What is hearing loss?

Hearing loss refers to a partial or complete inability to hear sounds in one or both ears. It can occur suddenly or gradually over time and may affect people of all ages. Hearing loss can vary in severity, ranging from mild to profound, and can have various causes and manifestations.


There are two main types of hearing loss:


  • Sensorineural hearing loss: This type of hearing loss occurs when there is damage to the inner ear (cochlea) or the auditory nerve pathways leading from the inner ear to the brain. Sensorineural hearing loss is often permanent and can be caused by factors such as aging (presbycusis), exposure to loud noise (noise-induced hearing loss), genetic factors, certain medications (ototoxic drugs), infections, head trauma, or diseases such as Meniere’s disease or acoustic neuroma.


  • Conductive hearing loss: This type of hearing loss occurs when there is a blockage or impairment in the outer or middle ear that interferes with the transmission of sound waves to the inner ear. Conductive hearing loss can be caused by factors such as earwax buildup, ear infections (otitis media), fluid accumulation in the middle ear (effusion), perforation of the eardrum, abnormalities of the ear canal or ossicles (small bones in the middle ear), or congenital malformations.


In addition to sensorineural and conductive hearing loss, a combination of both types, known as mixed hearing loss, can also occur. Mixed hearing loss involves damage to both the inner ear and the middle or outer ear structures.


What is the relationship between hearing loss and oxidative stress?

The relationship between hearing loss and oxidative stress involves complex interactions between various factors that contribute to damage and degeneration of the auditory system. Here’s how oxidative stress may be related to hearing loss:


  • Reactive Oxygen Species (ROS) Production: Oxidative stress occurs when there is an imbalance between the production of reactive oxygen species (ROS) and the antioxidant defense mechanisms in the body. The inner ear, including the cochlea, is particularly vulnerable to oxidative damage due to its high metabolic activity, exposure to oxygen, and susceptibility to environmental stressors. Excessive production of ROS can occur in the inner ear due to factors such as aging, noise exposure, ototoxic medications, infections, or ischemia-reperfusion injury.


  • Oxidative Damage to Cochlear Structures: Oxidative stress can cause direct damage to the delicate structures of the cochlea, including the sensory hair cells, supporting cells, and spiral ganglion neurons, which are essential for hearing function. ROS can oxidize lipids, proteins, and DNA within the cochlear cells, leading to cellular dysfunction, apoptosis (cell death), and degeneration of auditory structures. Damage to the sensory hair cells, in particular, can result in permanent hearing loss, as these cells are responsible for converting sound vibrations into electrical signals that are transmitted to the brain.


  • Inflammatory Response and Tissue Injury: Oxidative stress in the inner ear can trigger an inflammatory response and tissue injury, leading to further damage and dysfunction of auditory structures. Inflammatory mediators and cytokines released in response to oxidative stress can exacerbate tissue inflammation, disrupt cellular homeostasis, and impair the repair and regeneration of damaged cochlear cells. Chronic inflammation and oxidative stress in the inner ear are implicated in the progression of age-related hearing loss, noise-induced hearing loss, and other forms of sensorineural hearing loss.


  • Antioxidant Defense Mechanisms: The inner ear possesses antioxidant defense mechanisms to neutralize ROS and maintain redox homeostasis. However, in conditions of chronic oxidative stress or inadequate antioxidant protection, these defense mechanisms may be overwhelmed, leading to increased susceptibility to oxidative damage and hearing loss. Age-related declines in antioxidant enzyme activity, diminished levels of cellular antioxidants, or genetic variations in antioxidant genes may further exacerbate oxidative stress and contribute to age-related hearing loss and other forms of sensorineural hearing loss.


Overall, oxidative stress plays a critical role in the pathogenesis of hearing loss by promoting damage and degeneration of auditory structures, exacerbating inflammation and tissue injury, and impairing auditory function.