Intratympanic Gentamicin

Mechanism

Pharmacological Class and Group

Antibiotic from the group of aminoglycosides. Used in otoneurology for its selective vestibulotoxicity profile (chemical ablative agent).

Mechanism of Action

The goal of treatment is not antimicrobial control, but rather the controlled destruction of overactive vestibular hair cells in the posterior labyrinth of the affected ear:

Selective Vestibular Chemical Ablation Mechanism

1. Attachment to the Round Window Membrane and Penetration: After being instilled into the middle ear, gentamicin diffuses through the MVR in a dose-dependent manner.

2. Internalization in Vestibular Hair Cells: It selectively penetrates type I and type II vestibular hair cells located in the crests of the semicircular canals and in the macules of the utricle and saccule. Entry into the cell is carried out by apical endocytosis mediated by mechanoelectric transduction (MET) channels.

3. Induction of Mitochondrial Oxidative Stress: Once in the cytoplasm, gentamicin forms a complex with free intracellular iron, acting as a catalyst in the Fenton reaction:

Gentamicin-Fe Complex²⁺ + H₂O₂ Massive generation of Oxygen Free Radicals (ROS)

ROS cause lipid peroxidation of mitochondrial membranes, DNA fragmentation and activate the effector caspase cascade that triggers selective apoptosis of vestibular hair cells, drastically reducing aberrant signals reaching the cerebellum and vestibular nuclei.

4. Preservation of the Cochlea: Although gentamicin is intrinsically both vestibulotoxic and cochleotoxic, it has a preferential affinity for the vestibular epithelium. Using a low dosage protocol ("gentle titration"), the aim is to eliminate pathological vestibular function, reducing the risk of cochlear hearing loss.

Pharmacokinetics

High Resolution Pharmacokinetics

• Perilinph Concentration: Reaches peak concentrations in the perilymphatic system 12-24 hours after otic instillation.
• Perilymph Clearance: The cochlear clearance of gentamicin is notably slow compared to corticosteroids, maintaining an active deposit in the supporting cells of the inner ear for weeks or months, which explains the delayed appearance of the ablative response (late effect 2-3 weeks after the initial dose).
• Systemic Absorption: Minimal and undetectable. It is not associated with systemic nephrotoxicity or toxicity of the contralateral ear.

Indicators and dose

Clinical Indications and Off-Label Uses

• Refractory Unilateral Menière's Disease: Management of disabling vertigo attacks that do not respond to usual medical treatment (low-sodium diet, diuretics, betahistine or intratympanic corticosteroids). It is used in order to perform a unilateral chemical labyrinthectomy that ceases vestibular discharge fluctuations.

Dosage and Clinical Adjustment

Chemical Labyrinthectomy Protocol for Vestibular Titration:

• Solution of Choice: Pure injectable gentamicin, usually diluted to a final concentration of 20 mg/mL to 40 mg/mL with sodium bicarbonate or sterile physiological solution to regulate local osmotic acidity.
• Instillation Volume: 0.3-0.5 mL into the middle ear cavity by transtympanic puncture.
• Frequency and Titration: The safest method of administration is the "single or staged dose": a single dose is applied and a minimum period of 3 to 4 weeks is expected to evaluate the effects. A second dose is only given if attacks of disabling vertigo persist and there is no evidence of de novo hearing loss.
• Post-procedure instructions: The patient should remain lying down for 30 minutes without swallowing saliva, followed by early vestibular rehabilitation therapy to accelerate central brain compensation for the loss of peripheral vestibular function.

Security

Absolute and Relative Contraindications

Absolute Contraindications

• Hypersensitivity to aminoglycoside antibiotics.
• Bilateral Menière's disease: High risk of inducing bilateral deep oscillopsia and severe lifelong ataxia (Dandy syndrome due to bilateral ablation).
• Active infection of the outer or middle ear.
• Pregnancy in progress.

Relative Contraindications

• Pre-existing severe hearing loss in the contralateral ear (catastrophic risk of bilateral anacusis if accidental cochleotoxicity occurs in the only functioning ear).
• Pre-existing vestibular dysfunction incompletely compensated.

Adverse Effects (ADR) and Specific Toxicity

• Very common (>10%): Transient static or dynamic imbalance in the 2-4 weeks after the procedure (central compensation phase of induced vestibular hypofunction), transient irritative spontaneous nystagmus.
• Frequent (1%-10%): Cochleotoxic sensorineural hearing loss (occurs in 10-15% of treated cases, manifested by loss of high-frequency tones in tonal audiometry), persistent high-pitched tinnitus.
• Rare (<0.1%): Middle ear infection, transient peripheral facial paralysis due to fallopian canal dehiscence (exceptional, linked to extreme concentrations).

Critical Toxicity: Cochleotoxicity due to Aminoglycosides

The drug can migrate from the scala tympani to the scala media through the internal membranes, irreversibly damaging the outer and inner hair cells of the cochlea. This results in a permanent loss of hearing for high-frequency tones, which may progress to conversational frequencies. Close monitoring using high-frequency tonal audiometry and otoacoustic emissions (OAE) is mandatory before each instillation session.

Drug Interactions of Clinical Relevance

• Systemic aminoglycosides (Amikacin, Tobramycin): Cumulative effect that can trigger severe systemic vestibulotoxicity or nephrotoxicity.
• Systemic loop diuretics (Furosemide, Torasemide): They exponentially increase the risk of ototoxicity by direct alteration of ionic transport gradients in the stria vascularis.

Safety in Pregnancy and Breastfeeding

• Pregnancy: Absolutely contraindicated. Aminoglycosides cross the placental barrier and are directly associated with the risk of congenital bilateral anacusis due to fetal cochlear destruction during uterine development.
• Breastfeeding: Not recommended. Although local systemic absorption is low, it is preferred to suspend ablative otic treatment during this period.