Basic Audiology Review

Hearing Loss in Children 

Introduction 

  • Hearing loss (HL) can occur at any age, not just in older adults. 

  • Any degree of HL, even mild or slight, puts a child at high risk for learning disabilities. 

  • Early identification and intervention are crucial for minimizing the impact of HL on a child's development. 

 

Signs and Symptoms of HL in Children 

  • Babies:  

    • Lack of a startle reflex to loud noises. 

    • Inability to localize sound sources (especially Mom and Dad's voice) by six months. 

    • Absence of single words in vocabulary by one year. 

    • Failure to turn their head when they see you or their name is called. 

  • Children:  

    • Delayed speech development. 

    • Articulation problems. 

    • Difficulty following directions. 

    • Frequently asking for repetition or clarification ("huh?" or "what?"). 

    • Listening to the TV or headphones at a high volume. 

 

Screening and Diagnosis 

  • Newborn Hearing Screening Program:  

    • All infants should be screened for HL before one month of age. 

    • If a baby fails the screening, a full diagnostic evaluation should be conducted by three months. 

    • Diagnosis and treatment (if needed) should ideally be in place by six months. 

  • School-Aged Children:  

    • Children should have their hearing tested before entering school (around age five). 

    • If a child fails the school hearing screening, a full diagnostic evaluation should be conducted as soon as possible. 

 

Causes and Risk Factors for HL in Children 

  • Genetic Factors:  

    • Account for more than 50% of all cases of congenital HL. 

    • Can be autosomal dominant (one parent carries the dominant gene), autosomal recessive (both parents carry the recessive gene), or X-linked (mother carries the recessive trait on the sex chromosome - males). 

    • Genetic syndromes associated with HL include Down syndrome, Usher syndrome, Treacher Collins syndrome, Crouson syndrome, and Alport syndrome. 

    • The GJB2 gene (Connexin 26 gene) is responsible for a significant portion of genetic HL cases. 

  • Environmental Factors:  

    • Maternal infections during pregnancy (e.g., CMV, rubella, syphilis). 

    • Complications during birth or head trauma. 

    • Prematurity and extended stay in the NICU (more than five days). 

    • Severe jaundice requiring blood transfusion. 

    • Neurological disorders (e.g., Charcot-Marie-Tooth disease). 

    • History of meningitis or head injury requiring hospitalization. 

    • Exposure to ototoxic medications (e.g., aminoglycosides, chemotherapy drugs). 

  • Otitis Media (Middle Ear Infections):  

    • The most common cause of HL in young children. 

    • Causes a buildup of fluid behind the eardrum, dampening sound vibrations. 

    • Can lead to temporary conductive HL, making speech sound muffled. 

    • Repeated or untreated ear infections can cause permanent damage to the middle and inner ear. 

    • Risk factors include Eustachian tube dysfunction, large adenoids, exposure to secondhand smoke, and feeding infants in a lying down position. 

 

Additional Notes 

  • Incidental Learning: Children learn speech and language by hearing others talk. HL can disrupt this process, leading to delays. 

  • Craniofacial Anomalies: Malformations of the head, face, or ears can be associated with HL and other syndromes. 

  • Tinnitus: Ringing or buzzing in the ears can be a symptom of HL or other underlying conditions. 

  • Noise-Induced HL: Exposure to loud noises (e.g., firearms, heavy equipment, loud music) can cause permanent damage to the inner ear. 

  • Otosclerosis: A condition causing abnormal bone growth in the middle ear, leading to conductive HL. 

  • Meniere's Disease: A disorder of the inner ear characterized by fluctuating HL, vertigo, tinnitus, and a feeling of fullness in the ear. 

 

 

Hearing Loss in Adults 

Types of Hearing Loss 

  • Conductive Hearing Loss: Occurs when sound waves are unable to reach the inner ear due to a blockage or problem in the outer or middle ear. Common causes include earwax buildup, otitis media (middle ear infection), and otosclerosis (abnormal bone growth in the middle ear). 

  • Sensorineural Hearing Loss: Occurs when there is damage to the inner ear (cochlea) or the auditory nerve. This type of HL is often permanent and can be caused by various factors, including aging, noise exposure, ototoxic medications, and certain medical conditions. 

  • Mixed Hearing Loss: A combination of conductive and sensorineural HL. 

 

Causes of Hearing Loss in Adults 

  • Meniere's Disease: A disorder of the inner ear that causes fluctuating HL, dizziness, tinnitus (ringing in the ears), and a feeling of fullness in the ear. The cause of Meniere's disease is unknown, but it is thought to be related to an overproduction of fluid in the inner ear. 

  • Autoimmune Inner Ear Disease: An inflammatory condition in which the body's immune system attacks the inner ear, leading to sudden HL, vertigo, and tinnitus. If diagnosed and treated early with steroids, the HL can often be reversed. 

  • Ototoxic Medications: Certain medications, such as aminoglycoside antibiotics, high doses of aspirin, loop diuretics, and chemotherapy drugs, can damage the inner ear and cause HL. 

  • Noise-Induced Hearing Loss: The most common cause of HL in adults, caused by exposure to loud noises over time. The damage is cumulative and can lead to permanent HL. 

  • Acoustic Neuroma: A benign tumor that grows on the auditory nerve, causing HL, tinnitus, and balance problems. 

  • Presbycusis: Age-related HL that occurs gradually in both ears. It is a common condition that affects people over the age of 60. 

  • Other Causes: Head injury, otosclerosis, certain medical conditions (e.g., diabetes, hypertension), and genetic factors can also contribute to HL in adults. 

 

Impact of Hearing Loss 

Untreated HL can have a significant impact on an individual's quality of life, leading to: 

  • Communication Difficulties: Difficulty understanding speech, especially in noisy environments. 

  • Social Isolation: Withdrawal from social activities due to communication challenges. 

  • Depression and Anxiety: Feelings of frustration, loneliness, and low self-esteem. 

  • Cognitive Decline: Studies have shown a link between untreated HL and an increased risk of cognitive decline and dementia. 

  • Reduced Balance and Mobility: HL can affect balance and coordination, increasing the risk of falls. 

  • Medical Complications: Difficulty communicating with healthcare providers can lead to misunderstandings and potentially affect medical treatment. 

 

Management of Hearing Loss 

The management of HL depends on the type and severity of the condition. Options include: 

  • Medical Treatment: For conductive HL, addressing the underlying cause, such as removing earwax or treating an ear infection, can often restore hearing. 

  • Hearing Aids: Amplify sound and can significantly improve hearing and quality of life for people with sensorineural HL. 

  • Assistive Listening Devices: Can help people with HL hear better in specific situations, such as watching TV or talking on the phone. 

  • Cochlear Implants: Surgically implanted devices that can provide a sense of sound for people with severe to profound HL. 

  • Other Strategies: Lip-reading, communication strategies, and support groups can also help people with HL manage their condition and improve their quality of life. 

 

 

Basic Audiology Review: The Ear and Auditory System 

Outer Ear: 

  • Function: Collects and amplifies sound waves, particularly high-frequency sounds. The shape of the pinna helps with sound localization. 

  • Components: Pinna (auricle) and external auditory canal (ear canal). 

  • Clinical Significance: The outer ear can be affected by conditions like impacted cerumen (earwax), foreign bodies, and external otitis (swimmer's ear). 

Middle Ear: 

  • Function: Transfers sound vibrations from the outer ear to the inner ear. It acts as an impedance matching system, ensuring efficient sound transmission from the air-filled middle ear to the fluid-filled inner ear. 

  • Components: Tympanic membrane (eardrum), ossicular chain (malleus, incus, stapes), Eustachian tube. 

  • Eustachian Tube:  

    • Angled downwards at 30 degrees in adults. 

    • Shorter, wider, and more horizontal in infants, making them more susceptible to ear infections. 

    • Maintains atmospheric pressure in the middle ear space. 

    • Opens approximately once a minute when awake and every 5 minutes when asleep. 

  • Clinical Significance: Common middle ear disorders include otitis media (middle ear infection), otosclerosis (abnormal bone growth), and cholesteatoma (skin growth in the middle ear). 

Inner Ear: 

  • Function: Converts sound vibrations into electrical signals that are sent to the brain for processing. It also plays a crucial role in balance. 

  • Components: Cochlea (responsible for hearing) and vestibular system (responsible for balance). 

  • Cochlea:  

    • Contains the basilar membrane, which vibrates in response to sound waves. 

    • Different frequencies are coded at different locations along the basilar membrane (tonotopic organization). 

    • Hair cells on the basilar membrane convert vibrations into electrical signals. 

  • Clinical Significance: Inner ear disorders can lead to sensorineural hearing loss, tinnitus (ringing in the ears), and vertigo (dizziness). 

Auditory Nervous System: 

  • Function: Transmits auditory information from the cochlea to the brain for processing. 

  • Characteristics:  

    • Primarily afferent (sensory) pathway. 

    • Binaural processing (both ears work together). 

    • Crossover pathways: Information from the right ear is primarily processed in the left hemisphere of the brain, and vice versa. 

    • Left hemisphere dominance for language processing. 

  • Clinical Significance: Damage to the auditory nerve or brainstem pathways can result in auditory neuropathy spectrum disorder (ANSD). 

 

Audiological Assessment 

Pure Tone Audiometry: 

Purpose: Measures hearing sensitivity at different frequencies. 

Methods:  

  • Air conduction testing: Uses earphones to deliver sound to the outer ear. 

  • Bone conduction testing: Uses a bone vibrator placed on the mastoid bone behind the ear to bypass the outer and middle ear and directly stimulate the cochlea. 

Results: Presented on an audiogram, which plots hearing thresholds (in decibels) at different frequencies. 

 

Soundfield Testing: 

  • Purpose: Assesses hearing in a sound-treated environment using speakers. 

  • Applications:  

    • Infant hearing screening. 

    • Hearing aid verification. 

  • Limitations: Not ear-specific. 

 

Tympanometry: 

  • Purpose: Evaluates middle ear function by measuring the movement of the tympanic membrane in response to changes in air pressure. 

  • Procedure: A probe tip is inserted into the ear canal, and air pressure is varied. 

  • Results: Displayed as a tympanogram, which provides information about middle ear pressure, compliance, and volume. 

  • Tympanogram Types:  

    • Type A: Normal middle ear function. 

    • Type B: Middle ear dysfunction, often indicating fluid behind the eardrum (otitis media). 

    • Type C: Negative pressure in the middle ear space, possibly due to Eustachian tube dysfunction. 

    • Type As: Reduced compliance, suggesting stiffness in the middle ear system (e.g., otosclerosis). 

    • Type Ad: Hypermobile tympanic membrane, potentially indicating a disarticulation of the ossicular chain. 

 

Ear Canal Volume (ECV):  

  • Reflects the volume of the ear canal. 

  • Abnormal ECV values can indicate a perforation of the eardrum (large ECV) or a blockage (small ECV). 

  • Speech Audiometry: 

    • Purpose: Assesses an individual's ability to understand speech. 

    • Tests:  

      • Speech Reception Threshold (SRT): The softest level at which a person can correctly repeat spondee words (two-syllable words with equal stress). 

      • Word Recognition Score (WRS): Measures the percentage of correctly repeated phonetically balanced (PB) words at a comfortable listening level. 

    • Applications: Used to diagnose hearing loss, determine the need for amplification, and evaluate the effectiveness of hearing aids and cochlear implants. 

 

Otoacoustic Emissions (OAEs): 

  • Purpose: Measures sounds produced by the outer hair cells of the cochlea in response to sound stimulation. 

  • Applications:  

    • Newborn hearing screening. 

    • Early detection of hearing loss. 

    • Monitoring cochlear function. 

  • Interpretation:  

    • Present OAEs generally indicate normal cochlear function. 

    • Absent OAEs suggest a possible hearing loss. 

    • OAEs are typically absent in hearing losses greater than 25-30 dB. 

 

Auditory Brainstem Response (ABR): 

  • Purpose: Evaluates the function of the auditory nerve and brainstem pathways. 

  • Procedure: Electrodes are placed on the forehead and behind the ears to record brainwave activity in response to sound stimulation. 

  • Applications:  

    • Newborn hearing screening. 

    • Diagnosis of auditory neuropathy spectrum disorder (ANSD). 

    • Estimation of hearing thresholds in individuals who cannot participate in behavioral testing. 

  • Advantages: Non-invasive and objective.