Preface

Hearing loss affects 278 million people world wide and is one of the most significant health problems. Deafness can be acquired due to environmental insults including injuries and infections. Additionally, about 50 % of hearing loss is genetic and scientists have identified nearly 100 genes implicated in deafness, while many more remain to be identified. This book brings together different topics on our current understanding of hearing loss.

The first section on "Hearing" presents a brief overview about normal hearing as well as the technology and instruments involved in its measurement including audiometry and auditory brainstem response as well as methods to record otoacoustic emissions. Scientists have also investigated the listening scores of audio signals and the psychological impressions related to speech audibility. In the last chapter in this section, the suitability of the Mongolian gerbil is discussed, particularly as a model of structural and functional aspects of age-dependent hearing loss in humans.

The section on "Childhood Hearing loss" provides an overview of parental practices and behaviours related to children with and without hearing loss. The results indicated that the interactions established between mothers and children favour the acquisition and maintenance of social skills. Additionally, early detection of hearing loss and prompt intervention gives children with deafness a chance to develop spoken language comparable to those with normal hearing.

"Injuries & traumas" as cause of hearing loss are the focus of the third chapter in this book. This section includes a report on head injuries in India and the ensuing hearing loss. Symptoms of hearing loss were found in 10% of the patients with head trauma. Sensorineural hearing loss was the most common finding in the affected individuals. Additionally, perforation of the tympanic membrane is reported as a common finding in Nigeria due to trauma to the ear. The next two chapters explore the role of ototoxic agents such as solvents, pesticides and metals, and their interaction with noise in causing hearing loss. Research conducted in human subjects exposed to these chemicals indicates that these agents cause auditory dysfunction. The combined influence of noise and chemical pollutants on hearing loss confirmed the existence of an interaction between physical and chemical factors which influence the alteration of auditory function.

#### X Preface

The "Genetics" of hearing loss is discussed in a series of review articles in the next section. An overview is provided of different genes involved in nonsyndromic and syndromic hearing loss and reflects the unparalleled heterogeneity of deafness genes. Different modes of inheritance of deafness are also discussed. The genetics of recessively inherited, moderate to severe and progressive hearing loss in humans is covered in detail. Strategies are described about identification of modifiers of deafness genes. Common craniofacial abnormalities inherited together with hearing loss are discussed in the next chapter. The genes involved in causing Usher syndrome as well as different mouse models for each of these genes are also comprehensively reviewed.

The last section of the book presents the "Treatment" of hearing loss for both genetic and acquired cases of deafness. Cochlear implants are now accepted as the standard of care for children with severe to profound hearing loss. They have allowed many children to attend regular schools, and to develop their language, social and academic skills to levels that exceed those for their peers with severe to profound hearing loss using hearing aids. The efficacy of corticosteroid treatment through the eardrum and into the middle ear to treat various otologic disorders, such as Meniere's disease and sudden sensorineural hearing loss is also reviewed. The last chapter discusses an ancient Chinese technique of acupuncture for treatment of sudden sensorineural hearing loss. The authors' experimentally demonstrate improvement of the hearing loss in more than 20% of the individuals who underwent the treatment. Acupuncture has gained scientific ground after it was experimentally demonstrated that during this process adenosine is released and improves different symptoms in patients. Exactly how acupuncture may help in treating hearing loss remains to be determined.

It is hoped that the research and the reviews on various aspects of hearing loss described in this book will be of benefit to different students and the researchers working actively in this field. The coming years will see accelerated discoveries in many research areas discussed in this book, particularly in genetics of deafness due to the use of massively parallel sequencing technologies to identify disease genes. Therapies based on genetic findings for treatment of hearing loss may also become possible in future.

Ms. Martina Blecic, the publishing process manager, Ms Ana Pantar, editor relations consultant and Ms Nina Dundovic, the technical editor contributed to editing of this book.

> **Sadaf Naz**  School of Biological Sciences, University of the Punjab, Lahore 54590, Pakistan

X Preface

possible in future.

book.

The "Genetics" of hearing loss is discussed in a series of review articles in the next section. An overview is provided of different genes involved in nonsyndromic and syndromic hearing loss and reflects the unparalleled heterogeneity of deafness genes. Different modes of inheritance of deafness are also discussed. The genetics of recessively inherited, moderate to severe and progressive hearing loss in humans is covered in detail. Strategies are described about identification of modifiers of deafness genes. Common craniofacial abnormalities inherited together with hearing loss are discussed in the next chapter. The genes involved in causing Usher syndrome as well as different mouse models for each of these genes are also comprehensively reviewed.

The last section of the book presents the "Treatment" of hearing loss for both genetic and acquired cases of deafness. Cochlear implants are now accepted as the standard of care for children with severe to profound hearing loss. They have allowed many children to attend regular schools, and to develop their language, social and academic skills to levels that exceed those for their peers with severe to profound hearing loss using hearing aids. The efficacy of corticosteroid treatment through the eardrum and into the middle ear to treat various otologic disorders, such as Meniere's disease and sudden sensorineural hearing loss is also reviewed. The last chapter discusses an ancient Chinese technique of acupuncture for treatment of sudden sensorineural hearing loss. The authors' experimentally demonstrate improvement of the hearing loss in more than 20% of the individuals who underwent the treatment. Acupuncture has gained scientific ground after it was experimentally demonstrated that during this process adenosine is released and improves different symptoms in patients. Exactly

how acupuncture may help in treating hearing loss remains to be determined.

It is hoped that the research and the reviews on various aspects of hearing loss described in this book will be of benefit to different students and the researchers working actively in this field. The coming years will see accelerated discoveries in many research areas discussed in this book, particularly in genetics of deafness due to the use of massively parallel sequencing technologies to identify disease genes. Therapies based on genetic findings for treatment of hearing loss may also become

Ms. Martina Blecic, the publishing process manager, Ms Ana Pantar, editor relations consultant and Ms Nina Dundovic, the technical editor contributed to editing of this

**Sadaf Naz** 

Pakistan

School of Biological Sciences,

University of the Punjab, Lahore 54590,

**Part 1** 

**Hearing**

**Part 1** 

**Hearing**

**1** 

*Mexico* 

Josefina Gutierrez

*National Rehabilitation Institute* 

**Technology for Hearing Evaluation** 

The hearing system is composed of several components that, by means of a physiological process, break down the broad spectrum of frequencies and intensities of sounds from the environment (speech, music, signals and noise) into frequency components and temporal patterns. These acoustic signals are transmitted to the temporal lobes of the Central Nervous System by electric stimuli to generate the neural message (Salesa et al., 2005). The hearing process utilizes acoustic, mechanical and electrical principles, and in addition analyzes sound waves, removes noise and compares these with signals that have been previously registered in the memory of the subject. In this manner, we are able to know when a person is speaking, when we are hearing a musical note from a violin or a flute, or when a bird is

The auditory system possesses a wide dynamic range for perceiving sounds; humans with normal hearing detect tonal frequencies from 20 Hz to 20 kHz. Acoustic intensity is proportional to sound vibration amplitude. Commonly, sound intensity is measured in terms of decibels as dB = 10 log(*I*/*I*0), where *I*0 is the reference intensity, or equivalently for acoustic pressure, dB = 20 log(*P*/*P*0), where *P*0 is the reference pressure (in Pa). Tonality represents the number of vibrations in time and is measured in cycles/sec or Hz. Timbre characterizes all of the harmonics overheard in a clear sound, allowing differentiation

The dynamic range at 2-4 kHz*,* the span between threshold and pain, is approximately 120 dB. The minimum threshold for sound occurs between 2 and 5 kHz and is approximately 20μPa. At the low end of the auditory spectrum, the threshold is 80 dB higher, while at the high end, it is 70 dB higher. Intensity differences of 1 dB can be detected, while frequency

Sounds are normally transmitted both by Air conduction (AC), and by Bone conduction (BC). Conduction of sound starts in the pinna or auricle and passes through the external auditory canal on its way to the eardrum, where sounds are amplified to frequencies ranging from 5,000-6,000 Hz at 20 dB. Later, the pressure and strength of the sound wave vibration that reaches the tympanic membrane, particularly of low frequencies of up to 1,500 Hz, are amplified in the middle ear. The acoustic vibration of the sound is thus transformed into a mechanical vibration to be transmitted to the organ of Corti, located in the cochlear duct. The cochlea is a hydromechanical frequency analyzer whose major role is to turn the acoustic signal into a frequency map through which each frequency is assigned to certain

differences of 2-3 Hz can be detected at frequencies below 3 kHz (Aitkin, 1990).

between two sounds with the same tonality and intensity.

**1. Introduction** 

singing.
