History

A thorough history is one of the most important aspects of a hearing evaluation. Often, this gives the physician clues as to what to look for during the subsequent physical examination and audiological and/or radiographic tests in order to arrive at the correct diagnosis. The severity of the patient’s hearing loss can be assessed just by conversing with the patient in a normal or soft voice and observing whether the patient responds appropriately. If the patient speaks in a very loud voice, it may indicate a sensorineural cause of hearing loss, and if the patient speaks very softly, it may point to a conductive cause, as the patient’s voice may sound louder to the patient (just as a normal hearing person’s would if his or her ears were plugged). Sometimes, discrepancies between the patient’s behavior in conversation and during diagnostic tests can point to malingering as a possible diagnosis.

When taking a history of present illness, specific points should be emphasized. These include the patient’s perception of the degree of hearing loss, whether the hearing loss is unilateral or bilateral, and the onset of the hearing loss (sudden within 3 days, rapidly progressive within 1 week, slowly progressive over weeks to years, fluctuating, or stable). The patient may have associated symptoms, such as aural fullness, tinnitus, vertigo, disequilibrium, otalgia, otorrhea, headache, visual problems, and other neurological complaints (facial numbness or weakness, ataxia, oscillopsia, etc.), that may help point to specific causes of hearing loss. The past medical history is also very helpful: cardiovascular, renal, rheumatological, hematological, endocrine, and neurological conditions can predispose a patient to certain types of hearing loss.⁵ Past surgical history should also be obtained, with special emphasis on head trauma and previous otological or neurological surgery. A history of noise exposure is also important, as excessive noise exposure, either suddenly or over a period of time, can lead to hearing loss. A full account of the patient’s recent medications, including potentially ototoxic medications, should be taken. It is very important to know whether there is a family history of hearing loss, as there is a genetic predisposition for many types of hearing loss, and many genes associated with deafness and predisposition to hearing loss have been identified.^1,5

Physical Examination

A complete head and neck examination can give many clues to the cause of a patient’s hearing loss. The auricle and the postauricular area should be examined for deformities, surgical incisions, the presence of a hearing aid, and patency of the external auditory canal. Something as simple as cerumen impaction can be the cause of hearing loss in some patients, but other conditions, such as foreign bodies, exostoses, canal stenosis/atresia, and carcinoma of the external canal, can be more troublesome. Pneumatic otoscopy can then be used to examine the tympanic membrane and middle ear. Here, the presence of a tympanostomy tube, tympanosclerosis (scarring of the tympanic membrane), tympanic membrane perforation, retraction pocket, fluid in the middle ear, middle ear masses, or otorrhea can be assessed. It is important to obtain a good seal with the speculum in order to assess the mobility of the tympanic membrane. External and middle ear abnormalities usually point to a conductive component of hearing loss.

Tuning fork testing is an essential part of the physical examination and can help determine if the cause of hearing loss is conductive, sensorineural, or mixed. The three types of tuning forks that can be used are 256Hz (middle C), 512Hz (octave above middle C), and 1024Hz (two octaves above middle C). The Rinne test is useful in determining if there is a conductive hearing loss and is performed by striking the tuning fork and placing it on the mastoid bone (testing bone conduction). Once the patient stops hearing the sound, the tines of the tuning fork are then placed in front of the external canal (testing air conduction) with the tines oriented in the head-frontal plane, and the patient indicates whether he or she can hear the sound. If the patient can hear the sound, air conduction is greater than bone conduction, and the result is normal, or “positive.” If the patient cannot hear the sound, bone conduction is greater than air conduction, and the result is abnormal, or “negative.” The degree of conductive hearing loss can be estimated based on the results of the Rinne test. A test that is negative at 256Hz and positive at 512 and 1024Hz indicates a mild 20- to 30-decibel (dB) conductive loss. A test that is negative at 256 and 512Hz and positive at 1024Hz indicates a moderate 30- to 45-dB conductive loss. A negative test at all three frequencies indicates a severe 45- to 60-dB conductive loss.^6,7 The Weber test is a test used to lateralize the hearing loss. The tuning fork is struck and placed on the patient’s vertex, nasal bones, or maxillary teeth in the midline. The single most clinically useful fork used here is the 512-Hz variety, as the 256-Hz fork can be overly sensitive, leading to many false-positive results, and the 1024-Hz fork may not be sensitive enough.^7–9 Lateralization of sound to one ear during the Weber test indicates either a conductive hearing loss in that ear or a greater sensorineural loss in the opposite ear.⁷ Simple tuning fork tests using only a few frequencies are far from comprehensive. If both ears are symmetrically affected by a sensorineural hearing loss, both the Rinne and Weber tests will be normal, provided the patient is able to hear the tuning fork at all.

The physical examination should also include an assessment of any craniofacial deformities or stigmata that may be associated with hereditary causes of hearing loss or associated systemic diseases. Also, a full cranial nerve examination should be performed, as asymmetries in any of the cranial nerves may indicate that hearing loss is just one component of more severe or extensive disease, such as a skull base neoplasm. A decreased corneal blink reflex and hypesthesia of the external auditory canal (Hitselberger’s sign) can be suspicious for an acoustic neuroma. Finally, attention to the nose, nasopharynx, oral cavity, oropharynx, larynx, and hypopharynx can reveal other causes of hearing loss (e.g., the presence of nasopharyngeal carcinoma as the cause of serous otitis media).

Audiological Testing

Audiological testing has been available for decades, but developments over the years have advanced the field of audiology to include tests and procedures that can determine the site of lesion with far greater accuracy than before. Otolaryngologists and audiologists often need to rely on one another to diagnose accurately the cause of a patient’s hearing loss using a combination of the history, physical examination, and results of various audiological tests. The audiological test battery includes audiometry (pure tone and speech), acoustic immittance testing (tympanometry and acoustic reflex testing), electrophysiological testing (auditory brainstem response and electrocochleography), and otoacoustic emission testing.

Pure-Tone Audiometry

Pure-tone audiometry is the most commonly used test to measure auditory sensitivity. Pure-tone signals are delivered to the ear via air conduction and bone conduction at a variety of frequencies, and the patient responds to the sound by signaling the examiner with a button or by raising a hand. The response can be modified for pediatric patients or patients who lack the capacity to respond in the conventional manner. Although the entire range of human hearing is from 20 to 20,000Hz, the typical range of frequencies tested runs from 250 to 8000Hz, which is the range necessary to understand speech.¹⁰

The intensity of a sound presented is represented by a ratio of its sound pressure to a reference sound pressure, defined as the amount of pressure that can just be sensed by a normal human ear at its most sensitive frequency (0.0002dyne/cm²).¹¹ As the pressure level of a presented sound is often many times the reference sound pressure, the simplest way to present this ratio is to use the decibel, a logarithmic unit:

where P2 is the presented sound pressure and P1 is the reference sound pressure.

A sound referenced to the reference sound pressure is known as the absolute sound level, presented as decibels sound pressure level (dB SPL). The normal human ear is variably sensitive to different frequencies throughout its range, so clinically, the easiest reference level to use is the sound pressure level for each tested frequency that can be heard by a normal ear. The sound level is presented as decibels hearing loss, or dB HL.¹¹

For example, if a normal hearing patient responds to a sound P2 that is equal to P1 (what another normal person would hear), then that patient has 20log₁₀ 1 = 20(0) = 0dB HL. If a patient with hearing loss responds to a sound P2 that is 100 times what a normal person would hear, then that patient has 20log₁₀ (100/1) = 20(2) = 40dB HL. If a patient with hyperacusis (supersensitive hearing) responds to a sound P2 that is 1/10 what a normal person would hear, then that patient has 20log₁₀ (1/10) = 20(−1) = −20dB HL. These examples help to illustrate that dB is indeed a comparison between sound levels and that 0dB or negative dB does not mean that there is no sound—it just means that the sound is the same as or lower than the reference sound level, respectively.

Auditory threshold is defined as the lowest signal intensity at which the signal can be identified 50% of the time.¹² Air conduction thresholds are determined by presenting sound to the ears via headphones or insert earphones, and bone conduction thresholds are determined by vibrating the mastoid directly. Air conduction thresholds measure the sensitivity of the entire auditory system from the external ear to the auditory cortex. When analyzed alone, they do not provide much information regarding the etiology of hearing loss. However, when they are analyzed together with bone conduction thresholds, which measure the degree of sensorineural hearing loss, they can provide valuable information regarding both the type and severity of the hearing loss.¹² When air conduction thresholds are elevated relative to bone conduction thresholds, an “air-bone gap” exists, indicating a conductive hearing loss. Air conduction and bone conduction thresholds showing the same amount of hearing loss indicate a sensorineural hearing loss. A mixed hearing loss is present when both air and bone conduction thresholds are elevated, but air conduction thresholds are more elevated than bone conduction thresholds.

The normal region on the audiogram is from 0 to 20dB HL for adults and from 0 to 15dB HL for children. Mild hearing loss is 20 to 40dB HL, moderate loss is 40 to 55dB HL, moderately severe loss is 55 to 70dB HL, severe loss is 70 to 90dB HL, and profound loss is above 90dB HL. Hearing sensitivity within the speech frequencies is known as the pure-tone average (PTA) and can be calculated by adding the thresholds obtained at 500, 1000, and 2000Hz and dividing the result by 3.¹¹

For audiometric results to be valid, the patient must respond to stimulation of the ear being tested. When noninsert earphones are used, sounds greater than 40dB HL presented to one ear can cross over to the opposite ear, most likely with the vibration of the earphone against the skull acting as a bone conductor. The amount of sound needed for crossover to occur is known as the interaural attenuation, which for air conduction is about 50dB HL for lower frequencies and 60dB HL for higher frequencies. The interaural attenuation is considerably higher when insert earphones are used. For bone conduction, interaural attenuation is less than 10dB HL.¹¹ To correct for the presence of interaural attenuation when a true hearing loss is present, masking techniques are used. A narrow band “white” noise is presented to the nontest ear when the true stimulus is being given to the test ear, and with adequate masking, any sound crossing over to the nontest ear is masked by the noise. To work, the masking noise presented to the nontest must be greater than the threshold of hearing for the nontest ear.¹¹ This can be a problem when bilateral hearing loss (especially conductive) exists, as masking presented to the nontest ear can cross back over to the test ear. This is known as a “masking dilemma.”¹⁰ In air conduction testing, masking should be used when there is a difference between the air conduction presentation level to the test ear and the bone conduction threshold of the nontest ear of greater than 40dB for lower frequencies and greater than 60dB for higher frequencies. In bone conduction testing, masking should be used whenever there is any difference in the air and bone conduction thresholds.¹⁰

Speech Audiometry

Commonly measured speech tests include the speech detection threshold (SDT), the speech reception threshold (SRT), and speech discrimination or word recognition. The SDT is the softest level at which the patient can barely detect the presence of a speech signal 50% of the time.¹² The SRT is the softest level at which the patient can repeat 50% of balanced disyllabic words, or spondees (e.g. “hot dog,” “baseball”), correctly.^10,12 The SDT should correspond to the PTA, whereas the SRT is usually about 8 to 9dB higher than the PTA.¹² Both SDT and SRT can be measured with bone conduction testing and can be masked if necessary. Discrepancies between the PTA and the SDT or SRT can indicate malingering.

The speech discrimination score is a test of the patient’s ability to identify monosyllabic words, or phonemes, at a suprathreshold level, usually about 40dB above the SRT.¹⁰ The speech discrimination score is important in that it helps assess the patient’s ability to understand speech, to communicate effectively, and to benefit from amplification. It also provides some information regarding the patient’s central auditory function.¹²

In general, patients with conductive hearing loss tend to have excellent speech discrimination scores when presented with sounds loud enough for them to hear. Patients with cochlear sensorineural loss tend to have lower speech discrimination scores, and patients with retrocochlear sensorineural loss (from lesions of the eighth cranial nerve to the auditory cortex) have even lower speech discrimination scores. They may even have lower speech discrimination in the presence of normal pure-tone thresholds.¹²

Tympanometry

Acoustic immittance refers to either acoustic admittance (the ease with which energy flows through a system) or acoustic impedance (the blockage of energy flow through a system).¹² In tympanometry, acoustic immittance measures are used to determine the status of the tympanic membrane and middle ear. A probe is placed in the ear canal and an airtight seal is obtained. A tone is introduced into the ear canal and the pressure in the canal is varied. When the pressure in the ear canal is equal to the middle ear pressure, the tympanic membrane will be at its most compliant (highest admittance) and will absorb the sound. This results in a tympanometric peak.¹⁰

If eustachian tube function is normal, the middle ear pressure is equal to the atmospheric pressure and the peak occurs at 0 mm H₂O—this corresponds to a type A tympanogram. If there is negative middle ear pressure, the peak occurs at a negative pressure, corresponding to a type C tympanogram. If there is no peak (flat or type B tympanogram), there is no compliance of the tympanic membrane (no admittance), indicating a middle ear effusion, tympanic membrane perforation, or patent tympanostomy tube. These can be distinguished using ear canal volume measurements, with higher volumes corresponding to a hole in the tympanic membrane. Other types of tympanograms include A_s (shallow peak and low compliance at 0 mm H₂O), indicating ossicular chain fixation or middle ear effusion, and A_d (very high peak and high compliance at 0 mm H₂O), indicating ossicular chain discontinuity or a monomeric tympanic membrane.¹⁰

Acoustic Reflex

In acoustic reflex testing, acoustic immittance measures are used to assess the neural pathway surrounding the stapedial reflex, which occurs in response to a loud sound (70 to 90dB above threshold).¹⁰ The afferent limb of the stapedial reflex is the ipsilateral eighth nerve, which leads to the brainstem. Complex pathways in the brainstem involving the ipsilateral ventral cochlear nucleus, trapezoid body, and bilateral medial superior olives lead from the eighth nerve on the ipsilateral (stimulated) side to the motor nucleus of the facial nerve on both sides of the brainstem.^7,10–12 The efferent limb is the ipsilateral and contralateral facial nerves, which innervate the stapedius muscles. When the stapedius muscle contracts, the ossicular chain stiffens, causing a small change in compliance in the middle ear system that is detected by the probe.¹¹

Patients with mild to moderate cochlear sensorineural hearing loss have reflexes bilaterally at about the same intensity level as those with normal hearing, but patients with severe or profound hearing loss have absent reflexes when the affected ear is stimulated.¹⁰

A conductive hearing loss results in absent reflexes when the affected ear is stimulated, as sound will not be loud enough to stimulate the reflex. Even when the normal ear is stimulated, the ear with the conductive loss does not have a reflex, as the middle ear condition prevents the stapedius from contracting.¹⁰

A lesion of the eighth nerve should result in absent reflexes bilaterally when the affected ear is stimulated, but reflexes should be present bilaterally when the nonaffected ear is stimulated. This can be confused with the reflex result associated with profound unilateral hearing loss (>70dB) of cochlear origin. Lesions of the brainstem affecting the central crossed pathways may result in present ipsilateral reflexes when each ear is stimulated but absent contralateral reflexes. A facial nerve lesion results in an absent reflex on the affected side, no matter which side is stimulated, provided the lesion is proximal to the branching of the nerve to the stapedius muscle.¹⁰

Auditory Brainstem Response

The auditory brainstem response (ABR) is an electrophysiological recording of responses of the distal auditory pathway (eighth nerve and brainstem) to sounds.¹¹ The ABR involves placement of electrodes on the patient’s head and presentation of sound to the ear. When sound is presented to a normal ear, either in click form or frequency-specific tones, five to seven peaks occurring within 10 milliseconds make up the ABR.¹²