Human Auditory System diagnostic method

Introduction : the biophysics of hearing

hearing is the transduction of mechanical pressure waves into electrical nerve impulses.

• Outer/Middle Ear: Conducts and amplifies sound (impedance matching).
• Inner Ear (Cochlea): Frequency analysis via the Basilar Membrane.
• Neural Pathway: Transmission via the vestibulocochlear nerve (CN VIII) to the auditory cortex.

Subjective vs. Objective diagnostics

Subjective methods

Pure Tone Audiometry (PTA): The "gold standard." It determines the hearing threshold across different frequencies (125 Hz to 8000 Hz).

• Air Conduction: Tests the entire auditory pathway.
• Bone Conduction: Bypasses the outer/middle ear by vibrating the skull, testing the cochlea directly.

Speech Audiometry: Measures the patient's ability to hear and understand spoken words, not just tones.

Objective methods

Tympanometry: Measures the "compliance" (mobility) of the eardrum as air pressure is changed in the ear canal. It detects fluid in the middle ear or ossicular chain issues.

Otoacoustic Emissions (OAE): Measures the "echo" produced by the Outer Hair Cells in the cochlea. This is the standard test for newborn hearing screening.

Auditory Brainstem Response (ABR/BERA): Uses electrodes to record the electrical activity of the auditory nerve and brainstem in response to sound clicks.

Interpreting results: conductive vs. Sensorineural

Conductive Hearing Loss: The "gap" exists. Bone conduction is normal, but air conduction is poor. (Cause: Earwax, fluid, or perforated eardrum).

Sensorineural Hearing Loss: Both air and bone conduction are equally poor. (Cause: Damage to the cochlear hair cells or CN VIII).

Mixed Hearing Loss: Both are impaired, but air conduction is significantly worse.

Advanced biophysical tests

Weber Test: Tuning fork on the midline of the forehead. Sound lateralizes to the affected ear in conductive loss.

Rinne Test: Compares bone conduction to air conduction. Normally, Air > Bone.

Sources

• Katz, J. Handbook of Clinical Audiology. (The definitive textbook for diagnostics).
• Guyton & Hall. Textbook of Medical Physiology. (Chapter on the Sense of Hearing).
• StatPearls. Audiologic Testing. https://www.ncbi.nlm.nih.gov/books/NBK535357/