Hearing Loss and Deafness

(4-21-15)

Auditory Receptors and Hearing

Our sense of “sound” reflects changes in air pressure in our surrounding environment. We hear “loudness” in the amplitude of pressure changes, “pitch” in the frequency of pressure changes, and “timber” in the complexity of pressure changes.
- Amplitude: the intensity (energy) of sound–usually measured in decibels (dB)
- Frequency: the rate of vibration of sound waves–usually measured as cycles per second: “hertz” (Hz)
- Complexity: the mix of frequencies in a sound (what gives the sound it unique feeling)–while most sounds are a mix of frequencies (in contrast to a pure tone), we have no ready operationalization of the characteristic of sound. Some discussions refer to this aspect of sound as phase (Maltby & Knight, 2000)
Pressure waves produce responses in sensory receptors in the inner ear that generate action potentials, these action potentials are carried to the brain by the auditory nerve
- Outer ear: pinna & external ear canal (meatus)
- Middle ear: eardrum, ossicles (three bones: hammer, anvil, stirrup)
- Inner ear: oval window, cochlea (fluid, basilar membrane, organ of Corti [hair cells, cilia])
  - The basilar membrane is not uniform: near the oval window [base]it is narrow and thick (and responds to high frequency waves), at the other end [apex] it is wider and thinner (and responds to low frequency waves). The tonotopic theory states that different points on the basilar membrane represent different sound frequencies.
  - The transduction of sound into nerve impulses is accomplished by motion sensitive vibrissae (stereocilia) attached to neurons located between the plates of the cochlea. [The neurons and attached stereocilia resemble a hair and are often called “hair cells”] When the pressure wave shears the stereocilia in one direction, there is an influx of Ca⁺⁺: causing the cell to depolarize; when it moves the other direction there is an efflux of K⁺: causing hair cell to hyperpolarize. This generates an action potential (nerve impulse) and the membrane potential oscillates at precisely the frequency of the sound wave that passes through the ear.
  - Also in the inner ear is the vestibular mechanism involving the semicircular canals, which yield our sense of balance.
- Auditory pathway
  - axons from the hair cells leave the cochlea to form the auditory nerve (8th cranial nerve): projects to the medulla [lower brainstem], synapsing in either dorsal or ventral cochlear nuclei or superior olivary nucleus: axons of cells in these areas terminate in the inferior colliculus [the superior colliculus functions to orient the head toward the direction of a sound]. From the colliculus two pathways emerge: carrying impulses to the ventral and dorsal medial geniculate. The ventral medial geniculate projects to the primary auditory cortex in the Temporal Lobe (Broadman’s area 41), which projects to secondary auditory areas (42, 22). The dorsal medial geniculate projects to the secondary areas of the Temporal Lobe (42, 22) [multiple ascending pathways to the cortex is the usual pattern in the brain]. Approximately 90% of the projections from the medial geniculate are contralateral (to opposite side of brain) and 10% are ipsilateral (to same side of brain)–there is bilateral representation of input from each cochlear nucleus in each hemisphere.
  - The auditory system locates sounds in space by contrasting the time of arrival of each sound at each ear.
  - The tonotopic theory also applied to the auditory cortex: different locations represent different sound frequencies.

Hearing Loss/Hearing Impairment

Hearing loss can be temporary or permanent, and can occur suddenly (acute) or gradually
Degree of loss
- terminology:
  - Deafness: total, profound, severe, moderate, mild, insignificant
    - typically total or profound loss of auditory sensitivity and little or no auditory perception
  - Hard of Hearing: severe, moderate, mild
    - partial or residual hearing–able to process language through auditory reception (may use hearing aid)
    - The majority of the population with hearing loss are functionally “hard of hearing” rather than “deaf” (Flexer, 1993; Davis, Fortnum, & Bamford, 1998)
  - tinnitus: high-pitched throbbing or ringing sounds
Prevalence
- Prevalence estimates are affects by definitions used and sampling methodologies–be cautious consumers of information
- Approximately 1 in every 1000 babies is born deaf; 50% may not be identified until about age 3
- Estimates of hearing loss in the U.S. approach 1% of the total population (This includes deaf, “significant and irreversible hearing loss”, as well as less severe cases)
- Males are more likely to experience hearing loss over time
- Hearing loss (especially for high frequencies) occurs and increases with age
- Hearing loss can be exacerbated by exposure to loud and sustained sounds
Mechanism/Anatomical Site of Loss
- peripheral vs. central
  - Conduction hearing loss is due to physical loss of the sound wave transmission through the outer or middle ear to the oval window
  - Sensorineural hearing loss affects the reception/transmission of sensory signals from the chochlea via the auditory nerve to the auditory tracts and center of the brain
  - Mixed hear loss involves both conduction and sensorinural losses
- hearing loss may be bilateral or unilateral
  - Unilateral (monaural) hearing loss affects only one ear: child may have difficulty in noisy surrounding and with sound localization
- with hearing loss (as opposed to deafness), tinnitus may be a important associated limitation in functional hearing
Etiology:
- Etiology unknown in approximately 20% of cases of prelingual hearing loss
- Genetic factors: 200 types of deafness liked to hereditary
  - Dominant genetic deafness: approximately 15-20% of hereditary cases of hearing loss (Aros, Isreal, Devlin, & Wilson, 1966), usually inherited from affected parent, 50% chance of being passed on to each child
    - May arise from mutation: neither parent affect but individual is deaf from “new” dominant gene
    - Treacher-Collins syndrome is a dominant genetic disorder due to single gene which results in conductive hearing loss, malformations of external ears, downward sloping eyes, depressed cheekbones, notch or cleft of lower eyelid (coloboma); 60% of clinical cases are believed to arise from a new mutation and these cases are associated with older age of father (Aros, Isreal, Devlin, & Wilson, 1966)
  - Recessive genetic deafness: approximately 80% of hereditary cases of hearing loss (Aros, Isreal, Devlin, & Wilson, 1966), both parents usually unaffected “carriers” of recessive gene for deafness, 25% of children deaf, 50% “normal” carriers (like parents), 25% of children show normal hearing and are not carriers
    - X-linked recessive inheritance: a small number of genes for hearing loss are carried on X chromosome: more males than females will be found with expressed disorder
  - Multifactorial genetic deafness: in rare cases it is the interaction of a gene/genes with environmental factors which leads to a particular outcome:
    - Some cases of “aminoglycoside (drugs such as streptomycin) induced hearing loss” possess a gene which increases susceptibility to this drug–a mutation in mitochondrial DNA can cause hearing loss in members who receive doses of aminoglycoside antibiotics (Aros, Isreal, Devlin, & Wilson, 1966)
  - Congenital factors:
    - maternal infections are most common known cause of nongenetic deafness (rubella, cytomegalic inclusion disease, toxoplasmosis, chicken pox, syphilis), Rh incompatibility, anoxia, birth trauma
  - Constitutional factors:
    - postnatal disease: measles, mumps, influenza, typhoid fever, scarlet fever
    - environmental factors: loud noise, explosions, extreme changes in air pressure, physical abuse of cranium, foreign objects in the ear
- Age of Onset of loss
  - Congenital vs. Acquired
  - prelingual (<2 y/o) vs. postlingual
- Consequences:
  - speech and language
    - Speech development clearly lags in deaf and hard of hearing children, proportionate to the degree of hearing loss
    - Language development (vocabulary, fluency, complexity of communications) also lags behind hearing peers; language development is greatly facilitated by availability of a language system the child can participate in. Language development appears significantly greater in congenitally deaf children who are exposed early to a manual language. It remains controversial whether this completely eliminates differences in language. The relationship (facilitatory or inhibitory) between manual communication and speech also remains controversial.
  - educational progress
    - Education progress in deaf and hard of hearing children tends to lag behind that of hearing peers, secondary to their speech and language limitations. Addressing language deficiencies produces the most pronounced impact on academic progress.
  - intelligence
    - The relationship between language and intelligence continues to be the subject of much interest, research, theory, and controversy. The distribution of scores on nonverbal IQ tests in congenitally deaf children appears similar to that of hearing children, although methodological and sample issues make it quite difficulty to obtain a clear and convincing answer to this issue.
    - In the most commonly used measures of intelligence, such as the Wechsler Intelligence Scale for Children, 4th Edition (WISC-IV), subtests which emphasize visual-spatial analysis and reasoning (such asa Matrices, Block Design) are often introduced with verbal instructions, which raise issues as to how reliable, valid, and fair it is to use such tools with deaf children. There are tests which were specifically constructed to be administered without spoken instructions (an example is the Test of Nonverbal Intelligence, 3rd Edition; the TONI-3) but these will not correlate perfectly with more typical measures such as the WISC-IV (in part because they sample a narrower range of cognitive behavior). The most useful and fair measure(s) of general cognitive ability (intelligence) in children with limited hearing remains a major challenge for psychology.
  - social development
    - The role of language in social development, especially with respect to self- and impulse control, is hotly debated. Some theorists (Vygotsky) saw self control and civilization made possible through the internalization of verbal communications from our caretakers/society (private speech). A stereotype of deaf children/individuals is that they are more impulsive that “normal” children. It is very difficulty to separate out the multiple differences in the social, educational, and personal experiences of congenitally deaf children. It appears that language is an agent/tool of socialization–having access to a language system (for instance ASL) seems to reduces many/most of the possible personality/behavioral differences between deal and hearing children.
  - adjustment
    - The opportunities for educational, vocational, social success for children who are deaf or hear of hearing have dramatically increased over the past century. The majority of children with hearing impairments develop into successful and productive citizens (as is true of hearing children). Both significant challenges and questions still remain.
    - Common problems reported by adults with hearing loss (cited in Martin, 2001)
      - listening to TV/radio
      - general conversation
      - doorbell
      - group conservation
      - speech against background noise
      - telephone signal
- Intervention/habilitation approaches emphasis:
  - auditory: amplification, implants, oral communication
  - oral: lip-reading, speech training oral communication
  - manual: manual communication
  - total communication communication: use of all available tools
  - Selection criteria for pediatric cochlear implant (Boothroyd, 1998, p. 263):
    - “Two years of age and above”
      Profound bilateral sensorineural hearing loss (SNHL)
      No appreciable benefit from hearing aids
      No medical contraindications
      High motivation and appropriate expectations
      Enrolled in program that emphasizes development of auditory skills”

Deaf Community / Deal Culture

“For many people within the deaf community, ‘deaf’ is less about audiology than it is about life and culture . . . . Individuals who identify as culturally deaf (in contrast to audiologically deaf) perceive themselves as a linguistic minority group because of their use of American Sign Language (ASL), a visual language with its own grammar and syntax.”
(Christiansen & Leigh, 2002, pp. 254-255)

Who should speak for the “Deaf”?
- Cochlear implant controversy
- Homo sum: humani nil a me alienum puto – Terrence
  - Differences
    - When is a difference a disability?
    - How far should society go to remove a disability?
      - What function does human variability serve?
      - Who bears the cost?
Access to services issues
- communication barriers
- conceptual barriers
  - A study of deaf individuals in Great Britain who sustain head injuries found they were not to be referred for speech and language therapy, although this could be an issue in their rehabilitation (Marshall, Atkinson, Thacker, & Woll, 2003)
“Treatment Fads Versus Evidence-Based Practice” (Tharpe, 1998)
- Characteristics of alternative or “fad” treatments (Tharpe, 1998; based on Goldstein & Ingersoll, 1992)
- Concepts outside the mainstream
- Initial popularity burst prior to demonstrated effectiveness
- Non-refereed publications
- Claims of widespread effectiveness