Hearing Loss in Neonatal Intensive Care Unit Graduates
Jane E. Stewart
Aimee Knorr
I. DEFINITION.
Neonatal intensive care unit (NICU) graduates are at high risk for developing hearing loss. When undetected, hearing loss can result in delays in language, communication, and cognitive development. Hearing loss falls into four major categories:
Sensorineural loss is the result of abnormal development or damage to the cochlear hair cells (sensory end organ) or auditory nerve.
Conductive loss is the result of interference in the transmission of sound from the external auditory canal to the inner ear. The most common cause for the conductive hearing loss is accumulation of fluid in the middle ear or middle ear effusion. Less common are anatomic causes such as microtia, canal stenosis, or stapes fixation that often occur in infants with craniofacial malformations.
Auditory dyssynchrony or auditory neuropathy. In this less common type of hearing loss, the inner ear or cochlea appears to receive sounds normally; however, the transfer of the signal from the cochlea to the auditory nerve is abnormal. The etiology of this disorder is not well understood; however, babies who have a history of severe hyperbilirubinemia, prematurity, hypoxia, and immune disorders are at increased risk. There is also a reported genetic predisposition to auditory dyssynchrony.
Central hearing loss. In this type of hearing loss, despite an intact auditory canal and inner ear and normal neurosensory pathways, there is abnormal auditory processing at higher levels of the central nervous system.
II. INCIDENCE.
The overall incidence of severe congenital hearing loss is 1 to 3 in 1,000 live births. However, 2 to 4 per 100 infants surviving neonatal intensive care have some degree of sensorineural hearing loss.
III. ETIOLOGY
Genetic. Approximately 50% of congenital hearing loss is thought to be of genetic origin (70% recessive, 15% autosomal dominant, and 15% with other types of genetic transmission). The most common genetic cause of hearing loss is a mutation in the connexin 26 (Cx26) gene, located on chromosome 13q11—12. The carrier rate for this mutation is 3% and it causes approximately 20% to 30% of congenital hearing loss. Deletion of the mitochondrial gene 12S rRNA, A1555G, is associated with a predisposition for hearing loss after exposure to aminoglycoside antibiotics.
Other mutations, such as those of the SLC26A4 gene and connexin 30 (Cx30) gene, are associated with newborn hearing loss. Approximately 30% of infants with hearing loss have other associated medical problems that are part of a syndrome. More than 400 syndromes are known to include hearing loss (e.g., Alport, Pierre Robin, Usher, Waardenburg syndromes, and trisomy 21).
Nongenetic. In approximately 25% of childhood hearing loss, a nongenetic cause is identified. Hearing loss is thought to be secondary to an injury to the developing auditory system in the intrapartum or perinatal period. This injury may result from infection, hypoxia, ischemia, metabolic disease, ototoxic medication, or hyperbilirubinemia. Preterm infants and infants who require newborn intensive care or a special care nursery are often exposed to these factors.
Cytomegalovirus (CMV) congenital infection is the most common cause of nonhereditary sensorineural hearing loss. Approximately 1% of all infants are born with CMV infection in this country. Of these (˜40,000 infants/year), 10% have clinical signs of infection at birth (small for gestational age, hepatosplenomegaly, jaundice, thrombocytopenia, neutropenia, intracranial calcifications, and skin rash), and 50% to 60% of these infants develop hearing loss. Although most (90%) infants born with CMV infection have no clinical signs of infection, hearing loss still develops in 10% to 15% of these infants, and it is often progressive. Because there has not been an established treatment for CMV in the newborn, prevention of hearing loss is currently not possible. However, treatment with the antiviral agent ganciclovir (given intravenously) and valganciclovir (given orally) is being studied, and preliminary data indicate that these antiviral agents may prevent the development and/or progression of hearing loss.
Risk factors. The Joint Committee on Infant Hearing (JCIH) listed the following risk indicators associated with permanent congenital, progressive, or delayedonset hearing loss in their 2007 position statement.
Caregiver concerna regarding hearing, speech, language, or developmental delay
Family history of permanent childhood hearing loss
Neonatal intensive care of more than 5 days or any of the following regardless of length of stay: ECMOa, assisted ventilation, exposure to ototoxic medications (gentamicin and tobramycin) or loop diuretics (furosemide), and hyperbilirubinemia that requires exchange transfusion (some centers use a level of ≥20 mg/dL as a general guideline for risk)
In utero infections such as CMVa, herpes, rubella, syphilis, and toxoplasmosis
Craniofacial anomalies, including those that involve the pinna, ear canal, ear tags, ear pits, and temporal bone anomalies
Physical findings, such as a white forelock, that are associated with a syndrome known to include a sensorineural or permanent conductive hearing loss
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