Hearing loss can occur from two sources: maldevelopment of the hearing nerve and/or a blockage of sound due to an inadequate ear canal / eardrum / middle ear bones. Approximately 20% of patients with AM have an abnormality in the hearing nerve function causing a sensorineural hearing loss. All patients with an ear canal malformation have hearing loss from sound being blocked causing a conductive hearing loss. A conductive hearing loss can be reversed surgically in select cases whereas an existing sensorineural hearing loss cannot be improved surgically.
As an example, a child with normal hearing nerve function and a blocked ear canal with the following hearing test might have a score of H 6/57 in the affected ear
Your Pediatric Audiologist will use one or more of the following tests to determine if your child has a hearing loss. If a hearing loss is present he/she will define its exact characteristics and severity. In most cases, more than one test may be necessary to be 100% certain of the presence and type of hearing impairment. This is particularly true with tests designed to be screening evaluations. Each screening program has a small percentage of children referred for further testing who are found to have normal hearing.
Testing falls into one of two categories: Subjective Testing and Objective Testing. Subjective Testing relies on the response of an individual to a sound stimulus. In effect, the test subject must indicate to the examining Pediatric Audiologist when a sound is heard. Objective Testing does not rely on the subject being tested. The Pediatric Audiologist can rely on a measurement or waveform or electrical response generated by the hearing system and/or brain to determine if sound is being received and processed. As you may guess, very young infants need objective testing. Sedation or even general anesthesia is needed in many cases to complete objective testing. Subjective testing can be used with older children and adults and requires a participating, alert test subject as you will see.
The most commonly used testing is the audiogram. A complete audiogram includes testing of the following parameters:
To begin the test a device is placed firmly against the skin of the forehead or the bone behind the ear called the mastoid. Sounds are emitted as vibrations from this transducer and are carried through the bone of the skull directly to the inner ear. Inner ear nerve fibers (called hair cells for the small fibers that extend from their surface into fluid of the inner ear designed to pick up the small vibrations of sound) receive the sounds and turn them into electrical impulses before transmitting the signal to the brain. If the test subject hears the sound, he/she communicates this to the Pediatric Audiologist. Each ear is tested independently. Bone conduction measures the ability of the hearing nerve to receive sound. If the hearing nerve has a reduced or absent ability to receive sound, a sensorineural hearing loss is present.
Air conduction measures the sound heard through the ear canal. A sound-emitting probe is placed in the ear canal. By individual frequency, the Pediatric Audiologist identifies the smallest sound the test subject is able to hear at each frequency. Both ears are tested independently.
When the sound collecting system of the ear (the outer ear, ear canal, ear drum, and middle ear bones) are working properly, bone conduction and air conduction testing results are identical. When air conduction testing results show sound must be louder to be heard as compared to bone conduction, it is clear the sound collecting system of the ear is not working perfectly. This condition is known as a conductive hearing loss. Air conduction cannot be better than bone conduction. In summary, hearing loss can be sensorineural (due to a hearing nerve abnormality), conductive (due to dysfunction of the sound collecting system of the middle and outer ear), or mixed with both sensorineural and conductive components.
ABR: An Auditory Brainstem Response (ABR) also uses surface electrodes and sensitive recording equipment to record an electrical waveform produced in the hearing pathway. Once sound enters the inner ear, nerve stimulation begins in the cochlea and proceeds up the hearing pathway through several successive nerve centers to the conscious level where we become aware of the sound. The entire process takes fractions of a second. If several hundred broad band sound clicks are recorded and averaged as well as expanded so that a tenth of a second takes up the entire computer screen, a measurable waveform emerges. The ABR can test hearing up to a level of 90 dB. It fails, however, to give information about each frequency represented on the audiogram. If performed with special techniques using narrow band sound clicks, investigation can be focused on low and high frequency portions of the audiogram. The size, timing and onset of the waveform gives information of several types about the auditory pathway. For example, a coordinated firing of nerve cells has to occur in order to produce a clear and understandable representation of speech to the brain. Poor coordination of nerve signals is readily identifiable on an ABR. An ABR can help to gain information on mild, moderate, and some severe hearing losses. If hearing is over 90 dB, this technique will not allow a waveform to be generated.
Since the test measures tiny electrical impulses and since electrical impulses are also generated with muscle movement, the test subject must be very still during the test. Test time measures 1-2 hours and requires the Pediatric Audiologist to pick the correct waveforms. In most cases, very young children can have the test performed during a nap (we frequently coordinate the test with nap-time for infants). Children who would have trouble staying still will need sedation or sometimes general anesthesia for test accuracy. Fortunately, the testing technique still works even if the subject is asleep naturally or via anesthesia.
An ABR is the most commonly used investigation to determine if a hearing loss is present. Here’s a practical example of how the test works. During a nap in a 2 week old infant referred from infant hearing screening on the second day of life in the hospital a test is performed. Testing begins at 60 dB and let’s imagine a waveform which is normal in every way being produced. The Pediatric Audiologists turns down the sound clicks in 5 dB steps. The waveform decreases in size until it disappears between 30 and 35 dB. The test demonstrates that there is a hearing loss present and that the hearing impairment is in the mild range in the frequencies of the broad band click.
Tympanometry: [An ear canal and eardrum must be present for this type of testing] Immittance testing uses a pressure producing probe placed in the ear canal. By varying the pressure of the ear canal in both positive and negative directions while measuring sound bounced off the tympanic membrane (or ear-drum), the status of the middle ear space can be inferred. Testing results are recorded in a graph called a Tympanogram. If fluid is present in the middle ear, for example, a conductive hearing loss is produced and an abnormal Tympanogram is seen (see figures for Tympanograms: normal / flat (fluid) / retracted (negative middle ear pressure) / flat with TM perforation). The function of two small muscles attached to the middle ear bones can also be tested with the Immittance Probe adding information about the cause of certain types of conductive hearing loss.
Oto-acoustic emissions: [An ear canal and eardrum must be present for this type of testing] Nerve receptor cells in the cochlea take the vibrational energy of sound delivered to the inner ear by the ear drum and middle ear bones and turn it into electrical impulses. These cells are called hair cells. The hair cells transmit the electrical energy to the auditory pathway where it is carried into the brain for processing. As they receive and transform the vibrational energy, it turns out the hair cells “twitch”. This tiny twitch produces a miniscule sound. The sound is transmitted from the inner ear, through the middle ear bones and back to the ear drum where it is released into the ear canal! The sound is too tiny and too close to the sound going in to hear with our own ears. With very delicate instrumentation, this sound can be heard and measured. Think of an old submarine movie you have seen where the sonar man sends out a “ping” and waits for the sound to return. In a crude way, that’s what this test does. When the returning ping (or otoacoustic emission) is heard, it can be inferred the hair cells are alive and well. As most types of hearing loss involve the hair cells, this can be very helpful information. In a practical sense, if an otoacoustic emission is heard, we know the hearing is in the 0-25 dB range. The test is quick, painless and requires a still patient. It gives information at each frequency band.