We may perhaps best approach the problem of deafness as an increasing or decreasing phenomenon in the population, if we think of the deaf as composed of two great classes: those adventitiously deaf, that is, those who have lost their hearing by some disease or accident occurring after birth, and those congenitally deaf, that is, those who have never had hearing.[17] In regard to the former class, it follows that we are largely interested in the consideration of those diseases, especially those of childhood, which may affect the hearing, and in their prevention or diminution we can endeavor to ascertain how far there are possibilities of reducing the number of the deaf of this class. In the latter case we are called upon to examine some of the great problems involved in the study of heredity, especially in respect to the extent that the offspring is affected by defects or abnormalities of the parent, and to see what, if any, means are at hand to alter conditions that bring about this form of deafness. We shall first discuss the causes of adventitious deafness, together with the possibilities of its prevention and the likelihood of its diminution, and then consider the questions involved in congenital deafness.
Adventitious Deafness and its Causes
From three-fifths to two-thirds of the cases of deafness are caused adventitiously—by accident or disease. To accidents, however, only a very small part are due, probably less than one-fiftieth of the entire number.[18] Nearly all adventitious deafness results from some disease, either as a primary disease of the auditory organs, or as a sequence or product of some disease of the system, often one of infectious character, the deafness thus constituting a secondary malady or ailment. The larger portion is of the latter type, probably less than a fourth resulting from original ear troubles.[19] In either case deafness occurs usually in infancy or childhood, and does its harm by attacking the middle or internal ear.
From diseases of the middle ear results over one-fourth (27.2 per cent, according to the census) of all deafness, and from diseases of the internal ear, one-fifth (20.7 per cent), very little (0.6 per cent) being caused by disorders of the outer ear. Of the classified cases of deafness, according to the census, 56.3 per cent are due to diseases affecting the middle ear, and 42.7 per cent to diseases affecting the internal. Of diseases of the middle ear, 72 per cent are of suppurative character, often with inflammation or abscess, and 28 per cent non-suppurative, or rather catarrhal in character. Of diseases of the internal ear, 89 per cent are affections of the nerve, and 10 per cent of the labyrinth. It is to be noted that when the affection is of the internal ear, the result is usually total deafness.
By specified diseases, the leading causes of deafness are scarlet fever (11.1 per cent), meningitis (9.6), brain fever (4.7), catarrh (3.6), "disease of middle ear" (3.6), measles (2.5), typhoid fever (2.4), colds (1.6), malarial fever (1.2), influenza (0.7), with smaller proportions from diphtheria, pneumonia, whooping cough, la grippe, and other diseases. A large part of deafness is seen to be due to infectious diseases, the probabilities being that fully one-third is to be so ascribed, with one-fifth from infectious fevers alone.
After birth and under two years of age, the chief causes of deafness are meningitis, scarlet fever, disease of middle ear, brain fever, and measles. From two to five scarlet fever and meningitis are far in the lead, with many cases also from brain fever, disease of middle ear, measles, and typhoid fever. From five to ten scarlet fever alone outdistances all other diseases, followed in order by meningitis, brain fever and typhoid fever. From ten to fifteen the main causes are meningitis, scarlet fever, brain fever, and catarrh; from fifteen to twenty catarrh and meningitis; from twenty to forty catarrh, colds and typhoid fever; and from forty on, catarrh.
The following table[20] will show in detail the several causes of deafness and their respective percentages.
CAUSES OF DEAFNESS
In fairly approximate agreement with the returns of the census, are the records of the special schools for the deaf in respect to the causes of deafness in their pupils, with information also as to the amount from the minor diseases. The following table will give the causes by specific diseases, as found in one school, the Pennsylvania Institution, for two years:[21]