درک و پیشگیری از کم شنوایی ناشی از نویز Understanding and preventing noise-induced hearing loss

Hearing mechanisms The human auditory system consists of three main peripheral auditory components (outer ear, middle ear, inner ear) and acoustic nerve (8th cranial nerve). The outer ear gathers sound energy and transmits it to the middle ear through the ear canal and the eardrum (or tympanic membrane). The eardrum vibrates from the incoming sound energy and transmits these vibrations to the inner ear, through the tiny bones in the middle ear, known as the ossicles. The middle ear is filled with air and connects to the throat via the eustachian tube that is instrumental in equalizing pressure on either side of the eardrum. The inner ear has two parts, the vestibular system and the cochlea. The vestibular system is an organ of balance rather than of hearing and is primarily responsible for detecting movement of the head and, to a lesser extent, position in space. Vertigo is a main symptom of the damage to the vestibular system.8 The cochlea, shaped like a snail, contains thousands of delicate hair cells (auditory sensory cells) in the organ of Corti. There are two types of hair cells, called inner and outer hair cells. When the sound waves enter the inner ear from the middle ear, the outer hair cells help amplify sound vibrations. The inner hair cells convert these vibrations into electrical signals and send the signals to the brain through the auditory nerve. The brain, then, translates the signals into sound that we recognize and understand. More description on the anatomy and physiology of the auditory system is presented in the paper in this volume by Baiduc et al. The hair cells of the organ of Corti can be damaged by various factors including aging, loud noise, ototoxic chemicals (solvents, asphyxiants), and ototoxic medications (aminoglycoside antibiotics, Loop diuretics, cisplatin, etc.). More elaborate information on hearing loss related to ototoxic chemicals and medication are presented in the paper in this volume written by Campo et al. Among these, exposure to loud noise is the most common cause of irreversible damage to the hair cells, resulting in sensorineural hearing loss. Types of hearing loss The human ear and its neurological connections to the brain represent a complex and sensitive hearing mechanism that is vulnerable to damage by different illnesses, injuries, and toxic exposures.9 There are two main types of hearing loss: conductive and sensorineural hearing loss. Conductive hearing loss affects the outer or middle ear that includes the pinna, ear canal, eardrum, and the cavity behind the eardrum. There are various etiologies of conductive hearing loss, with the most common including cerumen impaction, middle ear infections such as otitis media, and tympanic membrane perforations. Conductive hearing losses are often temporary or correctable through increasingly sophisticated surgical techniques or medications as is the case with middle ear infections (otitis media). Sensorineural hearing loss affects the inner ear (sensory) or the auditory nerve (neural) that connects the inner ear to the origin of the nerve in the brain. In current clinical practice, sensory and neural hearing losses are not differentiated regularly and the presence of either is diagnosed as a sensorineural hearing loss. This is mainly due to limitations in the diagnostic techniques used historically to differentiate between these two causes of hearing loss. As techniques for assessing hearing loss are improving, differentiating the source of the loss is becoming more refined, thereby making determination of either sensory or neural loss possible.9 Sensory and neural loss can occur in isolation or be concurrent. Either or both can also co-occur with a conductive component, caused by a pathology-induced reduction in transmission of the sound energy through the outer and middle ears into the inner ear. A hearing loss is classified as a mixed loss when two or more (conductive, sensory, neural) stages of hearing are simultaneously affected.