Discussion
Nowadays there is an international consensus on diagnosis of MD while
there is still no general consensus on the most appropriate treatment to
be adopted1,4.Although treatment of medical refractory
unilateral MD with IT aminoglycoside injections has nowadays a standard
therapy, there is still no consensus on the doses and timing of
administration4,5. The IT aminoglycoside injections
was first introduced by Schuknecht using streptomycin and then by Lange
using gentamicin. Gentamicin has now become the most widely used
aminoglycoside for treatment of vestibular symptoms associated with MD.
It’s a relative selective vestibulotoxic aminoglycoside antibiotic that
is preferentially taken up by type I hair cells of the vestibular
neuroepithelium6.The ITG treatment was initially
conceived as an ablative method with complete vestibular ablation to
control vertigo1. However, with this approach there
were serious cochleotoxic effects with great hearing loss
risk1.Parnes reported 41,7% hearing worsening using
high doses of gentamycin(3 daily injections for 4
days)7. Similar results were reported by Cortsen who
performed 3 times daily ITG for 3 days8. It is likely
that the initial reversible effect of gentamicin on both the vestibule
and cochlea turns eventually to an irreversible stage due to the
accumulation of consecutive doses in the inner ear because of slow
clearance of gentamicin9.It has been demonstrated that
gentamicin is eliminated slowly from the inner ear and ototoxic effects
of ITG were delayed 2-3 days after the last
installation10,11.Low-dose treatment may produce
sufficient loss of vestibular function to provide relief from vertigo,
and with less risk of affecting the hearing level. Even though over the
last decade there has been a general trend to conserve vestibular and
consequently cochlear function in order to obtain vertigo control with a
very low hearing risks, some physicians are still concerned about
hearing loss.
In our study we used a low dose of gentamycin injected once daily for 3
consecutive days with high vertigo control (98.6%) and no case of
statistically significant hearing impairment. Similar results were
reported by Quaglieri et al, who achieved 96.5% of total effective
vertigo control with no cases of statistical hearing
impairment11. We agree with Yetiser who states that
titration methods or multiple injections on a daily basis can be used
only in patients with profound or non-serviceable hearing, since these
methods have significant incidence of hearing loss9.
In our series with a long follow-up, we noticed that the used protocol
(three injections in the following three days) provides effective
vertigo control without risk of anacusia and almost no risk of hearing
impairment. In one case of our series we have repeated the ITG treatment
five times(1,3, 6 and 7 years after the first ITG) without achievement
the full control on the vertigo symptoms but,however, without
significantly hearing impairment. It is likely that if the patient had
no hearing damage on the first treatment he probably does not have a
genetic predisposition for aminoglycoside
ototoxicity12. We are aware that the limit of our work
is that it is a retrospective study. For this reason, at the end of 2015
we have started a new prospected study to check if a further reduction
of the ITG administration times gives the same satisfactory results in
vertigo control.