Published on March 4, 2014
Applied anatomy and physiology of hearing. Approach to hearing loss. Audiometry , Evoked responses Management of hearing loss. by Prakash Harischandra Moderator- Unit 6
Main Components of the Hearing Mechanism: Divided into 4 parts (by function): • Outer Ear • Middle Ear • Inner Ear • Central Auditory Nervous System
Central Auditory System • VIIIth Cranial Nerve or “Auditory Nerve” – Bundle of nerve fibers (25-30K) – Travels from cochlea through internal auditory meatus to skull cavity and brain stem – Carry signals from cochlea to primary auditory cortex, with continuous processing along the way • Auditory Cortex – Wernicke’s Area within Temporal Lobe of the brain – Sounds interpreted based on experience/association
Auditory pathway to the brain • Organ of Cortispiral ganglion (in cochlear nerve)cochlear nuclei of medullasuperior olivary nucleus (pons/ medullary junction)along the lateral lemniscal tract to inferior colliculus (midbrain)medial geniculate body of thalamusauditory cortex in temporal lobe
Mechanisms of Hearing
SIX Steps of Hearing Process Figure 17–29
Cortical Processing • Pattern Recognition • Duration Discrimination • Localization of Sounds • Selective Attention Cerebral Dominance/Laterality • Language Processing in the left hemisphere. • Most people show a right-ear advantage in processing linguistic stimuli
Approach to hearing loss An algorithm for the approach to hearing loss. HL, hearing loss; SNHL, sensorineural hearing loss; TM, tympanic membrane; SOM, serous otitis media; AOM, acute otitis media; BAER, brainstem auditory evoked response; *, CT scan of temporal bone; , MRI scan.
Types of Hearing Disorders • 1. Nature of the loss: Sensitivity vs. Acuity Dysacusia – Deficit in discrimination or interpretation of sound: Acuity deficits sometimes due to disorders of the central auditory system. Disorders of sensitivity and acuity are not mutually exclusive. • 2. Functional Classification Conductive – Disorders involving the conduction of sound to the cochlea. Sensori-neural – Disorders involving the cochlea (usually the hair cells) or 8th N. Central – Disorders affecting the brain stem or auditory cortex. • 3. Cause or Etiology of the Disorder • Hearing disorders can be classified on the basis of the cause of the disorder. Some examples: • 1)Ototoxic drugs 2)Noise exposure 3)Old age (presbycusis) 4)Otitis media 5) 8th N tumors 9)Meniere’s Disease
Conductive Hearing Disorders 1. External Ear Congenital malformations. There are many of these. Most serious is congenital atresia – collapse or closure of the EAM (ear canal). May occur in isolation, but typically associated congenital malformations of the middle ear as well. Impacted wax (cerumen) – results in mild hearing loss; easily treated by removal of the wax. 2. Middle Ear a. Otitis Media B the most common cause of conductive hearing loss. Consequences • The hearing loss is typically mild (usually 20-30 dB) and often fluctuating. • The pain varies quite a bit but is often quite severe. It is not unusual for the pressure drop in the ME to become severe enough to cause the TM to rupture
Treatment of Otitis Media Most common treatment by far: Antibiotics (especially amoxicillin) Common treatment for recurrent or chronic OM: PE Tubes (PE = pressure equalization). This is a small plastic tube inserted into the TM. Why would such a tube be expected to treat OM?
b. Otosclerosis (note: topic here is still conductive HL, sorted by cause) Begins as a soft, spongy growth of new bone – may appear anywhere in the ME, but most often near oval window.>> Later hardens (i.e., becomes sclerotic) ->>In 90% of cases: No symptoms In unlucky 10%: Growth reduces mobility of stapes, causing a conductive HL. Progressive. Beginning in childhood. For that unlucky 10%, HL typically begins in late teens, early 20s. Maximum HL seldom worse than ~50-60 dB. Treatment: Stapedectomy (removal of stapes and replacement with an artificial stapes)
c. Cholesteatoma Cyst that invades the ME; usually grows rapidly Can: (1) destroy the ossicular chain, (2) invade the cochlea, or (3) break through the thin shelf of bone that forms the superior surface of the ME cavity, invading the meninges. This is extremely not good. HL is usually mild and not really the major concern.
Sensorineural Hearing Loss (SNHL) General: By far the most common underlying cause of SNHL is damage to the hair cell transducers. In these most common cases, the auditory nerve and central auditory pathway are intact, but stimulation of the auditory nerve is abnormal due to damaged hair cells. There are many possible reasons for hair cell damage. The various etiologies of SNHL consist mainly of a catalog of different causes of HC damage.
a. Presbycusis • Hearing loss associated with aging, Most common cause of SN HL – and most common cause of HL overall, Presbycusis begins in adolescence. Sad but true. • Presbycusis is listed here under the SN category since it is clear that this is the dominant component. The SN component may not be due exclusively to hair cell loss. Changes in the elasticity of the basilar membrane and metabolic changes in the stria vascularis may also play a There may also be a conductive component due to age-related changes in the mobility of tissues in the middle ear. There is sometimes a central component due to the loss of neurons in the CNS, (related primarily to arteriosclerosis).
b. Noise-Induced Hearing Loss Exposure to high levels of noise can damage HCs and cause SNHL. Two types: Acoustic trauma: Injury due to brief exposure to very intense sounds such as gun shots, artillery fire, explosions, etc. HL may be severe and permanent, but substantial recovery is common. Long-term noise exposure (more common): Damage results from long-term exposure to high levels of noise. Common in some occupational settings – heavy manufacturing and agriculture being the most common. Amount of inner-ear damage depends on the combination of: Intensity of the noise Length of exposure Pretty simple: High levels x long exposures=Bad news Low levels x brief exposures=Not so bad news
c. Ototoxic Drugs Certain drugs can cause SNHL. Toxicity effects vary from mild and temporary to severe and permanent. Some very common drugs such as aspirin (especially in large doses) can cause hearing loss (and/or tinnitus), but not in most people, and the loss is typically mild and temporary. An especially important group of antibiotics are notoriously ototoxic. Examples include neomycin, streptomycin, kanamycin.
ANTIBIOTICS WITH GOOD EVIDENCE FOR OTOTOXICITY Drug Vestibulotoxicity Toxic Level yes Erythromycin Hearing Toxicity High IV doses only Usually 2 weeks Gentamicin 8.6% minor Streptomycin very toxic minor dihydrostreptomicin minor toxic very toxic Tobramycin Yes minor in 6% Netilmicin Less toxic than Gentamicin 2.4% Amikacin not toxic 13.9% Neomycin minor very toxic Kanamycin minor very toxic Etiomycin moderate Vancomycin nontoxic none to moderate Metronidizole toxic (rarely) unknown Capreomycin yes In topical ear drops synergistic with gentamicin
d. Meniere’s Disease Serious, often debilitating disease of hearing and balance of uncertain cause. MD affects a single ear in about 75% of cases. 4 Major Sx: 1)Periodic episodes of rotary vertigo (the sensation of spinning) or dizziness (the “Meniere’s attack”) 2)Fluctuating, progressive, low-frequency hearing loss 3) Roaring or ringing tinnitus 4) A sensation of "fullness" or pressure in the ear. Cause – due to excessive and fluctuating pressure in the endolymphatic fluid that courses through the membranous labyrinth of the cochlea and vestibular systems. This causes the membranous labyrinth to balloon or dilate, Condition is known as endolymphatic hydrops. Result is progressive damage to the hair cells responsible for both hearing and balance. Underlying cause of the fluid imbalance (if that actually is what’s going on) is not known. Likely suspects – viral infection or autoimmune disorder affecting production or absorption of endolymph (duh). Treatment: Endolymphatic shunt, Vestibular nerve resection, Betahistine,
e. Infections • Bacterial or viral infections that invade the inner ear can cause SN HL and disruptions of vestibular function. Generic term for infections that invade the inner ear: labyrinthitis. Meningitis can sometimes spread to the inner ear and result in labyrinthitis. Other infectious diseases: Mumps, measles, meningitis, encephalitis, chicken pox, influenza, and syphilis can also invade the inner ear and cause SN HL and/or vestibular symptoms.
f. 8th N Tumors (acoustic neuroma) Benign (i.e., nonmalignant) tumor that exerts pressure on 8th N Almost always slow growing Most common symptom: hearing loss (mild initially), often accompanied by tinnitus Vestibular problems may also occur due - Continued tumor growth can be life threatening Treatment: Surgical removal or radiation, Early detection is really important: Small tumors can be removed with less risk of destroying the 8th N (and sometimes the 7th N as well). Early diagnosis is tough – early-stage symptoms are un-dramatic
f. 8th N Tumors (acoustic neuroma) • • • • Treatment: Surgical removal or radiation Early detection is really important: Small tumors can be removed with less risk of destroying the 8th N (and sometimes the 7th N as well). But, early detection is difficult – early-stage symptoms are not dramatic. Acoustic neuromas sometimes run in families (case in point to follow). Acoustic neuromas sometimes run in families (case in point to follow).
g. Congenital Causes Present at (or before) birth (1) Non-Hereditary Causes (a) Maternal rubella (German measles) When an expectant mother is exposed to rubella, the mother is not in any great danger, but the fetus is – especially in the 1st trimester. Effects can include: Heart defects, brain damage, various visual impairments SNHL, often profound (b) Anoxia (asphyxia) Insufficient oxygen during birth/delivery can cause all sorts of problems for the newborn. There’s hardly anything that’s not on the list of anoxia consequences. SN HL is on the list.
(2) Hereditary Causes Genetic factors are thought to cause more than 50% of all incidents of congenital hearing loss in children (NIDCD, 1989). Two patterns: (a) autosomal dominant One parent has a dominant gene for SNHL (and typically has a hearing loss). There is at least a 50% probability that the child will also have a hearing loss. Probability is higher if both parents have the dominant gene.
(b) autosomal recessive Both parents (typically with normal hearing) carry a recessive gene for SNHL. Each child will have a 1 in 4 chance of inheriting the bum gene. Approximately 80% of inherited hearing loss is autosomal recessive. This makes early detection tough since, with both parents hearing normally, the children are not considered at risk. Syndromes: Inherited hearing loss can also be associated with a complex of interrelated symptoms in the form of a syndrome. A few examples include: Klippel-Feil Syndrome-- The syndrome occurs in a heterogeneous group of patients unified only by the presence of a congenital defect in the formation or segmentation of the cervical spine. Klippel–Feil syndrome can be identified by shortness of the neck. Those with the syndrome have a very low hairline and the ability of the neck to move is limited
Syndromes with hearing loss Waardenburg Syndrome characterized by varying degrees of deafness, minor defects in structures arising from the neural crest, and pigmentation anomalies. Disruptions in myogenesis, particularly mutations in Pax3 Treacher-Collins Syndrome The typical physical features include downward slanting eyes, micrognathia (a small lower jaw), conductive hearing loss, underdeveloped zygoma, drooping part of the lateral lower eyelids, and malformed or absent ears.
• Normal Hearing: Audiometry PTAs < 25 dB • Hearing Impairment: PTAs 25-92 dB • Deaf: PTAs > 92 dB • The threshold of hearing is defined as the 'level of a sound at which, under specified conditions, a person gives 50 percent of correct detection responses on repeated trials'. 'Specified conditions' means the type of sound and ways of presenting the test sound. The normal test sound is pure tone pulses at standardized frequencies in the range of 125-8000 Hz and the normal presentation mode is monaurally by means of a standardized type of earphone. Pure tone audiometry has become the standard method for quantitative description of degree of hearing loss. It also provides information regarding the localization of the lesion that causes the hearing loss.
The Audiogram An audiogram is a picture of your hearing. It is a graph of the softest sounds you can hear. The yellow banana shows where all the speech sounds are heard when speaking at a normal level.
Audiogram for normal hearing Pitch Frequency in Cycles per Second (Hz) 125 250 500 1000 2000 4000 8000 0 10 Hearing Level in Decibels (dB) Volume 20 30 40 50 60 70 80 90 100 110 120 Right ear Left ear
Audiogram in CHL and SNHL bilateral conductive hearing loss Sensory Hearing Loss
Mixed Hearing Loss An audiogram of a bilateral mixed hearing loss
Understanding Degrees of Hearing Loss Mild (26 – 44 dB) • Understand conversation at 1 – 1.5m • May have delayed speech development • May miss up to 50% class discussion if speaker not visible • May need hearing aid • Will need special education attention Severe (60 – 89 dB) • • • • • • • May understand speech at <15cm Hears loud environmental sounds Will have delayed speech/language Will need hearing aid Requires auditory training Uses vision for additional cues Speech/language will not develop spontaneously if loss present before 1 year old Moderate (45 – 59 dB) • • • • • • Understand conversation at 0.5m Will have difficulty at school Likely to have language delay Will have poor speech clarity Will need hearing aid Will need special education assistance and probably special training for listening Profound (> 90 dB) • May only be aware of very loud sounds • Speech and language will be defective • Visual and gestured cues essential for learning • Needs full time special education assistance • Use of a hearing aid
Evoked Responses Electrical potentials that occur in the group of neuron in response to stimulation of a sense organ which can be recorded by surface electrodes is known as Evoked Potential. eg. SEP, ABR and VEP • Auditory brainstem response (ABR) is a neurologic test of auditory brainstem function in response to auditory (click) stimuli. • It’s a set of seven positive waves recorded during the first 10 seconds after a click stimuli. They are labeled as I - VII
Auditory brainstem response (ABR) typically uses a click stimulus that generates a response from the hair cells of the cochlea, the signal travels along the auditory pathway from the cochlear nuclear complex to the inferior colliculus in mid brain generates wave I to wave V. Wave I II III IV V VI VII Origin Cochlear nerve Dorsal & Ventral cochlear nucleus Superior olivary complex Nucleus of lateral lemniscus Inferior colliculus Medial geniculate body Auditory radiation(cortex
Interpretation Wave I : small amplitude, delayed or absent may indicate cochlear lesion Wave V : small amplitude, delayed or absent may indicate upper brainstem lesion I – III inter-peak latency: prolongation may indicate lower brainstem lesion. III – V inter-peak latency: prolongation may indicate upper brainstem lesion. I – V inter-peak latency: prolongation may indicate whole brainstem lesion. Shortening of wave the interval with normal latency of wave V indicate cochlear involvement.
APPLICATIONS • Identifying the hearing loss • Classification of type of deafness (conductive or sensorineural) Conductive Deafness Nerve Deafness Threshold of Hearing is High Threshold of hearing may be high WaveI to V are shifted to the Right Wave I is small and is delayed I-V interval is normal I – V interval is reduced Latency –intensity curce for wave V runs above the normal curve and is parallel to it The latency –intensity curve is of recruiting type (at high intensity, the curve is normal But of lower intesnity the wave V latency is
Interpretation • Normal : There is a shaded area for the normal person. If your points fall in this area then the person is having normal hearing. • Conductive Deafness: the latency – intensity graph plotted will be above & parallel to the shaded area. • Sensorineural Deafness: the graph plotted will be irregular & not forming a curve.
Medical Treatment • Glue ear and infections often clear up without treatment • Antibiotics will help with an infection but will not clear up the fluid build up that occurs with glue ear • Runny ear does not clear up quickly or easily. The recommended treatment is twice daily syringing with Betadine, followed by dry mopping, then drops, for 16 weeks.
Surgical Treatment Grommets are recommended for children who have had more than 6 infections in a year or one bout of Otitis Media lasting more than 3 months
More Surgical Treatment … • Myringoplasty / Tympanoplasty – patches the eardrum; not suitable for runny ears • Mastoidectomy – removal of part of mastoid bone and other parts of middle ear because of erosion by fluid over long period of time • Middle ear reconstruction
Other Amplification Options • Hearing hats/head bands – bone conduction • Personal FM systems • Sound Field Amplification Systems
Fact vs. Myth? Hearing aids will restore hearing to normal. • Hearing aids are designed to aid a person's hearing that is still intact. Hearing aids cannot restore hearing nor can they cure your hearing problem. They help to get the most out of the hearing that is left and are only part of hearing rehabilitation. Hearing aids may need to be supplemented by auditory training.
References • Harrisons Priniciples of Internal Medicine • The DeJong’s Neurologic examination • Understanding Hearing Aids –Dr Anirban Biswas • Review of Medical Physiology- William F Ganong • http://www.asha.org
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