The causes of profound deafness in childhood : a study of 3,535 individuals with severe hearing loss present at birth or of childhood onset / George R. Fraser.

  • Fraser, George R. (George Robert)
Date:
[1976], ©1976
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: The causes of profound deafness in childhood : a study of 3,535 individuals with severe hearing loss present at birth or of childhood onset / George R. Fraser. Source: Wellcome Collection.

    318/440 (page 298)
    unfortunate since the risk that they will give birth to deaf children is, of course, rather considerable. Some corroborative evidence concerning the presence of freshly arisen mutations may be obtained from a study of parental ages at the birth of the deaf individual, since these ages, especially paternal, tend to be raised in such cases (see Chapters VII and VIII). In principle, this leads to an increase above control values in the difference between the paternal and the maternal ages, which could be used to obtain an independent estimate of the number of cases contained in the group of unidentified cause which are due to freshly arisen mutations for autosomal dominant deafness; in the individual case, such evidence may be suggestive but, of course, not conclusive. The group of unidentified cause is, however, an extremely heterogeneous one, and deviations from normal parental age distributions are to be expected in several of the main etiological subgroups. Thus, the simplex recessive cases will tend to be drawn from smaller families and, therefore, to have lower birth ranks than expected, since the larger the family the more likely is the recessive nature of the hearing loss to be revealed by the birth of a second deaf child. These deviations of birth order and sibship size from the distributions applying in the general population will, of course, be accompanied by corresponding deviations in the distributions of parental age. These distributions are also somewhat deviant in the case of acquired deafness, both when the cause acts in the perinatal period (see Chapter XII) and subsequently (see Chapter XIII). Nevertheless, a study of parental age distributions does afford some indications that cases due to freshly arisen mutations are included in the group of unidentified cause, even though no good estimate can be made of their number. Thus, 722 children in this group, where data concerning parental ages are known, contain 71 (9.8 percent) where the age of the father is greater than that of the mother by 11 years or more, while this proportion is 24 of 474 or 5.1 percent among children deaf due to acquired causes acting in perinatal life or later. Thus, a very large difference in age of this type may afford some evidence concerning the presence of autosomal dominant deafness, due to a freshly arisen mutation, in cases who have no deaf relatives and in whom no indication of the cause of hearing loss can be obtained by clinical examination or through the anamnestic history. The study of the distribution of actual parental ages rather than their difference, however, throws little additional light on problems of causation in the group of unidentified cause, presumably, as mentioned above, because of the extreme etiological heterogeneity of this group. Thus, both paternal and maternal ages in the group as a whole, at 32.4 and 28.4 years, are very near the values for the entire school study (32.5 and 28.8 years). Fisch (1971) has claimed that the group of unknown cause in his survey of 600 cases of congenital deafness (corresponding to the group of unidentified cause here) contained a large excess of first-born children. However, this group contains a substantial proportion of simplex cases of recessive deafness which, as discussed above, will contain an excess of children of lower birth ranks, including those who are first-born. Thus, to obtain a birth rank distribution which can be compared with control data, either the cases subsumed under the hypothesis of recessive inheritance of Table XIV-4 must be removed, on the assumption that they are distributed randomly in the birth ranks of their sibships, or the multiplex cases of Table XIV-4 must be added to the group of unidentified cause. Table XIV-7 shows the results of these calculations. It is clear that the rather substantial apparent excess of first-born children in the group of unidentified cause has been reduced by both corrections to a figure which is very little higher than that applying to the school study as a whole. 22 The slight excess remaining is probably explicable in terms of the 122 22 Fisch (1971) found 47 first-born children among 151 with genetic causes for their deafness, or 29.9 percent, and 79 among 151 with unknown causes, or 52.3 percent. He claimed that this last figure was sig-