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Audiometric Requirements Prior to Ear Surgery: Pitfalls
Ear surgery differs from surgical procedures performed on most other organ systems, in that one of the outcomes of the procedure may be a change (for better or worse) in the special sensory function that is hearing. The three main purposes of ear surgery are to (1) cure disease (i.e., treat infection, remove a cholesteatoma, etc.), (2) prevent the spread or exacerbation of disease, and (3) restore or improve hearing function. Naturally, these three main purposes may interact and present in various combinations depending upon the disease process or the problem the surgeon is attempting to solve. Therefore, one needs to keep these surgical goals in mind in planning and executing preoperative auditory testing. It is important to note that, regardless of the purpose of the surgical procedure, it may have a direct or indirect effect on auditory function that will become evident post-operatively. Therefore, quantifying the hearing status preoperatively has tremendous clinical/medical as well as medicolegal importance. For instance, masking errors in pure-tone audiometry may induce the surgeon to either operate on the incorrect ear or attempt to close a nonexistent air–bone gap. Otologic clinicians need to be well aware of possible pitfalls in auditory testing and need to have the ability to recognize indicators of erroneous test results to identify possible errors.
■ Audiometry
Auditory Thresholds
One of the most basic, yet important and informative, components of the auditory evaluation is the determination of pure-tone (detection) thresholds, known as pure-tone audiometry. This patient-driven, adaptive, psychophysical (voluntary, behavioral) test, if accurate, provides detailed, frequency-specific information about auditory sensitivity. It also helps determine whether the patient’s hearing loss is conductive, sensorineural, or mixed as well as providing information regarding the presence of symmetry or asymmetry of hearing sensitivity between the left and the right ears. All of these clinical details help determine the need for further diagnostic testing and are essential for formulating an appropriate treatment plan. If this test is inaccurate and provides erroneous information, further diagnostic testing may not be considered, appropriate treatment may be delayed, and, in some cases, unnecessary or inappropriate surgery may be undertaken.
Auditory Sensitivity
Pure-tone audiometry by air conduction measures the function of the entire auditory system, including the external, middle, and inner ear as well as central auditory pathways. Bone conduction testing provides auditory threshold information when the cochlea is stimulated more or less directly, with stimuli delivered in such a way as to bypass external and middle ear structures. Thus, for practical purposes, these thresholds may be considered not to be affected by conductive pathology except in otosclerosis, characterized by the Carhart notch. In the presence of a conductive hearing loss, the difference between air conduction (elevated) and bone conduction (normal, if no sensorineural hearing loss) thresholds is referred to as the air–bone gap. This is an important indicator for the surgeon in planning corrective surgery for a conductive hearing loss.
■ Pitfalls of Audiometry
An inaccurate preoperative audiogram may be caused by several factors. These factors may be divided into technical, clinical procedural, and patient-related factors. Chief among the technical factors may be the use of an audiometer that is not appropriately calibrated.1, 2 This may cause discrepancies between air-conducted and bone-conducted thresholds resulting in either a false conductive component or a false sensorineural hearing loss. This factor comes into play quite rarely; however, it is important to ascertain that the audiometric equipment’s calibration is checked on at least a quarterly basis.
An additional issue that needs to be mentioned relative to preoperative audiometric testing is the utility (or lack thereof) of sound field audiometry. This is a technique often used with pediatric patients who, because of age or developmental stage, are unable to reliably respond to tones presented through earphones. Utilizing play audiometry or conditioning techniques, young patients age 2 to 2.5 years or younger are conditioned to respond to tones presented through a loudspeaker. In the hands of a skilled pediatric audiologist this can result in an accurate estimation of the patient’s overall auditory sensitivity; however, it is important to note that those results reflect hearing of the better ear. Thus a “normal” sound field audiogram may be obtained from a patient with one normal ear, whereas the other one may present with a severe to profound sensorineural hearing loss. A surgeon who unknowingly operates on the ear with the undocumented hearing loss could become the subject of medicolegal litigation when that hearing loss is identified later in the patient’s life. Therefore, it is strongly recommended that prior to ear surgery monaural thresholds be determined by the appropriate means, including electrophysiological [otoacoustic emissions (OAE), auditory brainstem response (ABR)] measures. This can be done in conjunction with the surgical procedure, under anesthesia just prior to undertaking the operation.
One of the main factors in the category of clinical/ procedural errors is the masking error. Masking errors may come in two different categories, either excessive or insufficient masking. Depending upon the actual hearing status, these masking errors may result in a unilateral sensorineural hearing loss masquerading as a conductive hearing loss or a true conductive hearing loss appearing as a sensorineural hearing loss. Patient-driven errors might also occur, given that the audiological evaluation requires a voluntary, conscious response. In some cases, driven by secondary gains or a psychological problem, patients might fail to respond at the actual threshold. This is commonly referred to as “pseudohypoacusis.” There are several indicators of an inaccurate audiogram: (1) the three-frequency pure-tone average and the speech reception threshold do not match within 5 dB;3 (2) an air -bone gap exceeds 60 dB; (3) there is an unexplained substantial, unilateral hearing loss (this always needs to be confirmed using the Stenger test discussed later in this chapter); and (4) there is a reasonable speech reception threshold with an extremely poor word discrimination score.
Masking
In either air conduction or bone conduction threshold testing, it may be necessary to mask the ear contralateral to the test ear to ascertain that the patient’s responses are indeed from the test ear. If the tone presented to the test ear is of sufficient intensity, it may cross the skull and be perceived by the nontest ear. This phenomenon is commonly referred to as crossover. This may occur at an intensity that is in fact lower than the intensity necessary to reach the test ear.4 Thresholds obtained under such conditions are referred to as a shadow curve because they in fact reflect the threshold of the better-hearing nontest ear when stimuli are delivered to the poorer, test ear. This relationship is defined by a phenomenon referred to as interaural attenuation, referring to the reduction in sound level when it crosses from one ear to another.
It is important to note that the crossover phenomenon and interaural attenuation do not occur due to acoustic radiation from under an earphone placed over the test ear, but are the result of sound crossing the skull by bone conduction. To determine masking levels necessary to avoid a shadow curve, one needs to be familiar with the sound levels associated with interaural attenuation. In bone conduction, interaural attenuation is essentially 0 dB, meaning that sound will cross over to the contralateral ear at very low levels. For air conduction, interaural attenuation ranges from 40 to 65 dB (lower for low frequency tones higher for high frequency tones) when sounds are delivered via supra-aural earphones.5 With the more contemporary transducers that utilize an ear canal insert plug, interaural attenuation increases to 60 to 65 dB because of the reduced area of contact with the skull associated with these transducers relative to headphones. Essentially, because insert transducers have less contact with the lateral temporal bone when compared with standard headphones, less sound energy is delivered contralaterally.6 This results in less crossover.
The following are examples of masking problems shown for a single frequency for the sake of simplicity and clarity.
Masking for Air Conduction
In the case of masking for air conduction (Fig. 2–1A) the normal hearing ear is the left ear with 10 dB air and bone pure-tone thresholds. The right ear presents with an unmasked air conduction threshold of 50 dB. Given the value of interaural attenuation for 1000 Hz, this threshold could represent a case of crossover. On the right side of the figure the left column is air conduction for the right, test ear, and the right-sided column lists masker levels delivered to the left ear. The following sequence is shown: with no masking the unmasked threshold is 50 dB; when 15 dB masking is presented to the left ear, the patient ceases to respond to the 50 dB tone (the unmasked threshold). The masker then is maintained at the 15 dB level and the test tone is increased to 55 dB, at which point the patient responds. The test tone remains at 55 dB and the masking level is raised by 5 dB steps to determine if a plateau may be accomplished. After several increments to a masker level of 30 dB, the masked threshold remains 55 dB and is thus considered to be the actual threshold for this patient at this frequency.
Masking for Bone Conduction
In masking for bone conduction (Fig. 2–1B) the patient presents with a normal, 5 dB threshold for both air and bone conduction in the right ear. The left ear presents with an unmasked air conduction threshold of 35 dB and an unmasked bone conduction threshold of 5 dB, identical to that of the right ear. The question here is whether the hearing loss in the left ear is conductive or sensorineural (i.e., is the 5 dB bone-conduction threshold accurate). To accomplish this, bone conduction needs to be retested with masking to the right ear. The table on the right hand side of the figure shows bone conduction thresholds for the left ear and masking levels for the right ear. With the masker set to 30 dB in the right ear, there is no response up to a level of 30 dB delivered to the left ear. With the masker held at 30 dB, the bone-conducted tone is raised to 35 dB in the left ear and the patient responds. Now the test tone is held at 35 dB and the masker level is raised in 5 dB steps up to 45 dB with the bone conducted, masked threshold in the left ear remaining stable at 35 dB, indicating that this is the true threshold for the left ear. Hence there is no conductive component in this case. The patient presents with a 35 dB sensorineural hearing loss. The 5 dB unmasked bone conduction threshold was a shadow curve.
Overmasking
In the case of overmasking (Fig. 2–1C) the patient presents with bilateral conductive hearing loss (air conduction thresholds are 35 dB bilaterally, unmasked bone conduction thresholds are 5 dB bilaterally). The question is whether this patient in fact has bilateral conductive hearing loss. This example shows testing bone conduction in the left ear while masking the right ear. As shown in the table on the right side of the figure, the left ear bone conduction threshold remains stable at 15 dB up to a masker level of 55 dB presented to the right ear. This is a somewhat difficult case because to effectively mask the right ear, one has to first overcome the conductive component in the nontest ear, and this increases the risk of overmasking. Beyond a 55 dB masker level, every 5 dB increment in masker level results in a 5 dB increment in the left ear bone conduction threshold. This is commonly viewed as an indication of overmasking; therefore, it appears that the true bone conduction threshold of the left ear is in fact 15 dB, and the hearing loss is therefore conductive.
Undermasking
The following is an example of undermasking (Fig. 2–2
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