Aminoglycosides That Target Thin-Walled Bacteria and Cause Sensorineural Hearing Loss
Streptomycin is the primary aminoglycoside antibiotic that targets thin-walled bacteria (such as Mycobacterium tuberculosis) and has a well-documented risk of causing sensorineural hearing loss as a side effect. 1
Mechanism and Risk Factors
Streptomycin belongs to the aminoglycoside class of antibiotics that work by binding to bacterial ribosomes, inhibiting protein synthesis. It is particularly effective against:
- Mycobacterium tuberculosis (thin cell wall)
- Other acid-fast bacilli
- Some gram-negative bacteria
Ototoxicity Mechanism:
- Causes damage to both vestibular and auditory branches of the eighth cranial nerve 1
- Generates free radicals within the inner ear, leading to permanent damage to sensory cells and neurons 2
- Vestibulotoxic potential exceeds its cochlear toxicity 1
- Hearing loss is usually irreversible and initially manifests as diminution of high-frequency acuity 1
Risk Factors for Ototoxicity:
- Dose and duration of treatment (directly proportional) 1
- Advanced age 1
- Impaired renal function 1
- Pre-existing auditory dysfunction 1
- Concurrent use of other ototoxic drugs 1
- Genetic predisposition - mitochondrial DNA variants (particularly m.1555A>G in the MT-RNR1 gene) 1, 3
- Dehydration 4
- Co-administration with loop diuretics (ethacrynic acid, furosemide, mannitol) 1
Other Aminoglycosides with Similar Effects
While streptomycin is primarily vestibulotoxic, other aminoglycosides that can cause sensorineural hearing loss include:
- Amikacin - primarily cochleotoxic 2, 5
- Kanamycin - primarily cochleotoxic 2, 5
- Gentamicin - primarily vestibulotoxic but can also affect hearing 2, 6
- Neomycin - primarily cochleotoxic 2
- Tobramycin - similar ototoxic potential to gentamicin 4
Monitoring and Prevention
Recommended Monitoring:
- Baseline audiometry before starting treatment 5
- Regular audiometric testing during treatment 5
- Monthly audiometry until treatment with aminoglycoside ceases 5
- Final audiometry review 2 months after the final dose 5
- Monitor for early symptoms: headache, nausea, vomiting, disequilibrium, tinnitus, vertigo 1
Definition of Ototoxicity:
Ototoxicity on audiogram is defined as:
- 20 dB loss from baseline at any one test frequency, OR
- 10 dB loss at any two adjacent test frequencies 5
Prevention Strategies:
- Use lowest effective dose for shortest duration possible
- Adjust dosing in patients with renal impairment 5
- Consider alternative antibiotics in patients with risk factors
- Avoid concurrent use of other ototoxic drugs when possible
- Consider genetic testing in patients with family history of aminoglycoside ototoxicity 1, 3
- Maintain adequate hydration during treatment 4
Special Considerations
Pregnancy:
Streptomycin readily crosses the placental barrier and can cause ototoxicity in the fetus. It should be avoided during pregnancy, especially in the second and third trimesters 1, 7.
Genetic Testing:
Consider genetic testing for mitochondrial DNA variants in patients with:
- Family history of aminoglycoside ototoxicity
- Maternal history of hearing loss after aminoglycoside use 1
Clinical Implications
When treating thin-walled bacteria like Mycobacterium tuberculosis, the risk of permanent hearing loss must be carefully weighed against the benefits of treatment. For tuberculosis specifically, alternative regimens that don't include aminoglycosides should be considered first-line when possible, especially in patients with risk factors for ototoxicity.
If an aminoglycoside must be used, implement rigorous monitoring protocols and consider anti-free radical agents as potential protective measures against ototoxicity 2.