Учебники / Genetic Hearing Loss Willems 2004

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Table 3 Prevalence of Hearing Impairment in Specific Subgroups
Author	Speech delay, % NICU, % VLBW + seizure
Douniadakis, 2001	27.4
Davis, 1992	3.5
Salamy, 1989	5.0
Bergman, 1985	28.6

patients the prevalence of PCHI is high. Hearing impairments are mostly mild, but severe and profound cases are also observed.

The incidence of hearing impairment in children with speech delay was higher than the one found in the general population (21).

2.Risk Factors

Concerning risk factors identified by the Joint Committee of Infant Screening (25) in recent studies, three factors were found to be significant: craniofacial abnormalities, low birth weight (less than 1500 g), and a familial history of hearing loss (22,23). The risk factor associated with the highest incidence of hearing loss was stigmata of syndromes (22). In this study, of 3000 children, a critical review of risk factors was presented. Only a small percentage of infants with a conventional risk indicator for hearing loss actually presented a hearing loss; in contrast, a significant number of hearing impairments without a risk indicator was found. In NICU population the four most common risk factors were ototoxic medications, very low birth weight, assisted ventilation > 5 days, and low APGAR scores. In contrast, in the well-baby nurseries only few risk factors were present: family history, craniofacial abnormalities, low APGAR scores, syndromes, ototoxic drugs, and congenital infections. These data suggest that risk factors, as identified by JCIH raccomendation, reflect the specific condition of the NICU population and do not correspond to actual causes of well-baby population.

B.Age of Identification

Although universal agreement exists about the need of an early identification of hearing impairment (20,26,27), identifying children with significant losses at an early age has proved to be di cult. In an EEC study of deafness on 1970–1980 birth cohorts, only 55% of children a ected by 50-dB permanent hearing loss were identified within 3 years of age (1,26).

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III.ETIOLOGY

Inherited hearing impairments account for more than 50–70% of PCHI (Table 4).

Acquired causes represent up to 40% of all factors (5,9). In some cases hearing impairment is not isolated, and additional disabilities in 23% are found. In some reports, 32% of all PCHI are progressive (29).

Thirty-five percent of all permanent hearing impairments in Germany are caused genetically and 20% are acquired (28). Causes are unknown in 45%.

Where the cause is known, genetic causes are most common, followed by infections. In these studies, genetic factors appear to be the most important cause of PCHI in developed countries, with greater incidence than was found in the last decade, probably owing to better identification. Mutations of the connexin 26 GJB2 gene have been found to account for about up to 50% of recessive nonsyndromal prelingual sensorineural hearing impairment (34–40).

About 30% of all genetic syndromes are associated with hearing impairment, characterized by di erent kinds of anomalies: craniofacial, cardiovascular, musculoskeletal, and others. In these syndromes, the eyes, central nervous system, and musculoskeletal system are often involved (41).

Much sensorineural hearing loss is of unknown cause (2–45% of all cases).

Serous otitis media is the most common cause of mild hearing loss in children, a ecting up to two-thirds of preschool children.

Table 4 Relative Prevalence of Causes (1,4,5,30–36)

		Syndromic,	Hereditary,	Perinatal,	Unknown,
Author	Genetic	%	isolated, %	%	%
Fraser, 1976	40.9			11.8	10.5
Martin, 1981	9			15	41
Parving, 1983	33			14	25
Upfold, 1988	23			4.8	42
Das, 1988	18.7			15.5	35.5
Dias, 1990	22.8			15.9	28.6
Fortnum, 1997	41			10	2
Uus, 2000	36	2		11	34
Gross, 2001	35	8.9			45
Neckham, 2001		8	22	19	36

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A.Changes

In longitudinal studies between comparable cohorts (32), born in 1969– 1977 and 1979–1987, a significant increase in congenital inherited hearing impairment, from 29% to 43%, was found. The perinatal acquired PCHI have shown a notable change during the last years regarding etiology and prevalence. Congenital rubella has been eliminated in developed countries and in those where national vaccination campaigns have been implemented.

In contrast, other infections, such as cytomegalovirus (CMV) and toxoplasmosis, and alcoholic fetal syndrom tend to increase, probably owing to an increased number of survivors in neonatal intensive care units (NICU) and better identification.

The age of identification of PCHI has improved over the years; in developed countries, the expansion of audiological services and implementation of universal screening have produced a significant reduction of age of diagnosis of moderate, severe, or profound bilateral hearing impairments. However, the age of identification of those children with unilateral hearing loss or with bilateral PCHI of mild degree remains over 5 years.

B.Perinatal Causes

1.Aminoglycosidis

Ototoxic drugs have been identified as causative factors of PCHI. In recent studies these do not appear to be strictly related to hearing damage, compared to previous reports. Among very preterm babies, the coexistence of risk factors for hearing loss may be more important than the individual factors themselves (42). Sensorineural (SNHL) was more likely if bilirubin levels coexisted with netilmicin use or if acidosis occurred when bilirubin levels were high. Aminoglycosidis is not an important risk factor when serum levels are monitored (26).

2.Prematurity

Many children born as extremely preterm (<25 weeks) have neurological and developmental disabilities (43). Prematurity causes a high risk of hearing impairment, often associated with additional handicaps; severe disability is common among children born as ‘‘fragile infant.’’ In a longitudinal study (44) on very preterm and very low-birth-weight babies (VLBW), the prevalence of sensorineural hearing loss present at 5 years of age was 15 times as high as in normal children of the same age. Birth weight between 1000 and 1500 g and gestational age <31 weeks, in particular the NICU

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population, are predictive indicators for hearing impairment (25). On the other hand, improved medical treatment in a NICU reduces the probability of hearing impairment.

3.CMV and Other Viral Infections

CMV infection is the most frequent cause of congenital infection in both phenotypes, symptomatic and asymptomatic, occurring in 0.4–2.3% of live births (45). In developed countries, CMV infection is the only relevant viral agent of sensorineural hearing loss, because of vaccination campaigns for rubella, measles, and mumps in the last decade (43). Hearing loss consequent to this infection can show a delayed onset or progressive deterioration of hearing, necessitating long-term clinical follow-up. Less than half of all congenital SNHL, CMV infection is present at birth (46). Late-onset hearing loss occurs throughout the first 6 years of life, with a cumulative incidence of 15.4%. Both asymptomatic and symptomatic infection develop SNHL, in 7.4% and 40.7% of cases, respectively (47,48). In the symptomatic group, CT scan abnormalities are present in 70% of cases, usually developing at least one neurological sequela, also associated with SNHL (46,49).

C.Meningitis

Meningitis is responsible in 4–10% of all cases of sensorineural hearing loss, 37% of perinatal cases (50–52) (4%, Newton, 1885; 4.2%, Drake, 2000, 7%, Parving, 1985; 10%, Derekoy, 2000; 37% among perinatal hearing losses in Martin, 1982), also depending on collection criteria (overall or selected population, date of reports, and timing of clinical evaluation). The infection causes hearing loss by a ecting the labyrinth, spreading from the meninges through aqueductus cochleae, or damaging the cochlear nerve as in cryptococcal meningitis (53). Common agents are Streptococcus pneumoniae,

Hemophilus influenzae, Escherichia coli, viral agents (mumps and measles), and rarely tuberculosis (54); 10–28% of patients with bacterial meningitis develop sensorineural hearing loss as a sequela (55). Hearing loss is usually bilateral and severe (55,57; 63% in Yeat, 58); there have been cases of unilateral and mild hearing loss. Negative prognostic indicators for hearing are considered coma and cranial computerized tomography alterations (52).

Labyrinthus ossificans is described as a sequela of meningitis and needs early intervention for cochlear implant. On the other hand, cases of spontaneous recovery of sensorineural hearing loss have been observed (59), with partial or complete recovery, suggesting the need for repeated audiological assessments in children a ected by bacterial meningitis. Deafness may be progressive, causing a long-term deterioration of hearing (60). Dexametha-

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sone may be beneficial in some cases of bacterial meningitis in adults; neurological complications with hearing loss are less frequent in patients treated with dexamethasone (61,62). Steroids play a role in preventing the development of labyrinthus ossificans even in children with pneumococcal meningitis. Recently the antipneumococcal vaccine has e ectively prevented this infection.

In ‘‘meningitis belt’’ countries, in middle, west, and central Africa, meningococcal infection represents the most common cause of severe to profound hearing loss (63–65). Epidemic meningitis is frequent in the overall population, causing death and neurological damage; Hemophilus influenzae is most dangerous in children under 12 months of age, causing up to 50% of death in a long-term period. In sub-Saharan Africa, many thousands of survivors annually su er severe to profound hearing loss. In Gambia, endemic childhood meningitis has many severe sequelae, including hearing loss, in 29% of pneumococcal survivors and 9% of Hemophilus survivors (65).

D.Unilateral Hearing Loss

Permanent unilateral hearing impairment is considered a risk factor concerning speech and learning acquisition. An early-onset, severe unilateral hearing loss in children may be associated with academic or behavioral problems in school (31%, Omaha study, 67). It may represent onethird of all PCHI (66) > 25 dB HL. Unilateral defect is often found to be severe or profound (10% in Brookhouser, 67). It is characterized by a delayed diagnosis (>5 years) and a di cult identification of causes (34– 60% unknown).

IV. EPIDEMIOLOGY AND ETIOLOGY OF HEARING IMPAIRMENT IN DEVELOPING COUNTRIES

The proportion of hearing impairment is estimated to be wider in developing countries than in developed countries (15–18). However, the number of people a ected in these countries is not yet known, because of a lack of valid epidemiological surveys. Little information is available about prevalence estimates and proportion of causes. In most cases, applied criteria for classification and diagnosis of hearing loss are not standard. WHO estimated that at least 78 million people have disabling hearing impairment in the developing countries (63). In a recent survey, conducted on a randomly selected sample of the WHO protocol in four developing countries, the prevalences are more than twice as great as the global result of 2.1% in the world. This result points out that prevalences of hearing impairment in the world could

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be largely underestimated. A total of 12–17% of children under 5 years of age were estimated to be a ected by permanent hearing loss, owing to poor conditions of life and lack of availability of ear care services. It has been estimated that 50% of the causes of hearing impairment, such as chronic otitis media (COM), could be fully prevented. Some causes of deafness are due to poor health intervention; other causes vary by region. In some regions meningitis is an endemic infection; in other regions consanguineous marriages are common, increasing the occurrence of inherited deafness. In the Indian school population inhertited deafness was found in up to 80%, owing to common marriage between first cousins. In Pakistan 70% of severe PCHI was the result of marriage between cousins. COM probably represents the major cause of mild to moderate hearing impairment, related to poverty in developing countries and to disadvantaged ethnic groups in developed countries. Congenital exogenous and perinatal problems, such as rubella, asphyxia, and use of ototoxic drugs, show a high to moderate frequency and are considered preventable causes. In the majority of undeveloped countries, vaccination has not been undertaken and rubella remains a significant cause of congenital problems. Syphilis is present in several part of the world and it is associated with a high risk of congenital hearing impairment. The lack of preventive care during pregnancy and delivery causes a globally high mortality and morbidity in newborns. Infections, birth trauma, and rhesus incompatibility remain frequent causes of perinatal hearing loss.

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