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Audiology in the OR

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Audiology in the OR

AUDIOLOGY IN THE OR

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By Robert Trace

While only a few hundred audiologists are providing intraoperative monitoring services on a regular basis, these clinicians have a significant impact on the overall safety and efficacy of surgical procedures involving monitored structures.

Intraoperative monitoring involves identifying and protecting structures at risk during surgical procedures. This includes cranial nerve monitoring which involves the cochlear, vagus, optic, extraocular, trigeminal, facial, spinal accessory and/or hypoglossal nerves; and spinal cord nerve monitoring, which involves sensorimotor functions.

Providing audiological services in the OR is unique, Joseph Danto, PhD, CCC-A, pointed out.

"Being capable of doing an auditory brainstem response (ABR) test in the comfort of your office is one thing," explained the president of Metropolitan Audiology and Physiologic Assessment Services, both in Englewood, NJ. "If you don't get the response you want, you can always have the person come back another day."

However, "when you must do an ABR in the operating room, you only have one chance to get everything right because your findings and recommendations may determine what the surgeon does," stated Dr. Danto, who is also director and coordinator of the speech and hearing program at Stern College for Women at Yeshiva University in New York, NY.

Intraoperative monitoring requires extensive knowledge of human anatomy and physiology. In cases involving brain tumors, for example, the audiologist may be asked to identify structures that might be affected depending on the area of the brain in which the surgeon is working.

A frequent lecturer on intraoperative monitoring issues, Dr. Danto uses the AMID Principle to identify the different intraoperative monitoring responsibilities of audiologists. AMID (Assessing, Minimizing, Identifying, Defining) involves assessing the function of various body structures, minimizing the risk to reduce the negative sequelae, identifying structures during the surgery, and defining the scope of how a surgeon's actions will impact the patient.

As part of the monitoring process, audiologists can identify particular structures--such as a partially obstructed spinal or cranial nerve--and bring it to the surgeon's attention. They also collect evoked responses through various monitoring instruments, such as an ABR computer, to help the surgeon know when the intended surgical objectives have been completed.

To determine when monitoring is appropriate, Dr. Danto said, audiologists should utilize the SAFE rationale for monitoring, which represents four essential clinical principles: system at risk, access to system, feedback, and expertise/equipment.

"A patient must be at risk for some neurologic or physiologic problem, or we really don't have a duty," he explained.

There must be a demonstrated need for the audiologist's expertise and equipment, he said. "If the risk involved has been minimized, there is no need for our expertise and equipment."

The surgeon must encourage feedback from the audiologist and other OR personnel to create a truly multidisciplinary environment.

"The surgeon must be flexible enough to accommodate my input," Dr. Danto stated. "If the surgeon isn't willing to consider my suggestions or allow me to give feedback regarding what is going on, I don't want to work with that person. The surgeon has to understand and appreciate what you can offer the surgical team."

"All monitoring should be done in accordance with the surgical team and what they feel is necessary in the monitoring process," said Douglas L. Beck, MA, CCC-A, F-AAA, director of audiology at St. Louis University Health Sciences Center, St. Louis, MO, and president of the Missouri Academy of Audiology. "Monitoring should be used to amend the real-time knowledge of what is occurring during surgery."

He added, "All devices you plan to use in the monitoring process should receive approval from the biomedical department at the hospital as a safety measure."

Beck prefers simpler, DC power-driven devices that don't produce the 60 Hz artifact often associated with AC power-driven units. However, "it isn't the quality of the instruments that is so important as the person using them. That cannot be emphasized enough."

Most audiologists aren't prepared to enter this area of practice right out of college, Beck noted. While a few universities are starting to implement graduate courses on intraoperative monitoring, "clinicians get most of their experience in this field through continuing education classes and mentorship programs," he said.

At St. Louis University Health Sciences Center, "we allow professionals to observe us at no cost because we feel that it is part of our professional responsibility," said Beck, who authored the Handbook of Intraoperative Monitoring (San Diego: Singular Publishing Group, Inc., 1994).

Currently, there is no licensing standard established for this area of practice.

Audiologists use a variety of monitoring equipment in the operating room. Instruments that measure visual evoked potentials (VEPs) provide a means of monitoring surgical procedures along the visual pathway, Beck explained.

VEPs are electrical signals recorded from the occipital cortical region in response to either flash or pattern light stimulation. An audiologist can monitor the status of the visual nerve in the OR by either fitting the patient with modified contact lenses or goggles that provide flash stimuli varying in wavelength, temporal frequency and intensity.

ABR tests are especially helpful in monitoring the status of the auditory system of patients, noted Beck. "We use ABR as a tool in a slightly different manner in the operating room than we would if we were in our office. In most audiological practices, ABR is used as a tool to estimate the degree and type of hearing loss. In the operating room, we use the tool to help maintain hearing throughout the surgery."

This is pertinent to surgery involving work done on or near the eighth cranial nerve, or cochlear nerve.

"We'll use ABR as a guide to make sure there are no changes in the patient's hearing as the surgeon, for example, removes an acoustic neuroma involving the eighth cranial nerve," he said. "ABR is used to monitor hearing and measure hearing loss that may result from surgery."

The audiologist should recognize when a surgeon may be getting too close to the cochlear nerve and report the potential problems the patient could experience if the nerve is damaged.

During the surgery, Beck may stimulate the patient's ears with clicks as part of the ABR testing. To measure brain activity, he places a needle or surface electrodes on the top of the head, at the center of the forehead, and at one or both earlobes.

Performing an intraoperative ABR is valid anytime monitoring is requested or a patient's hearing may be at risk during a surgical procedure, he said.

Some of the most common cases involving ABR monitoring are acoustic neuroma surgery with hearing preservation, endolymphatic sac surgery, vestibular nerve sections, revision mastoid cases, most posterior fossa cases, cochlear implants and microvascular decompressions.

An effective measurement of somatosensory function is evoked potential testing, reported Dr. Danto. The somatosensory evoked potential (SSEP) is elicited by stimulating either a sensory or sensorimotor peripheral nerve. Once this response has been elicited, the neural response can be recorded from various locations along the neural pathway.

This measurement includes near field recordings, with an electrode placed directly on the nerve, and far field recordings, which are made further along the neural pathway. Far field recordings can be made using either surface or subdermal electrodes.

For facial nerve monitoring, Beck prefers to use bipolar surface electrodes that he modifies rather than monopolar subdermal needle electrodes. "Especially for cases involving ear surgery, the bipolar electrodes are extremely sensitive and specific to what you are trying to look for," he said.

Electromyography (EMG) can be used as a component of a comprehensive intraoperative program, Dr. Danto said. Either a free-running or stimulated/triggered approach can be used to measure EMGs.

The free-running EMG collection method involves allowing the recording device, which is connected to a set of receptive and/or stimulatory electrodes, to run continuously. A signal is recorded when the nerve connected to the muscle(s) to which the electrode is attached is disturbed by the surgeon during the operation.

Using the triggered approach, an audiologist and surgeon may work together to identify a nerve or structure that cannot be seen easily, Dr. Danto said. He cited the example of patients with a "tethered" cord, whose spinal nerve is buried under a layer of fat. During the operation, a surgeon may be unable to differentiate the nerve from the fat.

"These are very tricky situations because you don't know what to cut," Dr. Danto said. "Even worse, you don't know if something you see could be fat or a nerve, and even moving the patient the wrong way could cause a problem."

The audiologist can insert the EMG electrode into the muscle area where fat and nerve cells combine. The EMG monitor is activated when the surgeon, who is working on nearby muscles, comes in contact with the spinal nerve.

EMG technology is especially useful when monitoring cranial motor nerves, explained Beck. "It is essential that we have a tool like EMG because the patient can experience facial paralysis as a result of iatrogenic injury during surgery."

Some instruments on the market today can accommodate both ABR and EMG functions, he said. While a multifunctional device saves on additional equipment and space, if it loses power, "you're stuck," Beck said. "If you have three different single-function machines and something goes wrong with one of them, you can always turn to another machine."

EMG also monitors nerve function of the upper and lower extremities in cases involving surgery of the arms, legs, upper or lower back, and neck, said Dr. Danto.

"If the surgeon is doing work on a patient's lumbar region, identifying and stimulating the patient's arm nerve won't necessarily help the surgeon with what he is doing," he stated. "However, it can provide you with a more accurate picture of how the nerves from both extremities are interconnected and are impacting each other."

Audiologists may be responsible for monitoring patients on numerous nonpathologic levels, including changes in the patient's temperature and overall condition as a response to the surgical intervention. They are also responsible for monitoring any physiological changes in the patient resulting from hemodilution, anesthesia, inhalation agents, bolus injections or positioning changes.

Monitoring patients for pathological effects of surgery includes ischemia, retraction, distraction and compression.

"We can use evoked potentials to tell if a positioning change, for example, is compressing a nerve or causing a retraction on the nerves or other structures," Dr. Danto said. "Some procedures place undue stress on the nerves, such as in hip operations where the spinal nerve(s) may be stretched. If we don't monitor carefully, the patient could be moved the wrong away and paralysis or other catastrophic problem could result."

Audiologists may be responsible for monitoring surgeries outside of the otologic and neurologic realm, such as in cardio thoracic cases and orthopedic surgeries.

"We may use a combination of evoked potential or electroencephalogram (EEG) monitoring in cardio thoracic cases such as aneurysms and angioplasties," he said. "We can assist the surgeon in the intraoperative process by identifying warning signs that a vessel may get blocked and cut off the blood supply to one part of the brain, causing a stroke."

Intraoperative monitoring as an area of practice is not for the faint of heart, according to Beck. "There is a lot of pressure in this job, and you have to maintain an excellent relationship with the surgeons. Most importantly, it is essential during preoperative consultation with surgeons that the audiologist explain the limitations and expectations of monitoring."

Believe it or not, he noted, "Many surgeons don't really understand what has to happen in monitoring to ensure safety for the patient. Misinformation can lead to a disaster. When used properly, effective monitoring can contribute to a surgeon's good performance."

"One thing that is strange about this practice area (of intraoperative monitoring) is the hours," stated Dr. Danto, who provides services to nearly 20 facilities. "As a private practitioner, I might get a call in the middle of the night to monitor an accident victim who has to receive emergency surgery."

He said audiologists are well suited to perform intraoperative monitoring because they have a thorough understanding of evoked potentials. In addition, "they have a natural inclination toward clinical interaction and problem-solving."

Effective monitoring is no substitute for a surgeon's skill and knowledge, however.

"We can't improve on a surgeon's skill, but our goal should be to help the surgeon's overall efforts by contributing what we can," Dr. Danto concluded.

For More Information

Douglas L. Beck, Department of Audiology, St. Louis University Health Sciences Center, Ste. 312, 3660 Vista Ave., St. Louis, MO 63110; (314) 577-6110

Joseph Danto, PhD, Physiologic Assessment Services, 214 Engle St., Englewood, NJ 07631; (201) 567-8408




     

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