The influence of a melanin-binding drug on temporary threshold shift in humans

Relationship Between Chloroquine or Hydroxychloroquine Use and Hearing Disorders: A Systematic Review and Meta-Analysis

BACKGROUND AND OBJECTIVES

Chloroquine and its analog hydroxychloroquine are derivatives of 4-aminoquinoline and are regularly used in the treatment of malaria and autoimmune diseases. Among the side effects of these drugs, alterations associated with the auditory system are frequently mentioned. Thus, the aim of this systematic review is to systematically review publications on hearing disorders and chloroquine or hydroxychloroquine use.

MATERIALS AND METHODS

Inclusion criteria were observational or interventional studies on audiological assessment in participants who were using chloroquine or hydroxychloroquine. The methodological quality was independently assessed by two reviewers using the Meta-Analysis of Statistics: assessment and review Instrument. The certainty of the evidence was assessed using the GRADE tool.

RESULTS

A total of 1,372 non-duplicate papers were screened, out of which 17 were included in the final qualitative synthesis, and 5 studies in the meta-analysis. The odds ratio for the two subgroups evaluated did not show significance with no heterogeneity between the effects observed between the different diseases (I2=0%) and obtaining the global estimate of 0.76 (95% confidence interval [CI]=0.41-1.39; p>0.05). Despite the inclusion of papers with different disease samples, the heterogeneity observed in the analysis was low (I2= 0%) and prediction interval (95% PI=0.32-1.80; p>0.05) remained close to that estimated by the CI (95% CI=0.41-1.39; p>0.05). The certainty of the evidence assessed by the GRADE tool was considered very low due to the risk of bias, indirect evidence, and imprecision.

CONCLUSIONS

The findings of this study suggest that the use of chloroquine/hydroxychloroquine is not associated with hearing disorders.

Mefloquine neurotoxicity: a literature review

A literature review revealed that mefloquine neurotoxicity has been demonstrated at both the preclinical and clinical levels, with nausea, dizziness, sleep disturbances, anxiety and psychosis, amongst other adverse neuropsychiatric events, reported in users. Females and individuals of low body mass index (BMI) are at apparent greater risk. Mechanisms of possible neurotoxicity may include binding to neuroreceptors and cholinesterases, inhibition of sarcoendoplasmic reticulum ATPase (SERCA) and interference with cellular Ca(2+) homeostasis, accumulation in the CNS, and reductions in CNS efflux in individuals possessing certain MDR1 polymorphisms. It may be prudent to avoid mefloquine in females and low BMI individuals, and in combination with other potentially neurotoxic agents such as the artemisinin antimalarials.

Absence of strial melanin coincides with age-associated marginal cell loss and endocochlear potential decline

Cochlear stria vascularis contains melanin-producing intermediate cells that play a critical role in the production of the endocochlear potential (EP) and in maintaining the high levels of K(+) that normally exist in scala media. The melanin produced by intermediate cells can be exported to the intrastrial space, where it may be taken up by strial marginal cells and basal cells. Because melanin can act as an antioxidant and metal chelator, evidence for its role in protecting the stria and organ of Corti against noise, ototoxins, and aging has long been sought. While some evidence supports a protective role of melanin against noise and ototoxins, no evidence yet presented has demonstrated a clear role for melanin in maintaining the EP during aging. We tested this by comparing basal turn EPs and a host of cochlear cellular metrics in aging C57BL/6 (B6) mice and C57BL/6-Tyr(c-2J) mice. The latter mice carry a naturally occurring inactivating mutation of the tyrosinase locus, and produce no strial melanin. Because these two strains are coisogenic, and because pigmented B6 mice show essentially no age-related EP decline, they provide an ideal test of importance of melanin in the aging stria. Pigmented and albino B6 mice showed identical rates of hearing loss and sensory cell loss. However, after two years of age, basal turn EPs significantly diverged, with 42% of albinos showing EPs below 100 mV versus only 18% of pigmented mice. The clearest anatomical correlate of this EP difference was significantly reduced strial thickness in the albinos that was highly correlated with loss of marginal cells. Combined with findings in human temporal bones, plus recent work in BALB/c mice and gerbils, the present findings point to a common etiology in strial presbycusis whereby EP reduction is principally linked to marginal cell loss or dysfunction. For any individual, genetic background, environmental influences, and stochastic events may work together to determine whether marginal cell density or function falls below some critical level, and thus whether EP decline occurs.

The effect of low level acoustic stimulation on susceptibility to noise in blue- and brown-eyed young human subjects

OBJECTIVE

The aim of this study was to investigate the influence of pigmentation on the reduction in temporary noise-induced threshold shift (TTS) due to low level acoustic stimulation (LLAS). A pigmentation-dependent LLAS effect on TTS could be interpreted as a strial melanocyte involvement in LLAS. It could not be explained by cochlear sensorineural structure changes only.

DESIGN

Teenagers were classified according to eye color (n = 6 + 6) and exposed to music at 70 dBA 6 h per day for 9 days (LLAS). TTS was measured before, during, and after the LLAS period.

RESULTS

It was shown that LLAS reduced TTS significantly more in blue-eyed than in brown-eyed subjects. The difference in TTS remained for at least 1 wk after the cessation of LLAS.

CONCLUSIONS

It is suggested that the observed difference in LLAS effect is due to strial melanocyte differences in free radical defense. It is also possible that other cochlear antioxidant enzyme systems, responsible for inactivation of harmful oxygen radicals and simultaneously involved in melanin synthesis such as the thioredoxin reductase/thioredoxin electron transfer system are activated.