This week at Hearing International we are pleased to have two guest authors, Mr. Arun Kumar Yadav and Mr. Himanshu Kumar Sanju from the Department of Audiology and Speech Language Pathology, Amity Medical School, Amity University, Gurgaon, Haryana, India.
Introduction
According to ASHA (2005), Central Auditory Processing Disorder (CAPD) refers to ‘difficulties in the perceptual processing of auditory information in the Central nervous system as demonstrated by poor performance in one or more of the auditory and temporal skills such as sound localization and lateralization, auditory discrimination, auditory pattern recognition and temporal aspects of audition:
- CAPD affects a diverse population and has been assumed to be associated with many different causes which include neuroanatomical anomalies, neuromaturational delay and neurologic insult of the central auditory nervous system (CANS). The main complaints and symptoms associated with CAPD are auditory, however, due to abnormal brain organization, functional discrepancies can commonly appear in areas of concentration, attention, cognition, language, communication, and learning (Boscariol et al, 2017).
- Temporal processes constitute the major component of auditory functions such as auditory discrimination, binaural interaction, pattern recognition, localization and lateralization, speech redundancy, and binaural integration but in CAPD populations, the temporal processing is poor (Schow et al, 2000). The physiology behind neural mechanisms for temporal processing have been evaluated by various behavioral and objective tests. Behavioral tests require many auditory processes such as attention, memory and perception.
- The Central Auditory Processing Disorders (CAPD) have acknowledged significant consideration over the last few years. In the present era, CAPD is not a novel condition under discussion especially in hearing science. For many decades, audiologists have been conscious that some persons with normal test findings for peripheral functions are delinquent in understanding speech. CAPD has always been associated with the problems concerning the processing of auditory signals, and audiologists have been consulted to undertake this diagnosis of APD based
upon a series of tests. A new perspective towards which the attention of the audience has been drawn is associated with the controversies regarding the definition of APD, the mixed nature of APD, and an appropriate test series for APD assessment (Hurley, 2004). - The controversy primarily relates to the characterization of APD with multi-sensory deficits. Some researchers claim that if multi-sensory deficits are existing, then the identification of APD with the discussed traits are unsuitable, and the diagnosis is only suitable where it involves only a single auditory deficit (Cacace and McFarland, 1988) (See Figure 1, right).
Symptoms of CAPD
A person with APD experiences difficulty in auditory discrimination and shows reduced abilities to differentiate between speech sounds. Persons with CAPD may experience difficulties in understanding speech amidst background noise. Individuals with CAPD have trouble in understanding what is being said to them in noisy situations like cafeteria, railway 
station, bus stop, road traffic and classroom. Individuals with CAPD may process thoughts and ideas slowly but may have difficulty in explaining them. They may experience discrepancies between expressive and receptive language skills and difficulties in auditory memory. Individuals with CAPD may be unable to remember auditory information or follow multiple instructions. Persons with CAPD may show poor musical abilities and not be able to recognize sound patterns or rhythms, and have poor vocal prosody in speech production.
Noise-induced hearing loss (NIHL)
Noise is the most common occupational and environmental hazard. Noise can be described in terms of intensity (perceived as loudness) and frequency (perceived as pitch). Together the intensity and duration of noise exposure determines the potential for damage to the hair cells of the inner ear. Even sound perceived as comfortably loud can be harmful. After presbycusis, noise-induced hearing loss (NIHL) is the second most common form of sensorineural hearing loss. Shearing forces caused by any form of sound may have an impact on the stereocilia of hair cells of the basilar membrane of the cochlea, when excessive, these forces can cause cell death. NIHL begins at the higher frequencies (3000 to 6000 Hz) and develops gradually because of chronic exposure to excessive sound levels (Rabinowitz, 2000). Prolonged exposure to noise at high intensity is associated with damage to the sensory hair cells of the inner ear and development of permanent hearing threshold shift, as well as poor speech in noise intelligibility. To be audible, sounds must apply a shearing force on the stereocilia of the hair cells lining the basilar membrane of the cochlea. When extreme, this force may lead to cellular metabolic overload, damage of the cell and cell death. NIHL therefore denotes excessive “wear and tear” on the delicate inner ear structures. Other than hearing loss, Temporary Threshold Shift (TTS) and tinnitus are two other common symptoms related to NIHL. TTS is a temporary change in hearing sensitivity following an exposure to noise. TTS exists for a short duration of time.
Literature reported that TTS on a regular basis can lead to Permanent Threshold Shift (PTS) i.e. a permanent change in hearing sensitivity (damage of cochlea). The noise exposure causing TTS changes the delicate micromechanics of the cochlea, which includes linkages between hair cell stereocilia, and indicates that the reversibility of such insults may not be 100%. The other warning sign for NIHL is tinnitus. Chronic tinnitus is experienced after a prolonged exposure to noise as a warning sign of cochlear damage. The ringing sound is considered to be caused by hair cells and neurons which are actually in the process of damaging the hair cells. Hair cells produce a neural injury discharge because the breakdown of cell membrane causes repeated depolarization (excitation) and uninhibited release of neurotransmitters. It is well established that the initial neural injury discharge extablishes (synaptic) connections in a network of auditory neurons at a more central (cortical) brain level, and these cells continue to fire spontaneously. There is also sufficient evidence supporting the fact that noise exposure frequently leads to tinnitus which might be due to alterations of the central auditory function (Henderson et al, 2011). Recently, a study done by Henry and Heinz (2016) reported distorted tonotopic coding of temporal envelop and fine structure among noise induced hearing loss subjects. They reported that coding of slower envelop structure lost the tonotopicity in cases of noise induce hearing loss. They also found that in apical fibers broadened tuning curve were observed without a significant shift in best frequency. These distortion of temporal fine structure interrupt tonotopicity, an essential principle of auditory processing required for robust signal coding. In the adult population, it may significantly influence the quality of life, and constitute a major limitation in relation to hearing-critical jobs, decreasing the potential worker’s chance of employment. Thus, NIHL not only affects
hearing , but is also a major social problem.
Conclusion
According to Henderson et al. in 2011, prolonged exposure to noise of high intensity is related with damage to sensory hair cells and frequent exposure leads to tinnitus which might be due to alterations in the central auditory system. They have investigated the influence of exposure to annoying quality of noise reduces the processing capacity and injures the auditory system. Existing literature suggests that damage to the peripheral auditory system is supported by evidences (Henderson et al, 2011). But, existing studies showed the chance of damage to the CANS in this population. There are no literature records till date, investigating central auditory processing evaluation among individuals working in noisy environment. So, there is a need to investigate central auditory processing in individuals working in noisy environment. Early diagnosis and intervention of CAPD could help in providing a better quality of life for individuals working in noisy environment. So, CAPD lies within the role and scope of practice of industrial audiologists to evaluate central auditory processing in the affected individuals on a regular basis other than a routine peripheral audiological evaluation, as prevention is better than cure.
About the Guest Authors:
Arun Kumar Yadav has post-graduate degree in Audiology and Speech Language Pathology from Amity University Haryana, India and is currently working as a faculty at Department of Audiology and Speech-Language Pathology at Amity Medical School, Amity University, Gurgaon, Haryana, India. Arun Kumar Yadav has expertise in noise induced hearing loss, hearing aids, implantable device and electrophysiology.
Himanshu Kumar Sanju has a post-graduate degree in Audiology from All India Institute of Speech and Hearing, Mysore-6, Karnataka, India and is currently working as an Assistant Professor-1 at Department of Audiology and Speech-Language Pathology at Amity Medical School, Amity University, Gurgaon, Haryana, India. His areas of special interest are vestibular assessment and management, electrophysiology, hearing aids, psychoacoustics, implantable devices and CAPD.
References:
American Speech-Language-Hearing Association. (2005) (Central) auditory processing disorder. www.asha.org. Retrieved August 22, 2017.
Boscariol M., Garcia V., Guimarães, C., Montenegro, M., Hage, S., Cendes, F., & Guerreiro, (2010).. Auditory processing disorder in perisylvian syndrome. Brain Dev.,
32(4):299-304. Retrieved August 22, 2017.
Cacace, A., Mcfarland, D.(1988). Central auditory processing disorder in school-aged children: a critical review. J Speech Lang Hear Res., 41(2):355-73. Retrieved August 22, 2017.
Henderson, D., Bielefeld, E., Lobarinas, E., & Tanaka, C. (2011). Noise-induced hearing loss: implication for tinnitus. In Textbook of Tinnitus (A. Moeller, Ed), Springer: New York.301-309.
Henry, K., Kale, S. & Heinz, M. (2016). Distorted Tonotopic Coding of Temporal Envelope and Fine Structure with Noise-Induced Hearing Loss. J Neurosci. Vol. 36(7):2227-37.
Hurley, A. (2004). Behavioral and Electrophysiological Assessment in Children with a Specific Temporal Processing Disorder (Doctoral dissertation submitted to the faculty at Louisiana State
University). Retrieved August 22, 2017.
Rabinowitz, P.(2000). Noise-induced hearing loss. American family physician. Vol. 61(9):2759-60. Retrieved August 29, 2017
Schow, R., Seikel, J., Chermak, G. & Berent, M.(2000). Central auditory processes and test measures: ASHA 1996 revisited. Am J Audiol. 9(2):63-8. Retrieved August 22, 2017
