Toxicology – Hearing Abnormalities

Definition

Toxic exposures and medications can produce auditory abnormalities, most commonly:

• Tinnitus – perception of ringing, buzzing, or other sound without an external source
• Hearing loss – partial or complete reduction in hearing

Drug-induced hearing loss is usually sensorineural, although conductive and central auditory disorders have other important causes.

Normal Hearing

Sound transmission occurs through:

Sound waves → tympanic membrane → ossicles → cochlear fluid movement → organ of Corti hair cells → cochlear nerve (CN VIII) → auditory cortex

Damage anywhere along this pathway can impair hearing.

Conductive vs. Sensorineural Hearing Loss

Conductive hearing loss

Results from impaired transmission through the:

• External auditory canal
• Tympanic membrane
• Middle ear

Examples include:

• Cerumen impaction
• Foreign body
• Otitis media
• Tympanic membrane injury
• Otosclerosis

Sensorineural hearing loss

Results from injury to:

• Cochlear hair cells
• Stria vascularis
• Cochlear nerve
• Central auditory pathways

Potential causes include:

• Ototoxic medications
• Excessive noise exposure
• Infection
• Aging
• Vascular disease
• Temporal bone injury
• Vestibular schwannoma
• Toxic chemical exposure

Major Ototoxic Drugs

Important medications associated with tinnitus or hearing impairment include:

Aminoglycosides

• Gentamicin
• Tobramycin
• Amikacin
• Streptomycin
• Neomycin

These can produce cochlear and/or vestibular toxicity.

Risk increases with:

• Higher cumulative exposure
• Renal dysfunction
• Concurrent ototoxic medications

Hearing injury can be irreversible.

Platinum Chemotherapy

Cisplatin is an important cause of:

• Bilateral sensorineural hearing loss
• High-frequency hearing impairment
• Tinnitus

Ototoxicity may be cumulative and permanent.

Salicylates

Salicylate toxicity commonly causes:

• Tinnitus
• Reduced hearing
• Nausea/vomiting
• Tachypnea
• Acid-base abnormalities

The auditory abnormalities are generally reversible after salicylate concentrations fall.

Key Point: Improvement or disappearance of tinnitus does not reliably prove that systemic salicylate toxicity has resolved.

Quinine and Quinidine

These may produce cinchonism, characterized by:

• Tinnitus
• Hearing impairment
• Headache
• Nausea
• Visual disturbances

Severe poisoning can additionally cause cardiovascular toxicity.

Loop Diuretics

Examples include:

• Furosemide
• Bumetanide
• Ethacrynic acid

Ototoxicity is particularly associated with:

• Rapid IV administration
• High exposure
• Renal dysfunction
• Concurrent ototoxic medications

Hearing impairment is often reversible but can occasionally persist.

Other Potentially Ototoxic Agents

Reported associations include:

• Macrolide antibiotics
• Chloroquine and related agents
• Deferoxamine
• Some NSAIDs
• Metronidazole
• Selected local anesthetics
• Certain industrial solvents
• Heavy metals

Not every reported association carries the same strength of evidence or risk.

Toxic Chemical Causes

Organic mercury

May produce:

• Hearing impairment
• Visual-field abnormalities
• Ataxia
• Dysarthria
• Tremor
• Sensory disturbances

Lead

Chronic exposure can produce neurologic abnormalities and has been associated with auditory dysfunction, particularly with significant or prolonged exposure.

Solvents

Some occupational solvents, including toluene, can contribute to sensorineural hearing impairment, particularly with chronic exposure and concurrent noise exposure.

Carbon Monoxide and Cyanide

Severe cellular hypoxia can injure auditory pathways.

Hearing abnormalities have occasionally been reported following recovery from severe:

• Carbon monoxide poisoning
• Cyanide poisoning

In these cases, auditory injury may become apparent after the initial life-threatening toxicity has resolved.

Bromate

Significant bromate poisoning can produce:

• Vomiting and diarrhea
• Acute kidney injury
• Sensorineural hearing loss

Hearing impairment may be permanent.

Clinical Assessment

Important history includes:

• Onset: sudden vs. gradual
• Unilateral vs. bilateral symptoms
• Tinnitus vs. hearing loss
• Medication exposure
• Recent overdose
• Occupational chemical exposure
• Noise exposure
• Renal impairment
• Ear infection or trauma
• Associated vestibular symptoms

Medication review is particularly important because multiple ototoxic drugs may have additive effects.

Associated Findings

Certain combinations provide diagnostic clues.

Tinnitus + tachypnea + vomiting + acid-base disturbance

→ consider salicylate toxicity

Hearing loss + renal injury during antimicrobial treatment

→ consider aminoglycoside toxicity

Hearing loss during cisplatin therapy

→ consider platinum-associated ototoxicity

Tinnitus + visual disturbances

→ consider quinine/quinidine toxicity

Hearing impairment + ataxia/tremor/visual abnormalities

→ consider organic mercury exposure

Physical Examination

Assess:

• External auditory canal
• Tympanic membrane
• Evidence of infection
• Cerumen or foreign body
• Neurologic examination
• Cranial nerves
• Vestibular findings when relevant

Bedside hearing tests can help distinguish conductive from sensorineural loss.

Weber Test

A vibrating tuning fork is placed on the midline of the skull.

Normal

• Sound is heard approximately equally in both ears.

Unilateral conductive hearing loss

• Sound lateralizes toward the affected ear.

Unilateral sensorineural hearing loss

• Sound lateralizes toward the better/unaffected ear.

Rinne Test

The test compares air conduction (AC) with bone conduction (BC).

Normally:

AC > BC

Conductive hearing loss

• Bone conduction becomes greater than air conduction in the affected ear.

Sensorineural hearing loss

• Air conduction generally remains greater than bone conduction, but overall hearing is reduced.

A 512-Hz tuning fork is generally preferred for modern bedside Weber and Rinne testing rather than the 256-Hz fork described in older references.

Audiologic Evaluation

Formal assessment may include:

• Pure-tone audiometry
• Speech audiometry
• Tympanometry
• Otoacoustic emissions
• Auditory brainstem response testing

Audiometry is especially useful for detecting and monitoring medication-associated ototoxicity.

Laboratory Evaluation

Testing should be directed toward the suspected cause.

Possible investigations include:

• Renal function
• Electrolytes
• Salicylate concentration
• Specific medication concentrations when clinically useful
• Heavy-metal testing when exposure is plausible
• Acid-base evaluation in systemic poisoning

Routine broad toxicology screening is usually less useful than targeted testing.

Imaging

Imaging is determined by the clinical pattern.

Unilateral or asymmetric sensorineural hearing loss

may warrant MRI evaluation for retrocochlear pathology such as a vestibular schwannoma.

Temporal-bone imaging may be appropriate when trauma or structural disease is suspected.

Sudden Sensorineural Hearing Loss

Sudden unexplained sensorineural hearing loss should be considered an otologic emergency.

Prompt ENT/audiology evaluation is important because time-sensitive treatment may be indicated for causes unrelated to poisoning.

Do not automatically attribute sudden hearing loss to a medication merely because the patient is taking a potentially ototoxic drug.

Management

Management centers on the underlying cause:

• Stop or modify the suspected ototoxic agent when medically appropriate.
• Treat the underlying poisoning.
• Correct renal or metabolic abnormalities.
• Avoid additional ototoxic exposures when possible.
• Obtain audiology/ENT assessment for persistent or significant abnormalities.

Medication changes should account for the clinical importance of the original treatment; an essential antimicrobial or chemotherapy agent should not simply be discontinued without appropriate specialist input.

Decontamination

Management depends on the route and specific toxicant.

For inhalational exposure:

• Remove the patient from the source.
• Provide appropriate supportive care.

For dermal contamination:

• Remove contaminated clothing when appropriate.
• Irrigate exposed skin according to the chemical involved.

Historical recommendations for routine ipecac or gastric lavage after ingestion are not part of standard modern management.

Prognosis

Recovery depends on the agent and extent of cochlear injury.

Often reversible:

• Salicylate-associated tinnitus/hearing impairment
• Many NSAID-related auditory effects
• Some loop-diuretic-associated hearing abnormalities

Potentially permanent:

• Aminoglycoside ototoxicity
• Cisplatin ototoxicity
• Bromate-associated hearing loss
• Severe cochlear injury from other toxicants

Key Points

• Toxicants most commonly produce sensorineural hearing abnormalities.
• Major ototoxic agents include aminoglycosides, cisplatin, salicylates, quinine/quinidine, and loop diuretics.
• Tinnitus is a classic manifestation of salicylate toxicity, but its disappearance does not prove that the poisoning has resolved.
• Aminoglycoside toxicity is more likely with renal dysfunction and greater cumulative exposure.
• Cisplatin commonly affects high-frequency hearing and may cause permanent injury.
• Weber and Rinne tests help distinguish conductive from sensorineural hearing loss.
• Weber: conductive → affected ear; sensorineural → unaffected ear.
• Rinne: normal/sensorineural = AC > BC; conductive = BC > AC.
• Sudden unexplained sensorineural hearing loss requires prompt specialist evaluation.
• Treatment primarily involves recognizing the responsible agent, treating systemic toxicity, and preventing further ototoxic exposure.