It is remarkable that deafness is a component of over 400 more complex different genetic syndromes (e.g., Waardenburg, branchio-oto-renal, Stickler, Pendred, Usher, Alport, Bartter, among many others listed in the table and those omitted because of their rarity). Among these, the finding of a mutation in a gene called PAX3 in the Waardenburg syndrome in the early 1990s began a flood of other gene defects underlying the many disorders that had been described on clinical grounds over the previous century. The mutations that give rise to some of these diseases, particularly the Usher syndrome, may also cause nonsyndromic congenital deafness. The syndromic forms of genetic deafness have been classified largely on the basis of their associated defects: retinitis pigmentosa, malformations of the external ear; integumentary abnormalities such as hyperkeratosis, hyperplasia or scantiness of eyebrows, albinism, large hyperpigmented or hypopigmented areas, ocular abnormalities such as hypertelorism, severe myopia, optic atrophy, and congenital and juvenile cataracts, cerebellar ataxia, myoclonus, and mental deficiency; skeletal abnormalities; and renal, thyroid, or cardiac abnormalities. Deafness is also a feature of several mitochondrial disorders, particularly the Kearns-Sayre syndrome and occasionally the MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) syndrome. The Wolfram syndrome, of which sensorineural deafness is a major feature, can have either a nuclear or mitochondrial genetic origin. Table 14-1 summarizes this and the other main hereditary syndromes. Chinnery et al have summarized the mitochondrial causes of deafness. Chapters 37 and 39 discuss further the association of neurosensory deafness with degenerative and developmental neurologic disease.
Differing from the degenerations is a group of acoustic aplasias. Four types of inner ear aplasia have been described: (1) Michel defect, a complete absence of the otic capsule and eighth nerve; (2) Mondini defect, an incomplete development of the bony and membranous labyrinths and the spiral ganglion; (3) Scheibe defect, a membranous cochleosaccular dysplasia with atrophy of the vestibular and cochlear nerves; and (4) rare chromosomal aberrations (trisomies) characterized by abnormality of the end organ and absence of the spiral ganglion.
It is possible to distinguish hysterical and feigned deafness from that caused by structural disease in several ways. In the case of bilateral deafness, the distinction can be made by observing a blink (cochleo-orbicular reflex) or an alteration in skin sweating (psychogalvanic skin reflex) in response to loud sound. Unilateral hysterical deafness may be detected by an audiometer, with both ears connected, or by whispering into the bell of a stethoscope attached to the patient’s ears, closing first one and then the other tube without the patient’s knowledge. The elicitation of the first two waves of the brainstem auditory evoked potentials provides indisputable evidence that sounds are reaching the receptive auditory structures and that the patient should be capable of hearing sounds. A brief episode of deafness with fully preserved consciousness may rarely be caused by seizure activity in one temporal lobe (epileptic suppression of hearing).