Chapter 17: Congenital Hearing Loss

Introduction

1. Deafness is the most common sensory defect (1 in 1000-2000 births; there are roughly 4 million births in the United States per year).

2. Early identification allows appropriate intervention as soon as indicated.

3. Fifty percent of congenital hearing loss is due to nongenetic factors.

   1. Environmental (25% of all congenital hearing loss cases)

      1. Infections (TORCH)

      2. Ototoxicity

      3. Prematurity

      4. Extracorporeal membrane oxygenation (ECMO)

      5. Hypoxia

   2. Idiopathic (25% of all congenital hearing loss cases)

4. Fifty percent of congenital hearing loss is due to genetic factors

   1. Seventy percent of genetic cases are nonsyndromic

      1. Seventy-five percent to 80% of nonsyndromic deafness is inherited through autosomal recessive (AR) patterns.

      2. Eighteen percent to 20% of nonsyndromic deafness is inherited through autosomal dominant (AD) patterns.

      3. One percent to 3% of nonsyndromic deafness is classified as X-linked, or chromosomal disorders.

   2. Thirty percent of genetic cases are syndromic

      1. The majority of these cases are passed on through AR or X-linked inheritance.

Environmental Factors

Infections

Toxoplasmosis

• Organism: Protozoan parasite Toxoplasma gondii

• Epidemiology: In United States, around 1/10,000 births

• Primary infection: Occurs through ingestion of oocytes and ova. Common sources include consuming undercooked meat or the handling of contaminated soils, water, and cat feces.

• Infant transmission: Vertical transmission or primary infection in neonatal period

• Symptoms: Asymptomatic in majority of patients. If symptomatic, triad includes (a) chorioretinitis, (b) hydrocephalus, and (c) intracranial calcifications

• Diagnostics:

  • Serology: IgM and IgG antibody tests

  • CSF studies: microscopic visualization of parasite

  • Amniocentesis: PCR

• Cochlear histopathology: encysted organisms within the inner ear causing tissue necrosis and inflammatory changes.

• Treatment: Pyrimethamine and sulfadiazine; treatment is capable of preventing hearing loss.

Rubella

• Organism: Togavirus

• Epidemiology: In vaccine era, rubella incidence below 1 per 1,000,000 births. Congenital rubella syndrome (CRS) incidence below 1case per 5,000,000 births

• Primary infection: Aerosol

• Symptoms (CRS): Congenital cataract, cardiovascular anomalies, mental retardation, retinitis, sensorineural hearing loss (SNHL)

• Infant transmission: Vertical transmission or primary infection in neonatal period

• Diagnostics: PCR identification of IgG and IgM

• Cochlear pathology: Degeneration of the organ of Corti, adhesion between the organ of Corti and Reissner membrane, rolled-up tectorial membrane, cochlea–saccule degeneration

• Treatment: Supportive care

Cytomegalovirus (CMV)

• Organism: Beta-herpesvirus

• Epidemiology: The most common viral cause of congenital SNHL. Congenital CMV (cCMV) affects up to 3% of all newborns. Eighty percent to 90% of infants with congenital cCMV are asymptomatic at birth. As many as 15% of these infants will develop late sequlae.

• Primary infection: Direct contact with urine, blood, or bodily fluids.

• Infant transmission: Vertical transmission or primary neonatal infection.

• Symptoms: Microcephaly, intrauterine growth restriction, hepatosplenomegaly, chorioretinitis, jaundice, petechiae, thrombocytopenia, hyperbilirubinemia, SNHL

• Prevention: CMV hyperimmunoglobulin in pregnant women with primary CMV

• Diagnostics:

  • Prenatal: More than 21 weeks of gestation and more than 6 weeks after infection → Nucleic acid PCR

  • Neonate: Less than 3 weeks → PCR of saliva, urine, or both

• Cochlear ...