(B) IMMEDIATE CAUSES.—According to recent statistics, in about one-half of the cases of acquired deaf-mutism the deafness is acquired during the first three years of infancy, the greater number of cases falling in the third (statistics from the -United States) or the second (Euro pean statistics) year of life; then comes the fourth, the first, the fifth, sixth, and so on.
Brain Diseases.—These play an im portant part in deafness acquired after birth and resulting in deaf-mutism. The Irish statistics of 1881 show the lowest figure, viz.: 11.9 per cent.; and the Pom eranian report the highest, viz.: 54.5 per cent. It will be seen that the im portance of brain diseases in the etiology of deaf-mutism varies considerably in the different countries; this is not only due to the circumstance that the expression "brain disease" includes different affec tions in the different reports, but also to the varying intensity with which cer ebral disease appears at different times and at different places. All modern in vestigators agree, however, that brain diseases are at present the predominant cause of acquired deaf-mutism.
There can be no doubt that the most frequent brain disease leading to deaf mutism is epidemic cerebrospinal menin gitis, the deleterious influence of which has been especially pointed out by Moos. We possess various clinical observations of partial or complete deafness caused by epidemic cerebrospinal meningitis, and post-mortem examinations of persons whose deafness is due to this disease or other similar brain diseases, which eluci date the manner in which cerebral affec tions act deleteriously upon the infantile organs of hearing. The great conformity which exists between the changes in the auditory organs caused by cerebrospinal meningitis and changes declared to be due to inflammation of the brain in gen eral, or to other diseases with pronounced cerebral symptoms, authorizes us to sup pose that the facts related in the follow ing paragraphs hold good for the ma jority of cases of deaf-mutism caused by acute brain disease.
Clinical experience teaches us that the very considerable defects in hearing which appear during epidemic cerebro spinal meningitis may have a dual origin, viz.: inflammation of the middle ear or an affection of the labyrinth. Loss of hearing from the former cause is, how ever, seldom so considerable or so lasting as to result in deaf-mutism. Deafness resulting from labyrinthine disease is more rare, but, at the same time, of more importance, since the loss of hearing is, as a rule, very considerable, often, indeed, total, generally affecting both sides, and nearly always permanent. According to Moos and Knapp, labyrinthine deafness in epidemic cerebrospinal meningitis generally appears suddenly, seldom grad ually. As a rule, it appears in the course of the first two weeks, but may also show itself later; Knapp reports a case where it appeared even six weeks after the com mencement of the disease.
Acute Infectious Diseases.—The im portance of this group of diseases in the etiology of deaf-mutism is doubtless at present less marked than that of brain diseases. If, however, epidemic cerebro meningitis is included among acute in fectious diseases,—to which group it doubtless belongs,—they immediately as sume a very prominent place, and there can be no hesitation in declaring that the great majority of cases of deaf-nautism caused by acquired deafness are the re sult of acute infectious diseases. The importance of the parts played by the different diseases varies greatly, as will be seen, scarlet fever predoininating.
Scarlet fever (scarlatina). This dis ease has always and in all countries been recognized as a very frequent cause of infantile deafness, and, consequently, of deaf-mutism. The influence of scarlet fever on deaf-mutism differs, however, in different countries and at different times, which is doubtless due to the varying intensity and character with which the disease appears. The lowest figures are represented by statistics from Italy (1.5 per cent.), the highest from Saxony (47.6 per cent.).