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The causes of congenital deafness
There
is a very small number of children born deaf because the
conducting apparatus has failed to develop in part or in 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 uncanalized 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. |
- 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 earliest hours or days after birth.
1. 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 carry the affected gene, the chances of the offspring being affected as as high as 50 per cent.On the other hand, in the recessive type in which both parents must be carriers of the particular gene, only 25 per cent of the offspring are affected. The former mode of transmission accounts for approximately 10 per cent of cases of hereditary congenital deafness, the latter for the remaining majority of about 90 per cent. And the hereditary group as a whole probably accounts for one-third of all cases of congenital deafness.
2. 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 are more or less fully developed and appear 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 syphillis is correctly included in this group because, although the symptoms of deafness is rarely present at birth, the noxious influences of the syphilitic disease are transmitted from the infected mother to her child during pregnancy. 3. 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. |
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