There are no official guidelines to assist in answering this question. I will search for relevant research papers instead.
From the FDA Drug Label
Symptoms include dizziness, vertigo, tinnitus, roaring in the ears and also hearing loss, which, as with the other aminoglycosides, may be irreversible. Hearing loss is usually manifested initially by diminution of high-tone acuity. The common presentation of gentamicin ototoxicity includes:
- Hearing loss, which may be irreversible
- Tinnitus (ringing in the ears)
- Vertigo (dizziness)
- Diminution of high-tone acuity (reduced ability to hear high-pitched sounds)
- Roaring in the ears 1
From the Research
Common Presentation of Gentamicin Ototoxicity
The common presentation of gentamicin ototoxicity includes:
- Imbalance and oscillopsia, which are symptoms of vestibular toxicity 2, 3
- Rarely, hearing loss and vertigo 2, 3
- Positive results on tests such as the bilateral clinical head-impulse test, vertical head-shaking test, and foam Romberg test 2
- Profoundly decreased gain and increased phase lead on sinusoidal rotational testing 3
Vestibular vs Cochlear Toxicity
- Gentamicin ototoxicity is primarily vestibular, rather than cochlear 2, 3
- However, some studies suggest that cochlear toxicity can occur, especially in patients with pre-existing hearing loss or other risk factors 4, 5
- The ratio of vestibular to cochlear toxicity is estimated to be around 2:1 4
Time Course and Risk Factors
- Symptoms of gentamicin ototoxicity can occur at any time during or after treatment, and can be immediate or delayed 2, 4
- Risk factors for gentamicin ototoxicity include high doses, prolonged treatment, and pre-existing renal or auditory dysfunction 2, 4, 5
- Even low serum levels of gentamicin do not rule out the possibility of ototoxicity 5