Protective effect of silymarin on noise-induced hearing loss in Guinea pigs.
BACKGROUND
Hearing capability plays a principal role on human's communication. Noise-induced hearing loss (NIHL) caused by exposure to high noise levels is a serious socio-economic problem in modern societies. NIHL can either be reversible, resulting in a temporary threshold shifts (TTS) or irreversible, resulting in a permanent threshold shifts (PTS). PTS is often confirmed in the time span of between 2 - 6 weeks. NIHL may be prevented by avoidance of excessive amounts of noise or reducing the sound energy entering the inner ear using hearing protective devices. However, there are some conditions that such prevention is not possible such as noise exceeding the protective capabilities of the hearing protection device, working in military or the person does not tolerate the protection device. Thus the protective agent for preventing NIHL would be useful.


OBJECTIVE
Free radical molecules and consequence oxidative stress have been shown to play a significant role in noise-induced hearing loss. Silymarin is an antioxidant flavonoid complex derived from the herb milk thistle has ability to mitigating the oxidative stress, scavenge free radicals. In the current study, we aimed to evaluate the protective effect of silymarin on noise induced hearing loss in guinea pig by auditory brain stem response.


METHODS
Twenty guinea pigs randomly divided into 2 groups. The animals in the experimental group were intraperitoneally injected with 100 mg/kg/day silymarin dissolved in propylene glycol for 6 consecutive days. The control subjects were intraperitoneally injected with propylene glycol for 6 consecutive days. All animals were exposed to 4 kHz octave band noise at 120 dB SPL for 6 hours. Auditory brainstem responses (ABRs) at frequencies of 2, 4, 6, 8, 12, 16 and 20 kHz were precisely recorded before intervention and then on intervals of 0, 3, 10 and 15 days after noise exposure. Data were analyzed using repeated measures ANOVA.


RESULTS
Threshold shifts for the experimental group at all frequencies immediately, 3, 10 and 15 days after noise exposure were significantly reduced compared to the control group (P < 0.01).


CONCLUSIONS
The findings indicate a protective effect of silymarin on temporary and permanent noise-induced hearing loss.
Department of Audiology, School of Rehabilitation, Tehran University of Medical Sciences, Tehran, IR Iran.
Ghassem
G
Mohammadkhani
Department of Audiology, Rehabilitation Research Center, School of Rehabilitation Sciences, Iran University of Medical Sciences, Tehran, IR Iran.
Akram
A
Pourbakht
Department of pharmacognosy and Traditional, Iranian Medicine Research Center, School of Pharmacy, Tehran University of Medical Sciences, Tehran, IR Iran.
Mahnaz
M
Khanavi
Department of Biostatistics and Social Medicine, Zanjan University of Medical Sciences, Zanjan, IR Iran.
Soghrat
S
Faghihzadeh
Auditory Brain Stem Implantation; Hearing Loss, Noise-Induced; Silymarin