The aetiology of congenital deafness and audiometric patterns

The Inheritance of Hearing Loss and Deafness: A Historical Perspective

If the term "genetics" is a relatively recent proposition, introduced in 1905 by English biologist William Bateson, who rediscovered and spread in the scientific community Mendel's principles of inheritance, since the dawn of human civilization the influence of heredity has been recognized, especially in agricultural crops and animal breeding. And, later, in familial dynasties. In this concise review, we outline the evolution of the idea of hereditary hearing loss, up to the current knowledge of molecular genetics and epigenetics.

The high-pitch notched audiogram: a cohort of patients without causative noise-induced hearing loss

BACKGROUND

One still encounters opinion that hearing loss with high-pitch notched audiogram is invariably due to noise-induced hearing loss. This paper tests this misapprehension.

METHODS

A study was conducted of patients identified in a prospective manner with notched audiograms but no history of noise exposure occurring in an otolaryngological practice over a 20-year period.

RESULTS

A cohort of 26 hearing loss patients, in whom notched audiograms were not associated with historical evidence of noise exposure, was documented.

CONCLUSION

The findings confirm that a notched audiogram is not pathognomonic of noise-induced hearing loss.

Genetics & Epigenetics of Hereditary Deafness: An Historical Overview

Hearing loss (HL) is one of the most common sensory impairments worldwide and represents a critical medical and public health issue. Since the mid-1900s, great efforts have been aimed at understanding the etiology of both syndromic and non-syndromic HL and identifying correlations with specific audiological phenotypes. The extraordinary discoveries in the field of molecular genetics during the last three decades have contributed substantially to the current knowledge. Next-generation sequencing technologies have dramatically increased the diagnostic rate for genetic HL, enabling the detection of novel variants in known deafness-related genes and the discovery of new genes implicated in hearing disease. Overall, genetic factors account for at least 40% of the cases with HL, but a portion of affected patients still lack a definite molecular diagnosis. Important steps forward have been made, but many aspects still have to be clarified. In particular, the role of epigenetics in the development, function and pathology of hearing is a research field that still needs to be explored. This research is extremely challenging due to the time- and tissue-dependent variability of the epigenetic changes. Multisystem diseases are expected to be investigated at first: specific epi-signatures have been identified for several syndromic disorders and represent potential markers for molecular diagnostics.

Audiometric patterns of genetic non-syndromal sensorineural hearing loss

Sixty-five families with non-syndromal sensorineural hearing loss (NS-SNHL) of genetic aetiology were subtyped according to Gorlin et al. Individual audiogram shapes were also classified in order to detect inter- and intra-familial variations. In 48 families with an Autosomal Dominant (AD) inherited form, 26 exhibited the features of (high-frequency) progressive NS-SNHL, 12 those of mid-frequency NS-SNHL, 5 were affected by congenital low-frequency NS-SNHL; 1 kindred showed a progressive low-frequency pattern and another 1 a unilateral NS-SNHL; only 3 kindreds were affected by severe congenital NS-SNHL. Autosomal Recessive (AR) inherited forms were composed of 9 kindreds with severe congenital NS-SNHL, and 7 with moderate congenital NS-SNHL. One X-linked form was identified. AD- and AR-inherited NS-SNHL differed significantly both in severity of hearing impairment and in audiogram shapes. With few exceptions, in each family classified according to Gorlin, most of the affected subjects shared the same audiogram profile. Intrinsic progression of the disease versus ageing was studied in the larger subtype of individuals with the high-frequency loss. Gorlin's classification still remains the best system to classify NS-SNHL, and can provide a broad base to separate a very heterogeneous group of disorders. Results obtained in gene mapping in single large human families or in homologous gene search could be tested in our families. For some of them, namely those with high frequency progressive and low-frequency NS-SNHL, testing should already be feasible.

Nonsyndromic hearing loss: an analysis of audiograms

We examined features of the audiograms of 136 individuals, from 28 families, affected by nonsyndromic genetic hearing loss. There were 83 (12 families) with autosomal dominant (AD) loss, 50 (15 families) with autosomal recessive (AR) loss, and 3 (1 family) with X-linked recessive loss. The main audiogram shapes found were sloping (50.3%), residual (26.5%), and flat (21.0%). Specific shapes (ascending and U-shaped) only occurred in 3.7% of AD cases. Audiogram shapes were found to be significantly different between AD and AR families, and showed intrafamilial and interfamilial variability. In the AR group, the main shapes were residual and sharply sloping, and in the AD group, sharply sloping, flat, and gently sloping. There was a significant difference in the degree of hearing loss between AD and AR types, with AD being milder than AR. It has been shown that there is more marked intrafamilial variation in the degree of hearing loss in AD families than in AR ones. The results suggest that the audiograms of nonsyndromic hearing loss are usually nonspecific and that counseling of family members would be better based on the specific family's condition rather than on group information.

Parental suspicion and identification of hearing impairment

The contribution of parental suspicion in the original identification of a 16 year cohort of 171 children with varying degrees of hearing impairment who were screened and identified in childhood was studied. Only a quarter of the children with permanent hearing loss were identified as a result of parental concern. The presence of parental suspicion preceding the audiological diagnosis was also measured. Of the children with severe or profound deafness, the parents only suspected the presence of hearing loss in 44%. Parental suspicion was even lower for those with a mild or moderate permanent hearing loss, and for those with an otherwise symptomless conductive hearing loss caused by otitis media with effusion. Some parents did identify hearing impairment in their children, and parental suspicion should never be professionally disregarded. Most parents, however, experienced initial difficulty in recognising their children's hearing loss, even when the children were comparatively old. The study confirms the need to continue to identify deafness early by both parental vigilance and sensitive hearing screening programmes.

Pathology of congenital deafness. Present status and future priorities

The pathology of congenital hearing loss is as varied as the etiologies. Severe external ear anomalies, such as microtia, and middle ear defects have a significant incidence of accompanying inner ear aplasias, more often of the vestibular than of the cochlear end-organs. Classic inner ear aplasias are discussed, including Michel, Mondini, Alexander and Scheibe. A case illustrating close resemblance between a genetically induced Scheibe malformation and rubella inner ear deformities is present. A variety of ear disorders acquired during intrauterine or neonatal life, including cytomegalic inclusion disease, hypoxia, erythroblastosis fetalis and later-onset genetic disorders are also discussed. Illustrative case histories are presented as well as priorities for future investigations into the pathology of congenital and genetic hearing loss.

Hearing loss in very low birthweight infants treated with neonatal intensive care

The hearing of 111 perinatal intensive care survivors of birthweights 1500 g or less was assessed at a mean age of 6 1/2 years (range 4--12). These 111 infants included 86% of the long-term survivors of this birthweight cared for in the newborn unit of University College Hospital, London, during the years 1966--72. All these infants were nursed in commercially available incubators for periods ranging from 2 to 80 days (mean 37) in which the mean noise threshold was 65 dB. Ten (9%) had sensory neural nearing losses, one (1%) infant had a congenital conductive hearing loss, and 21 (19%) infants had exudative otitis media with a mean loss of 25 dB. Apnoeic attacks in the neotal period were the most significant predictors of hearing loss in these infants (P less than 0.05) and an indirect serum filirubin level of at least 170 micromol/l (10 mg/100 ml) in the neonatal period had an additive effect (P less than 0.05). There was no evidence that ambient noise had affected the hearing of these very low birthweight infants.

The etiology of neurosensory hearing defects in preterm infants

We retrospectively investigated the influence of gestational age, perinatal risk, and the duration of incubator care periods in 193 surviving preterm infants with a gestational age between 28 and 36 weeks raised in our intensive care nursery incubators from 1965--1967. 24 (12.4%) of the children showed definite neurosensory hearing defects, which were particularly encountered in the high frequencies. No correlation could be substantiated between hearing difficulties and hyperbilirubinemia, streptomycin application and gestational age. This study does not support the assumption that the duration of noise exposure in currently used incubators is a major determinant for the development of deafness in otherwise healthy preterm infants. Our study did show, however, a strong correlation between the sum of all perinatal risk factors and neurosensory hearing loss.

Effects of incubator noise on the cochlea of the newborn

The possible effects of incubator noise on the hearing of premature babies have long been debated. The type of hearing loss found in 12 low-birthweight children was examined; and the variable noise level in regularly used incubators was measured. This noise, applied to guinea pigs continuously during their second week after birth, was shown histologically to destroy a proportion of the sensory cells in the cochlea. Adult guinea pigs, however, were not vulnerable in this way. The conclusion is that there is definite circumstantial evidence of the damaging effect of many incubators on the hearing of premature infants.

Degeneration patterns in human ears exposed to noise

Various forms of sensorineural degeneration patterns related to noise exposure are illustrated in six pairs of temporal bones selected from a group of 33 male patients with histories of noise exposure. For the entire group the commonest form of lesion, associated with a 4-kHz dip in the audiogram, was a relatively diffuse degeneration in the second quadrant of the basal turn, in the 9-13 mm area. An advanced form of this lesion had a wide gap of more or less complete sensorineural degeneration affecting the entire second quadrant and displaying various degrees of extension toward the apex and base. The pattern associated with an "abrupt high-tone loss," with more or less complete hair cell and nerve degeneration in both the second and first quadrants and extending to the basal end of the cochlea, was rare. In one case this pattern appeared to have evolved from the first type of lesion as the remaining nerve fibers in the first quadrant had degenerated. The protective effect of the acoustic shadow of the head for the right ear, in shooting from the right shoulder, is demonstrated for the higher frequencies. Two almost identical cases of sharply-circumscribed single areas of degeneration in the first quadrant and one case with two such areas represent the third type of lesion. In one of these cases there was a history of firearm usage. It is postulated that this type of lesion is caused by impulse noise. In most of the material the degeneration pattern differed markedly from the diffuse degeneration seen with presbycusis. Degeneration patterns with knife-sharp transitions between completely degenerated and apparently undamaged areas appear to be characteristic of noise-induced injuries.