Important Cervical myotomes:
Muscle | Myotome | Nerve |
Deltoid | C5-6 | Axillary |
Biceps | C5-6 | Musculocutaneous |
Brachioradialis | C5-6 | Radial |
Triceps | C7 | Radial |
Pronator teres | C6-7 | Median |
Extensor digitorum communus EDC | C7,8 | Radial |
First dorsal interossious FDI | C8-T1 | Ulnar |
Abductor Pollicis Brevis APB | C8-T1 | Median |
Extensor indecis propius EIP | C8-T1 | Radial |
Rhomboids | C4-5 | Roots/plexus |
Important Lumbosacral myotomes:
Muscle | Myotome | Nerve |
Tibialis anterior TA | L5 | Peroneal |
Peroneus Longus | L5 | Peroneal |
Tibialis posterior TP | L5 | Tibial |
Tensor facia lata TFL | L5 | Superior gluteal |
Gluteus medius | L5 | Superior gluteal |
Biceps femoris (short head) | L5 | Peroneal |
Vastus lateralis | L3,4 | Femoral |
Gastrocnemius | S1 | Tibial |
Iliopsoas | L2,3 | Femoral (proximal to inguinal ligament) |
Adductor Longus | L2,3,4 | Obturator |
Approach to Nerve conduction studies NCS & Electromyography EMG:
Nerve conduction studies NCS:
Organisation:
- Studies
- Technique
- Interpretation
- Basic patterns
- Notes
Assess/Studies (things you do):
Action potentials:
- Compound motor action potential CMAP (motor nerves)
- Sensory nerve action potential SNAP (sensory nerves)
- Compound nerve action potentials CNAP (sensorimotor nerves)
Late responses:
- F-waves= F-response
H-reflex:
- The electrical equivalent of monsynaptic reflex of a tendon jerk
- Stimulate the tibial nerve and record over the soleus muscle.
- Blink reflex
- Repetitive Nerve Stimulation RNS (RepStim)
Technique (how to do it):
CMAP Technique:
- G1 (-) recording electrode on muscle belly (proximally)
- G2 (+) reference electrode on tendon (distally)
- GND ground between G1 & G2 for sensory studies
- Set:
- Duration 0.2ms, Current 20-50 mA, gain 2-5mV per division
- Stimulate at 8cm (medialn, ulnar, tibial, peroneal), and then proximally, supramaximally
- Record:
- CMAP amplitude, duration, distal latency, conduction velocity
SNAP Technique:
- G1 (-) recording electrode (proximally)
- G2 (+) reference electrode (distally)
- GND ground between G1 & G2 for sensory studies
- Set:
- Duration 100-200ms, Current 5-30mA, Gain 10-20 micoV
- Stimulate the nerve distally, supramaximal stimulation
- Record:
- SNAP amplitude, duration, distal latency, conduction velocity
F-Wave Technique:
- Set:
- Gain 200 microV, Sweep 5-10ms,
- Supramaximal stimulation, Cathode (+) proximal, +/-Jendrassik maneuver
- Record: shortest F-wave latency on either side.
H-reflex Technique:
- Set:
- Gain 200-500 microV
- Sweep 10 ms
- Duration 1 ms
- G2 Achilles tendon
- G1 Soleus (5-6 eights the way from popliteal fossa to medial malleoulus)
- Stimulate tibial nerve in the popliteal fossa, submaximally, cathode (+) proximally, +/-Jendrassik maneuver
- Record: shortest H-reflex latency on either side.
Blink reflex Technique:
- Eye open or gently closed
- G1 lateral & inferior to pupil, bilaterally
- G2 lateral to lateral canthus, bilaterally
- GND chin or forehead
- Set:
- Sensitivity 100-200 microV/division
- Sweep speed 5-10 ms/division
- Motor filter 10 Hz & 10KHz (same as for CMAP)
- Stimulate:
- Superior orbital nerve (depression on the medial superior orbital ridge)
- 4-6 times (wait seconds in between), rastered
- Duration 0.1ms, Current: 3-25mA, supramaximal
- Record: shortest R1, ipsilateral R2 & contralateral R2 responses.
Repetitive nerve stimulation Technique:
- Hold acetylcholinesterase inhibitors prior to the study
- Select nerve e.g. median, ulnar, accessory
- Temperature >33 C
- Perform baseline CMAP at supramaximal stimulation. Immobilize electrode placement.
- Perform low frequency RNS: at 3 Hz, train of 5-10. Repeat X3, 1 min apart.
- Exercise: maximal contraction X 30 seconds.
- Repeat low frequency RNS immediantely and at 1,3,4 min postexercise.
- Consider high frequency RNS 30-50 Hz if unable to exercise.
- If decrement occurs, perform 10seconds exercise & repeat 3 Hz RNS to demonstrate repair.
- If abnormal perform EMG of affected muscle to assess for denervation.
Describe (things you should mention i.e. like a report):
Compound motor action potentials CMAP:
- Amplitude: reduced vs. normal
- Motor Conduction:
- Motor conduction velocity:
- Velocity: Reduced vs. normal
- Motor distal conduction latency a.k.a. Distal motor latency:
- Normal vs. prolonged
F-waves:
- Present vs. absent
H-reflex:
Sensory nerve action potential SNAP:
- Amplitude: reduced vs. normal
- Sensory conduction velocity:
- Velocity: Reduced vs. normal
Findings (things you notice):
- Temporal dispersion= increased duration AP with smaller amplitude. Area under the curve of CMAP should be unchanged. Indicates some axons are slower than others i.e. demyelination
- Conduction block= loss of conduction across a lesion. Neurapraxia is the term used when a lesion causes conduction block. This type of lesion results in a decrease in amplitude of AP by >50% (comparing proximal & distal amplitude)
Causes of Conduction block:
- Focal compression/entrapment of the nerve
- Focal acquired demyelination (inflammatory, early phase of vasculitis)
- Conduction slowing is a sign of demyelinating lesions.
Interpret (things you figure out):
CMAP Interpretation:
- Causes of low CMAP amplitude:
- Axon loss (anterior horn disease, radiculopathies, peripheral nerve disease with Wallerian degeneration)
- Conduction block
- NMJ disorder
- Myopathy
- CMAP Onset latency & conduction velocity CV only reflect the fastest nerve fibres (nerve, NMJ, muscle depolarization)
- CMAP duration: is a measure of synchrony. Is useful in assessing demyelination.
- CMAP conduction velocity CV (m/s): reflects only the fastest nerve fibres.
- CMAP Distal latency: prolonged in demyelination e.g. useful in HNPP that can otherwise look axonal but will have prolonged distal latency.
SNAP Interpretation:
- SNAP amplitude: is low in peripheral nerve disorders
- SNAP is abnormal in peripheral nerve disease but not radiculopathies.
- SNAP Not used for assessing demyelination (distal latency etc)
- SNAP onset latency reflects only the fastest nerve fibres.
- SNAP duration is used to distinguish between SNAP (1.5ms) and CMAP (5-6ms)
F-Wave Interpretation:
- Ulnar f-wave latency < 33 (C8-T1)
- Median f-wave latency <32 (C8-T1)
- Peroneal/tibial f-wave latency <57 (L5-S1)
- Peroneal f-waves may be absent in normal individuals
- F-wave abnormalities:
- Prolonged minimal f-wave latency
- Absent f-waves
- Impersistent f-waves (compare side to side)
- F-waves are pure motor
H-reflex Interpretation:
- Compare shortest H-reflex latency to Age-height matched controls. This is prolonged in proximal lesions.
- Compare to contralateral side, >1.5ms difference is abnormal
Blink reflex Interpretation:
- Electricial correlate of the corneal reflex (V1, VII, midpons & medulla)
- R1 repsonse: disynaptic, ipsilateral, stable, bi- or tri-phasic (midpons)
- R2 response: multisynaptic, bilateral, habituates, polyphasic (pons & medulla)
Normals:
- R1 <13ms, side to side difference <1.2ms
- Ipilateral R2 <41ms, side to side difference <5ms
- Contralateral R2 <44ms, side to side difference <7ms
Patterns:
- Unilateral V nerve lesion: delayed/absent R1, ipsilateral R2 & contralateral R2. Stimulation on unaffected side shows intact R1, ipsilateral R2 & contralateral R2. e.g. connective tissue disorders & toxic neuropathies.
- Unilateral VII nerve lesion: delayed/absent R1, ipsilateral R2 & contralateral R2. Stimulation on unaffected side shows normal R1, ipsilateral R2 but delayed/absent contralateral R2
- Unilateral midpons lesion: delayed/absent R1 but normal ipsilateral R2 & contralateral R2. Stimulation of unaffected side shows normal R1, ipsilateral R2 & contralateral R2
- Unilateral medulla lesion: normal R1, delayed/absent ipsilateral R2 & normal contralateral R2. Stimulation of unaffected side shows normal R1, ipsilateral R2 but delayed/absent contralateral R2
- Demyelinating neuropathy: delayed/absent R1, ipsilateral R2 & contralateral R2 on both sides.
Repetitive nerve stimulation Interpretation:
- In NMJ disorders, look for decrement on low frequency RNS, postexercise exhaustion that corrects with exercise, or increment postexercise.
- CMAP decrement on RNS: U shaped, >10% decrement between first & fourth (lowest) response. % Calculated= change in amplitude/baseline amplitude X100
- CMAP increment on RNS: >40% increase between first & last (highest) response. % Calculated= change in amplitude/baseline amplitude X100
- Pseudofacilitation: postexercise increase CMAP amplitude <40% with out increase in area.
Causes of CMAP decrement on RNS:
- Myasthenia gravis
- Lamert-Easton Myasthenic Syndrome (LEMS)
- Denervating Peripheral neuropathy
- Motor neuron disease
- Myopathy
Basic NCS patterns:
- CMAP is abnormal in Axon loss (anterior horn disease, radiculopathies, peripheral nerve disease with Wallerian degeneration), Conduction block, NMJ disorder, Myopathy
- SNAP is abnormal in peripheral nerve disease but not radiculopathies.
Axonal loss:
- Reduced CMAP, SNAP amplitude compared to normal controls, baseline or opposite side.
- CV & latency are preserved. If large fibres are involved, mild CV slowing (>75% of normal), mild latency prolongation (<130% of normal) occurs
- In hyperacute axon loss e.g. nerve transection, CMAP, CANP, CV & latency are normal for ~3 days (Wallerian degeneration takes time to occur). This is if nerve is tested distal to transaction site.
Demyelination:
- Conduction block, decreased CMAP amplitude (or area) by >20-50% between distal & proximal stimulation sites.
- Temporal dispersion on CMAP, increased duration of >15% between distal & proximal stimulation sites.
- Reduced CV <75% of normal
- Prolonged distal latency >130% of normal
- CV <35m/s in arms or <30ms/ in legs indicated unequivocal demyelination
- SNAP amplitude may be reduced due to exaggerated temporal dispersion & phase cancellation.
- When stimulating at Erb’s point. >40% drop in amplitude is needed for conduction block & >30% prolongation in duration for temporal dispersion
Acquired demyelination:
- Conduction block & temporal dispersion
- Inherited demyelination:
- Reduced CV, increased latency, no conduction block or temporal dispersion
Entrapment:
- Conduction block or slowing across the entrapment segment
- Axonal loss pattern if secondary axonal loss occurs.
Myopathy:
- SNAP: normal
- CMAP: amplitude is normal or reduced (depending on affected muscle)
- CV & latency: normal
Myasthenic gravis (MG):
- normal CMAP
LEMS: reduced CMAP. CV & latency is normal
Notes:
- Depolarization occurs under Cathode (+)
- The fastest myelinated fibres conduct at 65m/s
- The slowest myelinated fibres conduct at 35m/s. Therefore in purely axonal disease, CV is always >30m/s i.e. 75% of normal.
Notes, terminology:
- Orthodromic conduction (in the direction of the axon)
- Antidromic (opposite to the direction of the axon)
- M-response a.k.a. M-wave is the normal muscle response a.k.a. CMAP
- Late responses (H-reflex & F-waves). F/waves= F-response
- H-reflex: electrical equivalent of tendon jerk with orthodromic conduction in Ia afferents from muscle spindle to spinal cord and then orthodromic via anterior horn cells to muscles
- F-wave/F-response: antidromic stimulation of the anterior horn cell leads to it firing and a orthodromic action potential.
Anomalies:
Martin Gruber anastomosis:
- Median nerve gives a motor branch to the ulnar nerve in the forearm.
Riche-Cannieu anastomosis
- Connections between deep ulnar & median nerves in the hand.
Routine Electromyography EMG:
About EMG:
Parts:
- Insertional activity
- Resting
- Mild Voluntary contraction
- Maximal voluntary contraction
Out put:
- Visual: oscilloscopic or visual display
- Auditory: speaker
Norms, Muscle potentials (motor unit potentials):
- Waveforms
- Duration 5-15s,
- 2-4 phases
- Amplitude: 0.5-3 mV
Assess (things you do):
Insertional activity:
- Insert the needle at 4 different depth in each of 4 different quadrants
Spontaneous activity (resting):
- Normal: no activity
- Abnormal: (see below)
Motor unit action potentials MUAP:
- Ask the patient to minimally contract the muscle
- See below for description of:
- Amplitude
- Rise time
- Duration
- Phases
Recruitment:
- Assess MUAP recruitment (normal, reduced)
Repetitive nerve stimulation test RNST:
- Response= incremental vs. decremental vs. normal
Describe (things you mention i.e. like the bones of a report):
- Insertional activity (see below)
- Spontaneous activity (resting)
- Motor unit action potentials MUAP:
- Amplitude: from most positive to most negative peak. normal 2-5mV
- Rise time: time from first positive peak to first negative peak
- Duration: time from beginning of first positive peak to final return to baseline. Normal is 5-15 MiliSec.
- Phases: count the number of positive & negative peaks normal 4 or less
- Recruitment
- Repetitive nerve stimulation test RNST
Findings & abnormalities (Things you notice):
Insertional activity:
- Reduced (<300msec) in atrophied muscle.
- Increased (>300msec) in muscle pathology & may be associated with transient sharp waves & fibrillation potentials.
Abnormal resting muscle pattern e.g.:
Fasciculation potentials:
- Morphology: 0.5-1 Hz, irregular.
- Significance: firing of a single motor unit (a single neuron & all its innervated muscle fibres). Neurogenic or myogenic disorders
Fibrillation potentials (fibs): involuntary contraction of a single muscle fibre.
- Morphology: Regular 1-10Hz, short, small potentials, looks like a single MUAP
- Sounds like: static, or cooking bacon. “rain on roof” sound.
- Significance: firing of a single muscle fibre. Neurogenic or myogenic disorders
- Always abnormal: denervation of single muscle fibre
- In axonal injury it takes 10-14 days to appear (usually 3 week delay) , suggests active/persistent dennervation as they disappear with reinnervation
Positive sharp waves PSW: spontaneous discharges from groups of denervated muscle fibres.
- Morphology: Downward monophasic waveforms
- Sounds like: dull popping sound
- Significance: firing of a single muscle fibre. Neurogenic or myogenic disorders
- Always abnormal: denervation of groups of muscle fibres (usually 3 week delay), suggests active/persistent dennervation as they disappear with reinnervation
Complex repetitive discharges CRDs:
- Morphology: Groups of spontaneous action potentials. Abrupt onset and offset. Irregular.
- Sounds like: Machine like sound
- Significance: Single muscle fibre depolarization & ephaptic transmission to other fibres. Neurogenic or myogenic disorders
- Suggests injury >6 months old
Myokymic discharges (Multiplets and doublets):
- Morphology: groups of MUAP that are regular & rhythmic. Continuous or discontinuous/bursts
- Sounds like: soldiers marching,
- Significance:
- Facial: bell’s palsy, Multiple sclerosis, polyradiculopathy
- Limbs: chronic nerve lesions, radiation radiculopathy
Myotonic discharges:
- Morphology: High frequency 150-250Hz. Prolonged action potential firing after activation. PSW or biphasic/triphasic morphology
- Sounds like: Dive-bomber sound. Pinging sound. Changes in amplitude and frequency
- Significance: Myotonic conditions such as
- Myotonic dystrophy, myotonia congenital, paramyotonia congenita
- Hyperkalemic periodic paralysis
- Acid maltase deficiency
- Myotubular myopathy
- Polymyositis, chronic radiculopathy, neuropathies
Continuous motor unit activity:
- Significance: spasticity (myelopathy or other UMNL), voluntary in some, stiff person syndrome
Other patterns:
- Cramps
- Neuromyotonic discharges
- Tremours
- Multiples (doublets or triplets i.e. multiple MUAP)
End plate Noise:
- Morphology: 20-40Hz, low amplitude monophasic negative potentials.
- Sounds like: seashell, hissing
- Significance: painful, normal feature
End plate spikes:
- Morphology: brief irregular spikes. biphasic, initially negative
- Sounds like: crackling, buzzing, splattering
- Significance: normal feature
Motor unit action potentials MUAP:
Amplitude: Increased (5-15mV): reinnervation after nerve injury
- Decreased amplitude: myopathies
- Size index >2 in large MUAP
- Rise time: aim for 0.5 miliSeconds or less (adequacy of recording, how far the tested motor unit is)
Duration: assesses synchrony
- Increased: reinnervation
- Decreased: myopathies
- Duration >10ms is long MUAP
Phases:
- >4 is polyphasia
- Concentric needle: >20% is considered abnormally polyphasic
- Monopolar needle: >40% is considered abnormally polyphasic
- Polyphasic units occur in:
- Neuropathies & myopathies
Recruitment:
- Normal MUAP frequency is 10 Hz
- Decreased recruitment: increase in the firing rate of a motor unit prior to another motor unit starting to fire. Indicates neuropathic process
- To calculate MUAP frequency (for recruitment):
- 1. Look at the page=20s
- 2. Page speed set at 10ms
- 3. Frequency= number of times MUAP appears on the screen/page X5
- Neuropathic/neurogenic recruitment: MUAP is typically >20 Hz
- Recruitment typically tests Type 1 fibers so may be falsely normal in myopathies that affect type 2 fibers only
- Recruitment ratio= rate of firing of fastest motor unit/number of units firing.
- If >0.8 it suggests neuropathic process.
- If <0.3 it suggests a myopathic process.
Interpretation (things you figure out):
Myogenic, potentials:
- Shorter duration. Lower amplitude
- Usually polyphasic
Neurogenic, potentials:
- Longer duration. Higher amplitude
- Usually polyphasic
EMG evidence of denervation (axonal):
- Insertional activity: increased insertional activity, pseudomyotonic runs
- Resting:
- Fibrillation potentials
- Positive sharp waves
- Fasciculation potentials
- Voluntary contraction:
- Polyphasic MUAP
- Large amplitude MUAP & sometimes Giant units
- Reduced interference pattern
EMG evidence of myopathy:
- Insertional activity:
- May have fibrillation potentials
- Resting
- Voluntary contraction:
- Small polyphasic MUAP (crackling noise)
- Normal interference pattern
- Other notes:
- High amplitude (large) MUAP may be normal in quadriceps
NCS/EMG tips & patterns:
Demyelinating neuropathies:
- NCS:
- Findings differ by the pattern of demyelination:
- Uniform, segmental, focal, conduction block
- CMAP (conduction block, temporal dispersion, uniform velocity slowing, focal conduction velocity slowing)
- CMAP in temporal dispersion, amplitude will be lower but area under the curve is normal
- Increased latency & reduced velocity in some patterns
- EMG is normal in demyelinating neuropathies unless conduction block is present, in those cases reduced recruitment may be seen
In axonal injury:
- NCS:
- Reduced amplitude SNAP & CMAP (distal & proximal to the lesion). It takes 11 days for this to occur.
- Reduced amplitude >50% compared with other side
- <20% increase in latency or decrease in conduction velocity
- EMG:
- Fibrillation potentials, Positive sharp waves PSW
- If early <14 days, fibrillation potentials & PSW may be absent
- If late, fibrillation potentials & PSW may be absent (due to reinnervation)
- MUAP evidence of reinnervation: polyphasic, increased amplitude, prolonged duration
- Reduced MUAP recruitment (normal configuration) suggests it in early phase.
- Increased MUAP firing frequency
Lower limb lesions:
- The tibial & peroneal parts of the sciatic nerve are completely separate from origin to muscle. Thus partial sciatic lesions can mimic peroneal nerve lesions
- Needle EMG of short head of biceps femoris (the only peroneal innervated hamstring muscle) distinguishes between peroneal lesions above & below the fibular head.
- The sural nerve (sensory) receives the sural communicating nerve (from the peroneal nerve) in many people & thus peroneal nerve lesions can lead to abnormal sural nerve NCS.
- Use paraspinal EMG for root & plexus problems:
- Paraspinal muscles are abnormal in radiculopathy, but normal paraspinal muscles doesn’t rule radiculopathy out (especially if early)
Others:
- If low CMAP & ?denervation: suspect Lambert-Eaton Syndrome
Other notes regarding approaching EMGS:
- How regular is the discharge?
- Regular
- Semi rhymic
- Irregular
- Regular with exponential change
- Burst firing
- Semi rhymic= motor unit potential
MUAP parameters:
- Rise time tells you how far you are from MUAP, shorter time the closer you are
- Recruitment: assess at which rate the second unit starts to fire:
- A. if the rate of the first unit is <10 without a second unit, this is poor activation.
- B. If only one unit is firing >12hz this is reduced recruitment (>12 is mild >15 is moderate >20 is severe).
- C. the different MUAP are all firing at a very similar rate for every give time.
- Stability: stable vs unstable:
- You can use the trigger sweep function on the EMG machine.
- ALS, Radiculopathy, MG, LEMS,
- Spikes, turns, phases:
- Triphasic vs. polyphasic
Turns are like phases but don’t cross the baseline. Many turns is called complex MUAP.
- Triphasic vs. polyphasic
- Duration: normal vs. long vs. short duration