- What is the difference between congenital, and acquired hearing loss?
- What are the causes of congenital hearing loss?
- What are the causes of acquired pediatric hearing loss?
What is the difference between congenital, and acquired hearing loss?
Congenital refers to patients who were born with hearing loss, regardless of the cause. It may or may not be due to genetic/hereditary causes, as we shall see above. Acquired pediatric hearing loss occurs after a child is born with normal hearing. Such hearing loss is most likely due to external factors (such as infections or trauma) after a baby is born with normal hearing. It is important to recognize that some syndromes or hereditary factors may result in hearing loss later in life.
What are the causes of congenital hearing loss?
Genetic causes are responsible for approximately half of all congenital hearing loss. This means that the child has inherited abnormal genes from either one, or most commonly, both his parents, which results in a change in the ear. Half of the time, if genetic testing is performed, medical professionals are able to determine which gene is responsible for the changes that have caused the hearing loss. The inner ear is made of proteins. Changes in the structure of these proteins can cause inner ear malfunction and deafness. At present about 110 specific protein defect related causes of deafness have been identified.
Although genetic testing is unlikely to change the management of your child’s hearing loss, it may be helpful in order to avoid additional testing as well as with the prognosis of treatment.
An example of a case in which knowledge of the cause of your child’s deafness can help manage the condition is connexin deafness. Connexin deafness, is caused by a change in the structure of an inner ear protein called connexin, and accounts for nearly 50% of genetic congenital deafness. Connexin deafness is the most common cause of genetic deafness. In these children, in whom the deafness is most often complete, the inner ear is normally formed and the auditory nerve is unaffected. As a result, these children are often excellent candidates for cochlear implantation. This knowledge can be very reassuring for parents.
Hearing loss can sometimes be part of a syndrome, which might include other features. The most common syndromes associated with hearing loss are:
- Waardenburg syndrome. Patients with Waardenburg syndrome have widely space eyes of different colors, a white forelock in their hair, in addition to other features, along with the hearing loss.
- Usher syndrome. There are many types of Usher syndrome, that have varying degrees of hearing loss, imbalance, and eye issues (retinitis pigmentosa). We encourage all children with congenital hearing loss to have their eyes evaluated by an ophthalmologist to rule out syndromes like Usher syndrome.
- Jervell-Lange-Nielson syndrome. These patients have a predisposition for a cardiac arrhythmia (prolonged QT), which may cause fainting spells or instant death, in addition to the hearing loss. We encourage patients with congenital hearing loss to have an EKG (electrocardiogram) to rule out this syndrome.
- Pendred syndrome. These patients have a goiter (enlarged thyroid gland in the neck) with hearing loss. These patients typically also have inner ear abnormalities such as “enlarged vestibular aqueduct”.
Although these are the most common syndromes associated with hearing loss, there are many others including Stickler syndrome, Treacher-Collins syndrome, Goldenhar syndrome, and CHARGE syndrome.
Infection of the mother with certain organisms during critical months of gestation may cause deleterious effects on babies’ normal growth and development, and the ear is no exception. The typical infections that can cause congenital hearing loss are referred to as the TORCH infections, and include:
- Toxoplasma: a parasite found in cat feces
- Other (such as syphilis, chickenpox, mumps, HIV)
- Rubella: a virus responsible for German measles
- CMV (cytomegalovirus): a very common virus which is most commonly benign and has infected most people on the planet, but could cause multiple birth defects in newborns.
- Herpes simplex: the virus responsible for cold sores, or genital sores.
When suspected, these infections can be diagnosed with blood tests.
Inner Ear Defects
The inner ear is a very complex structure and requires multiple functioning parts to allow hearing and speech recognition. As such, anomalies affecting the different parts of the inner ear may cause congenital hearing loss. These can range from the complete absence of the inner ear, to minimal malformations, which can result in equally varying effect on hearing, ranging from mild hearing loss to complete deafness. Common terms you may encounter are Michel aplasia, Mondini deformity, or Enlarged Vestibular Aqueduct(EVA). These anatomical malformations are typically diagnosed with a CT or an MRI. It is important to recognize that some malformations(Michel, common cavity) compromise the success of cochlear implantation because the spiral tunnel of the cochlea, for which cochlear implant electrodes are designed, is not present. Still others(EVA), are associated with more hearing loss later in life.
The ability to hear requires a functional inner ear and a functional auditory pathway, which refers to a railway of communication from the inner ear to the part of the brain responsible for sound perception. In auditory neuropathy, the cochlea functions normally, but the signal is unable to travel to the brain. Many patients with auditory neuropathy have risk factors such as prematurity or a neonatal intensive care unit admission, while others have a hereditary form of auditory neuropathy. This problem is best diagnosed with a test called an ABR, which is a special hearing test, most often performed under sedation. It is important to recognize that auditory neuropathy is often missed with simple newborn hearing tests.
Absence of the Auditory Nerve
In some children with complete deafness, the auditory nerve that connects the inner ear to the brain is completely absent. This absence is sometimes suspected on a CT scan of the inner ear and confirmed with an MRI, which can show the individual nerves leaving the inner ear. This is crucial information since cochlear implants will not work if no auditory nerve is present. The family is counseled to begin learning sign language in these cases and to place the child in an early signing pre-school environment, to be better prepred for the sign- based curriculum of a Deaf school.
What are the causes of acquired pediatric hearing loss?
The term acquired hearing loss refers to hearing loss that occurs after a child is born with normal hearing.
About 5-7 percent of patients with bacterial meningitis will develop profound hearing loss. Although early changes in the cochlea can be identified on an MRI, calcifications in the cochlea may only be seen on CT in the late stages.. Since scarring and calcification can prevent full electrodie insertion in the inner ear, a cochlear implant should be performed early before these changes occur.
Pre-maturity and Intensive Care Unit(ICU)
Care Babies who are born premature or spend some time in the neonatal ICU have many risk factors for hearing loss. These include a low birth weight and jaundice. The use of life saving antibiotics in these infants can cause hearing loss.
In rare cases, Mumps can cause hearing loss. However, as a result of widespread mumps vaccination in children, the incidence of mumps- related hearing loss has decreased in recent generations.
Some life-saving drugs used in chemotherapy or to fight neonatal infections can injure the hair cells of the inner ear and cause hearing loss or deafness. Although ototoxicity may be temporary, it is often permanent. The most common drugs that cause permanent hearing loss in children are chemotherapy drugs (such as cisplatin), and antibiotics (such as gentamycin). If your child is on medications that have a high risk of causing ototoxicity, they may undergo multiple hearing tests during treatment to detect ototoxicity and to protect your child’s hearing as much as possible.