Severe hearing loss begins at 51 decibels (dB), with the sound measured at frequencies of 500, 1,000, and 2,000 hertz (Hz) in both ears1 (see box). The category of severe hearing loss includes the more severe conductive, sensorineural, and mixed types of hearing loss. Even smaller deficits in infancy and early childhood may have significant effects on language development, so the child's development and neurologic status need to be taken into consideration in determining whether he is at risk for hearing loss.
How frequent is early severe hearing loss? Oftenquoted statistics state that profound congenital hearing loss occurs in between one in 1 ,000 and one in 2,000 live births.2 Recently developed intensive neonatal detection methods find that one in 600 to 800 otherwise normal neonates has a profound congenital hearing loss and that one in 60 of the infants in neonatal intensive-care units has such a loss.3
Early-childhood hearing loss has its most serious effects in the first three years of life, when language basics are being formed. The causes of early-childhood hearing loss are many; there is, however, a simple, orderly classification by which one can recognize infants who are at risk, formulate reasonable diagnostic possibilities, and go about ruling them in or out.
GENETIC AND NONGENETIC CAUSES
Studies suggest that early-childhood hearing loss is of genetic origin in about 25 percent of the cases; in 42 percent the causes are nongenetic, and in the remaining 33 percent they are unknown.4 10 Ideally, the cause of a hearing loss should be determined in every child so that he may receive the best possible management, including family counseling. But it is likely that the unknown causes are either of recessive genetic etiology or due to exogenous factors, and ascertainment of the latter is usually difficult, since many children are two or three years old by the time there is a diagnostic work-up.
Why is this so? Most nongenetic early-childhood hearing losses occur during the first 15 months of life - that is, at some time during the nine months of intrauterine life or the first six months of postnatal life. It is during the six postnatal months that the cause of a hearing loss can be most easily found: viral cultures may be positive; titers and immune-globulin M more accurately reflect intrauterine infections at that period of life. Also, prenatal and perinatal records are most easily retrieved than at a later date, and the mother's recollection of her pregnancy is still fresh.
Prenatal etiologies are most likely to have occurred during the first trimester: maternal rubella and ingestion of teratogenic, ototoxic, or neurotoxic drugs are the best-known prenatal causes. Women who receive irradiation or who have diabetes - at least to the extent that these factors are teratogenic - are at risk for giving birth to children with ear malformations. Laboratory-proved rubella during the second or third trimester of pregnancy also may result in congenitally impaired hearing11,12 in between 3 and 8 percent of the live infants born to such mothers.
Toxic drugs; poor intrauterine environment due to maternal diabetes, thyrotoxicosis, uterine bleeding, and toxemia; and viruses crossing the placenta! barrier are other factors that may set the stage for deafness to occur later - prenatally, perinatally, or in the neonatal period. They may also result in an infant with extremely low birth weight (under 1,500 gm.), hypoxia, hyperbilirubinemia, and/or congenital infections. Erythroblastosis fetalis and hyperbilirubinemia are generally managed well, but occasionally infants are still seen who were severely damaged by these factors or by amniotic infection as a cof actor.13
The TORCH complex of Toxoplasma, Other, Rubella, Cytomegalovirus, and Herpesvirus type H organisms produces various defects of central nervous system and sense organs.* Rubella may also cause middle-ear defects.14,15 There is evidence to suggest that early prenatal influenza and chickenpox may be teratogenic and that the ear may be affected.16 Neonatal sepsis due to infection of the mother's genital tract occasionally, results in men-, ingitis. Infants born out of hospital or in substandard institutions may present with this complication and may already be deafened as a result.
Postnatal causes of severe conductive-hearing loss include the frequent and destructive bouts of otitis media to which the infant who has had a stormy neonatal course seems somewhat prone. Head trauma, either accidental or caused by child abuse, probably accounts for an unknown but small incidence of early-childhood hearing loss for which no cause is ever assigned. Recurrent meningitis, as a sequel of head trauma or facilitated by an anomalous communication between middle ear, inner ear, and subarachnoid spaces, is a rare cause of hearing loss.17
ASSOCIATED DEFECTS AS CAUSES
Associated defects are the reason why as many as one-fourth of all children with hearing loss are originally identified.4-10 These defects may be classified as oiobranchial, cranio/acini, neuroectodertnal and pigmentary, mesodermal, or entodermal.
The otobranchial classification encompasses visible defects of the external ear, seventh-nerve innervation, and mandible. It includes some firstbranchial-cleft cysts; the familial syndrome of preauricular pits, appendages, and cervical fistulas18,19 and either sensorineural or conductive-hearing loss; microtia with atresia of the external canal in which conductive loss and sometimes superimposed sensorineural hearing loss are seen. In addition, sensorineural and/or conductive-hearing loss occurs in about 25 percent of "normal" opposite ears.15,20 Isolated external-auditory-canal atresia sometimes is a manifestation of deletion of chromosome 1 6 or 18. 2I Other external-ear defects, too many to enumerate, may occur with congenital conductive-hearing loss and other defects, sometimes in families. Severe micrognathia, as in Pierre Robin syndrome, may have as high as a 70 percent correlation with congenital hearing loss, usually conductive.15
In craniofacial anomalies or syndromes, the hearing loss may be conductive or sensorineural. Conductive-hearing loss is likely if the auricle or the mandible is affected, as in Treacher Collins or Goldenhar's syndrome. In the cranial synostoses, in which the skull base may be affected, congenital stapes fixation and/or sensorineural hearing loss may occur. Digital defects, as in Apert's acrocephalosyndactylia, seem particularly associated with stapes fixation. Other disorders in which hearing loss has been reported are Crouzon's, SaethreChotzen's, Pfeiffer's, and cleidocranial dysplasia.22,23
Neuroectodermal and pigmentary defects affect organs and systems sharing a common origin: embryologie surface ectoderm (skin and its appendages, nails, and teeth), and its invaginations (ear, otic pit; eye, optic cup; central nervous system, neural groove), and neural crest (some endocrine glands and pigment cells). Hearing loss is usually sensorineural but need not be congenital.
There are several ectoderm al-dy spia si a syndromes in which sparse or abnormal hair; fine, dry skin; nail dysplasia; few teeth; and hearing loss occur in various combinations. Usher's syndrome is perhaps the prototype of genetic oculo-otic disease. Hearing loss is congenital, and blindness due to retinitis pigmentosa occurs later, but there are variations on that theme in which blindness occurs early and deafness and ataxia late, some with associated skin disorders. Type of heredity may vary from syndrome to syndrome. Congenital defects of other ocular structures may be associated with deafness. Abnormalities or degeneration of some part of the central nervous system also may be seen.24 A striking congenital ocular defect is cryptophthalmos, in which the lids are fused over abnormal eyes; the pinnae, digits, and genitalia are abnormal; and ossicular defects cause conductive hearing loss.25 In Möbius* syndrome there is facial and abducens nuclear dysgenesis causing congenital facial and abducens-nerve palsy associated with pinna defects and severe middle and inner-ear anomalies.26
Pigmentary defects include Waardenburg's autosomal-dominant syndrome, in which a white forelock is a striking but inconstant feature. Lateral displacement of the medial canthi of the eyes is typical, and there may be heterochromia of the irises and spotty depigmentation of the skin. Hearing loss and inner-ear vestibular anomalies occur in 20 percent of cases, occasionally unilaterally.27,28 Piebald and albino syndromes with deafness are also known. In other syndromes, excessive pigment deposition occurs, as in the lentigines, or leopard, syndrome.29
Mesoderma) defects include a wide range of cardiac, urogenital, vascular, muscular, cartilaginous, and osseous dysplasias. Examples are the cardioauditory syndrome, characterized by congenital deafness and cardiac dysrhythmias;30 infantile renal-tubular acidosis;31,32 the syndrome of renal, genital, and middle-ear anomalies;33,34 osteopetrosis;35 craniometaphyseal dysplasia;36 the KlippelFeil anomaly;37 proximal symphalangism;38 and Fanconi's pancytopenia.39
Entodermal defects involve derivatives and outpouchings of the embryonic fore-, mid-, and hindgut. Some examples are Pendred's recessive syndrome of juvenile goiter and deafness;40 athyreotic cretinism;41,42 and DiGeorge's third-and-fourthpharyngeal-pouch syndrome.43,44
Of the genetically acquired deafness syndromes discussed above, as many as half are autosomal-recessive, and another 15 to 40 percent are autosomal dominant, with only 1.5 to 3 percent being X-linked. About 1 percent are polygenic. Chromosomal disorders account for less than 0.5 percent of all cases of congenital hearing loss.4- 10
Only the smaller chromosomes (numbers 13 through 22) and the sex chromosomes have been associated with hearing loss. The pathology, degrees, and types of hearing losses vary considerably within each chromosome syndrome.45 so In autosomal and X-linked genetic deafness, however, the type and severity of hearing loss as well as the age of onset and frequencies of sound affected may be specific to the genetic disorder.51 Thus, it is possible to refer (for example) to congenital, mild, midfrequency autosomal-dominant hearing loss in a particular child.
Ossicular defects may occur in isolation and sporadically and may elude both screening and high-risk approaches but are usually suspected or detected at about the time of entry into school. Some congenital ossicular defects occur in genetic patterns and may be detected much earlier.
Gross labyrinthine anomalies may occur in conjunction with gross somatic defects or as isolated defects detectable only by tomography of the ear. Some of these defects are genetic.
Thus, while early-childhood deafness may have many causes, the pediatrician can unravel the apparent complexity of the problem by reducing it to a relatively small number of concepts. These can be used to identify the infant at risk for early childhood hearing loss and thus make it possible for prompt consultation and testing. ?
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