Basics of RNST,VEP ,BAER and EMG

Dr Sankalp MohanSenior ResidentNeurologyGMC, KotaBasics of RNST,VEP ,BAER

Repititive Nerve Stimulation TestVariant of theNerve conduction StudyFirst Described by German Neurologist Jolly in 1895Harvey and Masland(1941) reported electrical decremental muscle response on repetitive motor nerve stimulation.electrical stimulation is delivered to a motor nerve repeatedly several times per second- observing the change in the CMAP

DefinitionsQuantum.- A quantum is the amount of Ach packaged in a single vesicle (5000-10000 molecules )Miniature EPP Presynaptic terminals spontaneously release Ach quantum Causing MEPPEnd plate potential -EPP is the potential generated at the postsynaptic membrane following a nerve action potential and neuromuscular transmission

Muscle action potential (MAP) If EPP exceeds threeshold generated MAPCMAP Sum of MAPs generated by no of fibresSafety Factor Amplitude of EPP above threshold needed to generate MAP

Physiology of RNS Ach stores: immediately available (primary) store and secondary (or mobilization) store Primary or immediately available store 1000 quanta- beneath presynaptic nerve terminal membrane.Secondary or mobilization store 10,000 quanta- supplies the primary stores after few seconds.Tertiary or reserve store. More than 10,000 quanta in the axon and cell body

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- Low Rate RNS (40%

Protocol For Evaluating Disorder Of NMJWarm the extremity (33 degree centigrade)Immobilize the muscle as best as possiblePerform Routine NCS first to ensure that the nerve is normalPerform RNS at rest. After making sure that the stimulus is supramaximal, perform at 3 Hz RNS, normally there is a less than 10% decrement b/w the first and the fourth response.

If more than 10% decrement occurs and is consistently reproducible Patient Has patient perform maximal voluntary exercise. Immediately repeat 3 Hz RNS post exercise If less than 10% decrement or no decrement: Has patient perform maximal voluntary exercise for 1 min and perform 3 Hz RNS immediately and at 1,2,3 and 4 mins.

Perform RNS on one distal and one proximal muscles especially the weak muscles.If no decrement is found with a proximal limb muscle, a facial muscle can be tested.If the compound muscle action potential is low at baseline, have patient perform 10 sec exercise, then stimulate the nerve supramaximally immediately post exercise

Rapid RNSoptimal frequency is 2050 Hz,for 210 secondsbrief (10-second) period of maximal voluntary isometric exercise has,the same effect as rapid RNSDepletion of quanta vs calcium accumulationIncremental response in LEMS

MG is considered to be a reproducible 10% decrement in amplitude when comparing the first stimulus to the forth or fifth, which is found in at least 1 muscle.Abnormality in LEMS is considered to be a reproducible postexercise increase in amplitude of at least 100% as compared to preexercise baseline value.

Myasthenia GravisClassic Findings

1. Normal CMAP2. Decremental Response at low rate RNS3. Normal or minimal post exercise facilitation4.Normal or decremental response at High rate RNS5. Post Exercise or post tetanic Exhaustion

LEMSClassic Findings

Low Normal CMAPDecremental Response at Low Rate RNSPost exercise facilitationHigh Rate RNS 100 % increment in two muscles ,400 % increment in one muscle

Utility of RNSMost commonly used test, easy.RNS is relatively insensitive,10-50% in ocular myastenia,75% in generalised MGRNS is relatively specific(90%)SFEMG is Most sensitive.(90% in ocular,95% in MG)

Visual Evoked Potentials

Evoked Potential Electrical potentials that occur in the cortex after stimulation of a sense organ which can be recorded by surface electrodes is known as Evoked Potential.eg. SEP, BAER and VEP

VEPVEPs are electrophysiologic responses to stimulation by either patterned or unpatterned visual stimuli.Stimulation at a relatively low rate (up to 4/s) will produce transient VEPs Stimulation at higher rates (10/s or higher) persist for the duration of the stimulation and are referred to as steady-state VEPs.Responses evoked by patterned stimuli are pattern VEPs Responses evoked by unpatterned stimuli are flash VEPs

Choice of StimulusPatterned visual stimuli elicit responses that have far less intra- and interindividual variabilitygreater sensitivity and accuracyCheckerboard pattern reversal is the most widely Unpatterned stimuli are generally reserved for patients who are unable to fixate or to attend to the stimulus

Physiologic basis

The generator site for VEPs is believed to be the peristriate and striate occipital cortex

Pretest EvaluationTest should be explained Ability to fixate important throughoutAvoid Hair Spray or OilCycloplegics generally should not be used Subjects with refractive errors should be tested with appropriate corrective lenses

P100 amplitude decreased and latency prolonged when pupils constricted . 26

Electrode PlacementStandard Disc EEG electrodes usedeActive/Recording Electrode Placed at Oz in midline 4cm above InionReference Electrode FPz 12 cm above Nasion.Ground Electrode placed at vertex Cz

Pattern Reversal Visual Evoked Potential TestingNegative and positive polarities are designated N and P, respectively. Peak latencies are expressed in millisecondsPeaks N75, P100, and N145 are recorded over the occiput Wave Nl00 is recorded from the midfrontal regionN145 is highly variable and is not used for standard test interpretation Type of pattern.- Checkerboard ,Bar and sinusoidal grating stimuli

Stimulus field typespattern that extends equally to both sides of the fixation point is referred to as a full-field stimulus A pattern presented to one side of the fixation point in one-half Half field stimuluspattern presented to a small sector of the visual field is designated a partial-field stimulus

half-field or partial-field stimuli are used, the fixation point should be displaced to the nonstimulated visual field by a small amount, to prevent stimulation of both retinal hemifields

Test Protocol for Full-Field StimulationFull-field PVEP testing is most sensitive in detecting lesions of the visual system anterior to the optic chiasm should be performed monocularly, black-and-white checkerboard pattern, at a reversal rate of 4/s or less. The subject should be placed no closer than 70 cm to the stimulus screen. Small checks (1216) and small fields (2-4) selectively stimulate central vision. These responses are particularly sensitive to defocusing and decreased visual acuityRecommended recording time window (ie, the sweep length) is 250 msec; 50-200 responses are to be averaged. A minimum of 2 trials should be given,

Electrode placement-Montages International federation of Clinical Neurophysiology (IFCN) recommends 2 channels minimumChannel1 Oz FpzChannel 2 Oz Linked ear Four Channel montage Channel 1 : Oz FpzChannel 2- Pz- FpzChannel 3 L5-FpzChannel 4 R5 -Fpz

Factors Affecting VEPThe size of the checks Pupillary sizeGender (women have slightly shorter P100 latencies), Age: below 1 yr of age P100 may be 160ms, & above 60 yrs. also it get delayed- upto 120Sedation and anesthesia abolish the VEP.Visual acuity deterioration up to 20/200 does not alter the response significantly . Drugs.

Waveforms(The NPN complex)The initial negative peak (N1 or N75)A large positive peak (P1 or P100) Negative peak (N2 or N145)

N75P100N145

Clinically Significant Abnormality changes in latency, amplitude, topography, and waveform P100 latency prolongation is the most reliable indicatorWaveform abnormalities are generally subjective in nature and difficult to quantify Amplitude affected by technical Factors wide individual variation Hence interoccular amplitude ratio used

P100 is 110 milliseconds (ms) in patients younger than 60 years .

Flash Visual Evoked Potential Testing limited to: (1) subjects with severe refractive errors or opacity of ocular media subjects who are too young or too uncooperative results should demonstrate reproducible peak positive responses to flash stimulation consist of up to six major peaks in the first 250 ms after flashUnpatterned visual stimuli commonly consist of brief flashes of light with no discernible pattern or contour(LED) board can be viewed from a distance or LED goggles can be placed directly over the eyes. Goggles have the advantage of producing a very large field of stimulation that minimizes the effect of changes in direction of gaze

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Clinical Applications of VEPVEPs are most useful for testing optic nerve function and less useful for assessing postchiasmatic disordersNon Specific for etiology Partial-field studies may be useful for retrochiasmatic lesions; however, they are not performed routinely

VEP may be abnormal ( low amplitude ) in Refractive error severe ,Retinal diseases

Optic neuritis-MS P100 latencies prolonged with or without amplitude lossNMO unrecordable P100 waveform with reduced amplitude more likelyIschemic optic neuropathy Attenuation of amplitude earlier than latencyVit B12 deficency Bilateral asymmetric prolonged p100 latenciesPapilledema only VEP not affected

VEP in cortical blindnessSome reports suggest that VEP may show a varied result OR normal VEPother reports suggest prognostic importance of VEP with absent VEP response foretelling poor prognosisINCONSISTENT PATTERN

Brainstem Auditory Evoked Responses

BAER

BAER are recorded from the ear and vertex in response to brief auditory stimulation Actually a misnomer as responses from responses of the auditory nerve, brainstem, and, perhaps, higher subcortical structures

AEPShort latency AEPMiddle latency AEPLong latency AEPThe short latency AEP include peak of up to 10 msec and amplitude of about 0.2uv, they are generated in brainstem.The middle latency AEP have several variable peaks with latency of 10-50 msec and with amplitude of about 1 uv, they probably reflect early cortical excitation. The long latency AEPs beginning after 50sec and having peak of 1-10uv, represent later cortical excitation

Auditory pathway

Electrode placement

Typically are placed at the vertex (location Cz of the International 1020 System) and at both ear lobes (Ai and Ac) Electrodes at the mastoids (Mi and Mc) may be substituted, although wave I tends to be smaller because of muscle noise

Montages Cz-AiCz - AcAc- AiMay assist in the identification of wave I

Stimulusbroad-band clicks, the acoustic energy of which is spread over a wide range of audio frequencies 100usec rectangular pulse (single monophasic square wave)

Stimulus Polarity -clicks in which the first and major acoustic wave applies negative pressure in front of the earspeaker diaphragm are referred to as rarefaction clicksfirst and most prominent acoustic wave applies a positive pressure in front of the earspeaker diaphragm are referred to as condensation clicks

Stimulus Rate 10 -70 times/s . Most common 11-31 HzStimulus Intensity- about 70 dbClick should be delivered monaurally, i.e., to one ear at a timecontralateral (nonstimulated) ear be masked by white noise at 60 dB SPL to eliminate crossover responses, i.e., bone-conducted responses

Waveform components Wave I - wave I response -in the distal portion of cranial nerve (CN) VIIIProminent initial upgoing peak

Wave II Poorly defined in some adults and most neonates. More prominent in contralateral channel .Proximal VIII nerve /Cochlear nucleus

Wave III: Prominent peak followed by trough .Arises from Superior Olivary nucleus

Wave IV: The ABR wave IV, which often shares the same peak with wave V, arise from pontine third-order neurons mostly located in the Lateral lemniscus, but additional contributions may comeWave V: Most prominent peak appearing 5.5 ms after stimulus. Wave v may fuse with wave IV . Origin Inferior colliculus

Wave VI and VII: Thalamic (medial geniculate body) and cortical region. Not clinically significant

Abnormalities of BAERAbsence of waveformsAbnormal absolute or interpeak latenciesAmplitude ratio abnormalityRight to left asymmetry latencyof > 0.5 ms

- Wave II is difficult to distinguish ,Wave IV absence may not be pathological

Abnormalities of Wave IReflect peripheral auditory dysfunction, either conductive or cochlear, or pathology involving the most distal portion of the eighth nervePoorly formed or absent wave I but a clear wave V may reflect high-frequency hearing loss.May reflect intracranial pathology because the cochlea receives its blood supply from the intracranial circulation via the internal auditory artery

Abnormalities of the IIII Interpeak IntervalProlongation reflects an abnormality within the neural auditory pathways between the distal eighth nerve on the stimulated side and the lower pons . Upper limit- 2.5 msSeen in acoustic neuromas, demyelinating disease, brainstem tumors, or vascular lesions of the brainstem,meningitis ,Sub

Abnormalities of the IIIV Interpeak IntervalReflects an abnormality between the lower pons and the midbrain most often, although not always, ipsilateral to the lesion.Upper limit- 2.4 msProlongation not an abnormality if the IV interpeak interval is normal.Seen in a variety of disease processes involving the brainstem, including demyelination, tumor, and vascular disease

Abnormalities of the I-V Interpeak latenciesUpper limit 4.5 msVariety of disorders demyelination ,ischemia

Wave V to I amplitude ratio If ratio less than 50 % suggests central impairmentVery high ratio suggests Peripheral impairment

Clinical ApplicationMost Important is for Hearing assessment in newborns and childrenNeurologic Conditions- Multiple Sclerosis , Stroke,Coma Retrocochlear Hearing loss Acoustic Neuroma

Clinical Application- Usually non specific and should be correlated clinically and with other investigations

CP angle tumor 1. Unrecordable BAER2. Only wave I recordable3. Prolongation of I III and I-V Interpeak latencies4. Right to left asymmetry in wave V latency - MRI is gold standard for evaluating retrocochlear hearing lossBAER 71 % sensitivity ,74 % specificity

Multiple Sclerosis Most frequent Absence of Wave VProlongation of III-V interpeak latencyProlongation of I-V interpeak latencyReduction of V/I amplitude ratio

Diagnostic yield is higher in definite MS- 67%,Probable-41% ,Higher in those with brainstem signs Diagnostic yield Lower compared to VEPCan be used to detect silent brainstem lesions ,follow up and response to treatment

Coma and Brain DeathPrognostic predictor of comaBAER is normal in toxic or metabolic cause of comaAbsence of III or IV waves associated with vegetative state Better Predictor of outcome following head injury than GCS

In brainstem stroke abnormal BAEP correlated with unstable clinical course

Role of BAER in pediatrics

Newborn Hearing Screening Hearing loss occurs in 1/1000 births .Early detection to improve language skillsHyperbilirubinemia/kernicterusChildren with intellectual impairmentSpastic Cerebral Palsy

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ReferencesElectromyography and Neuromuscular disorders David Preston ,Barbara Shapiro -3rd EditionClinical Neurophysiology UK Mishra & J Kalita 3rd EditionPractice parameter for repetitive nerve stimulation AANEM Guidelines on Visual Evoked Potentials 2008 American Clinical Neurophysiology Society (ACNS)ACNS guidelines for Auditory evoked potentials