Hearing Loss:

Hearing is the transduction of sound (mechanical energy) into neural impulses and the interpretation of those impulses by the central nervous system. Hearing loss can result from a defect at any level in this system. The proper management of patients with hearing loss requires an understanding of the normal mechanisms.

I. Nature of Sound: Vibration of Air

• Loudness is determined by the amplitude of pressure changes in the alternating compression and rarefaction of air,and expressed in units called decibels (dB).
• Pitch is determined by the frequency of cycles of vibration and is expressed in cycles per second or Hertz (Hz).

II. Conversion of Sound into Neural Energy

• The external ear shelters the eardrum and plays a role in localization of sound.
• The middle ear transmits the vibration of air into vibrations of the fluid in the inner ear. The difference in impedance(resistance to vibration) of air and water causes much sound energy to be reflected from an air-fluid interface. The middle ear compensates for this impedance mismatch.
  • Transmission of sound
    • Eardrum (tympanic membrane, or TM ) vibrates in response to sound
    • Malleus, embedded in the TM, transmits vibrations to incus and stapes.
    • Stapes, in the oval window, transmits vibrations to fluid investibula.
  • Impedance mismatch compensation
    • Area of eardrum is 10 times that of oval window
    • Mechanical advantage of 2 to 1 due to leverage of ossicles.

Middle ear transformer system. Note in the diagram above that the handle of the malleus (1) compared to the long process of the incus (2) adds an advantage of 3-to-1, allowing a gain in sound energy of only 2.5 decibels. However, the area ratio of the tympanic membrane footplate is much greater. The effective ratio is 14:1 and corresponds to a 23-decibel gain.

• The inner ear transduces sound waves of inner ear fluid into afferent impulses in the auditory nerve by stimulation of the hair cells in the cochlea.

Physiology:

• Motion of stapes sets perilymphatic fluid in vestibule into motion.
• Vibrations travel through Reissner's membrane to endolymphatic fluid, and produce a traveling wave in the basilar membrane.
• As fluid is non-compressible, there must be another mobile wall to permit vibration. Round window serves this function.

Hair cells transform mechanical energy to electrical energy.

• Intensity coding: Perceived "loudness" is a function of the number of auditory nerve fibers firing and their discharge rate
• Frequency coding:
• Place coding - hair cells at maximal displacement of basilar membrane are maximally stimulated.

• Volley coding - hair cells fire at same frequency as sound
• Telephone place coding - currently most popular theory: Low-frequency sounds are volley coded; high-frequency, place coded; and at mid-frequency, both mechanisms are operative.
• Impulses transmitted to brain via acoustic nerve with projections to both sides
• Central perception and interpretation.

II. Evaluation of Hearing Loss

• History
  • Age of patient
  • Severity of loss
  • Duration
  • Onset - rapid vs. gradual (sudden hearing loss is an emergency),constant vs. fluctuating.
  • Precipitating or exacerbating factors: trauma, noise, drugs,prenatal infection, etc.
  • Associated symptoms: Vertigo, tinnitus, pain or fullness in the ear, headache
  • Family history
• Physical - emphasize the following:
  • Otologic exam
    • Systematic otoscopy
    • Tuning forks to grossly assess hearing and to differentiate conductive vs. sensorineural
  • Exam of nasopharynx
  • Neurologic exam
  • Inspection, palpation, and auscultation of neck
  • Look for associated anomalies
• Tests
  • Basic audiogram - all patients
  • Diagnostic audio, ENG, internal auditory canal, x-rays, and/or CT scan if vertigo present or neural lesion suspected
  • Electrocochleography if Meniere's suspected
  • Appropriate blood tests. All patients with sensorineural hearing loss should have VDRL and FTA-ABS.

V. Differential Diagnosis

• External ear:
  • Cerumen impaction
    • One of commonest causes of sudden hearing loss
    • Treat by removing wax
  • External otitis - inflammation and swelling of canal skin
  • Tumors of external canal
  • Congenital aural atresia
• Middle ear - some further discussion in chapter on external and middle ear disorders
  • Otitis media
    • Acute - infectious or serous
    • Chronic - serous
    • Must always rule out possibility of nasopharyngeal carcinoma
  • Tympanic membrane perforation or cholesteatoma
  • Normal tympanic membrane with conductive hearing loss - suspect ossicular abnormality: otosclerosis, ossicular dislocation, etc.
• Sensorineural hearing loss - often associated with poor discrimination out of proportion to degree of pure tone sensitivity loss - this is due to distortion of sound by cochlea or nerve
  • Congenital
    • Hereditary
      • Isolated sensorineural hearing loss
        • Normal inner ear
        • Abnormal inner ear (Scheibe, Mondini-Michelle, etc.)
      • Hearing loss with associated anomalies
    • Acquired
      • Prenatal infection, especially syphilis, rubella, CMV
      • Prenatal drugs
      • Birth trauma
      • Developmental anomaly
    • Hereditary but delayed onset
      • Dominant or recessive
      • Numerous syndromes, some with associated anomalies (example: Waardenburg's syndrome with white forelock, hypertelorism, etc.)
  • Acquired
    • Noise induced - very common
      • Due to single blast or repeated or prolonged exposure to loud noise (hunting, rock music)
      • Affects high frequencies first (4 kHz); often progressive
      • Frequently associated with tinnitus
      • No known treatment. Counsel patient to avoid noise in future
      • PREVENTION is key to reducing incidence
    • Presbycusis - hearing loss of old age
      • Not universal, etiology not known
      • Central interpretation deficit complicates peripheral sensitivity loss
      • No known cure
      • Amplification can help, but hearing aids must be carefully fitted Cochlear distortion and central processing may preclude us
    • Head trauma - temporal bone fracture, labyrinthine concussion,central damage
    • Meniere's disease or syndrome
      • Fluctuating hearing loss
      • Characteristically associated with bouts of vertigo
      • Anatomically correlated with endolymphatic hydrops
      • Electrocochleograph (ECOG) frequently shows elevated summating potential
      • ) In active phase, glycerol may improve hearing
      • Treatment
        • Medical: low-salt diet, diuretics, avoidance of caffeine,anti-vertigo medication, psychological support
        • Surgical: for selected patients with progressive disease
    • Luetic hearing loss (syphilis)
      • Usually a fluctuating hearing loss - may mimic Meniere's
      • Treponemas may remain in endolymph after eradicated from other sites in the body.
      • Treatment - steroids and antibiotics (penicillin)
    • Ototoxic drugs
      • Reversible: aspirin - associated with tinnitus
      • Permanent: aminoglycosides, anti-neoplastic drugs, etc.
      • Treat by prevention:
        • Careful monitoring of blood levels of toxic drugs
        • Monitor hearing and vestibular function
    • Oval or round window rupture
      • Sudden onset of hearing loss, usually fluctuating, often accompanied by vertigo. Definitive diagnosis can only be made by surgical exploration.
      • Usually associated with sudden pressure change: flying,Valsalva, scuba diving, sneeze, etc.; but may be idiopathic
      • Treatment - initially, bed rest for suspected patients. If no recovery, explore and repair leak if found
    • Idiopathic sudden sensorineural hearing loss
      • Sudden hearing loss with no apparent cause
      • Etiology obscure, could be viral, autoimmune, vascular,or allergic, to name a few suspected causes
      • Diagnostic evaluation - should be extensive to rule out other causes
      • Treatment - many therapies suggested - few are statistically proven except for bed rest and possibly 95% O2:5% CO2 inhalation and steroid therapies. Patient is usually admitted to the hospital for treatment.
    • Acoustic nerve tumor
      • Uncommon tumor. Usually arises in vestibular nerve and is schwannoma, or less often, neurilemmoma
      • Usually present with hearing loss. Progression of vestibular nerve involvement is so slow that it is not noticed by patient.
      • Characteristic audiometric results with abnormal acoustic reflex, poor discrimination, and/or abnormal ABR.
      • X-rays or CT show flaring of IAC in large tumors. Small tumors are seen with air contrast CT
      • ) Treatment - surgical excision
      • Infections
        • Viral infection
        • Bacterial infection - labyrinthitis, meningitis, etc.
      • Otosclerosis
        • Sensorineural hearing loss often seen in association with otosclerosis of foot plate, but occurrence of pure sensorineural hearing loss is controversial
        • Treatment with fluoride may be helpful
      • Surgical trauma
    • Central hearing loss - normal basic tone audiogram and impedance,impaired understanding and processing

V. SIGNIFICANCE OF HEARING LOSS

• Acquired in adulthood
  • Social and occupational handicap
  • Loss of monitoring of environmental warning sounds
  • Loss of pleasure of music, etc.
• Congenital
  • Severe language development handicap
  • Early recognition, prompt amplification, and/or special early management vital for normal development. Therefore, high risk infants should be screened.
• Acquired in childhood
  • Can cause language delay or learning problems
  • Audiometric evaluation is mandatory in all children with speech delay and/or learning problems
  • Frequent screening of school children is advised.