Week 15: electroneuronography

Question 1

what portion of the VII nerve is motor and what is sensory

Answer 1

2/3 motor and 1/3 sensory so mainly a motor nerve

Question 2

what muscles does the facial nerve innervate

Answer 2

facial muscles, stapedius, stylohyoid, and digastrics muscles

Question 3

what are the two main branches of the facial muscle?

Answer 3

temporal and cervical

Question 4

what are the 5 divisions of the facial muscle

Answer 4

temporal
zygomatic
buccal
mandibular
cervical
*note that the buccal branch allows communication between the zygomatic and mandibular branches

Question 5

what do we look for visually to cue for a facial nerve lesions

Answer 5

• forehead= corrugation
• eyes= orbicularis occuli
• nasolabial fold
• mouth= purse lips
• cheeks= inflate cheeks
• symmetry of both sides

Question 6

what are the 4 ways we can look for facial nerve functioning

Answer 6

• clinically with facial grading system
• imaging:
• –MRI with IV gadolinium contrast to see VII nerve tumor in the CPA and temporal bone, or parotid gland tumors
• –CT scan for cholesteatomas and temporal bone trauma
• electrophysiologically
• –stimulation and recording techniques
• –interpretation and reporting guidelines
• histologically

Question 7

house-brackman system for facial nerve paralysis grading

Answer 7

• 1=none=normal function in all areas
• 2=mild= slight weakness on close inspection, synkinesis
• 3= moderate= obvious but not disfiguring asymmetry
• 4=moderately severe= obvious, disfiguring asymmetry; normal appearance at rest
• 5= severe= barely perceptible motion; asymmetry at rest
• 6=total= no perceptible motion

Question 8

histological sunderland classifications of 7th nerve damage

Answer 8

*grade 1= neuropraxia= conduction block and is just a bit of compression on the axon, no real injury
*grade 2= axonotmesis= axonal discontinuity/injury to the axon
*grade 3= neurotemisis type 2= the tube (endoneurem) and axon are disrupted neurotemisis which has grades
*grade 4= neurotemeisis type 3 which is where the epineurium is also affected
grade 5= neurotemisis type 4 which is where is entire nerve is injured and is broken
—wallerian generation degeneration is anything more than grade 1

Question 9

what electrophysical tests are used to see VII nerve injury

Answer 9

• nerve excitability test (NET)
• maximal stimulation test (MST)
• electromyography (EMG)
• electroneurography (ENoG)
• determining prognosis
• –rarely in differential diagnosis
• classifying nonsurgical vs surgical management
• IOM of VII nerve function (usually with EMG)

Question 10

NET (nerve excitability test)

Answer 10

*useful only during the first 2-3 weeks of complete paralysis, before complete degeneration has occurred to determine whether a pure conduction block exists or whether degeneration is ocurring
*the stimulating electrode is placed on the skin over the stylomastoid foramen or over one of the peripheral branches of the nerve
*beginning with the healthy side, electrical pulses steadily increasing current levels until a facial twitch is noted
*the lowest current eliciting a visable twitch is the threshold of excitation
*next the process is repeated on the paralyzed side
*a difference of 3.5 milliamps or more is a sign of severe degeneration and an indicator for surgical
decompression
—simple conduction block would have no difference between the sides

Question 11

MST (maximal stimulation test)

Answer 11

• stimulating all intact axons
• nerve-stimulating equipment are the same as the NET. *Maximal stimuli or supramaximal stimuli are used
• –start at 5mA and increase to the level of the patient’s tolerance
• on the unaffected side, the stimulus current intensity is increase until the maximum stimulation level
• this level is then used to stimulate the affected side
• –the degree of facial contraction is subjectively assessed as either equal, mildly decreased, markedly decreased, or without response compared to that of the normal side (0%, 25%, 50%, 75%, 100%)

Question 12

the beginning of ENoG

Answer 12

*developed by Esslen and Fisch in 1979

Question 13

what does ENoG show

Answer 13

the distal VII nerve function
*we record the facial nerve function from the peripheral (aka from the muscles of the face) and we record from each side and compare the latency and amplitude of the N1, P1, N2 wave

Question 14

overview of ENoG testing

Answer 14

• used in Bell’s palsy, trauma, otitis media
• –not useful in Ramsay Hunt because the virus causing this gives severe nerve problems and multiple sites of facial nerve lesions
• VII nerve is stimulated transcutaneously (as in NET) or using a bipolar stimulating electrode
• comparing responses from both sides to maximal electrical stimulation (as in MST)
• –compound muscle action potential (CMAP)
• normally: the average difference in response amplitude between the two sides is only 3%

Question 15

why is Enog measured

Answer 15

• to see the extent of VII nerve degeneration (wallerian degeneration=WD=% of neural fibers that are no longer responding
• comparing the response amplitude (and latency) of both sides

Question 16

how would a tumor affect your ENoG response

Answer 16

increase latency and decrease amplitude

—decrease amplitude= increasing tumor size

Question 17

how to assess abnormal amplitude responses

Answer 17

*if the response on the paralyzed side is 10% of that on the normal side then 90% of the fibers are severely degenerated and so on

Question 18

when do you do ENoG (timing)

Answer 18

• initial/baseline ENoG most useful between 4-21 days post insult
• –not before 72 hrs post-insult because WD takes 3 days to occur
• –not after 3 wks; false positive from deblocking
• serial ENoG, every 3-5 days
• –document improvement or further degereration
• response is affected by neuroblockers–which cause chemical paralysis of VII muscles

Question 19

what do you have to do to your evoked response system to do ENoG

Answer 19

*add an additional unit that is an electric generating device to deliver electric stimulus and the software

Question 20

electrode placing for ENoG

Answer 20

place on the face to monitor the muscle activity of various portions of the face

• –use the same surface electrodes
• noninverting (+) at the angle of the mouth (ipsi)
• inverting (-) :nasolabial fold (ipsi)
• ground :forehead (fz)
• again stimulating distally at the stylomastoid foramen with a bipolar stimulus electrode
• –which means it has an anode and a cathode

Question 21

recording parameters of ENoG

Answer 21

• analysis time: 12-20 ms
• filter settings: 3-300 Hz
• notch filter: no
• amplification: 5000 or less
• impedance: <5 kohms
• sweeps: 1-20

Question 22

stimulus parameters for ENoG

Answer 22

• transducer: pair of electrodes
• site: stylomastoid foramen (main trunk of the nerve)
• orientation: anode + is the anterior and cathode - is the posterior
• –red is front and black is back
• type: constant current pulse
• mode: continuous
• duration: 0.2 ms (200 microseconds)
• laterality: unilateral (uninvolved side first)
• intensity: >10 mA

Question 23

ENoG waveform analysis by response amplitude

Answer 23

• intensity >10mA to produce supra-maximal P1 response
• biphasic waveform w/in the first 5-7 ms after electrical stimulation
• –N1, P1, N2 deflection
• different methods of calculation amplitude leads to high variability in the amplitude of p1
• limitation: abnormal ENoG in individuals with normal clinical VII function
• **note a response earlier than 4 ms is not true response, you pros stimulated the massiter muscle

Question 24

ENoG waveform analysis

Answer 24

• documentation of normal vs abnormal side is important:
• –ask pt to smile to identify the stronger side of the face
• –or use medical records
• response analysis to determine abnormal asymmetry in amplitude (or latency) between both sides
• if one side is 750 and the other is 40 mV then do the calculation to know the % of wallerian degeneration
• ——-40/750=5% or 95% degeneration
• –this test is not to diagnose, but is to determine the amount of asymmetry to see the amount of wallerian degeneration

Question 25

at what degree of asymmetry means surgical intervention is helpful

Answer 25

*response of <10% (90% wallerian degeneration) may suggest a surgical intervention (no spontaneous recovery)

Question 26

how to interpret ENoG results

Answer 26

* significant degeneration requiring surgical intervention - --response <10% (or >90% WD) of the normal side * spontaneous recorvery--wait and see - --any response >10% (or less than 90% wallerian degeneration * note if you increase the intenstiy and dont get a bigger response, this is a problem and could be due to demyelination - --note that you want to use the same intensity between ears

Question 27

ENoG waveform analysis of latency of the response

Answer 27

* VII Tumors: - --delayed latency and decreased amplitude - --decreased amplitude shows increasing tumor size * delayed latency and normal amplitude - --distal nerve demyelination - --reduction in the synchrony of neural response

Question 28

how to monitor with ENoG

Answer 28

*serial testing every 3-5 days until normal, stable, or improving ENoG indicates good prognosis

Question 29

test instructions for ENoG

Answer 29

* patients are anxious and apprehensive about the test procedure--they are in pain - --take time to explain the procedure - --avoid terms that will cause alarm or concern * procedure leads to some discomfort - --patient will tolerate some discomfort to reach the diagnosis - --patient can stop the test if it becomes too uncomfortable - -----start at low presentation level and build up if necessary

Question 30

troubleshooting no ENoG response, bilaterally

Answer 30

* chemical paralysis-- in the OR or ICU - --reverse neuromuscular blocking agent medically (physician can give the pt an anti-inflammatory and then have them come back) * improper electrode placement - --verify correct electrode placement

Question 31

troubleshooting poor ENoG response, bilaterally

Answer 31

* edema at stimulation side - --relief pain and anti-inflammatory * inadequate stimulus electrode pressure (ie) tenderness.pain, obese patient) - --apply more pressure---relief pain - --needle electrode * bilat dysfunction or injury - --compare patient ENoG amplitudes to normal data

Question 32

troubleshooting ENoG when there is a poor response unilaterally

Answer 32

* inappropriate stimulus site - --relocate stimulating electrodes - --increase stimulus intensity to supra-max level

Question 33

troubleshooting ENoG when there is an early response

Answer 33

* <6ms, but 4 ms is what she said was the cutoff for early earlier * masseter muscle response - --move stimulating electrode posteriorly - --if stimulating the masseter muscle, the you need to move the stimulating electrode back towards the back of the head direction

Question 34

troubleshooting ENoG when you have excessive artifact rejection (to the point where you cannot average)

Answer 34

* stimulus artifact - --increase distance between the stimulating and recording electrodes - --avoid crossing of stimulus and recording electrode wires - --decrease amplification (gain) to 3000

Question 35

what is the main indication that you want to do IOM of the VII nerve

Answer 35

if there is at least some preoperative neural function, not total degeneration ---you wouldnt want to monitor if there is a total degeneration or mostly degeneration of the nerve

Question 36

the 5 IOM objectives (for the 7th nerve)

Answer 36

* distinguishing the VII nerve from other cranial nerves, tissue, etc * facilitation of tumor excision by verifying that regions of the tumor are remote from the VII nerve * early recognition of surgical trauma to the VII nerve * confirmation of nerve stimulation following tumor removal--post-op prognosis * identification of site and degree of neural degeneration in nerve exploration cases for suspected VII tumors or decompression for acute VII palsy

Question 37

what are 5 otologic surgeries you would want to do VII nerve monitoring for

Answer 37

* IAC and CPA tumors * vestibular neurectomy * cochlear implantation * cholesteatoma * congenital aural atresia

Question 38

what are 4 non-otological surgeries you would want to do VII nerve monitoring for

Answer 38

* VII nerve surgeries - --decompressing, mobilizing nerve * parotid gland tumors * posterior fossa tumors compressing VII nerve * hemifacial spasm (HFS)

Question 39

what is HFS

Answer 39

* hemifacial spasm * in hemifacial spasm, the nerve has an offending loop of blood vessel, compressing the facial nerve at the root entry zone - --observe the VII nerve root as tit leaves the brainstem * the vascular loop could also be around the cochlear nerve causing tinnitus and SNHL

Question 40

why do we monitor the VII nerve (3 reasons)

Answer 40

* cosmetic consideration * loss of VII nerve function is costly * functional implications to the eye - --lack of tears - --improper closure - --injury to the cornea * study actually showed lowest quality and higher cost associated with decision NOT to provide facial nerve monitoring because the surgery to repair the facial nerve is very costly

Question 41

what are the two electrodes for the IOM VII nerve monitoring

Answer 41

* monopolar stimulators - --for tumor mapping but some current spread away from the tip of the stimulating probe - --look like a needle electrode and it is placed on the nerve, the current goes through the electrode and you record from the electrodes on the muscles of the face * bipolar stimulators - --to separate the facial nerve from adjacent neural structures - --reduces false-positive response form the surrounding neural tissue - --concenteric=bipolar and it has two tips, where you can put the facial nerve in between the two tips which would help separate the facial nerve from other neural structures (reduces splatter of the stimulating current)

Question 42

what happens with VII stimulation with IOM

Answer 42

* the facial muscles will fire off - --will give visual waveforms - --and audible sounds * **use needle electrodes and we are looking at the peripheral effect of the stimulation of the facial nerve

Question 43

how does the VII nerve elicit sound acitvity

Answer 43

* intentionally electrical stimulation - --present electrical current and record direct VII nerve stimulation - --this electrical stimulation will cause precisely timed locks * direct surgical/mechanical manipulation - --nerve it touched, compressed, or stretched by fluid irrigation causing bursts, electrocautery, or laser devices - -----direct surgical manipulation will cause bursts at several clicks at the time of manipulation - -----traction, pressure, or calorics will cause training which would be repetitive asynchronous clicks like pop corn * **can hear audible responses when the surgeon is purposely stimulating the nerve so you hear pulses that are precisely time locked with the stimulus; this is a good sign the facial nerve is functioning

Question 44

what is the deal with bursts of EMG activity with VII nerve monitoring

Answer 44

* fluid irrigation, manipulation of the nerve | * suggests functional integrity of the nerve distal to the point of stimulation

Question 45

what is the deal with train of EMG activity with VII nerve monitoring

Answer 45

* dissector device, electrocautery, drilling, nerve ischemia, stretching of the nerve * suggesting some neural trauma and post op decrease in VII motor function - -this means you must inform the surgeon

Question 46

how can more audiologists be involved in IOM

Answer 46

* education and training | * specialty certification