March 30, 2004 Feature

Intraoperative Neurophysiologic Monitoring

A Contemporary Brief

Audiologists who perform intraoperative neurophysiologic monitoring (IOM) use electromyography (EMG) and sensory and motor evoked potentials to identify functional changes in a patient's peripheral and central nervous system during surgery to reduce the risk of iatrogenic injury-injury originating during surgery-and avoid significant postoperative impairment.

IOM has been part of the professional practice of audiologists for more than two decades and is included in the Scope of Practice documents of our professional organizations. IOM is provided for otolaryngology, neurosurgery, and vascular and orthopedic surgical services in academic and private medical institutions by hospital-based audiologists. Moreover, some private practitioners focus their practices on IOM. For instance, in 2002 Surgical Monitoring Associates, Inc. was recognized as one of the fastest growing privately held companies in Philadelphia, PA.

Over the course of 20 years, the number of surgical procedures calling for IOM by audiologists at the University of Michigan Health System (UMHS) has grown to the point that two full-time equivalent positions are used daily in otolaryngology or neurosurgery cases (Figure 1). Among a wide variety of cases, over 500 patients with facial and acoustic nerve tumors have been treated. From a staff that includes 22 audiologists, five are qualified to perform IOM. Notably, this level of commitment to monitoring is not unique to UMHS. An informal telephone survey of several Otolaryngology-Head & Neck Surgery departments listed in the 2003 U.S. News & World Report "Best ENT Departments," found that 11%-26% of the audiology staff complement in those university-based teaching programs provide monitoring (see Edwards and Spak).

IOM Uses and Necessary Skills

During surgical procedures, IOM is used to continuously assess the functional integrity of a patient's nervous system, identify neural structures in the operative field and avoid damage to those structures. Importantly, an audiologist must learn to quickly and correctly determine whether a change in monitored activity relates to a surgical (iatrogenic injury) or non-surgical (introduction of pharmacological agents by anesthesiologists) event and should be able to suggest ways to avoid or reverse those effects. To realize the goals of monitoring, many technical concepts must be mastered. These critically important matters are far beyond the scope of this article, but suffice it to say that effective, useful monitoring follows specific basic and clinical science education, relevant experiences, and substantial training under direct supervision.

Justification for electrophysiologic monitoring is a complex medicolegal concept and may be viewed through economic, altruistic, or educational lenses. For example, a simple economic calculation exists that delineates the cost effectiveness of IOM when compared to physical, emotional, and monetary costs resulting from unforeseen neurological compromise, prolonged postoperative care, and lost income in the absence of monitoring (see Schwartz and Komesar; Wilson et al.; and sidebar 2 on page 16).

A substantial volume of retrospective studies confirm that monitoring leads to a reduction in iatrogenic injury and improved postoperative outcomes for patients. Several recent reports describe its efficacy in primary parotidecomy (see Terrell et al.) and in neurotologic surgery for vestibular schwannomas (see Isaacson et al.), and detail its cost effectiveness in otologic surgery (see Wilson et al.). In all cases, a sense of "situational appropriateness" should apply by answering the simple question, "What is best for the patient?"

The educational value of monitoring in a medical teaching institution may be unbounded. In our institution, an otolaryngology resident learns surgical techniques and develops skills that last a career and IOM is available to support that training. As stated earlier, continuous monitoring helps to identify neural elements in the surgical field, a particularly important function in revision surgery when anatomic landmarks are missing or obscured by scar tissue.


The education of graduate audiologists and working professionals who wish to add IOM to their complement of skills is a complex topic that is not well addressed in academia, in private settings, or in revised requirements for entry-level audiologists. Primarily, this results from the non-medical orientation of many academic audiological training institutions and a dearth of medical off-sites where monitoring experiences may be acquired. The end-result is a lack of knowledge, skills, and practice opportunities for audiologists who wish to develop IOM expertise.

Nonetheless, some institutions offer IOM experiences to interested graduate students. At the UMHS, audiology Fellows rotate through the monitoring service observing and assisting their senior colleagues in, for example, the placement of monitoring leads and insertion of earphones. By doing so, they learn how to function in this complex, technical environment. These audiologists tend to have an interest in electrophysiological measures and gain useful knowledge in the techniques used in the operating room. In most cases, however, their monitoring experience is insufficient to permit them to claim competence or to independently provide monitoring.

Working audiologists who wish to gain similar skills may find it difficult to obtain the requisite supervised practice, and this component of practice does not lend itself to distance learning. Nonetheless, for those who can arrange such experiences, supportive and/or certifying bodies exist outside of audiology. These include the American Society of Neurophysiologic Monitoring, American Board of Registration of Electroencephalographic and Evoked Potential Technicians, American Board of Neurophysiologic Monitoring, and American Society of Electroneurodiagnostic Technologists. The stated goals of these organizations include developing and promoting high standards for monitoring and advocating within the medical/surgical communities on behalf of IOM practitioners.

Case Study

A brief case study illustrates the value of IOM by describing a rather common event associated with a neurotologic surgical procedure: A patient with a medium-sized vestibular schwannoma expressed a strong desire that her hearing and facial function be preserved after surgery. Her neurotologic surgeons suggested a suboccipital approach to the mass lesion. The seventh and eighth cranial nerves were continuously monitored using facial EMG and the auditory brainstem response (ABR). Following the craniotomy, surgeons used a device to retract the cerebellum in order to facilitate access to the tumor. The ABR deteriorated immediately, characterized by a rapid reduction in wave V amplitude and a shift in the absolute latency of nearly one millisecond (Figure 2). This dramatic change in auditory function was immediately reported by the audiologist in an effort to minimize permanent auditory nerve injury. The retractor was removed and then repositioned. The ABR gradually improved to near-baseline morphology and absolute values. The schwannoma was successfully resected, and postoperatively the patient was found to have hearing levels equal to the preoperative findings. Central to the assertion that IOM has value is the fact that seemingly innocuous surgical maneuvers can lead to permanent postoperative hearing loss in the absence of continuous monitoring.

The recognition of the value and efficacy of IOM bodes well for the future of audiologists who wish to practice in this domain. Long-established techniques of evoked potential and electromyography are being joined by other methods that more directly evaluate nervous system function. These include transcranial motor evoked potentials, microdoppler flow studies, speech cortex mapping, and cerebral oximetry (see Beacham). As the field of IOM matures, many exciting opportunities can be anticipated.

cite as: Edwards, B. M. , Kileny, P. R. , Kovach, D. J. , McCue, J.  & Spak, C. (2004, March 30). Intraoperative Neurophysiologic Monitoring : A Contemporary Brief. The ASHA Leader.

Additional Information

Refer to the following references in the area of IOM:

Early Investigations

  • changes in the middle latency response during open-heart surgery (Kileny)
  • effects of lidocaine infusion or various anesthesia agents on auditory evoked potentials (Ruth)

Experiences in Surgical Settings

  • Dennis and Earley; Schwartz et al.; Beck and Benecke 

Textbooks or Chapters of Instruction and Advice

  • Hall; Schwartz and Rosenberg; Kileny and Niparko; Beck, 1994; Edwards and Kileny.

(Most of these forward-thinkers and leaders of the profession continue to practice in this important area.)


Economic Calculation for Justifying the Need for IOM

C < P x L (where C = Cost of monitoring, P = Probability of injury, L = Loss in dollars). In otologic revision surgery, "P" is estimated to be 4%-10% (Wilson et al, 2003). If "L" is set at $100,000, IOM is cost effective if its charge is $4,000-$10,000. Estimates of "L" in this example may not reflect actual costs associated with iatrogenic injury during otologic surgery; these vary widely by individual case and by geographic region.


American Academy of Audiology. (1993). Audiology: Scope of practice. Audiology Today, 5. 16-17, 1993.

American Speech-Language-Hearing Association. (1996). Scope of practice in audiology. Asha, 38 (Supplement 16), 12-14.

Beacham, S. G. Kathleen Mears lecture: Intraoperative neurophysiological monitoring-a contemporary perspective. American Journal of Electroneurodiagnostic Technology, 41, 99-115.

Beck, D. (1994). Handbook of intraoperative monitoring. San Diego, CA:Singular Press.

Beck, D., & Benecke, J. (1990). Intraoperative facial nerve monitoring: Technical aspects. Otolaryngology-Head and Neck Surgery, 102, 270-272.

Dennis, J., & Earley, D. (1988). Monitoring surgical procedures with the auditory brainstem recording. Seminars in Hearing, 9, 113-125.

Edwards, B. M., & Kileny, P. R. (2000). Intraoperative monitoring of cranial nerves. In R. F. Canalis & P. R. Lambert (Eds.), The ear: Comprehensive otology (pp.279-294). Baltimore, MD: Lippincott Williams & Wilkins.

Edwards, B. M., & Spak, C. Intraoperative monitoring: Some history and contemporary information. Instructional course, ASHA Convention, Chicago, IL, November 2003.

Hall, J. (1992). Intraoperative monitoring. In J. W. Hall III, Handbook of auditory evoked responses. Needham Heights, MA: Allyn & Bacon.

Isaacson, B., Kileny, P. R., & El-Kashlan, H. (2003). Intraoperative monitoring and facial nerve outcomes after vestibular schwannoma resection. Otology Neurotology 24, 812-817.

Kileny, P., Dobson, D., & Gelfand, E. (1983). Middle latency auditory evoked responses during open heart surgery with hypothermia. Electroencephalography and Clinical Neurophysiology, 55, 268-76.

Kileny,  P. R., & Niparko, J. K. (1992). Future challenges in intraoperative neural stimulation and recording. In J. Kartush & K. Bouchard (Eds.). Neuromonitoring in otology and head and neck surgery (pp. 229-39), New York: Marcel Dekker, Inc.

Mishler, E. T. (1999). Clinical report. Audiologists in surgery. Audiology Today, 11, 14-16.

Ruth, R. A., Gal, T. J., DiFazio, C. A., & Moscicki, J. C. (1985). Brain-stem auditory-evoked potentials during lidocaine infusion in humans. Archives of Otolaryngology, 111, 799-802.

Schwartz, D., & Rosenberg, S. (1992). Facial nerve monitoring during parotidectomy. In J. Kartush, & K. Bouchard (Eds.), Neuromonitoring in otology and head and neck surgery (p. 121). New York: Marcel Dekker, Inc.

Schwartz, D. M., Gennarelli, T., Young, M., Fedder, S., & Schwinn, D. (1989). Intraoperative monitoring of brainstem auditory evoked potentials following emergency evacuation of a cerebellar vascular malformation. Journal of Clinical Monitoring, 5, 116-118.

Schwartz, W. B., & Komesar, N. K. (1978). Doctors, damages, and deterrence: An economic view of medical malpractice. New England Journal of Medicine, 298, 1282.

Terrell, J. E., Kileny, P. R., Yian, C., Esclamado, R. M., Bradford, C. R., Pillsbury, M. S., & Wolf, G. T. (1997). Clinical outcome of continuous facial nerve monitoring during primary parotidectomy. Archives of Otolaryngology-Head and Neck Surgery, 123, 1081-1087.

Wilson, L., Lin, E., & Lalwani, A. (2003). Cost of facial nerve monitoring in middle ear or mastoid surgery. Laryngoscope, 113, 1736-1745. 


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