between the sounds of the words cow and now or cat and can. A hearing loss can lead to elevated level and frequency difference thresholds, making it difficult for the person with a hearing loss to differentiate the small differences in level and frequency that often accompany changes in the speech waveform.

Hearing Loss Hearing Loss Hearing loss can be categorized into the following ranges based on PTAs (PTA 512): Slight (16-25 dB hearing loss). Mild (26-40 dB hearing loss). Moderate (41-55 dB hearing loss). Moderately severe (56-70 dB hearing loss). Severe (71-90 dB hearing loss). Profound (greater than 90 dB hearing loss).

The loss can be caused by damage to any part of the auditory pathway. Three major types of hearing loss have been defined: 1) Conductive: damage to the conductive system of the ear that is, the ear canal, TM, and ossicles and can include fluid filling the inner ear space. 2) Sensorineural: a problem in the inner ear, auditory nerve, or higher auditory centers in the brainstem and temporal lobe. 3) Mixed: designates that the hearing loss has both a conductive and sensorineural component.