Physiology Practical Audiometry And Deafness

Objectives of the Practical At the end of the session, the students should be able to:  Determine the type, degree, and configuration of hearing loss.  Describe the techniques of Tuning Fork tests.  Plot the frequency-intensity recording in a procedure called audiometry and construct the audiograms.  Interpret the audiograms.

Characteristics of Sound Waves Sound is a mechanical wave (travelling vibration of air) Sound waves are alternating regions of compression and rarefaction (expansion) of air molecules Sound intensity (loudness) is measured in decibels (dB)

Characteristics of Sound Waves

properties of sound Pitch. It is the psychological perception of the sound frequency; the higher the frequency, the higher the pitch. The entire audible range extends from 16 Hz to 20, 000 Hz (1 hertz = 1 cycle/sec). Infrasound refers to frequencies below 16 Hz Ultrasound refers to frequencies above 20, 000 Hz. The human ear is most sensitive to frequencies between 500 and 5000 Hz. The average conversation voice frequency is 120 Hz in the males and 250 Hz in the females. The sounds from a distant plane range from 20 to 100 Hz.

Pitch discrimination is possible because different frequencies cause vibrations in different regions of the basilar membrane. Each segment of this membrane is thus “tuned” for a particular pitch— high-pitched sounds near the base of cochlea, and low-pitched sounds near the apex. While the human ear cannot perceive (“hear”) ultrasounds, bats, dogs, and other animals can. Ultrasound is used extensively to study the internal organs of the body. The inaudible sounds are reflected from the organs and analyzed by a computer to provide a picture on the display screen.

Intensity (loudness). The intensity or loudness of a sound is the psychological term referring to the amplitude of the sound vibrations. The sound intensity is measured in units called decibels (Db; db). Timbre (quality or pattern). This property refers to the sensation perceived when we hear a mixture of related frequencies, i.e. harmonics or overtones. (The same note played on different musical instruments is “perceived” or “sounds” differently). Direction of sound. The ability to detect the position of the source of sound is called binaural effect

Anatomy of the Ear

Organ of Corti (hearing sense organ) with hairs of cells (stereocilia) Rests on basilar membrane Contains inner & outer hair cells These are attached to tectorial membrane • Supporting cells

The Auditory Pathway ❶Spiral ganglion neurons (Cochlea) ❷Cochlear nerve (VIII) ❸Cochlear nuclei (Medulla) ❹Superior olivary complex (Pons) (bilateral) ❺Lateral lemniscus ❻Inferior colliculus (Midbrain) ❼Medial geniculate nucleus (Thalamus). ❽Primary auditory cortex (Temporal lobe).

The Auditory Cortex is Mapped According to Tone  Primary and secondary auditory cortex is tonotopically organized  Each region of the basilar membrane is linked to specific region of primary cortex  Specific cortical neurons are activated only by particular tones  Secondary auditory cortex - Wernicke's area (Detection of language sounds)  Auditory agnosia - can hear but can't recognize sounds

Hearing Disorders Conductive hearing loss: - Inadequate transmission of sound through external or middle ear due to:  Blocked auditory canal (wax, fluid)  Rupture or perforation of tympanic membrane  Otitis media - middle ear infection /inflammation  Restriction of ossicular movements (e.g. by fibrosis or calicification)  Osteosclerosis (pathological fixation of stapes on the oval window)  Bone conduction is better than air conduction

Sensorineural (nerve) deafness – Hearing Disorders Sensorineural (nerve) deafness – Hearing loss caused by disruption anywhere in pathway from hair cells to the auditory cortex due to:  Loss of hair cells (explosion, chronic loud noise)  Damage to vestibulocochlear nerve (VIII)  Damage to nuclei / tracts to the cortex.  Weber`s hearing test: (lateralization to better ear) Neuronal presbycusis: degenerative age related process occurs as hair cells wear out with use (loss of ~ 40% of hair cells by age 65) Cochlear implants have become available (do not restore normal hearing!)

I. Rinne’s Test This test compares the AC hearing with his BC hearing, in each ear separately. 1. Hold the stem of the tuning fork with the thumb and finger and set it into vibration by striking one of its prongs on the heel of your hand. 2. Place its base on the mastoid process. The subject will hear a sound. Ask him to raise his hand when the sound stops. Note the time for which the sound is heard 3. When the sound stops, bring the prongs in front of the ear—the sound will become audible once again. Note the time for which it lasts (E.g., for another 10 seconds; total= 45 seconds).

Results In normals. For example, sound heard on mastoid process = 35 seconds. Sound heard in front of ear = 35+ 10= 45 seconds Thus, AC > BC (Rinne positive). In conduction deafness. BC sound remains normal at 35 seconds, but AC sound not heard after BC sound stops. Thus, AC < BC (Rinne negative). In nerve deafness. Hearing will be impaired in both BC and AC sounds. For example, BC becomes 15 seconds, AC becomes 20 seconds. Thus, AC > BC, if nerve deafness is partial.

Weber’s Test This test compares the bone conduction of the subject in his two ears. 1. Set the tuning fork into vibration and place its base in the midline on the top of the subject’s head or on his forehead. Ask the subject if he hears the sound equally well in both the ears, or louder on one side. In a normal subject. Bone conducted sounds are heard equally well on the two sides. In conduction deafness. Sound is louder/better heard in deaf or deafer ear because of masking effect of environmental noise is absent on diseased side. In nerve deafness. The sound is louder /better heard on the healthy side i.e. the patient lateralizes the sound to healthy side.

Audiometry: An audiometer is an apparatus in which selected pure tones of 125–800 Hz can be fed into each ear separately through headphones. The threshold is determined at each frequency and is then plotted as a percentage of normal hearing. Audiometry is thus the only reliable method to determine the nature and degree of deafness in a patient.

Procedure An earphone connected to an electronic oscillator capable of emitting pure tones ranging from low frequencies to high frequencies, (Pure Tone -a musical tone of a single frequency produced by simple harmonic vibrations and without overtones) If the loudness must be increased to 30 decibels above normal before it can be heard, the person is said to have a hearing loss of 30 decibels at that particular frequency. Test about 8 to 10 frequencies covering the auditory spectrum, and the hearing loss is determined for each of these frequencies. Then the audiogram is plotted, as shown in Figures

Audiogram in Nerve Deafness. Includes damage to the cochlea, the auditory nerve, or the central nervous system circuits from the ear—the person has decreased or total loss of ability to hear sound as tested by both air conduction and bone conduction. In audiogram , the deafness is mainly for high-frequency sound. This type of deafness occurs to some extent in almost all older people.

Other patterns of nerve deafness- (1) deafness for low-frequency sounds caused by excessive and prolonged exposure to very loud sounds (a rock band or a jet airplane engine), because low-frequency sounds are usually louder and more damaging to the organ of Corti, and (2) deafness for all frequencies caused by drug sensitivity of the organ of Corti—in particular, sensitivity to some antibiotics such as streptomycin, kanamycin, and chloramphenicol.

Audiogram for Middle Ear Conduction Deafness Caused by fibrosis in the middle ear following repeated infection or by fibrosis that occurs in the hereditary disease called otosclerosis. The sound waves cannot be transmitted easily through the ossicles from the tympanic membrane to the oval window. Audiogram Bone conduction is essentially normal, but conduction through the ossicular system is greatly depressed at all frequencies, but more so at low frequencies. In some instances the faceplate of the stapes becomes “ankylosed” by bone overgrowth to the edges of the oval window. - the person becomes totally deaf for ossicular conduction but can regain almost normal hearing by the surgical removal of the stapes and its replacement with a minute Teflon or metal prosthesis-

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