**2.1 Etiology**

Based on the cause of sensorineural deafness, HL is categorized into three major forms as acquired, genetic and unknown. There are many causes for hearing loss:

1. Acquired HL: infectious and pharmaceutical agents known as teratogens would affect the sense of hearing. HL could occur by physiological, biochemical or infectious factors. However, genetic background has an important effect on its occurrence. The risk factors that may affect the hearing process are as follows:

Guilford et al., 1994; Kelsell et al., 1997; Lynch et al., 1997; Eudy et al., 1998; Gorlin, 2004;

Sixteenth century Reports indicating the prevention of the deaf from marrying

Nineteenth century "The most frequent causes of congenital deafness are hereditary.. ."

1975 The forms of HL, the need for research and genetic counseling were

1992 The first locus for ADNSHL\*, DFNA1, was mapped in a Costa Rican

1994 The first and second loci for ARNSHL■, DFNB1 and DFNB2, and

1997 *GJB2* and DIAPH1 genes were discovered for DFNB1 and DFNA1

\*Autosomal domiant non syndromic hearing loss, ■Autosomal recessive nonsyndromic hearing loss

One of the main programs of the World Health Organization (WHO) is to encourage countries for the prevention of deafness [Emery, 2003]. Understanding the molecular and genetic mechanisms of HL may lead to development of new therapy and treatment approaches. Here, we review major causes leading to either syndromic or nonsyndromic

Approximately two-thirds of the HL affected children show the problem at birth and unfortunately it may not be diagnosed before the age of 3 years. HL has several classification criteria which are important for diagnosis, prognosis and treatment [Mahdieh

Based on the cause of sensorineural deafness, HL is categorized into three major forms as

1. Acquired HL: infectious and pharmaceutical agents known as teratogens would affect the sense of hearing. HL could occur by physiological, biochemical or infectious factors. However, genetic background has an important effect on its occurrence. The risk

acquired, genetic and unknown. There are many causes for hearing loss:

factors that may affect the hearing process are as follows:

1988 An X-linked form of deafness was mapped in a large Mauritian

1990 The first locus for syndromic HL, *USH2A*, was mapped

DFNA2 loci were mapped

loci, respectively

Table 1. Chronological events regarding hereditary HL.

et al., 2010a]. These criteria are summarized in table 2.

**2. Classification of hearing loss** 

Seventeenth century The mode of recessive and dominant HL

described

family

family

1968 One of the genetic forms of deafness was described

Dror & Avraham 2009].

HL.

**2.1 Etiology** 

Time Event

