There is a very small number of children born deaf because the conducting apparatus has failed to develop in part or as a whole. At one end of the scale, there may be only a simple plug of solid tissues occluding the outer part of the external auditory canal; at the other extreme, it may remain un canalized throughout the whole life of its length and the ossicles may be ill-developed or malformed.
For practical purposes, however, the vast majority of all children born deaf are born with some defect of the sensorineural apparatus and such defects may caused either by failure of development before birth or by damage to the cochlea or the nerve at or about the time of birth.
It is customary to consider the cause of congenital sensorineural deafness in a chronological sequence and they fall into three main groups:
Main Causes of congenital deafness
- The hereditary group, due to genetic influences.
- The pre-natal group, due to a variety of noxious influences.
- The peri-natal group, due to any of one of a number of 'accidents' at the time of birth itself, shortly before birth, or within the earliest hours or days after birth.
The hereditary group of congenital deafness - the 'bad seed'
The incidence of hereditary congenital deafness is determined by Mendelian law, and transmission of this type of deafness may be either dominant or recessive. In dominant transmission in which only one of the parents need to carry the affected gene, the chances of the offspring being affected as high as 50 percent.
On the other hand, in the recessive type in which both parents must be carriers of the particular gene, only 25 percent of the offspring are affected. The former mode of transmission accounts for approximately 10 percent of cases of hereditary congenital deafness, the latter for the remaining majority of about 90 percent. And the hereditary group as a whole probably accounts for one-third of all cases of congenital deafness.
The prenatal group of congenital deafness - the 'damaged embryo'
Rubella (German measles) will cause congenital deafness, not infrequently progressive, only if it is contracted by the mother during the first three or four months of her pregnancy. Thereafter, the neural structure of the embryonic cochlea is more or less fully developed and appears to be almost immune from the toxic effects of the rubella virus.
Other viral infections, notably influenza, are thought to account for a small number of cases and it is also possible that certain drugs may damage the immature cochlea during the first three critical months of embryonic life.
Congenital syphilis is correctly included in this group because, although the symptoms of deafness are rarely present at birth, the noxious influences of the syphilitic disease are transmitted from the infected mother to her child during pregnancy.
The perinatal group of congenital deafness - the 'hazardous birth'
The perinatal period is a somewhat ill-defined period of a few days - possibly a week or so - around and about the time of birth.
It includes the period immediately before birth, the moment of birth itself and a short period immediately after birth. Toxaemias in the later stages of pregnancy; prematurity; birth injury, instrumental or accidental; anoxia, or lack of oxygen; and neonatal jaundice are all numbered amongst the perinatal causes of congenital deafness.
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