Hearing Loss & Tinnitus
HEARING LOSS & TINNITUS
Causes & Risk Factors
Hearing loss is one of the most common chronic conditions in older adults. Next to arthritis, it is the second most common handicapping condition (Bielefeld 2010; NIHSenior Health 2012). Although hearing loss is more common with age, approximately 8.5% of American adults aged 20 to 29 have significant hearing loss, a number that appears to be rising (Agrawal 2008).
Hearing loss and a related condition, tinnitus or “ringing in the ears”, can become severe obstacles in communicating and interacting with others, contributing to poor quality of life. Moreover, hearing loss can lead to reduced neurologic activity in the parts of the brain that process speech, and atrophy in the parts that process sound in general (Samson 2001; Peelle 2011; Dalton 2003).
Hearing loss can be conductive, sensorineural, or mixed, which is a combination of conductive and sensorineural. The type of hearing loss is correlated with the anatomic part of the ear affected (outer, middle, or inner ear). Generally, damage to the outer and middle ear causes conductive hearing loss, whereas inner ear damage results in sensorineural hearing loss (Medwetsky 2007).
Conductive Hearing Loss
Outer and middle ear conductive hearing loss could be caused by infections, trauma, congenital malformations or tumors in the outer ear. Otitis media, a common childhood disease that can also affect adults, is one of the most common types of ear infections to cause hearing loss; similarly, viral infections of the upper respiratory tract can affect the ear and cause temporary hearing loss. Trauma to the tympanic membrane, one of the middle ear structures that help translate sound waves into interpretable neurologic signals, can also result in conductive hearing loss. The tympanic membrane can become damaged by direct trauma, which can be caused by a foreign body such as a cotton swab (e.g., Q-tip®), infection, and sudden changes in air pressure (middle ear barotrauma) (Weber 2012).
Sensorineural Hearing Loss
Damage to the inner ear is usually responsible for hearing loss that progresses over time. Presbycusis, or age-related deterioration of hearing ability, is marked by the gradual loss of high frequency hearing on both sides in elderly individuals (Huang 2010). Presbycusis is also associated with tinnitus (i.e., ringing in the ears). Excessive noise can also cause sensorineural hearing loss that can gradually increase over time. Loud noise damages the delicate structures in the ear both due to trauma and accumulation of free radicals & excess glutamate, as well as altering intracellular magnesium and calcium levels (Prasher 1998). Infections and a condition called Meniere’s diseasecan also lead to inner ear damage and sensorineural hearing loss (Weber 2012; Mayo Clinic 2010).
Closely linked to hearing loss is a condition known as tinnitus, characterized by a persistent ringing sensation in the ears. Although tinnitus can be triggered by a variety of causes, the majority of cases are associated with hearing loss (Roberts 2010). Researchers are still working to understand the process behind tinnitus. One popular hypothesis is when the hair cells (specialized nerve cells that help translate sound waves into interpretable signals for the brain, not to be confused with hair follicles) in the cochlea are damaged, some of the associated neurons partially lose the inhibitory regulation that keeps them from firing when no sound is present. As a result, these neurons send signals that the brain perceives as persistent noise. Supporting this hypothesis is that many people who suffer from tinnitus perceive the “ringing” in their ears to be of the same or similar frequency to their hearing deficits. Consequently, similar processes that lead to hearing loss may also lead to tinnitus; thus, interventions that prevent hearing loss may also prevent tinnitus (Roberts 2010).
How Hearing Loss Occurs
Over the years, scientists have gained a better understanding of how noise can damage the auditory system, particularly a part of the inner ear known as the cochlea. The cochlea contains specialized nerve cells, known as hair cells, which help translate sound waves into interpretable signals for the brain. Loud sounds damage hair cells through direct mechanical trauma and secondary metabolic damage. Direct mechanical trauma typically causes immediate structural damage to cochlear hair cells and can potentially cause immediately detectable hearing loss. The metabolic effects of loud noise, however, can accumulate for days or even weeks after initial sound exposure (Oishi 2011).
Loud noise affects metabolism in hair cells by decreasing oxygen supply and increasing energy demands. Loud noise can disrupt the flow in blood vessels that supply oxygen to hair cells, depriving these cells of nutrients needed to function and leading to cell damage through a process known as ischemia. At the same time, the increased stimulation due to noise forces the hair cells to be metabolically more active. The end result is that, during this period of intense stimulation, these hair cells burn through their energy reserves, resulting in the formation of reactive oxygen species (ROS). These ROS have the ability to damage proteins and lipids and can ultimately lead to death of the hair cells (Henderson 2006).
Hair cells may also be damaged by inflammatory mediators known as cytokines. Animal studies have found an increase in certain pro-inflammatory cytokines in response to loud noise. These cytokines include interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α), two compounds that can be toxic to nerve cells at high levels (Fujioka 2006). In addition, overstimulation of hair cells can cause them to release large amounts of the neurotransmitter glutamate. Although glutamate release is needed to help translate sounds into neurological signals, too much glutamate can result in significant “excitotoxicity”, in which excessive stimulation damages nerve cells (Pujol 1999).
CAUSES & RISK FACTORS
A number of risk factors can predispose a person to hearing loss. Although advancing age is the most important risk factor, people with heart disease, high blood pressure, diabetes and an extensive smoking history are more likely to develop hearing loss (Helzner 2005; Bielefeld 2010). Otosclerosis, a condition involving abnormal bone growth within the middle ear, is associated with both conductive and sensorineural hearing loss (Liktor 2012; Ealy 2011; Bloch 2012; Deggouj 2009). In addition, hearing loss is more common in men (Agrawal 2008).
Repeated exposure to loud noises from occupational sources, recreational activities, or firearms strongly correlates with an increased risk of unilateral (hearing loss in one ear), bilateral (hearing loss in both ears), and high-frequency hearing loss (Agrawal 2008). According to a 2007 report, approximately 30 million Americans are exposed to dangerous levels of noise every day, with 10 million adults and 5.2 million children affected by irreversible hearing loss due to excessive noise exposure (Seidman 2010).
The National Institute of Occupational Safety and Health considers noise levels above 85 decibels to be harmful (Marsh 2011). Although sustained levels of loud noise are dangerous, impulse noise (i.e., large bursts of loud noise) can also damage hearing. In fact, research suggests that short exposure to very loud noise, such as that experienced by soldiers, can be more damaging to the auditory system than continuous noise (Clifford 2009).
Some drugs have the potential to cause hearing loss or tinnitus because they are toxic to the ear or “ototoxic”. Examples of ototoxic drugs include high doses of aspirin, some antibiotics, some chemotherapy drugs, and some anti-inflammatory medications (Verdel 2008; Ligezinski 2002; Rybak 2007; Wecker 2004; Puel 2007). For example, high doses of aspirin in the range of 2,000 to 4,000 mg daily can cause tinnitus and hearing loss via peripheral effects on the cochlea and central effects on nerves involved in hearing. These effects usually subside within one to three days of discontinuing aspirin (Stolzberg 2012; McFadden 1984; Carlyon 1993; Day 1989). Risk of developing drug-induced hearing loss is greater in those with impaired kidney health or inner ear disorders (Ligezinski 2002).
Antioxidants are compounds that have the ability to neutralize damaging reactive oxygen species (ROS). Since ROS are involved in the development and progression of tinnitus and hearing loss, antioxidants represent a promising therapeutic strategy (Sergi 2004; Savastano 2007; Joachims 2003).
Mitochondria are the energy powerplants of the cell. They are also the site of ROS production, especially when the cell is under stress. In cochlear hair cells, mutations in mitochondrial DNA and declining function of the mitochondria have been found to cause age-induced hearing loss (Yamasoba 2007). As a result, compounds that help maintain mitochondrial health, such as acetyl-L-carnitine, may help protect cells from damage. Animal studies have found that acetyl-L-carnitine is able to protect the cochlea from both continuous and impulse noise damage as well as prevent loss of hair cells (Kopke 2002; Kopke 2005). Acetyl-L-carnitine was also found to reduce mutations in mitochondrial DNA, suggesting that it could prevent not only noise-induced hearing loss, but also age-related hearing loss (Seidman 2000). Much like NAC, acetyl-L-carnitine appears to be effective even when administered after exposure to loud noise(s) (Coleman 2007; Du 2012). In one animal study, acetyl-L-carnitine was shown to protect against ototoxicity induced by the chemotherapeutic drug cisplatin (Gunes 2011).
Lipoic acid has been found to reduce age-related hearing loss (Seidman 2000). Preliminary animal studies have also found that lipoic acid can help protect against noise-induced hearing loss and preserve inner-ear mitochondrial function (Diao 2003; Peng 2010). This may be partly due to the effect it has on glutathione (i.e., a naturally occurring antioxidant in the body). Studies have found increasing glutathione levels help protect the cochlea from damage due to loud noises (Le Prell 2007). In one laboratory study, lipoic acid was shown to increase glutathione levels in nerve cells, protecting them from damage (Jia 2008). Lipoic acid may also be able to counteract the action of toxins (e.g., carbon monoxide) that aggravate the effects of noise and make normally safe levels of volume harmful to the ear (Pouyatos 2008). In a clinical trial among 46 elderly subjects with hearing loss, 8 weeks of treatment with lipoic acid (60 mg/day) combined with two other free radical scavengers (vitamin C [600 mg/day] and rebamipide [300 mg/day]) significantly improved hearing at all frequencies tested (Takumida 2009).
Dietary supplementation with vitamins that have antioxidant capabilities can help protect the hair cells of the cochlea. One animal study showed that a 35-day pretreatment regimen of vitamin C may be able to protect against noise-induced hearing loss (McFadden 2005). Similarly, supplementing animals with certain forms of vitamins A and E have shown significant protective effects (Hou 2003; Ahn 2005). The length of time vitamins need to be taken prior to noise exposure may vary depending on the vitamin. For example, vitamin E appears to be effective with three days of pretreatment, vitamin A may only require two days to be effective, and Vitamin C may require a longer pretreatment period. In addition, taking vitamins in combination may be more effective than any one of them alone (Le Prell 2007). For example, a combination of B-vitamins, vitamins C & E, and L-carnitine protected rodents from cisplatin ototoxicity (Tokgoz 2012).
Folate and Vitamin B12
Folate and vitamin B12 are important for the functioning of many cells in the body, including nerve cells. They also help reduce levels of homocysteine, a potentially toxic compound found in the body. Elevated homocysteine levels are linked to an increased risk of hearing problems (Gok 2004; Gopinath 2010). Vitamin B12 injections (1 mg for 7 days followed by 5 mg on day 8) protected against noise-induced hearing loss in healthy volunteers aged 20 to 30 years (Quaranta 2004). Researchers have found that patients with low levels of folate in their blood are more likely to develop hearing loss (Gok 2004; Lasisi 2010; Gopinath 2010), and that low vitamin B12 levels are associated with hearing loss (Gok 2004) and tinnitus (Shemesh 1993).
Because loud noise impairs blood flow to the cochlea, researchers have also examined compounds that could help improve circulation to the hair cells and prevent their death. Magnesium is known to help expand blood vessels and improve circulation; it also helps control the release of glutamate, one of the major contributors to noise-induced hearing loss (Le Prell 2011). Animal studies have found that magnesium deficiency increases the risk of noise-induced hearing loss (Sendowski 2006b; Scheibe 2002). A combination of magnesium and other antioxidants may synergistically prevent hearing loss, potentially because magnesium’s ability to increase blood flow also helps transport the protective antioxidants (Le Prell 2011). Magnesium’s benefits have been demonstrated in human trials as well; magnesium supplementation (122 mg daily for ten days) reduced noise-induced hearing loss in men aged 16-37 years (Attias 2004). Studies have also found that both intravenous magnesium and oral magnesium supplementation may be beneficial for other types of hearing loss, such as sudden sensorineural hearing loss (Gordin 2002; Coates 2010).
Melatonin, a hormone critical for healthy sleep (Wurtman 2012), has powerful antioxidant properties. Animal studies have found that it is effective at preventing hearing damage after exposure to loud noises (Karlidag 2002; Bas 2009). It is also effective at treating other types of hearing loss caused by ROS, such as due to the chemotherapy drug cisplatin (Lopez-Gonzalez 2000). Researchers have discussed the potential for melatonin to act as a protectant against age-related hearing loss (Martinez 2009). For example, it was noted in a study that low plasma levels of melatonin were associated with significant high-frequency hearing loss among elderly subjects (Lasisi 2011).
Additionally, melatonin has been tested as a treatment for tinnitus, both in combination with the medication sulpiride (an atypical antipsychotic) and on its own. On its own, melatonin provides relief from tinnitus, especially in people with significant sleep problems (Rosenberg 1998; Megwalu 2006; Reiter 2011).
Ginkgo biloba, a commonly used herbal supplement, has attracted interest as a means of protecting against hearing loss as well as a treatment for tinnitus. Early animal studies found that when a standardized preparation of Ginkgo biloba extract was given as a supplement to animals, it reduced behavioral manifestations of tinnitus (Jastreboff 1997). This extract, at a dose of 160 mg daily over a 12 week period, was also effective at reducing symptoms in humans (Morgenstern 2002). However, other studies have found negligible or no effect (Hilton 2010; Canis 2011); therefore, more research is needed in this area. Ginkgo biloba may also be effective at preventing hearing loss that causes tinnitus; an animal study found that a Gingko biloba extract was able to reduce drug-induced oxidative damage to hair cells in the cochlea (Yang 2011).
Coenzyme Q10(CoQ10) supports mitochondrial function and has significant antioxidant properties (Quinzii 2010). Animal studies have found that supplementation with CoQ10 reduced noise-induced hearing loss and the death of hair cells (Hirose 2008; Fetoni 2009, 2012). Human studies have also yielded promising results, as 160-600 mg of CoQ10 daily was found to reduce hearing loss in people with sudden sensorineural hearing loss and presbycusis (Ahn 2010; Salami 2010; Guastini 2011). Also, a small preliminary trial found that CoQ10 supplementation alleviated tinnitus in those whose CoQ10 blood levels were initially low (Khan 2007). Another small trial found CoQ10 may slow progression of hearing loss associated with a mitochondrial genetic mutation (Angeli 2005).
Zinc, a mineral involved in many physiological processes (including nervous system function), has antioxidant and anti-inflammatory properties (Frederickson 2000; Prasad 2008). Evidence suggests that inadequate zinc intake may be associated with impaired hearing (Kang 2012). Researchers have found that zinc supplementation may be helpful in treating some forms of hearing loss (Yang 2010). In addition, low levels of zinc correlate with perceived loudness of tinnitus in afflicted individuals (Arda 2003).
Omega-3 fatty acids
Long-chain omega-3 (n-3) polyunsaturated fatty acids, long recognized as important for health, may also affect hearing loss; a preliminary study found that participants with the highest blood levels of these beneficial fats suffered the least amount of hearing loss over time (Dullemeijer 2010). In another study, greater fish or fish oil consumption was associated with less hearing loss among nearly 3,000 subjects over 50 years of age.
Taurine plays a vital role in hearing. In fact, studies have found that in some cases, taurine can reverse the biochemical processes behind hearing loss (Liu 2006; Liu 2008a). Other studies have demonstrated that taurine can almost completely eliminate the ringing in the ears associated with tinnitus (Brozoski 2010).
Much of the damage to hearing occurs not in the mechanical parts of the ear, but rather in the nerve cells that convert sound waves into the electrical energy that is perceived in our brains. Like other nerve cells, these so-called “hair cells” depend on the flow of calcium ions into and out of the cell. Taurine helps restore and control normal calcium ion flow in auditory cells (Liu 2006; Liu 2008b).
Taurine improves the hearing ability in animals exposed to drugs like the antibiotic gentamicin, which is notoriously toxic to hearing (Liu 2008a). Animal studies using human equivalent doses of 700 mg to 3.2 grams per day of taurine over the course of several weeks demonstrate near-complete resolution of tinnitus with taurine supplementation (the animals had been trained in tasks that are sensitive to distraction by tinnitus) (Brozoski 2010).
- Acetyl-L-carnitine: 1000 – 2000 mg daily
- R-Lipoic acid: 240 – 480 mg daily
- Vitamin C: 1000 – 2000 mg daily
- Vitamin A: 5000 IU (as 90% beta-carotene and 10% acetate) daily in divided doses
- Vitamin E: 100 – 400 IU alpha-tocopherol and 200 mg gamma-tocopherol daily
- Zinc: 30 mg daily
- Coenzyme Q10 (as ubiquinol): 100 – 300 mg daily
- Fish oil (with olive polyphenols): providing 1400 mg EPA and 1000 mg DHA daily
- Ginkgo biloba; standardized extract: 120 mg daily
- Magnesium: 144 mg daily as magnesium-L-threonate; 320 mg daily as magnesium citrate
- Melatonin: 0.3 – 5 mg before bed (sometimes up to 10 mg)
- Folate: 400 mcg – 1000 mcg daily
- Vitamin B12: 300 mcg – 5000 mcg daily
- Taurine: 1000 – 4000 mg daily
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