The Integrative Nature of Audiological Assessment
The assessment of the human auditory system is a complex intersection of acoustics, neurophysiology, and behavioral psychology. It goes beyond simple sound detection to evaluate a sensory system extending from the external pinna through the middle ear, cochlea, and neural pathways to the auditory cortex.
The clinical mandate is to determine not only the degree of hearing loss but also its site of lesion—differentiating conductive pathologies from sensory deficits and retrocochlear abnormalities. Audiologists employ a battery of tests that cross-check each other for diagnostic accuracy.
Pre-Test Foundations: Environment & Otoscopy
Reliable diagnostic data hinges on a precise test environment and the patient's physical ear status. Valid audiometric thresholds are inextricably linked to controlled acoustic conditions.
Acoustic Environment
Standard practice mandates sound-treated booths to dampen external noise. Ambient noise, especially low-frequency signals, can artificially elevate hearing thresholds.
Equipment Standards
Audiometers and transducers require rigorous quality control. Annual electroacoustic calibration is mandatory to verify signal frequency and intensity accuracy.
Otoscopy Findings & Interpretation
Otoscopy screens for conditions that invalidate audiometric results or require medical referral:
Cerumen Impaction
Excessive earwax can occlude the canal, causing temporary conductive hearing loss.
Infection & Inflammation
Edema, erythema, or drainage contraindicates the insertion of headphones or probes.
Structural Anomalies
Inspection for atresia, stenosis, or preauricular pits linked to syndromic hearing loss.
Middle Ear Pathology
TM appearance (retraction, perforation, fluid bubbles) provides clues for conductive loss.
Pure Tone Audiometry: The Gold Standard
Pure Tone Audiometry (PTA) quantifies auditory sensitivity by determining the “threshold”—the lowest intensity level at which a patient detects a stimulus 50% of the time.
Air vs. Bone Conduction
- Air Conduction (AC): Sound travels through the entire system (outer, middle, inner ear). Measures overall hearing sensitivity.
- Bone Conduction (BC): Bypasses outer/middle ear to stimulate the cochlea directly. Measures sensorineural reserve (inner ear potential).
Interpreting the Audiogram
The relationship between AC and BC thresholds determines the type of loss. The "Air-Bone Gap" (ABG) is the key diagnostic indicator.
1. Conductive Hearing Loss (CHL)
- AC: Elevated (>25 dB)
- BC: Normal (≤25 dB)
- ABG: Present (>10 dB)
- Implication: Problem in outer/middle ear (e.g., wax, fluid). Inner ear is healthy.
2. Sensorineural Hearing Loss (SNHL)
- AC: Elevated (>25 dB)
- BC: Elevated (within 10 dB of AC)
- ABG: Absent
- Implication: Problem in cochlea or neural pathway (e.g., aging, noise).
3. Mixed Hearing Loss
- AC: Elevated
- BC: Elevated
- ABG: Present
- Implication: Combination of both issues (e.g., Presbycusis + Ear Infection).
Severity & Configuration
Hearing loss severity is classified based on AC thresholds, following ASHA guidelines. The colors below represent the "Audiometric Heatmap."
- Normal Hearing -10 to 15 dB
- Slight Loss (Pediatric) 16 to 25 dB
- Mild Loss 26 to 40 dB
- Moderate Loss 41 to 55 dB
- Moderately Severe 56 to 70 dB
- Severe Loss 71 to 90 dB
- Profound Loss 91+ dB
Pathological Patterns (Configuration)
The shape of the audiometric curve offers etiological insights:
📉 Presbycusis
Typically a sloping high-frequency SNHL, reflecting basal-to-apical cochlear degeneration due to aging.
📣 Noise-Induced
Characterized by a distinct "Notch" at 4000 Hz, often with recovery at 8000 Hz.
🦴 Otosclerosis
Bone growth fixing the stapes. Manifests as conductive loss with "Carhart’s Notch" (BC dip at 2000 Hz).
⚠️ Asymmetry
Significant difference between ears. A primary red flag for retrocochlear pathology (e.g., vestibular schwannoma).
Speech Audiometry
Pure tones measure sensitivity; speech audiometry measures communication ability.
SRT (Reception Threshold)
The softest level to repeat 50% of spondaic words (e.g., "baseball"). Used to cross-check the pure tone average (PTA). A mismatch suggests functional loss.
WRS (Recognition Score)
Percentage of single words understood at a comfortable volume. Measures clarity.
Conductive: Excellent scores (90%+)
Cochlear: Reduced scores (distortion)
Otoacoustic Emissions (OAE)
OAEs are low-intensity sounds generated by the movement of healthy Outer Hair Cells (OHCs) in the cochlea. They are objective and require no behavioral response.
Clinical Utility
- Screening: Primary tool for Universal Newborn Hearing Screening (UNHS).
- Cross-Check: Verifies behavioral results in difficult-to-test populations.
- Differential Diagnosis: OAEs are present in Neural/Auditory Neuropathy Spectrum Disorder (ANSD) but absent in cochlear hearing loss.
TEOAE (Transient Evoked)
Stimulated by a broadband click. Best for assessing cochlear function in the 500–4000 Hz range. Highly sensitive to mild hearing loss.
DPOAE (Distortion Product)
Stimulated by two simultaneous tones (f1 and f2). Allows for frequency-specific evaluation up to 8000 Hz or higher. More robust against mild background noise.
Auditory Brainstem Response (ABR)
The ABR is an electrophysiological test that measures neural synchrony from the auditory nerve to the brainstem upon sound stimulation.
Key Applications
- Threshold Estimation: Estimating hearing levels in infants or non-compliant adults (e.g., finding the "Wave V" threshold).
- Neurological Assessment: Detecting retrocochlear pathologies like acoustic neuromas by analyzing the latency (timing) between Wave I, III, and V.