Amplification

Author Credentials 

Alexander Luryi, MD
Yale University School of Medicine, Div. of Otolaryngology
333 Cedar St. PO Box 208062
New Haven, CT 06511

Christopher Schutt, MD
Michigan Ear Institute 
30055 Northwestern Hwy
Farmington Hills, MI 48334

Module Summary

Hearing amplification devices are powerful tools that can significantly improve quality of life for patients who have mild to moderate hearing loss or bothersome tinnitus. Amplification technology has progressed significantly in recent decades and programming allows for compression and frequency-modulated amplification which can personalize devices. While hearing aids are versatile tools, careful patient selection and device customization are key to success.

Module Learning Objectives 
  1. Review the indications and contraindications for hearing amplification.
  2. Describe the basics of how hearing amplification devices work.
  3. Discuss the features of hearing aids other than amplification, including noise reduction, feedback reduction, acoustic compression, etc.
  4. Recognize different hearing aid styles and their respective advantages and disadvantages. 
  5. Cite metrics of success in hearing aid amplification. 

 

Anatomy

Learning Objectives 
  1. Review the anatomy and physiology of the auricle and external auditory canal, as relating to hearing aid devices and potential for conductive hearing loss. 
References 
  1. Kelly KE, Mohs DC. The external auditory canal. Anatomy and physiology. Otolaryngol Clin North Am 1996;29(5):725-39.

Pathogenesis

Learning Objectives 
  1. Recognize that presbycusis and noise-induced hearing loss are the most common causes of hearing loss in adults and disproportionately affect high-frequency hearing. 
  2. State that excessive noise exposure damages outer and inner hair cells in the Organ of Corti, likely via damage to rootlets of stereocilia.
  3. Explain that GJB2 mutations (encoding protein connexin 26) are the most common cause of congenital hearing loss and hearing loss in children, and that congenital cytomegalovirus is the most common infectious cause of congenital hearing loss. 

 

References 
  1. Korver AM, Smith RJ, Van Camp G, et al. Congenital hearing loss. Nat Rev Dis Primers 2017; 3:16094.
  2. Kurabi A, Keithley EM, Housley GD, Ryan AF, Wong AC. Cellular mechanisms of noise-induced hearing loss. Hear Res 2017; 349:129-137

 

Basic Science

Learning Objectives 

  1. Recognize the measurement of differences in sound intensity using decibels, dictated by the equation:

where dB is decibels, J is the intensity of the sound of interest (typically expressed in watts per meter squared), and J­r is the intensity of a reference sound. Jr is equal to 10-12 W/m2 (a typical threshold for noise detection by the healthy human ear) by convention.

  1. Review that sound pressure level (SPL) is a local change in atmospheric pressure caused by a sound wave and is related to the square root of sound intensity: 
  2. Explain the importance of binaural hearing in sound localization.
  • Interaural time difference, or time between sounds reaching each of two ears, is important for localization of low frequency sound.
  • Interaural intensity difference is important for localization of high frequency sound.  
  • The “head shadow effect” is the decrease in sound intensity due to travel through the head to reach the far ear and is more pronounced in high frequency sounds. 
  1. State the importance of binaural hearing. 
  • Binaural squelch is the ability of the auditory system to filter background noise in noisy environments.
  • Binaural summation is a phenomenon in which a stimulus presented to both ears is interpreted as louder than one presented to a single ear.

 

References 
  1. Avan P, Giraudet F, Buki B. Importance of binaural hearing. Audiol Neurootol 2015;20 Suppl 1:3-6.
  2. Kileny PR, Zwolan TA. Diagnostic Audiology. In: Flint PW, Haughey BH, Lund V, Niparko JK, Robbins KT, Thomas JR, et al., editors. Cummings Otolaryngology. Philadelphia, PA: Saunders; 2015. p. 2051-2070.

 

Incidence

Learning Objectives 
  1. Review the prevalence of hearing loss in children and adults, and that only approximately 10-20% of adults with a hearing loss of 25 dB or greater use hearing aid devices. 
References 

Chien W, Lin FR. Prevalence of hearing aid use among older adults in the United States. Arch Intern Med 2012;172(3):292-3.

Patient Evaluation

Learning Objectives 
  1. Recognize that no firm guidelines exist for use of hearing aids, and evaluation of patient candidacy depends on multiple factors. 
  • Degree and configuration of hearing loss, extent of communicative difficulty, patient motivation, and attitude towards hearing aids are all important factors.
  • The vast majority of patients with fitted for amplification have a sensorineural or mixed hearing loss, although hearing aids may be appropriate in some cases of conductive hearing loss.
  1. Identify hearing loss factors which influence hearing aid decision-making.
  • Mild to moderate hearing loss is more easily treated with amplification, while maximal amplification may be insufficient for severe or profound hearing loss.
  • Patients with poor speech discrimination and/or retrocochlear pathology are usually poor candidates for amplification.
  • Slope of audiometric configuration: flat vs. high frequency vs. other hearing loss. Sharply sloping high frequency hearing loss may be difficult to correct with amplification.
  • Fluctuating hearing loss does not preclude amplification but requires a device with flexible programming.
  • Microtia or other anatomic abnormalities may limit amplification options.
  • Patients with poor supra-threshold speech discrimination, suggestive of cochlear or retrocochlear pathology, may derive less benefit from amplification, particularly in ability to communicate.
  1. Discuss evaluation of the extent of patients’ communicative difficulty.
  • Patients with slow-onset hearing loss often develop mechanisms for compensation.
  • Other patient factors, including occupation and degree of anxiety, also play a role.
  • Largely a subjective determination.
  1. Recognize the importance of patient motivation and attitudes toward amplification
  • Stigma for hearing aids exists; success relies on patient motivation towards hearing and amplification

 

References 
  1. Stach BA, Loiselle LH, Jerger JF. Special hearing aid considerations in elderly patients with auditory processing disorders. Ear Hear 1991;12(6 Suppl):131S-138S.
  2. Wilson C, Stephens D. Reasons for referral and attitudes toward hearing aids: do they affect outcome? Clin Otolaryngol Allied Sci 2003;28(2):81-4.

 

Measurement of Functional Status

Learning Objectives 
  1. Define how to interpret audiologic data.
  2. Recognize that success in hearing amplification is often subjective, defined as reaching the goal of reducing or eliminating a patient’s communicative disorder. 
  3. Gain awareness of validated self-assessment tools for hearing, including the Hearing Handicap Inventory for the Elderly, the Abbreviated Profile of Hearing Aid Benefit, and the International Outcome Inventory-Hearing Aids. 
  4. Indicate that hearing amplification is an effective treatment for bothersome tinnitus associated with hearing loss.
  • Can be used in conjunction with tinnitus retraining therapy or ear-level masking.

 

References 
  1. Gatehouse S. Glasgow hearing aid benefit profile: derivation and validation of. J Am Acad Audiol 1999;10(80):103.
  2. Cox RM, Alexander GC. The International Outcome Inventory for Hearing Aids (IOI-HA): psychometric properties of the English version. Int J Audiol 2002;41(1):30-5.
  3. Dillon H, James A, Ginis J. Client Oriented Scale of Improvement (COSI) and its relationship to several other measures of benefit and satisfaction provided by hearing aids. J Am Acad Audiol 1997;8(1):27-43.

 

Imaging

Learning Objectives 
  1. Realize that imaging studies have limited utility in hearing amplification. Imaging may be necessary to rule out treatable middle ear pathology, or in cases of asymmetric or otherwise atypical hearing loss without a clear diagnosis.
References 

None. 

Treatment

Learning Objectives 
  1. Explain that a hearing aid consists of a microphone, an amplifier, and a speaker. The microphone records ambient sound, the amplifier makes this sound louder according to programming, and the speaker plays the amplified sound into the ear canal.
  2. Cite different types of hearing aids that are currently available, and characteristics by which they can be distinguished.
  • Styles
    • Behind-the-Ear: microphone and amplifier sit behind the ear, connected via tube to a speaker sitting in the ear canal.
      • May be contraindicated in aural atresia or other deformities.
    • In-the-Ear: All components in custom-fitting case sitting in ear canal.
      • Occlusion of ear canal can lead to problems such as cerumen impaction or otitis externa.
    • Venting: Placement of a passageway for exchange of air and sound around or through an in-the-ear hearing aid mold
      • Improves aeration and reduces risk of otitis externa.
      • Can manipulate hearing aid response; for example, low-frequency amplification can be reduced or eliminated by venting.
      • However, leads to increased acoustic feedback (see feedback reduction, below).
  • Programming
    • Target gain: how much to amplify based on hearing loss
      • Varies by frequency, so patients with high-frequency hearing loss can have greater amplification at higher frequencies.
    • Noise reduction circuitry
      • Reduces / does not amplify constant background noise to provide better discrimination.
    • Compression: Selectively amplifying quiet sounds and not loud sounds.
      • “Compresses” a wide range of sound intensity into a narrower range accessible but comfortable to the hearing impaired ear.
    • Feedback reduction: Programming does not amplify sound produced by the device speaker itself (prevents feedback loop).
  • Patients with asymmetric hearing loss
    • Monaural amplification: For patients with hearing loss in one ear
      • Helpful for speech recognition and sound localization; generally indicated if possible.
    • Contralateral routing of signals (CROS): For patients with hearing loss in one ear which cannot be amplified.
      • Microphone in poorer hearing ear connected to speaker in healthy ear.
    • Bilateral contralateral routing of signals (BiCROS): For patients with bilateral hearing loss with one ear which cannot be amplified.
      • Microphone in poorer hearing ear connected to amplifier and speaker in healthy ear; microphone in healthy ear also amplified to speaker in the same ear.
    • CROS and BiCROS aids work best in quiet environments; in noisy environments, they may simply amplify ambient sound to the healthy ear and worsen overall function.
    • Bone-anchored hearing aids are a potential alternative to CROS and BiCROS systems.
References 
  1. Bishop CE, Eby TL. The current status of audiologic rehabilitation for profound unilateral sensorineural hearing loss. Laryngoscope 2010;120(3):552-6.
  2. Cook JA, Hawkins DB. Hearing loss and hearing aid treatment options. Mayo Clin Proc 2006;81(2):234-7.
  3. Hill SL, 3rd, Marcus A, Digges EN, Gillman N, Silverstein H. Assessment of patient satisfaction with various configurations of digital CROS and BiCROS hearing aids. Ear Nose Throat J 2006;85(7):427-30, 442.
  4. Plomp R. Noise, amplification, and compression: considerations of three main issues in hearing aid design. Ear Hear 1994;15(1):2-12.

 

Case Studies

  1. A 74-year-old gentleman presents to your clinic noting difficulty with conversations in noisy environments, such as during dinner at restaurants with his family. He inquires about hearing amplification. What is the most likely pathophysiology of his complaint? What further information do you wish to elicit? What diagnostic studies would be helpful?
  2. A 45-year-old woman presents to you for follow up for worsening hearing. She had a right acoustic neuroma removed through the translabyrinthine approach 15 years ago and has no remaining hearing in that ear. She reports significant difficulty understanding conversations in noisy environments and occasionally in quiet environments when her family is speaking quickly. An audiogram reveals no function in the right ear, and a moderate sensorineural hearing loss in the left ear. What is the best treatment option for management of her hearing dysfunction? What type of hearing aid configuration would provide her with the most benefit? 

 

References 
  1. What are the most common causes of hearing loss in children and adults?
  2. What factors are important to elicit in history for a patient with hearing loss who is considering amplification?
  3. What are target gain, compression, and noise reduction circuitry and how do they help optimize hearing with amplification?
  4. What are options for amplification in patients with asymmetric hearing loss or one non-hearing or non-amplifiable ear?
  5. How is success of hearing amplification measured?

 

Resources 

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