Ototoxicity

1. 0:00Ototoxicity is the most preventable cause of bilateral vestibulopathy. The mechanism is identical — bilateral, symmetric, hair-cell-level VOR loss — but the cause is iatrogenic, dose-related, and visible if you look for it. Aminoglycoside antibiotics are the principal offenders. Platinum-based chemotherapy is a growing second source. DVA is one of the surveillance tools that catches the vestibular deficit before it becomes catastrophic.
2. 0:35Aminoglycosides cross into the inner ear and concentrate in vestibular hair cells through the mechanotransduction channels. Type I hair cells in the central crista of each semicircular canal are most vulnerable. Type II cells and the otolith organs are relatively spared. The damage starts within days of exposure, accumulates with cumulative dose, and is usually permanent — hair cells do not regenerate in humans.
3. 1:15Among the aminoglycosides, gentamicin is the most vestibulotoxic agent in widest clinical use, followed by streptomycin and tobramycin. Amikacin and kanamycin are preferentially cochleotoxic rather than vestibulotoxic. This matters because audiometric monitoring alone — which detects cochleotoxicity — completely misses gentamicin vestibulotoxicity. About 90% of patients with gentamicin vestibulotoxicity have no measurable hearing loss.
4. 1:55Clinically the presentation is identical to bilateral vestibulopathy from any other cause. Symmetric oscillopsia with head motion, ataxia worsening in darkness and on uneven ground, no rotational vertigo because both sides are equally affected, no spontaneous nystagmus. The DVA pattern is the same: symmetric loss often greater than 0.4 logMAR, no directional asymmetry, profoundly impaired bedside head-impulse testing.
5. 2:30The surveillance argument is what makes this chapter different. If you screen DVA and bedside head-impulse testing at baseline and at weekly intervals during treatment, you can stop the drug before the damage becomes complete. The dynamic-illegible-E test takes two minutes at the bedside, requires only a Snellen chart, and has been recommended in the Canadian Medical Association Journal practice review for hospital surveillance of patients on aminoglycoside therapy. Most institutions do not currently do this, which is why this chapter exists.
6. 3:05Three practical points. First, treatment duration over seven days, renal impairment, and elevated trough levels are the major risk factors — but vestibulotoxicity occurs at normal trough levels and normal renal function in some patients. Surveillance is not optional for at-risk patients. Second, once detected, stopping the drug is the only effective intervention; vestibular function rarely recovers. Third, DVA-tracked rehabilitation works the same way as in other bilateral vestibulopathies — central compensation through covert saccades. Refer early.

What is ototoxicity?

Ototoxicity is drug- or chemical-induced damage to the inner ear, affecting the cochlea (cochleotoxicity, causing hearing loss), the vestibular system (vestibulotoxicity, causing imbalance and oscillopsia), or both. Aminoglycoside antibiotics are the principal cause of vestibulotoxicity in clinical practice; platinum-based chemotherapy is the second major class. Loop diuretics, salicylates, quinine, and several other agents can contribute, usually reversibly.^43,45

The clinical picture of established aminoglycoside vestibulotoxicity is indistinguishable from bilateral vestibulopathy of any other cause — and indeed, aminoglycoside ototoxicity is the most commonly identified single cause of bilateral vestibulopathy in clinical practice.^26,44 What makes ototoxicity worth its own chapter is the surveillance argument: the damage develops over days to weeks, and if anyone is looking, drug discontinuation during the surveillance window can prevent the catastrophe.^43,45

Agents and mechanism

Aminoglycosides differ in their preferential target within the inner ear:⁴³

• Predominantly vestibulotoxic: gentamicin, streptomycin, tobramycin, netilmicin. Gentamicin is the most vestibulotoxic agent in widest clinical use.
• Predominantly cochleotoxic: amikacin, kanamycin, neomycin, dihydrostreptomycin. These cause hearing loss with relative vestibular sparing.

The mechanism is well characterised. Aminoglycosides enter vestibular hair cells via the mechanotransduction channels at the stereocilia tips — the same channels used in normal vestibular signalling. Once inside, the drug accumulates and damages the cell through oxidative stress, mitochondrial dysfunction, and apoptosis. Type I hair cells in the central crista of each semicircular canal are the first and most severely affected; type II cells and the otolith organs are relatively spared. Damage is dose-dependent and cumulative; once established, it is essentially permanent because human vestibular hair cells do not meaningfully regenerate.⁴³

Platinum chemotherapy

Cisplatin and related platinum-based chemotherapeutic agents have a well-established cochleotoxic profile (high-frequency sensorineural hearing loss in a high fraction of patients) and a less well-characterised but increasingly recognised vestibulotoxic profile. Animal studies show preferential utricular hair-cell loss with cisplatin, contrasting with the canal-crista predilection of aminoglycosides. Human data are heterogeneous, with abnormal vestibular testing reported in 0–50% of patients depending on assessment protocol and cumulative dose; pre-existing vestibular loss increases the risk of further vestibulotoxicity from cisplatin.

Risk factors

• Treatment duration ≥ 7 days is a key threshold. Beyond this, vestibulotoxicity risk rises substantially.⁴⁴
• Renal impairment — slows drug clearance, raises steady-state levels.
• Elevated trough drug levels — though vestibulotoxicity can develop at normal trough levels too.
• Older age, pre-existing hearing loss,concurrent loop diuretic use, previous aminoglycoside exposure.
• Genetic susceptibility— the mitochondrial m.1555A>G variant predisposes to aminoglycoside-induced hearing loss; vestibular equivalents are less well-characterised but similar mechanisms are suspected.

The surveillance workflow

The clinical case for surveillance is straightforward: drug discontinuation during the early surveillance window can limit the final deficit, while detection only after symptom onset usually means established bilateral vestibulopathy. The Canadian Medical Association Journal practice review and the 2018 vestibulotoxicity strategies paper both recommend bedside DVA as a simple, institutional surveillance tool requiring only a Snellen chart and two minutes of clinical time.⁴⁵

A pragmatic surveillance protocol:

1. Baseline (before first dose): bedside DVA, bedside head-impulse test, pure-tone audiogram. Record absolute values, not just "normal."
2. During treatment, days 5 onwards: repeat bedside DVA every 3–4 days. Flag any deterioration of ≥0.2 logMAR (two chart lines) from baseline as a surveillance threshold event.
3. Threshold event triggers: formal vHIT and caloric testing if available, discussion with prescribing team about drug substitution or dose adjustment, and ENT/vestibular referral.
4. Post-treatment (≥6 weeks after final dose): repeat DVA, vHIT, caloric, audiogram for documentation of final deficit and rehabilitation planning.

The DVA pattern in detail

Established aminoglycoside vestibulotoxicity produces the same DVA signature as bilateral vestibulopathy of any other cause — symmetric loss without directional asymmetry, profound impairment of the bedside head-impulse test, near-normal subjective visual vertical. The distinctive feature is not the pattern but the trajectory:

• Days 0–4: DVA typically unchanged from baseline. Hair-cell uptake of the drug is occurring but functional consequence is below the bedside threshold.
• Days 5–14 — the surveillance window: DVA begins to deteriorate, typically asymmetrically at first as the cristae become differentially affected. Loss accelerates around day 7–10. Drug discontinuation in this window limits the final deficit substantially.⁴³
• Days 14–28: the pattern becomes profoundly bilateral. Patient symptoms — oscillopsia, unsteadiness — typically emerge in this window if surveillance has not detected the problem earlier. By this point much of the damage is done.
• Beyond day 28: the deficit plateaus and becomes essentially permanent. Drug cessation prevents further accumulation but does not reverse what has already happened.

What head-impulse saccades look like in vestibulotoxicity

A practical observation worth knowing: bedside head-impulse testing in established aminoglycoside vestibulotoxicity produces unusually large overt saccades — substantially larger than in normal subjects or even in some unilateral vestibulopathies. The cumulative amplitude of overt catch-up saccades has been measured at approximately 5.6 times greater than in healthy controls. Covert saccades, which can mask the bilateral deficit during vHIT interpretation, are only about half as common as in unilateral patients because there is no remaining functional side to provide predictable input.⁴³ Practically, this means the bedside head-impulse test in this population is one of the easier peripheral vestibular signs to observe — the saccades are large and unmissable.

Management once detected

• Stop the offending agent if at all clinically feasible. Discuss with the prescribing team — many infections can be managed with a non-aminoglycoside regimen with comparable efficacy.^43,45
• Refer to vestibular rehabilitation early. DVA-tracked gaze-stabilisation exercises drive the same compensatory-saccade development as in bilateral vestibulopathy of any aetiology — the Herdman 2007 evidence applies directly.²⁵
• Document the deficit formally with vHIT and calorics post-treatment, both for clinical management and because medico-legal documentation can be relevant.⁴⁴
• Address occupational and fall-risk implications. Driving, working at height, working with moving machinery, and walking outdoors in darkness all become higher-risk activities.
• Genetic counsellingfor the m.1555A>G variant and first-degree relatives, particularly if the patient developed ototoxicity at unexpectedly low cumulative doses.

Differential diagnosis

Once vestibulotoxicity is established the picture is identical to bilateral vestibulopathy of any other cause. The clinical task is attribution rather than discrimination:

• Pre-existing bilateral vestibulopathy — the patient may have had subclinical loss before aminoglycoside exposure. Baseline DVA documentation is what distinguishes ototoxicity-caused loss from ototoxicity-revealed loss.
• Presbyvestibulopathy — older patients on aminoglycoside therapy may have age-related decline contributing to baseline thresholds.⁴¹
• CANVAS — the cerebellar-ataxia-neuropathy- vestibular-areflexia syndrome can mimic aminoglycoside vestibulotoxicity. Look for cerebellar oculomotor signs and chronic cough.²⁶
• Other ototoxic agents in combination — patients on multiple potentially ototoxic drugs (aminoglycoside + loop diuretic, aminoglycoside + cisplatin) carry additive risk.

Reading the report

A patient who developed bilateral vestibular loss during or after systemic aminoglycoside or platinum-based chemotherapy exposure, with vHIT gain < 0.6 bilaterally and/or caloric sum < 6 °/s per side, meets the Bárány criteria for bilateral vestibulopathy attributed to ototoxicity.^24,43 The DVA finding quantifies the functional consequence and provides the right outcome measure for both initial documentation and rehabilitation tracking. The single most useful clinical move at the time of diagnosis is the audit question: could this have been caught earlier with surveillance?⁴⁵