TSH.8 Hearing Impairment and Hearing Aids_updated

Auditory Localisation

Definition

  • Process of determining the source of a sound.

  • Utilizes both monaural (one-eared) and binaural (two-eared) cues.

Mechanisms

  • Sound reaches one ear before the other, indicating direction (interaural time differences - ITD).

Interaural Time Differences (ITD)

  • Specifically used for localising sound by recognizing time differences in sound arrival at each ear.

  • The sound has a different phase in each ear.

Interaural Level Differences (ILD)

  • Sound localisation that depends on the sound's intensity; the head creates an “acoustic shadow.”

  • The intensity decreases depending on the direction of the sound source.

Frequency Dependence of Auditory Localisation

  • ITD and ILD effectuality varies with frequency:

    • Below 1000 Hz: ILD is ineffective due to wavelength size.

    • Above 4000 Hz: ITD is not well-defined anymore.

Pinna Direction Filtering

  • The pinna amplifies sounds differently from above and below, aiding in identifying elevation of sounds.

Audiometry Overview

  • Hearing function and loss measured in units of dBHL (decibel hearing level).

  • Audiometer produces audiogram; tones sent through headphones at increasing volume.

Audiometry Procedure

  • A specific frequency (e.g., 125 Hz) is selected and intensity increased until detection.

  • Threshold of hearing defined as the intensity at which sound is detected expressed in dBHL.

  • Procedure repeated across various frequencies to create an audiogram.

Audiogram

  • Displays threshold of audibility at different frequencies with hearing loss shown further down the chart.

  • Y-axis is upside down to represent hearing loss clearly.

Hearing Impairment

Causes of Hearing Loss

  1. Conductive Path (outer & middle ear) issues

  2. Cochlea/inner ear (sensorineural) issues

Conductive Hearing Loss

  • Result from disorders in the outer/middle ear.

  • Sounds may be faint or distorted.

  • Tested via air conduction method.

Common Causes of Conductive Hearing Loss

  • Ear infections, wax build-up, foreign objects, fluid accumulation, eardrum injuries, ossicle dislocation, tumors.

Bone Conduction

  • Sound conducted into cochlea through bone in the skull, bypassing outer ear.

Sensorineural Hearing Loss

  • Hearing loss testing can’t localize problems effectively using air conduction alone; requires testing using bone conduction.

Distinction Between Hearing Loss Types

  • Conductive Hearing Loss: Damage to outer/middle ear.

  • Sensorineural Hearing Loss: Damage to inner ear.

  • Both tests identify the source and extent of hearing impairment.

Audiogram Markers

  • Different symbols indicate air conduction (x,o) and bone conduction ([,] or <,>) on audiograms.

Sensorineural Hearing Loss: Presbycusis

  • Age-related hearing loss primarily affecting high frequencies.

Other Causes of Sensorineural Hearing Loss

  • Causes: injury, noise exposure, viral infections, ototoxic drugs, meningitis, stroke, high fever, acoustic tumors, heredity, diabetes.

Mixed Hearing Loss

  • Combination of conductive and sensorineural components.

Hearing Aids

Indications

  • Required for significant loss (>55 dBHL) in conversation frequency ranges (100-3000 Hz).

  • Care must be taken when designing hearing aids in 120-130 dBHL

Types

  • Deliver sound via ear canal, acting as amplifiers.

Surgical Options

  • Bone anchored hearing aids, cochlear implants.

Personalisation of Hearing Aids

  • Digital electronics allow tailoring of frequency response to the individual user's needs.