ABSTRACT
Age-related hearing loss (AHL), also known as presbycusis, is one of the most common features of mammalian aging and is the most common sensory disorder in the elderly (Schuknecht and Gacek, 1993; Jennings and Jones, 2001; Gates and Mills, 2005; Liu and Yan, 2007; Van Eyken et al., 2007; Yamasoba et al., 2007; Vrijens et al., 2008). AHL is characterized by an age-dependent decline of auditory function and loss of hair cells and spiral ganglion (SG) neurons in the cochlea of the inner ear. Neither of these long-lived cell types is thought to regenerate in mammals, and extensive cell loss leads to permanent hearing impairment. Schuknecht and coworkers de ned sensory, neural, and strial types of AHL based on correlations between cochlear pathology and audiogram (Schuknecht and Gacek, 1993); approximately 75% of cases of AHL represent a pure sensory, neural, or strial type, or a mixture of two or more types, and in the remaining 25% of cases the cochlear pathology does not correlate with the audiogram. Therefore, the major cause of AHL is the loss of hair cells, SG neurons, and stria vascularis cells in the cochlea of the inner ear. However, despite intensive studies in the AHL eld, the molecular mechanisms of AHL remain unclear. In the United States, people over 65 years of age numbered 37.9 million in 2007, representing 12.6% of the U.S. population, and this elderly population is expected to grow to 71.5 million (20% of the U.S. population) by 2030 (Administration on Aging, 2009). A study performed in Wisconsin showed that AHL affects nearly half the population between 48 and 92 years of age (Schuknecht and Gacek, 1993), and it is estimated that more than 40% of individuals over 65 years of age (15.2 million) have AHL in the United States (Seidman, 2000; Gates and Mills, 2005; Yamasoba et al., 2007). Since the overall number of people suffering from this age-associated disorder is rapidly growing as the lifespan
8.1 Introduction .......................................................................................................................... 113 8.2 α-Lipoic Acid ....................................................................................................................... 115 8.3 Acetyl-l-Carnitine ................................................................................................................ 115 8.4 Vitamin C ............................................................................................................................. 116 8.5 Vitamin E .............................................................................................................................. 117 8.6 Folic Acid .............................................................................................................................. 117 8.7 Melatonin .............................................................................................................................. 118 8.8 Caloric Restriction ................................................................................................................ 119 8.9 Potential Mechanisms for Cochlear Protection
and the Retardation of AHL by Antioxidants and CR ......................................................... 119 8.10 Summary .............................................................................................................................. 121 References ...................................................................................................................................... 122
increases in industrialized nations, AHL is considered a major social and health problem. However, there is no treatment for the disorder at this time.
