CHAPTER ONE

Literature review and Introduction

1.1 Hearing loss

The ability to detect vibrations and perceive sound through ears is termed as “Hearing”. Any individual unable to hear within the normal hearing threshold (≥ 25 dB) is said to have the condition of hearing loss (http://www.who.int/mediacentre/factsheets/fs300/en/, 2017).

Table: Classification of Severity of Hearing Impairment (http://www.rehabcouncil.nic.in/writereaddata/hi.pdf, 2012)

Niparko (2012), classified hearing loss as: –

1. conductive hearing loss- caused due to lesions in the tympanic membrane, external auditory canal, or middle ear, prevents the reception of sound to the inner ear;
2. sensorineural hearing loss- caused due to lesions are present either in the inner ear or the auditory nerve; and
3. mixed loss- caused due to chronic infection, extreme head injury, genetic disorders, or during a state of transient hearing loss accompanied with sensorineural hearing loss (Listen Hear! New Zealand, 2017).

Hearing loss is amongst a frequent sensory impairment affecting 1 in 500 new-borns and 1 in 300 children by the age of 4. Approximately, 70% of genetic hearing loss is non-syndromic (i.e., hearing loss is the only phenotype), and 30% syndromic (e.g., additional clinical findings such as changes in pigmentation of the hair, skin, and eyes or individuals with Waardenburg syndrome) (Peters et al., (2002)).

In New Zealand, hearing loss is relatively common as it affects one in six New Zealanders. In 2016, the frequency of hearing loss was estimated to be 880,350 people. This figure represented ~18.9%. Also, it prevalence amongst males was greater (472,961 people) than females (407,388) (Listen Hear! New Zealand, 2017) (https://www.nfd.org.nz/our-work/education-and-prevention/).

Table: The prevalence of hearing loss among the population aged ≥65 years, by Region (DJ Exeter, B Wu, AC Lee, & Searchfield, 2015).