GENERAL

1. Audiograms are used to diagnose and monitor hearing loss.
2. Audiograms are created by plotting the thresholds at which a patient can hear various frequencies.
3. Hearing loss can be divided into two categories: conductive or sensorineural. 
4. The results of an audiogram can help direct medical and surgical interventions to improve and/or preserve hearing function.

BACKGROUND INFORMATION

INTENSITY:

1. Intensity of sound is measured in decibels (dB) which can be thought of as the ‘loudness' of the sound.
2. The decibel scale is logarithmic (based on a factor of 10):
   1. An increase of 10 dB means a 10-fold increase in sound intensity.
   2. An increase of 20 dB means the sound is 100-fold more intense.
3. Standard audiograms test between 0 and 110dB.
   1. For reference, normal conversation is around 60 dB

FREQUENCY:

1. Frequency is measured in Hertz (Hz), which is often thought of as the “pitch” of the sound.
2. The average human can hear between 20 and 20,000 Hz 
3. Audiograms typically test frequencies between 250Hz and 8000Hz.
   1. Human speech usually falls between 250Hz and 6000Hz.

AUDIOMETRIC TESTING:

1. During testing, the audiometer delivers various “pure tone” sounds at particular frequencies and intensities, from low to high.
2. The patient’s ability to hear these tones is plotted on a graph to create an audiogram.
3. In children testing varies according to age:
   1. Evoked otoacoustic emissions (EOAE): test function of outer hair cells, often used as a newborn hearing screen
   2. Auditory brainstem response (ABR): use electrodes to monitor brain activity response to sound stimulus, can be done at any age but will often need to be sedated after 6mo
   3. Behavioral observation audiometry (BOA)(0 to 5 mo): observes child response to sound stimulus. Does not assess laterality
   4. Visual reinforcement audiometry (VRA) (6mo to 2yo): child turns to visual cue in response to sound stimulus
   5. Conditioned play audiometry (CPA)(2yo to 5yo): child interacts with toy or object in response to sound stimulus. Can assess laterality.
   6. Conventional pure-tone audiometry (5yo+): raising hand in response to sound stimulus.

TYPES OF HEARING LOSS

1. Hearing loss can be divided into two types: 
   1. conductive
   2. sensorineural
2. Conductive and sensorineural hearing losses can occur alone or in combination.
3. A combination of conductive and sensorineural hearing loss is referred to as a “mixed hearing loss.”

CONDUCTIVE HEARING LOSS (CHL):

1. A conductive hearing loss occurs when sound from the environment is unable to be ‘conducted’ to the structures of the inner ear.
2. Differential diagnosis includes:
   1. Cerumen impaction
   2. Perforated tympanic membrane
   3. Fluid in the middle ear space
   4. Otosclerosis
3. Conductive hearing losses are more likely to be correctable with surgical intervention than sensorineural losses. 
4. Air conduction refers to conduction through the entire outer ear mechanism: including auricle, external ear canal, tympanic membrane and ossicles/middle ear. Bone conduction refers to soudn vibration transmitted to the inner ear through the skull.
5. Conductive loss can be assessed with the Rinne and Weber test
   1. Weber: Place the tuning fork in the midline and determine which ear its heard louder. Normal: heard equally loud in both ears (also equal in symmetric bilateral hearing loss). Unilateral conductive hearing loss: lateralize to affected ear. Unilateral sensorineural hearing loss: lateralize to contralateral ear.
   2. Rinne: Place the tuning fork in front of the ear and over the mastoid and determine in which position it is heard louder. Normal: air conduction > bone conduction (positive Rinne). Conductive hearing loss: bone conduction > air conduction (negative Rinne). Sensorineural hearing loss: air conduction > bone conduction (positive Rinne).
   3. A flipped 256 Hz fork corresponds to a 15 dB hearing loss. Whispered voice is about 20 dB and normal spoken voice is 50 to 60 dB.

SENSORINEURAL HEARING LOSS (SNHL):

1. Sensorineural hearing loss occurs when there is damage to the structures of the inner ear or nervous pathways between the ear and brain 
2. SNHL is the most common type of permanent hearing loss. 
   1. The most common cause of SNHL in the United States is chronic noise exposure. 
3. SNHL is often not as amenable to surgical intervention compared to conductive hearing loss.
4. Differential Diagnosis for SNHL:
   1. Infectious: meningitis, mumps, measles, syphilis, etc. 
   2. Autoimmune
   3. Ototoxic Medications: aminoglycosides, platinum chemotherapeutics, methotrexate, furosemide, aspirin, etc. 
   4. Familial 
   5. Presbycusis
   6. Head trauma: temporal bone fractures
   7. Congenital malformations of the inner ear structures 
   8. Noise-induced hearing loss 
   9. Neoplastic: acoustic neuroma or meningioma

AUDIOGRAM

1. The hearing test results are plotted on a graph with the y-axis representing hearing threshold and the x-axis representing frequency.
2. The right ear is generally plotted with a O and the left ear with a X.
3. Bone conduction is also plotted (to allow for differentiation of conductive and SNHL). The right ear is plotted as < and the left ear as >.
4. Common measures:
   1. Threshold = the lowest level of sound that can be heard 50% of the time.
   2. Speech reception threshold (SRT) = Softest intensity bisyllabic spondee (balanced syllable) words can be repeated 50% of the time
   3. Word recognition score = % of words discerned at threshold
   4. Speech discrimination = % single syllabic words identified and repeated at suprathreshold levels (generally 30 dB above SRT)
   5. Acoustic reflex = muscle contraction in middle ear in response to high intensity stimulus (contralateral and ipsilateral reflexes tested)
   6. Tympanometry = assessing external auditory canal volume and tympanic membrane mobility with air pressure.
      1. Type A: normal
      2. Type B “flat”: limited mobility, fluid or TM damage
      3. Type C: negative pressure from retraction
5. Patients will often ask what their “percentage of hearing loss” is. There is no evidence-based formula to convert the logarithmic dB scale to a percentage of hearing loss with any meaning.
   1. There IS a formula to calculate percent disability for disability pension eligibility:
      1. Take the thresholds for four frequencies (500,1000,2000,3000) for each ear and average them
      2. Increase by 1.5% for each dB above 25dB for each ear
      3. Multiply the better ear by 5 (to weight it more heavily). Add that number with the worse ear and divide by 6 to get your hearing handicap.
      4. This formula has many problems and is NOT the percent hearing loss

CHRONIC NOISE EXPOSURE: 

1. Noise-Induced Hearing Loss (NIHL) typically demonstrates a “knoch” on the audiogram at 4000k.
2. Sounds around 85 dB for prolonged periods of time can cause hearing loss
   1. If you have to raise your voice to be heard, (normal conversation is around 60dB) you are most likely in an environment with at least 80 dB of noise. 

Image above taken from Wikimedia commons without alteration

REFERENCES

How to Read an Audiogram and Determine Degrees of Hearing Loss. The National Hearing Test. http://www.nationalhearingtest.org/wordpress/?p=786. Accessed March 13, 2019.

Walker JJ, Cleveland LM, Davis JL, Seales JS. Audiometry Screening and Interpretation. American Family Physician. https://www.aafp.org/afp/2013/0101/p41.html. Published January 1, 2013. Accessed March 13, 2019.