Cytomegalovirus (CMV) is a common virus that can affect individuals of all ages. While often harmless, CMV infection carries a notable connection to hearing loss, particularly in infants. This association is a leading infectious cause of non-genetic hearing loss in children. Hearing impairment is among the most frequent and impactful long-term effects for affected newborns.
Cytomegalovirus Basics
Cytomegalovirus is a widespread virus belonging to the herpes family, similar to those causing chickenpox and mononucleosis. Most individuals become infected with CMV at some point in their lives, often without realizing it. The virus typically spreads through direct contact with bodily fluids, including saliva, urine, blood, breast milk, and semen.
In healthy people, CMV infection usually presents with mild or no symptoms, sometimes resembling a common cold or flu. Once acquired, the virus remains in the body for life, often in a dormant state, but it can reactivate. While generally benign for most, its effects can be more serious for individuals with weakened immune systems or developing fetuses.
CMV’s Impact on the Developing Ear
CMV poses the greatest concern when transmitted from a pregnant individual to their fetus, known as congenital CMV infection. The virus can cross the placenta, infecting the developing baby before birth. This transmission is especially likely if the pregnant individual experiences a primary CMV infection during early pregnancy.
Congenital CMV affects approximately 1 in 200 babies born each year. While many infants with congenital CMV show no symptoms at birth, a significant number will still develop hearing loss. About 10% to 15% of asymptomatic newborns will experience some degree of hearing loss later in life. For babies who are symptomatic at birth, around 75% will develop hearing impairment.
Hearing loss due to congenital CMV can be present at birth or manifest later in childhood, sometimes progressing over time. This auditory impairment can affect one ear (unilateral) or both ears (bilateral), and its severity can vary widely. The impact underscores the particular vulnerability of the developing auditory system to CMV during gestation.
Mechanisms of Auditory Damage
Once CMV infects the developing ear, it can directly damage structures within the inner ear, such as the cochlea and the auditory nerve. The cochlea, a snail-shaped organ containing hair cells, is frequently affected. Damage to these delicate hair cells, which convert sound vibrations into electrical signals, disrupts the normal hearing process.
CMV can also induce inflammation within the inner ear, termed labyrinthitis. This inflammatory response, driven by the immune system reacting to the viral presence, can contribute to tissue damage and scarring, further impairing auditory function. CMV can infect specific cells like perilymphatic epithelial cells and spiral ganglion neurons, which are crucial for transmitting sound signals to the brain.
Furthermore, CMV infection can disrupt the delicate balance of ions within the inner ear, which is essential for proper hair cell function. Damage to the stria vascularis, a structure involved in maintaining this ionic balance and producing endocochlear potential, can lead to impaired hearing. This interplay of direct viral effects, inflammatory responses, and functional disruptions contributes to the range of hearing loss observed in congenital CMV.