I. DEFINITION. Neonatal intensive care unit (NICU) graduates have an increased risk of developing hearing loss. When undetected, hearing loss can delay language, communication, and cognitive development. Hearing loss falls into four major categories:

A. Sensorineural loss is the result of abnormal development or damage to the cochlear hair cells (sensory end organ) or auditory nerve.

B. 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 of conductive hearing loss is 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.

C. Auditory dyssynchrony or auditory neuropathy is a rare type of hearing loss accounting for only 10% of children diagnosed with severe permanent 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 extreme prematurity, hypoxia, severe hyperbilirubinemia, and immune disorders are at increased risk. In approximately 40% of cases, there is a genetic basis for their auditory dyssynchrony.

D. Central hearing loss occurs despite an intact auditory canal and inner ear and normal neurosensory pathways because of 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 per 1,000 live births. However, 20 to 40 per 1,000 infants surviving neonatal intensive care have some degree of sensorineural hearing loss.

A. Genetic. Approximately 50% of congenital hearing loss is thought to be of genetic origin (30% syndromic and 70% nonsyndromic). Of the nonsyndromic, 75% to 85% are autosomal recessive, 15% to 24% autosomal dominant, and 1% to 2% X-linked. The most common genetic cause of nonsyndromic autosomal recessive hearing loss is a mutation in the connexin 26 (Cx26) gene, located on chromosome 13q^11-12 (at least 90 deletions have been associated with hearing loss). The carrier rate for a Cx26 mutation is 3%, and it causes approximately 20% to 30% of all congenital hearing loss. Deletion of the mitochondrial gene 12SrRNA, A1555G, is associated with a predisposition for hearing loss after exposure to aminoglycoside antibiotics. 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., Robin sequence, Usher, Waardenburg syndrome, neurofibromatosis type 2, branchio-oto-renal syndrome, trisomy 21). To see a full review of the genetics of hearing, please see Smith et al.¹

B. Nongenetic. In approximately 25% of childhood hearing loss, a nongenetic cause is identified. Hearing loss is thought to be secondary to injury to the developing auditory system in the intrapartum or perinatal period. This injury may result from infection, hypoxia, ischemia, metabolic disease, hyperbilirubinemia, or ototoxic medication. Preterm infants and infants who require newborn intensive care or a special care nursery are often exposed to these factors.

1. 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 per year), 10% have clinical signs of infection at birth (small for gestational age, hepatosplenomegaly, jaundice, thrombocytopenia, neutropenia, intracranial calcifications, or 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. Some promising new data indicate that treatment with the antiviral agent valganciclovir given orally for 6 months after birth is associated with improved long-term hearing function as well as improved neurodevelopmental outcomes at 2 years of life. Prompt diagnosis of congenital CMV is essential to determine if the infant is a possible candidate for treatment; ideally, treatment is initiated within 1 month after birth. Screening for CMV with urine or saliva in all babies who fail their newborn hearing screen has been implemented by some hospitals to facilitate making this diagnosis. Educating women on strategies to avoid CMV exposure during pregnancy is equally important.

C. Risk factors. The Joint Committee on Infant Hearing (JCIH) listed the following risk indicators associated with permanent congenital, progressive, or delayed-onset hearing loss in their 2007 Position Statement:

1. Caregiver concern^* regarding hearing, speech, language, or developmental delay

2. Family history of permanent childhood hearing loss

3. Neonatal intensive care of >5 days or any of the following regardless of length of stay: extracorporeal membrane oxygenation (ECMO),^* assisted ventilation, exposure to ototoxic medications (gentamicin, 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)

4. In utero infections such as CMV,^* herpes, rubella, syphilis, and toxoplasmosis

5. Craniofacial anomalies, including those that involve the pinna, ear canal, ear tags, ear pits, and temporal bone anomalies

6. Physical findings, such as a white forelock, that are associated with a syndrome known to include a sensorineural or permanent conductive hearing loss

7. Syndromes associated with progressive or late-onset hearing loss such as neurofibromatosis, osteopetrosis, and Usher syndrome. Other frequently identified syndromes include Waardenburg, Alport, Pendred, and Jervell and Lange-Neilsen.

8. Neurodegenerative disorders,^* such as Hunter syndrome, or sensory motor neuropathies, such as Friedreich ataxia and Charcot-Marie-Tooth syndrome