Noise-Induced Hearing Loss and its Impact on the Ear

Noise Noise & The Ear & The Ear Prof Ehab Taha Yaseen, FICMS, FRCS Head of Al-Yarmouk center for postgraduate study in Otolaryngology Supervisor of the Master s degree in Clinical Audiology and Balance Consultant Otolaryngologist

Introduction Introduction - Noise is defined as an undesirable or unwanted sound with potential to harm the ear . - Noise-Induced Hearing Loss (NIHL) is hearing loss caused by prolonged exposure to noise. - It is characterized as sensorineural hearing loss and is usually bilateral, irreversible, and progressive while the exposure to noise continues. - Is a predictable and preventable disease with an epidemiologically relevant prevalence in urban communities.

- The hearing loss begins and predominates in the frequencies of 3, 4, and 6 kHz and eventually progresses to 8, 2, 1, 0.5, and 0.25 kHz. - A notch at 4 kHz (4K notch) in the audiogram is a basic characteristic of noise-induced hearing loss (NIHL). According to the resonance theory, the classic 4K notch in NIHL is caused by the physiological properties of human external auditory canal. - NIHL is the 2ndcause (age related HL as the most prevalent). - NIHL: is a complex condition that is influenced by environmental and genetic factors.

- Genetic association studies have identified genetic factors primarily related to oxidative stress that influence an individual's susceptibility to NIHL. - Current research on the administration of certain antioxidants before or after noise exposure shows promise.

Measurement of Noise Measurement of Noise - The temporal patterns of environmental noise are typically described as continuous, fluctuating, intermittent, or impulsive (impact). - Sound Pressure Level (SPL) is the amount of noise, measured by a sound- level meter in decibel (dB) units using a frequency-weighting formula called the A-scale. - The dBA-scale metric of sound level essentially mimics the threshold- sensitivity curve for the human ear, so the low-frequency and high- frequency components are given less emphasis as auditory hazards. - Types of Sound-level meters: 1) Standard sound-level meters have electronic networks designed to measure noise magnitude automatically in dBA.

2) Peak reading sound level meter: measure impulse noise. 3) The personal noise dosimeter is typically used to measure noise exposure in the workplace. - It provides readout of the noise dose, or the percent exposure experienced by a single worker, typically over a specific shift. - The logging dosimeter integrates a function of sound pressure over time and calculates the daily (8-hour) dose with respect to the current permissible noise level for a continuous noise of less than or equal to 85 dBA lasting 8 hours. - More recently, personal noise dosimeters have been offered to the consumer as a portable, compact, and affordable device that can be used as hearing protectors.

Nature of the Hearing Loss Nature of the Hearing Loss It depends on: 1. Level of sound 2. Duration 3. Re-exposure before recovery Damage to the peripheral auditory end-organs depends on the above risk factors and will results in: 1. Temporary Threshold shift 2. Permanent Threshold shift

Temporary Threshold Shift Temporary Threshold Shift - Results from exposures to moderately intense sounds, such as might be encountered at orchestra concert. - Associate hearing problems include: 1) Reversible HL 2) Elevated thresholds (particularly for the higher midfrequency region that includes the 3- to 6-kHz frequencies). 3) Tinnitus. 4) Loudness recruitment. 5) Muffled sounds. 6) Diplacusis. 7) Delayed recovery, depending on the duration of the exposure, can occur over periods ranging from minutes to hours and days.

Permanent Threshold Shift Permanent Threshold Shift - There is irreversible elevation in hearing thresholds due to acoustic over stimulation. - PTS separated in two distinct classes: 1) Acoustic trauma, is caused by a single, short-lasting exposure to a very intense sound (e.g., an explosive blast), and results in a sudden, usually painful, loss of hearing. Violent changes in air pressure can produce direct mechanical damage to the delicate tissues of the peripheral auditory apparatus, including components of the middle ear (tympanic membrane, ossicles) and inner ear (organ of Corti) 2) Chronic NIHL chronic exposure to less intense levels of sound. It involves the insidious destruction of cochlear components that eventually and unavoidably leads to an elevation in hearing levels, along with other common symptoms of hearing impairment

NIHL NIHL - Is a specific pathologic state exhibiting a recognized set of symptoms and objective findings. - Includes (1) Permanent SNHL with damage principally to hair cells, and primarily to OHCs. (2) History of a long-term exposure to dangerous noise levels (i.e., >90 dBA for 8 hours/day). (3) Gradual loss of hearing over the first 5 to 10 years of exposure. (4) Initially involve the higher frequencies from 3 to 8 kHz before including frequencies less than or equal to 2 kHz. (5) Speech-recognition scores that are consistent with the audiometric loss; (6) Hearing loss that stabilizes after the noise exposure is terminated.

- The patient will suffer from difficulties in hearing and understanding ordinary speech, especially in the presence of background noise. - Involves the sensitive midfrequency range (3-6 kHz), classically 4-kHz notch. This pattern of maximal hearing loss, with little or no loss at less than 2 kHz, typically occurs regardless of the noise-exposure environment. - The sensorineural aspect of NIHL in that thresholds for bone-conducted stimuli are essentially identical to the thresholds for air conduction. - The profile of noise-induced threshold hearing is usually symmetric for both ears, particularly for individuals who have been working in noisy industrial settings in which there are surround sounds

The development of a hearing loss caused by habitual exposure to moderately intense levels of noise typically consists of two stages: - As the length of time of exposure to loud noise increases, hearing loss becomes greater and begins to affect adjacent higher and lower frequencies Hearing levels are increased rarely beyond about 70 to 90 dB of hearing loss, on average regardless the duration of exposure.

Cochlear Damage Cochlear Damage - The primary site of anatomic damage is at the level of the mechanosensory receptors of the auditory system's end organ. - Loud sound damages the inner hair cells and OHCs of the organ of Corti. In instances involving very intense acoustic stimulation, supporting-cell elements also can be directly affected. - Damage can vary from total destruction to effects evident only in the ultrastructure of specialized subcellular regions depending on the physical attributes of the exposure stimulus: - Time-varying - Intensity, - Frequency, - Duration, - Schedule

Pharmacologic Protection from NIHL Pharmacologic Protection from NIHL This topic is Self Learning as Homework

Susceptibility Susceptibility Some ears are more readily damaged: 1) Genetic based imperfection in the physical characteristics of the cochlea (e.g., stiffness of the cochlear partition) and variability in cochlear ultrastructure (e.g., density of hair cells). 2) Age 3) Sex (> men) 4) Race (> in white) 5) Previous damage to the cochlea 6) Efficiency of the acoustic reflex 7) Smoking 8) Influence of certain disease states hypertriglyceridemia, diabetes, and cardiovascular disease represented by hypertension 9) Simultaneous uses of ototoxic drugs such as hypercholesterolemia,

Early Detection of NIHL Early Detection of NIHL Identification of individuals that are vulnerable to the long-term damage caused by continuous noise exposure. Methods: 1) Routine PTA ( Note: by the time such a loss is identified, permanent cochlear damage has occurred). 2) OAE levels (are more sensitive than PTA in detecting the early states of permanent noise-induced cochlear damage)

Other Adverse Effects Caused by Noise Other Adverse Effects Caused by Noise Noise Induced Vestibular damage is due to: 1) Nearby to the cochlea 2) Common blood supply 3) Similar ultrastructures