1. Aetna considers evoked potential studies medically necessary for the following indications:

   1. Somatosensory evoked potentials (SEPs, SSEPs) or dermatosensory evoked potentials (DSEPs) are considered medically necessary for any of the following indications:

      1. Unexplained myelopathy; or
      2. To localize the cause of a central nervous system deficit seen on exam, but not explained by lesions seen on CT or MRI; or
      3. To identify clinically silent brain lesions in multiple sclerosis suspects in order to establish the diagnosis, where multiple sclerosis is suspected due to presence of suggestive neurologic symptoms plus one or more other objective findings (brain plaques on MRI, clinical lesions by history and physical examination, and/ or positive CSF (determined by oligoclonal bands detected by established methods (isoelectric focusing) different from any such bands in serum, or by an increased IgG index));
      4. To manage persons with spinocerebellar degeneration (e.g., Friedreichs ataxia, olivopontocerebellar (OPC) degeneration); or
      5. To assess any decline which may warrant emergent surgery in unconscious spinal cord injury persons who show specific structural damage to the somatosensory system, and who are candidates for emergency spinal cord surgery; or

      6. To evaluate persons with suspected brain death.

      SEPs and DSEPs are considered experimental and investigational for all other indications.

   2. Intraoperative somatosensory evoked potentials (SSEPs) performed either alone, or in combination with motor evoked potentials (MEPs) are considered medically necessary for monitoring the integrity of the spinal cord to detect adverse changes before they become irreversible during spinal, intracranial, orthopedic, or vascular procedures, when the following criteria are met:

      1. The evoked potential monitoring is performed in the operating room by dedicated trained technician; and
      2. A specially trained physician who is not a member of the surgical team contemporaneously interprets the intraoperative evoked potentials during the operation; and

      3. The physician who performs the interpretation provides direct, immediate communication of intraoperative evoked potential results to the technician and surgeon during the operation.

      Intra-operative SEP monitoring, with or without MEPs, may be appropriate for the following types of surgery (not an all-inclusive list):

      1. Spinal Surgeries:

         1. Decompression of the spinal cord where function of the spinal cord is at risk
         2. Removal of spinal cord tumors
         3. Surgery as a result of traumatic injury to the spinal cord
         4. Surgery for arteriovenous (AV) malformation of the spinal cord

         5. Correction of scoliosis or deformity of the spinal cord involving traction on the cord

      2. Intracranial Surgeries:

         1. Correction of cerebral vascular aneurysms
         2. Deep brain stimulation
         3. Endolymphatic shunt for Meniere's disease
         4. Microvascular decompression of cranial nerves (e.g., optic, trigeminal, facial, auditory nerves)
         5. Oval or round window graft
         6. Removal of cavernous sinus tumors
         7. Removal of tumors that affect cranial nerves
         8. Resection of brain tissue close to the primary motor cortex and requiring brain mapping
         9. Resection of epileptogenic brain tissue or tumor
         10. Surgery as a result of traumatic injury to the brain
         11. Surgery for intracranial AV malformations
         12. Surgery for intractable movement disorders

         13. Vestibular section for vertigo

      3. Vascular Surgeries:

         1. Arteriography, during which there is a test occlusion of the carotid artery
         2. Circulatory arrest with hypothermia (does not include surgeries performed under circulatory bypass such as CABG, and ventricular aneurysms)
         3. Distal aortic procedures, where there is risk of ischemia to the spinal cord

         4. Surgery of the aortic arch, its branch vessels, or thoracic aorta, including carotid artery surgery, when there is risk of cerebral ischemia.

      SSEPs with or without MEPs are considered experimental and investigational for all other indications.

      Note: Depending on the clinical condition being investigated, it may be medically necessary to test several nerves in one extremity and compare them with the opposite limb.

      Note: Upper extremity SEPs or MEPs are not necessary for lumbar surgery.

      Note: Pre- and/or post-operative SEP or MEP monitoring is not considered medically necessary for individuals who will undergo or have undergone intra-operative SEP or MEP monitoring because baseline values may be obtained intraoperatively. 
      
      
      Note: The NIM-Spine System received 510(k) clearance from the FDA in June 2003.  It offers 2 types of monitoring modalities: (i) electromyography, and (ii) MEP.

      Note on documentation requirements: The physician's SEP report should note which nerves were tested, latencies at various testing points, and an evaluation of whether the resulting values are normal or abnormal.

2. Visual evoked potentials (VEPs) are considered medically necessary for any of the following indications:
   
   
   
   1. To identify persons at increased risk for developing clinically definite multiple sclerosis (CDMS); or
   2. To diagnose and monitor multiple sclerosis (acute or chronic phases); or
   3. To localize the cause of a visual field defect, not explained by lesions seen on CT or MRI, metabolic disorders, or infectious diseases; or

   4. To evaluate signs and symptoms of visual loss in persons who are unable to communicate (e.g., unresponsive persons, etc.).

      VEPs are considered experimental and investigational for routine screening of infants; evidence-based guidelines from leading medical professional organizations and public health agencies have not recommended VEP screening of infants. VEPs are considered experimental and investigational for all other indications.

3. Brain stem auditory evoked response (BAER)** is considered medically necessary for any of the following:

   1. To diagnose suspected acoustic neuroma; or
   2. To assess recovery of brainstem function after a lesion compressing the brainstem has been surgically removed; or
   3. To localize the cause of a central nervous system deficit seen on exam, but not explained by CT or MRI; or
   4. To diagnose and monitor demyelinating and degenerative diseases affecting the brain stem (e.g., central pontine myelinolysis, olivopontocerebellar (OPC) degeneration, etc.); or
   5. To evaluate infants and children who have suspected hearing loss that cannot be effectively measured or monitored through audiometry; or
   6. To screen infants and children under age 5 for hearing loss. Note: For purposes of neonatal screening, only limited auditory evoked potentials or limited evoked otoacoustic emissions are considered medically necessary. Neonates who fail this screening test are then referred for comprehensive auditory evoked response testing or comprehensive otoacoustic emissions. Comprehensive auditory evoked response testing and comprehensive otoacoustic emissions are considered experimental and investigational for neonatal screening because there is a lack of evidence of the value of comprehensive testing over the limited auditory evoked potentials or limited otoacoustic emissions for this indication; or
   7. To assess brain death or profound metabolic coma in selected cases where diagnosis or outcome is unclear from standard tests (e.g., EEG); or

   8. To diagnose post-meningitic deafness in children.

   BAERS are considered experimental and investigational for all other indications.
   
   ** Also known as auditory evoked potentials (AEPs), brainstem auditory evoked potentials (BAEP), BERA, BSER, and BSRA.

Aetna considers the following studies and indications to be experimental and investigational because they have not been proven necessary to aid in diagnosis or alter the management of the member:

• Intra-operative visual evoked potentials;

• Motor evoked potentials, other than for intraoperative use with SSEPs;

• Auditory evoked potentials to determine gestational age or conceptual age in pre-term neonates;

• BAERs as a test to identify persons at increased risk for developing clinically definite multiple sclerosis (CDMS);

• SEPs in conscious persons with severe spinal cord or head injuries (the standard neurologic exam is the most direct way to evaluate any deficits);

• SEPs in the diagnosis or management of amyotrophic lateral sclerosis (ALS);

• SEPs in diagnosis of cervical spondylytic myeloradiculopathy;

• SEPs in the diagnosis or management of acquired metabolic disorders (e.g., lead toxicity, B12 deficiency);

• SEPs in the diagnosis of thoracic outlet syndrome;

• Cognitive evoked potentials (also known as auditory or visual P300 or P3 cognitive evoked potentials) to diagnose cognitive dysfunction in persons with dementia (e.g., Alzheimer's disease and Parkinson's disease) or to identify the etiology of depression in persons with chronic demyelinating disease;

• Event-related potentials for the diagnosis of attention deficit/hyperactivity disorder (see CPB 426 - Attention Deficit/Hyperactivity Disorder) or post-traumatic stress disorder, or assessment of brain injury, or evaluation of comatose persons;

• Gustatory evoked potentials for diagnosing taste disorders. See CPB 390 - Smell and Taste Disorders - Diagnosis;

• SEPs for radiculopathies and peripheral nerve lesions where standard nerve conduction velocity studies are diagnostic. See CPB 502 - Nerve Conduction Velocity Studies;

• SEPs for the diagnosis of carpal tunnel syndrome/ulnar nerve entrapment;

• Cortical auditory evoked response (CAER) for the diagnosis of depression, attention deficit/hyperactivity disorder, autism, or any other indication;

• Vestibular evoked myogenic potentials (VEMP).

Evoked potentials measure conduction velocities of sensory pathways in the central nervous system using computerized averaging techniques.  Three types of evoked potentials are routinely performed: (1) somatosensory; (2) visual; and (3) brainstem auditory. In each of these tests a peripheral sense organ is electrically stimulated and conduction velocities are recorded for central somatosensory pathways located in the posterior columns of the spinal cord, brain stem, and thalamus, and the primary sensory cortex located in the parietal lobes.

For patients with symptomatic nerve root compression, the accurate identification of the particular nerve root(s) that are causing symptoms is an essential prerequisite to surgical intervention.  The history and physical examination may be helpful in identifying the particular peripheral nerve root that is affected, but these are often inconclusive.  Patients often have difficulty defining the distribution of pain or sensory symptoms, and the physical examination may be completely normal even in patients with severe pain.  Imaging studies may be helpful, but are frequently normal or reveal abnormalities of uncertain clinical relevance.  Moreover, because structural abnormalities are commonly seen in imaging studies of normal asymptomatic middle-aged or elderly subjects, it is difficult to determine whether any such abnormalities that are identified in pain patients are related to their symptoms.  In addition, imaging studies may show equivocal changes or anatomic abnormalities at multiple levels, making it impossible to determine which nerve root is responsible for the patient's symptoms.  In these circumstances, evoked potentials may be used to measure nerve root function and thereby more accurately identify the precise nerve roots responsible for the patient’s symptoms.

Somatosensory evoked potentials (SEPs or SSEPs) (also known as cerebral sensory evoked potentials) augment the sensory examination and are most useful in assessing the spinal nerve roots, spinal cord, or brain stem for evidence of delayed nerve conduction. Dermatomal somatosensory evoked potentials (DSEPs) are elicited by stimulating the skin "signature" areas of specific nerve roots.  Both techniques involve production and recording of small electrophysiological responses of the central nervous system that follow sequential electrical stimulation of peripheral nerves.  These small electrophysiological responses are extracted from the background noise of electroencephalography (EEG), usually by signal averaging techniques. Delays in signal propagation suggest lesions of the central sensory pathways.  Although controversial, evoked potentials have been used to assess the prognosis of children with spinal cord lesions, brain malformations, and neurodegenerative diseases, as well as young children who are at risk for brain injury, such as preterm infants.  SSEP measurements have been used to predict outcome in spinal cord injury; however, signal changes on MRI actually may be more useful in determining the severity of injury.  Hemorrhage within the spinal cord is readily identified on MRI, and such hemorrhage is predictive of injury severity.  Intraoperative SSEP measurements are useful in complex neurologic, orthopedic, and vascular surgical procedures as a means of gauging nerve injury during surgery (e.g., resection of cord tumors).

SEPs are altered by conditions that affect the somatosensory pathways, including both focal lesions (such as strokes, tumors, cervical spondylosis, syringomyelia) and diffuse diseases (such as hereditary systemic neurologic degeneration, subacute combined degeneration, and vitamin E deficiencies).

SEPs may detect clinically silent brain lesions in multiple sclerosis suspects. Although SEP abnormalities alone are insufficient to establish the diagnosis of multiple sclerosis, the diagnosis can be established when there is also other objective findings (brain plaques on MRI, clinical lesions by history and physical examination, and/ or positive CSF (determined by oligoclonal bands detected by established methods (isoelectric focusing) different from any such bands in serum, or by an increased IgG index)).

Fifty to 60 percent of multiple sclerosis patients have other concurrent demyelinating lesions that may not be clinically evident, and SSEP may be helpful in documenting these abnormalities. SSEP abnormalities are also produced by other diseases affecting myelin (adrenoleukodystrophy and adrenomyelo-neuropathy, metachromatic leukodystrophy,  Pelizaeus-Merzbacher disease). In adrenoleukodystrophy and adrenomyeloneuropathy, SSEP abnormalities may be present in asymptomatic heterozygotes.  Abnormally large amplitude SEPs, reflecting enhanced cortical excitability, are seen in progressive myoclonus epilepsy, in some patients with photosensitive epilepsy, and in late infantile ceroid lipofuscinosis.

Studies have demonstrated a statistically significant association between abnormal visual evoked potentials (VEPs) and an increased risk of developing clinically definite multiple sclerosis (CDMS).  In these studies, patients with suspected MS were 2.5 to 9 times as likely to develop CDMS as patients with normal VEPs. VEP sensitivities ranged from 25% to 83%.  VEPs improved the ability to predict which MS suspects will develop CDMS by as much as 29%.

Measurement of visual evoked responses (VERs) is the primary means of objectively testing vision in infants and young children suspected of having disorders of the visual system, where the child is too young to report differences in color vision or to undergo assessment of visual fields and visual acuity. A flashing stroboscope or an alternating checkerboard pattern is presented and the wave patterns are recorded. In an infant, vision may be reliably tested using a flashing light during quiet sleep. Lesions affecting the visual pathways can be localized by noting the presence of decreased amplitudes or increased latencies of VERs, and by determining whether VER abnormalities involve one or both eyes. VERs are also useful for testing vision in other persons who are not able to communicate.

Brain stem auditory evoked responses (BAERs) are electrical potentials that are produced in response to an auditory stimulus and are recorded from disk electrodes attached to the scalp. Depending on the amount of time elapsed between the "click" stimulus and the auditory evoked response, potentials are classified as early (0 to 10 msec), middle (11 to 50 msec), or late (51 to 500 msec).  The early potentials reflect electrical activity at the cochlea, eighth cranial nerve, and brain stem levels; the latter potentials reflect cortical activity.  In order to separate evoked potentials from background noise, a computer averages the auditory evoked responses to 1000 to 2000 clicks.  Early evoked responses may be analyzed to estimate the magnitude of hearing loss and to differentiate among cochlea, eighth nerve, and brainstem lesions.

The clinical utility of BAER over standard auditory testing is due to several of BAER's characteristics: (1) BAER's resistance to alteration by systemic metabolic abnormalities, medications or pronounced changes in the state of consciousness of the patient; and (2) the close association of BAER waveform abnormalities to underlying structural pathology. BAER has been proven effective for differentiating conductive from sensory hearing loss, for detecting tumors and other disease states affecting central auditory pathways (e.g., acoustic neuromas, subclinical lesions in multiple sclerosis), and for noninvasively detecting hearing loss in patients who cannot cooperate with subjective auditory testing (e.g., infants, comatose patients). BAER is the test of choice to assess hearing in infants and young children. It is most useful for following asphyxia, hyperbilirubinemia, intracranial hemorrhage, or meningoencephalitis or for assessing an infant who has trisomy. BAER also is useful in the assessment of multiple sclerosis or other demyelinating conditions, coma, or hysteria.  Audiometric analysis using multiple sound frequencies is usually preferred over BAER for testing hearing in cooperative patients who are able to report when sounds are heard.

Studies of cognitive evoked potentials (also known as the P300 or P3 cognitive evoked potentials) have been used in research settings to correlate changes in cognitive evoked potentials with clinical changes in cognitive function in patients with dementia (e.g., Alzheimer's disease and Parkinson's disease) and identify the etiology of depression in patients with chronic demyelinating disease.  However, there is insufficient evidence regarding the effectiveness of cognitive evoked potential studies in diagnosing or rendering treatment decisions that would affect health outcomes.  Furthermore, there is a lack of studies comparing cognitive evoked potential studies with standard neuropsychiatric and psychometric tests used in diagnosing cognitive dysfunction.

The American Academy of Pediatrics (AAP) Task Force on Newborn and Infant Hearing and the Joint Committee on Infant Hearing (JCIH) endorse the implementation of universal newborn hearing screening. Screening should be conducted before discharge from the hospital whenever possible. Physicians should provide recommended hearing screening, not only during early infancy but also through early childhood for those children at risk for hearing loss (e.g., history of trauma, meningitis) and for those demonstrating clinical signs of possible hearing loss.

The U.S. Preventive Services Task Force (USPSTF) recommends screening for hearing loss in all newborn infants.  All infants should be screened before 1 month of age.  Those infants who do not pass the newborn screening should undergo audiologic and medical evaluation before 3 months of age for confirmatory testing.  Because of the elevated risk of hearing loss in infants with risk indicators (e.g., neonatal intensive care unit admission for 2 or more days; syndromes associated with hearing loss, such as Usher syndrome and Waardenburg syndrome; family history of hereditary childhood hearing loss; craniofacial abnormalities; and congenital infections such as cytomegalovirus, toxoplasmosis, bacterial meningitis, syphilis, herpes, and rubella), an expert panel recommends that these children undergo periodic monitoring for 3 years. The USPSTF found good evidence that newborn hearing screening leads to earlier identification and treatment of infants with hearing loss and improves language outcomes.  However, additional studies detailing the correlation between childhood language scores and functional outcomes (e.g., school attainment and social functioning) are needed.

Two types of tests are commonly used to screen for congenital hearing loss: otoacoustic emissions (OAEs) and auditory brainstem response (ABR) (Helfand, et al., 2001). OAE testing evaluates the integrity of the inner ear (cochlea). In response to noise, vibrations of the hair cells in a healthy inner ear generate electrical responses, known as otoacoustic emissions. The absence of OAEs indicates that the inner ear is not responding appropriately to sound. Transient evoked otoacoustic emissions (TEOAEs) are generated in response to wide-band clicks, while distortion product otoacoustic emissions (DPOAE) are a response to tones. Both stimuli are presented via a light-weight ear canal probe. A microphone picks up the signal, and multiple responses are averaged to get a specific repeatable waveform. OAEs are used in screening and diagnosis of hearing impairments in infants, and in young children and patients with cognitive impairments (e.g., mental retardation, dementia) who are unable to respond reliably to standard hearing tests. OAEs are also useful for evaluating patients with tinnitus, suspected malingering, and for monitoring cochlear damage from ototoxic drugs.

The ABR is an electrophysiological response generated in the brainstem in response to auditory signals and composed of either clicks or tones. The stimulus is delivered via earphones or an inserted ear probe, and scalp electrodes pick up the signal. ABR evaluates the integrity of the peripheral auditory system and the auditory nerve pathways up to the brainstem and is able to identify infants with normal cochlear function but abnormal eighth-nerve function (auditory neuropathy). For purposes of neonatal screening, a limited ABR is performed in the nursery using a significantly low intensity level (35 to 40 dB) to rule out marked hearing loss (Schwartz & Schwartz, 1990; Scott & Bhattacharyya, 2002). If testing at this level fails to elicit a response, the infant is referred to an audiologic laboratory for a comprehensive ABR, involving testing at many different intensity levels.

Typically, screening programs use a 2-stage screening approach (either OAE repeated twice, OAE followed by ABR, or ABR repeated twice). Criteria for defining a "pass" or "fail" on the initial screening test vary widely. Comprehensive (diagnostic) OAEs or ABRs are used to diagnose hearing impairments identified by limited (screening) tests.

ABR and OAE have limitations that affect their accuracy in certain patients. Both require a sleeping or quiet child. Middle-ear effusion or debris in the external canal can compromise the accuracy of these tests.  OAE and ABR test the peripheral auditory system and eighth nerve pathway to the brainstem, respectively.  They are not designed to identify infants with central hearing deficits.  Therefore, infants with risk factors for central hearing deficits, particularly those who have congenital Cytomegalovirus infection or prolonged severe hypoxia at birth, may pass their newborn hearing screens with either OAE or ABR, but develop profound hearing loss in early infancy.

The newer generation of automated screeners are easy to use and do not require highly trained staff.  However, equipping hospitals with equipment and sufficient staff can be costly, the staff must be trained to understand the limitations of the techniques, and ongoing quality control is essential to achieve accurate, consistent test results.  The importance of technique is illustrated by the results of multicenter studies of universal screening, in which the rates of false positive and technically inadequate examinations varied ten-fold among sites.

There are differences between the guidelines with respect to the screening technology that is endorsed.  JCIH recommends that all infants have access to screening using a physiologic measure (either otoacoustic emissions [TEOAE or DPOAE] and/or auditory brainstem response [ABR]). AAP states that although additional research is necessary to determine which screening test is ideal, EOAE and/or ABR are presently the screening methods of choice. AAP defers recommending a preferred screening test. USPSTF recommends a 1- or 2-step validated protocol, stating that OAEs followed by ABR in those who failed the first test is a frequently used protocol.  Well-maintained equipment, thoroughly trained staff, and quality control programs are also recommended to avoid false-positive tests.

Cortical auditory evoked responses (CAERs) measure the later-occurring auditory evoked potentials reflecting cortical activity in response to an auditory stimulus (UBC, 2005).  Cortical auditory evoked responses have a long latency, compared to the short latency auditory evoked responses.  CAERs have been used in clinical research to evaluate the timing, sequence, strength, and anatomic location of brain processes involved with the perception of sounds. Current research underway concerns the use of CAERs to understand the brain processes underlying basic hearing percepts such as loudness, pitch, and localisation, as well as those processes involved with speech perception (UBC, 2005).

Vestibular evoked myogenic potentials (VEMP), also known as click evoked neurogenic vestibular potentials, are presumed to originate in the saccule.  They are recorded from surface electrodes over the sternocleidomastoid muscles, and can be activated by means of brief, high-intensity acoustic stimuli.  Papathanasiou, et al. (2003) stated that VEMP testing is a possible new diagnostic technique that may be specific for the vestibular pathway.  It has potential use in patients with symptoms of dizziness, sub-clinical symptoms in multiple sclerosis, and in disorders specific for the vestibular nerve. There is a lack of reliable evidence from well controlled, prospective studies demonstrating that VEMP testing alters management such that clinical outcomes are improved. Current evidence-based guidelines on the management of neurological disorders from leading medical professional organizations have not incorporated VEMP testing in diagnostic and treatment algorithms. The American Academy of Neurology considered VEMP as an investigational technique (Fife, et al., 2000). Guidelines prepared for the State of Colorado (DLE, 2006) state that VEMP "is currently a research tool and is not recommended for routine clinical use." In a review of the literature, Rauch (2006) states that VEMP holds great promise for diagnosing and monitoring Ménière's disease and some other neurotologic disorders. Rauch notes, however, that the methods, equipment, and applications for vestibular evoked myogenic potential testing are not yet standardized, and many aspects of vestibular evoked myogenic potential and its use have not yet been adequately studied or described.

Akkuzu, et al. (2006) examined the role of VEMP in benign paroxysmal positional vertigo (BPPV) and Meniere's disease, and ascertained if this type of testing is valuable for assessing the vestibular system. The 62 participants included 17 healthy controls and 45 other subjects selected from patients who presented with the complaint of vertigo (25 diagnosed with BPPV and 20 diagnosed with Meniere's disease). VEMP were recorded in all subjects and findings in each patient group were compared with control findings. VEMP for the 30 affected ears in the 25 BPPV patients revealed prolonged latencies in 8 ears and decreased amplitude in 1 ear (9 abnormal ears; 30 % of total). The recordings for the 20 affected ears in the Meniere's disease patients revealed 4 ears with no response, 6 ears with prolonged latencies (10 abnormal ears; 50 % of total). Only 2 (5.9 %) of the 34 control ears had abnormal VEMP. The rate of VEMP abnormalities in the control ears was significantly lower than the corresponding rates in the affected BPPV ears and the affected Meniere's ears that were studied (p = 0.012 and p < 0.001, respectively). The results suggested that testing of VEMP is a promising method for diagnosing and following patients with BPPV paroxysmal positional vertigo and Meniere's disease.

Brantberg, et al. (2007) studied VEMP in response to sound stimulation (500 Hz tone burst, 129 dB SPL) in 1000 consecutive patients. Vestibular evoked myogenic potentials from the ear with the larger amplitude were evaluated based on the assumption that the majority of the tested patients probably had normal vestibular function in that ear. Patients with known bilateral conductive hearing loss, with known bilateral vestibular disease and those with Tullio phenomenon were not included in the evaluation. It was found that there was an age-related decrease in VEMP amplitude and an increase in VEMP latency that appeared to be rather constant throughout the whole age span. Vestibular evoked myogenic potentials data were also compared to an additional group of 10 patients with Tullio phenomenon. Although these 10 patients did have rather large VEMP, equally large VEMP amplitudes were observed in a proportion of unaffected subjects of a similar age group. Thus, the findings of a large VEMP amplitude in response to a high-intensity sound stimulation is not, per se, distinctive for a significant vestibular hypersensitivity to sounds.

Muyts, et al. (2007) provided an overview of vestibular function testing and highlights the new techniques that have emerged during the past 5 years. Since the introduction of video-oculography as an alternative to electro-oculography for the assessment of vestibular-induced eye movements, the investigation of the utricle has become a part of vestibular function testing, using unilateral centrifugation.  Vestibular evoked myogenic potentials have become an important test for assessing saccular function, although further standardization and methodological issues remain to be clarified. Galvanic stimulation of the labyrinth also is an evolving test that may become useful diagnostically. The authors concluded that a basic vestibular function testing battery that includes ocular motor tests, caloric testing, positional testing, and earth-vertical axis rotational testing focuses on the horizontal semicircular canal. Newer methods to investigate the otolith organs are being developed. These new tests, when combined with standard testing, will provide a more comprehensive assessment of the complex vestibular organ.

Magnetic stimulation of the brain and spine elicits so-called motor evoked potentials (MEPs) (Goetz, 2005).  The latency of the motor responses can be measured, and central conduction time can be estimated by comparing the latency of the responses elicited by cerebral and spinal stimulation.  Abnormalities have been described in patients with a variety of central disorders including multiple sclerosis, amyotrophic lateral sclerosis, stroke, and certain degenerative disorders.  An assessment by the McGill University Health Centre on use of intraoperative neurophysiological monitoring during spinal surgery stated that there is sufficient evidence to support the conclusion that intraoperative spinal monitoring using SSEPs and MEPs during surgical procedures that involve risk of spinal cord injury is an effective procedure that is capable of substantially diminishing this risk (Erickson, et al., 2005).  The report explained that intraoperative spinal cord injury during spinal surgery generally compromises both motor and somatosensory pathways; therefore the use of both of these independent techniques in parallel has been proposed and is seen as a safeguard should one of the monitoring techniques fail. Combination of SSEP monitoring with MEP monitoring is also proposed to reduce false–positive results, and eliminate the need for the wake-up test.  The assessment identified 11 studies, all case series, of the combined use of SSEPs and MEPs in neurophysiological monitoring during spinal surgery.  The assessment found that, in several reports, combined SSEP and MEP monitoring was shown to have greater sensitivity than SSEP alone.  The report also noted that the addition of MEP monitoring where SSEP monitoring is already being performed is considered to be relatively straightforward, adding little to the overall effort and resources employed in intraoperative neurophysiological monitoring.

The clinical utility of MEPs outside of the operative setting, however, is unclear and at the present time the magnetic stimulation of central structures is regarded as investigational (Goetz, 2003; Miller, 2005).