D6 High Frequency Audiometry and Ototoxicity Monitor

Topic

An ototoxic drug—such as the chemotherapy agent cisplatin, aminoglycoside antibiotics, or the diuretic furosemide—circulating in the bloodstream of a patient undergoing treatment also reaches the cochlea. Inside the cochlea, it preferentially damages the outer hair cells at the cochlear base—the end of the cochlea closest to the oval window—before affecting outer hair cells in any other region. As exposure to the ototoxic drug continues, the damage that began at the cochlear base gradually progresses toward the opposite end of the cochlea, moving away from the base with each subsequent dose.

Since the cochlear base is the region that responds to the highest-pitched sounds, the damage an ototoxic drug initially causes there manifests first as a loss of the highest frequencies the cochlea can process—frequencies well above those required to understand everyday speech. Conventional audiometry, the standard test for measuring hearing, assesses a frequency range that covers speech-relevant frequencies but does not extend to the ultra-high frequencies where ototoxic damage first appears; consequently, a conventional audiogram fails to detect such damage as long as it remains confined to the cochlear base.

High-frequency audiometry extends the tested frequency range far beyond the limits of conventional audiometry—reaching frequencies several times higher than the upper limit of standard tests—thereby covering the specific frequency range associated with the cochlear base. By measuring the hearing threshold at these higher frequencies prior to the start of ototoxic drug treatment—and re-measuring periodically during treatment—high-frequency audiometry can detect damage to the cochlear base while it is still confined to the highest frequencies, well before it progresses to affect the frequencies assessed by conventional audiograms.

To determine whether a change observed between the initial and subsequent high-frequency threshold measurements indicates actual ototoxic damage—rather than mere normal measurement variation—a "significant change" criterion is applied: a result is considered positive if the hearing threshold rises by at least twenty decibels at a single frequency, or by at least ten decibels at two or more distinct frequencies. This criterion requires either a substantial shift at one frequency or a more moderate yet repeated shift across several frequencies; this is because a large, isolated change at a single frequency could be due to measurement error or chance, whereas a smaller change consistently repeated across multiple frequencies is unlikely to be attributable to chance alone.

When high-frequency audiometry detects that the significant change criterion has been met at the cochlear base of a patient receiving an ototoxic drug, the medical team still has an opportunity to adjust the dosage or switch medications before the damage spreads from the cochlear base to the frequencies measured by conventional audiograms—frequencies that are crucial for understanding everyday speech. Thus, this high-frequency audiometry model transforms what would otherwise be a silent, undetectable onset of hearing loss at the cochlear base into an early warning, allowing for the protection of speech-range frequencies before the ototoxic drug affects them.

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