Audiometry

Audiometry

Audiometry is a diagnostic test used to evaluate hearing ability and detect hearing loss. It is a cornerstone of otological assessment and an essential tool for diagnosing conditions like sensorineural, conductive, or mixed hearing loss.

Types of Audiometry

1. Pure Tone Audiometry (PTA):
   • Assesses hearing sensitivity across a range of frequencies.
   • Differentiates between conductive and sensorineural hearing loss using air conduction and bone conduction thresholds.
2. Speech Audiometry:
   • Evaluates the ability to hear and understand speech.
   • Tests include speech recognition threshold (SRT) and word recognition score (WRS).
3. Tympanometry:
   • Assesses middle ear function by measuring tympanic membrane compliance in response to air pressure changes.
4. Otoacoustic Emissions (OAEs):
   • Detects sound waves produced by the outer hair cells of the cochlea.
   • Useful for newborn hearing screening and early detection of cochlear dysfunction.
5. Auditory Brainstem Response (ABR):
   • Measures neural activity in the auditory pathway in response to sound stimuli.
   • Used for diagnosing retrocochlear pathologies like acoustic neuroma.

Indications

1. Screening:
   • Routine newborn hearing screening.
   • Occupational screening for noise-induced hearing loss.
2. Diagnostic:
   • Unilateral or bilateral hearing loss.
   • Tinnitus.
   • Balance disorders.
   • Monitoring ototoxicity in patients on ototoxic medications (e.g., aminoglycosides).
3. Pre- and Post-Treatment Assessment:
   • Hearing improvement after hearing aids or cochlear implants.
   • Baseline and follow-up for surgical procedures (e.g., tympanoplasty).

Procedure

1. Pure Tone Audiometry:
   • The patient is placed in a soundproof room and wears headphones.
   • Pure tones at varying frequencies (250 Hz–8,000 Hz) are delivered through air conduction (headphones) and bone conduction (bone oscillator).
   • Thresholds are plotted on an audiogram to determine hearing levels.
2. Speech Audiometry:
   • Words or sentences are presented at different intensities.
   • The patient repeats back the words to determine their understanding.
3. Impedance Audiometry (Tympanometry):
   • A probe is inserted into the ear canal, and the tympanic membrane’s response to pressure changes is measured.

Interpretation

1. Pure Tone Audiogram:
   • Normal Hearing: Thresholds ≤25 dB across all frequencies.
   • Conductive Hearing Loss:
     • Air-bone gap >10 dB with normal bone conduction thresholds.
     • Causes: otitis media, otosclerosis, cerumen impaction.
   • Sensorineural Hearing Loss:
     • No air-bone gap with elevated thresholds for both air and bone conduction.
     • Causes: presbycusis, noise exposure, acoustic neuroma.
   • Mixed Hearing Loss:
     • Air-bone gap with elevated thresholds for both air and bone conduction.
2. Speech Audiometry:
   • Reduced speech discrimination scores suggest retrocochlear pathology (e.g., acoustic neuroma).
3. Tympanometry:
   • Type A: Normal middle ear function.
   • Type B: Flat curve, suggesting fluid in the middle ear or perforation.
   • Type C: Negative pressure, indicating eustachian tube dysfunction.

Common Findings in Specific Conditions

• Presbycusis: Bilateral high-frequency sensorineural hearing loss.
• Noise-Induced Hearing Loss: “Notch” at 3,000–6,000 Hz on the audiogram.
• Acoustic Neuroma: Unilateral sensorineural hearing loss with reduced speech discrimination.

Limitations

• Requires patient cooperation; not suitable for infants or uncooperative individuals.
• Cannot differentiate specific causes of sensorineural hearing loss.

Complications

• None directly associated with audiometry.
• Misinterpretation may lead to missed diagnoses (e.g., retrocochlear pathologies).

Key Points for GP Exams

• Initial Test: PTA is the most common first-line investigation for hearing loss.
• Referral: Patients with red flags (e.g., asymmetric hearing loss, sudden hearing loss, poor speech discrimination) should be referred for further evaluation.
• Diagnostic Value: Essential for identifying conductive vs. sensorineural hearing loss, aiding in conditions like otitis media, Meniere’s disease, and acoustic neuroma.
• Integration: Use in combination with history, examination, and imaging for comprehensive otological assessment.