1. Clinical Overview
Gentamicin (0.3% w/v) is a topical ophthalmic/otic formulation of a broad-spectrum aminoglycoside antibiotic. It is bactericidal and is primarily used for the treatment of superficial bacterial infections of the eye and external ear canal. It is effective against a wide range of Gram-negative bacteria (including Pseudomonas aeruginosa) and some Gram-positive bacteria. In the Indian context, it is a first-line, cost-effective option for bacterial conjunctivitis, blepharitis, and otitis externa.
| Onset | Duration | Bioavailability |
|---|---|---|
| Therapeutic concentrations are achieved at the site of infection within minutes of topical application. | Approximately 4 to 6 hours per dose, necessitating multiple daily applications. | Negligible systemic absorption (<1%) following topical application to intact skin or non-inflamed ocular/otic surfaces. Absorption can increase with application to inflamed, denuded, or ulcerated tissues. |
2. Mechanism of Action
Gentamicin binds irreversibly to the 30S ribosomal subunit of susceptible bacteria. This binding interferes with the initiation complex and causes misreading of the mRNA template, leading to the incorporation of incorrect amino acids into the growing polypeptide chain. This results in the production of non-functional or toxic proteins and ultimately bacterial cell death.
3. Indications & Uses
- Bacterial Conjunctivitis
- Bacterial Keratitis (Corneal Ulcers)
- Bacterial Blepharitis
- External Otitis (Otitis Externa)
- Prophylaxis of infection following minor ocular injuries or surgeries
4. Dosage & Administration
Adult Dosage: **Ophthalmic:** Instill 1-2 drops into the affected eye(s) every 4 to 6 hours. For severe infections, may be used hourly initially, tapering as infection improves. **Otic:** Instill 3-4 drops into the affected ear 3-4 times daily.
Administration: **Eye Drops:** Wash hands. Tilt head back, pull lower eyelid down to form a pouch. Instill drops without touching dropper to eye or any surface. Close eye gently for 1-2 minutes, apply gentle pressure to the nasolacrimal duct (inner corner of eye) to minimize systemic absorption. **Ear Drops:** Lie on side with affected ear upward. Pull the auricle upward and backward (in adults) to straighten the ear canal. Instill drops. Remain in position for 5 minutes. Do not insert any device into the ear. Use sterile cotton plug if needed.
5. Side Effects
Common side effects may include:
- Transient ocular/otic stinging or burning on instillation
- Itching or irritation at the application site
- Mild erythema (redness) of conjunctiva or skin
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Other Ototoxic Drugs (e.g., IV Furosemide, Cisplatin) | Increased risk of hearing loss or vestibular damage. | Major |
| Other Nephrotoxic Drugs (e.g., IV Vancomycin, NSAIDs, Amphotericin B) | Increased risk of kidney damage if systemically absorbed. | Moderate |
| Neuromuscular Blocking Agents (e.g., Succinylcholine, Vecuronium) | Potentiation of neuromuscular blockade, leading to prolonged apnea. | Major |
| Other Topical Corticosteroids (e.g., Dexamethasone, Prednisolone) | May mask signs of infection or potentiate spread of infection. Combined formulations are used judiciously. | Moderate |
7. Patient Counselling
- Do complete the full course of treatment even if symptoms improve.
- Do not touch the dropper tip to your eye, ear, fingers, or any other surface.
- Do not share your medication with others.
- Do not use leftover medication for a new infection.
- Do not wear contact lenses while having an eye infection unless directed by your doctor.
- Wait at least 15 minutes after instilling eye drops before inserting soft contact lenses.
8. Toxicology & Storage
Overdose: **Topical Overdose:** Unlikely to cause systemic toxicity unless applied to very large denuded areas. Symptoms of systemic overdose include ototoxicity (tinnitus, hearing loss, vertigo), nephrotoxicity (rising serum creatinine, oliguria), and neuromuscular blockade (muscle weakness, respiratory depression).
Storage: Store at room temperature (15°C to 30°C). Protect from light. Do not freeze. Keep the container tightly closed. Keep out of reach and sight of children.