General
- AKA: “needle exam”
- Measures the electrical activity of muscle membranes from surface electrodes or needle electrodes
- Technique
- Inserting needle electrodes into muscles and analysing muscle electrical activity at rest and under conditions outlined below.
- CI
- Relative
- Anticoagulants or antiplatelet drugs
- Risk of hematoma → compartment syndrome
- Pacemaker
- Level localization
- Precise localization can be difficult due to the overlap in innervation of the various nerve root levels.
- Using paraspinal muscles
- Nerve root damage will tend to produce abnormalities in
- Paraspinal muscles AND
- Muscles of the limbs
- Helping to distinguish a radiculopathy from a plexopathy or peripheral neuropathy (example)
There are 3 phases of an EMG exam
Phase 1 - insertional activity
- The electric response of the muscle to mechanical irritation caused by small movements of the needle.
Phase 2 - spontaneous activity. In muscle at rest.
- Normal: silent with stationary needle once insertional activity has subsided
- Spontaneous activity: independently produced electrical activity.
- Usually abnormal
- After denervation (secondary to a nerve injury) or muscle injury:
- Positive sharp waves (PSW)
- Fibrillation potentials (AKA fibrillations or fibs):
- Action potentials arising from single muscle fibers
- Detectable on EMG
- Not visible to the naked eye (aka fasciculations)
- Earliest onset 7–10 days after denervation, sometimes not for 3–4 weeks.
- If the nerve recovers, it may reinnervate the muscle, but with larger motor units resulting in longer duration and decreased numbers
- Myotonic discharges (“dive bomber” sound on speaker monitor)
- Complex repetitive discharge (CRD):
- Ephaptic conduction of groups of adjacent muscle fibers
- Occurs in neuropathic or myopathic disorders
- Fasciculation potentials:
- Nonspecific, but typically associated with motor neuron disease (ALS)
- To be considered abnormal must contain
- Fibrillation and/or positive sharp waves
- Other less common spontaneous activity includes: myokymic, neuromytonic and cramp discharges
Phase 3 - volitional activity.
- Motor unit action potential (MUAP)
- Represents the summated action potentials of the muscle fibres of one motor unit
- The more the muscle contract the more Motor units it recruits
- Amplitude of the MUP is dependent on the density of the muscle fibers attached to that one motor neuron (also to the proximity of the MUP).
- Technique
- Assessed only with voluntary muscle contraction by the patient.
- Myogenic MEPs record the muscle response (M response) of a specified muscle by using electromyography (EMG) recording
- EMG signal capture
- Input from two different points on the muscle
- Close (usually 1-2 cm)
- Electrode alignment with the direction of muscle fibers - increased probability of detecting same signal
- Subtracts the two inputs
- Amplifies the difference and EMG pattern recording
- Evaluation of motor unit
- Amplitude
- Duration
- Neurotonic discharge is prolonged and indicates ongoing nerve injury.
- Polyphasia
- Polyphasic potentials:
- MUAPs with > 4 phases
- Normally comprise <15% of MUAPs
- Following a nerve injury, abnormally increased polyphasic potentials can be seen 6–8 weeks after reinnervation begins, gradually increase over several months, and then begin to wane (as firing becomes more synchronous)
- Stability
- Evaluated with minimal volitional effort and maximal effort.
- With minimal volitional effort.
- Two possible abnormal findings
- Reduced recruitment (or fast firing)
- Indicative of a neuropathic process
- Early or increased recruitment:
- Indicative of a myopathic process
- With maximal effort
- During EMG assessment of a muscle during contraction, the electrical activity should fully obscure the baseline (termed a full interference pattern).
- Incomplete interference pattern is considered to be a reflection of loss of motor units in a muscle, though it can also be seen with diminished effort.
- Spontaneous benign discharges
- Produce short non-sustained discharges (likened to the sound of ‘popcorn’) indicate proximity of the stimulus to the nerve
- Pathology
- Increased amplitude and duration suggest a disorder of the LMN
- Reduced amplitude and duration suggest a primary myopathic disorder
Pathology
- If there is axonal loss within a nerve, full voluntary muscle contraction will generate fewer units, or even a single unit, firing at high rates due to abnormal recruitment
- The presence of even a single firing unit = the presence of at least one axon in continuity with the muscle.
- This is valuable in assessment of plexus injuries, or as evidence that reinnervation is occurring
- Spontaneous discharge
- Muscle denervation hypersensitivity
- Complete denervation lasting for around three weeks → muscle membrane develops ‘denervation hypersensitivity’ → recordings show spontaneous activity such as fibrillation and positive sharp waves
- Best Nerve conduction test for radiculopathy:
- Most reliable and objective test that there is for damage to motor axons to the muscle after 1 week at least up to 12 months after the damage.
- Myotonia:
- There are a number of myotonic conditions, including myotonic dystrophy in which there is sustained contraction of the muscle
- Classic EMG finding: “dive bomber” sound due to myotonic discharges
- Myositis
- Variation of firing of different muscle fibres within a motor unit gives a measure of the ‘jitter’.
- This increases with unstable end plates (e.g. in myasthenia, or with reinnervating unstable terminal axons)
- Nerve compression findings on EMG
- Positive Sharp waves
- Insertional activity
- Fibrillations
- Spontaneous rest activity
- Decreased recruitment
- EMG patterns
- Myopathic has a lower amplitude
- Neurogenic because axonal loss then body compensate to get remaining nerves to innervate more motor units so you get a large but few EMG
- EMG firing
- Rarely neural damage
- Serious neural insult
- Often post-op deficit
Phasic/burst pattern:
Tonic/train activity:
Results
Muscle | Insert Activity | Fibs | Pos Waves | Fasc | Amp | Dur | Poly | Pattern |
Cervical paraspinals L. | Inc | 2+ | 2+ | None | Norm | Norm | None | Full |
Supraspinatus L. | Norm | None | None | None | Norm | Norm | None | Full |
Infraspinatus L. | Norm | None | None | None | Norm | Norm | None | Full |
Serratus anterior L. | Inc | 1+ | 1+ | None | Norm | Norm | None | Full |
Deltoid L. | Norm | None | None | None | Norm | Norm | None | Full |
Biceps brachii L. | Norm | None | None | None | Norm | Norm | None | Full |
Triceps L. | Inc | 3+ | 3+ | None | Norm | Norm | None | Red |
Flexor carpi radialis L. | Inc | 2+ | 2+ | None | Norm | Norm | None | Red |
Flexor carpi ulnaris L. | Norm | None | None | None | Norm | Norm | None | Full |
Extensor digitorum communis L. | Inc | 2+ | 2+ | None | Norm | Norm | None | Red |
Extensor indicis proprius L. | Inc | 2+ | 2+ | None | Norm | Norm | None | Red |
Pronator teres L. | Inc | 2+ | 2+ | None | Norm | Norm | None | Full |
Pronator quadratus L. | Norm | None | None | None | Norm | Norm | None | Full |
1st dorsal interosseous L. | Norm | None | None | None | Norm | Norm | None | Full |
- This is an EMG data for C7 nerve palsy: C7 Root rather than proximal radial because the cervical paraspinal EMG increased insertion activity as the paraspinal muscle innervation occurs proximal to the formation of the brachial plexus
- One caveat to this rule is that for C5 root injury can be confirmed with increased insertion activity of the muscles innervating the dorsal scapular nerve (Levator scapulae and rhomboid muscles)
Muscle | Insert Activity | Fibs | Pos Waves | Fasc | Amp | Dur | Poly | Pattern |
Cervical paraspinals R. | Norm | None | None | None | Norm | Norm | None | Full |
Rhomboids R. | Inc | None | 2+ | None | Norm | Norm | None | Full |
Supraspinatus R. | CRDs | None | None | None | Norm | Norm | None | Full |
Infraspinatus R. | Inc | None | None | None | Norm | Norm | None | Full |
Deltoid R. | Inc | None | 1+ | None | Norm | Norm | None | Full |
Biceps brachii R. | Inc | None | 2+ | None | Norm | Norm | None | Red |
Flexor carpi radialis R. | Norm | None | None | None | Norm | Norm | None | Full |
Flexor carpi ulnaris R. | Norm | None | None | None | Norm | Norm | None | Full |
Extensor digitorum communis R. | Norm | None | None | None | Norm | Norm | None | Full |
1st dorsal interosseous R. | Norm | None | None | None | Norm | Norm | None | Full |
- The left peroneal nerve had normal motor conduction velocity and distal and F-wave latencies, but the CMAP response on knee stimulation was larger than ankle stimulation. The left tibial motor nerve had normal conduction velocity, CMAP amplitude, distal latency, and F-wave latency. The right peroneal conduction test, left sural SNAP, and both H-reflexes were normal. Electromyography showed denervation potentials in the left tibialis anterior, tensor fasciae latae, and tibialis posterior muscles; some large motor unit potentials were seen in the extensor hallucis longus. These muscles are innervated mainly by L4 and L5 roots. The lack of denervation in the vastus lateralis suggests that the L4 was not affected, and the negative findings in the gastrocnemius ruled out a S1 root involvement. Thus all denervated muscles originate from the L5 root, particularly the tensor fasciae lata which is innervated primarily by this root. Although the paraspinal muscles were normal, the electrophysiologic diagnoses were a left L5 radiculopathy, and an accessory deep peroneal nerve (larger CMAP when stimulated above the branch point at the knee; no longer activating accessory nerve when stimulating at ankle).
- Nerve and site, latency, amplitude, and conduction velocity
- Nerve latency normal values
- Sural nerve conduction study
- EMG data lower limb muscles
Nerve and site | Latency (ms) | Amplitude (mV) | Conduction velocity (m/s) |
Peroneal nerve L. | Normal ≤ 5.7 | Normal ≥ 3 | Normal ≥ 40 |
Ankle | 3.5 | 11 | - |
Fibular head | 9.9 | 15 | 52 |
Lateral malleolus | 3.7 | 7 | - |
Tibial nerve L. | Normal ≤ 5.3 | Normal ≥ 4 | Normal ≥ 40 |
Ankle | 2.9 | 18 | - |
Pop. fossa | 11.2 | 18 | 49 |
Peroneal nerve R. | Normal ≤ 5.7 | Normal ≥ 3 | Normal ≥ 40 |
Ankle | 3.7 | 9 | - |
Fibular head | 10.1 | 9 | 52 |
Nerve | Latency (ms) | Normal latency ≤ (ms) |
Peroneal nerve L. | 54.0 | 54 |
Tibial nerve L. | 47.6 | 54 |
H-reflex L. | 30.3 | 34 |
H-reflex R. | 30.1 | 34 |
Nerve | Onset latency (ms) | Normal onset latency ≤ (ms) | Peak latency (ms) | Normal peak latency ≤ (ms) | Amp (μV) | Normal Amp ≥ (μV) | Conduction velocity (m/s) | Normal conduction velocity ≥ (m/s) |
Sural nerve L. | 2.8 | 3.5 | 3.3 | 4.0 | 17 | 11 | 50 | 40 |
Muscle | Insert Activity | Fibs | Pos Waves | Fasc | Amp | Dur | Poly | Pattern |
Lumbar paraspinals L. | Norm | None | None | None | Norm | Norm | None | Full |
Tensor fasciae latae L. | Inc | None | 1+ | None | Norm | Norm | None | Full |
Vastus lateralis L. | Norm | None | None | None | Norm | Norm | None | Full |
Biceps long head L. | Norm | None | None | None | Norm | Norm | None | Full |
Tibialis anterior L. | Inc | None | 1+ | None | Norm | Norm | None | Red |
Tibialis posterior L. | Inc | None | 1+ | None | Norm | Norm | None | Red |
Peroneus longus L. | Inc | None | None | None | Norm | Norm | None | Full |
Gastrocnemius L. | Norm | None | None | None | Norm | Norm | None | Full |
Extensor hallucis longus L. | Norm | None | None | None | Lg | Norm | None | Full |
Examples
Carpal tunnel syndrome
- Local entrapments of nerves commonly produce localized slowing of conduction
- Severity depends on the degree of median nerve slowing compared to the equivalent ulnar velocity across the wrist
- Sensory velocity slowing is the most sensitive finding, with motor latency only increased in more severe disease
Ulnar nerve compression syndrome
- Reduced motor velocity around the elbow and amplitude drop (50% drop = conduction block),
- ‘Inching’ technique can localize ulnar entrapment preoperatively
EMG for radiculopathy
General principles
- If a reliable motor exam can be done, the EMG will not likely add any information.
- Normal motor exam = normal EMG
- EMG is not extremely sensitive for radiculopathy
- e.g. especially with sensory-only radiculopathy, which is more common in the cervical region than lumbar.
- However, when abnormal, EMG is very specific
- EMG is best reserved for cases with documented weakness where additional localizing or prognostic information is needed, or when the patient’s strength cannot be reliably assessed (inability to cooperate, functional overlay…)
- Timing
- It takes about 3 weeks after onset of radiculopathy for the EMG to reliably show any findings
- Acute changes begin at about 3 weeks and can last up to about 6 months
- Chronic changes can be seen starting at about 6 months, and may persist indefinitely
- Cervical EMG:
- EMG is most helpful for nerve roots C5–T1.
- There are no good muscles to reliably test C3–4, and compression here may cause findings in lower nerve roots
- Lumbar EMG:
- If lumbar MRI is normal in a patient with evidence of motor weakness (e.g. foot drop), do an EMG to look for peripheral neuropathy (again, a good motor exam can give the same info).
- If the EMG is negative for peripheral neuropathy (e.g. peroneal nerve palsy) then do an MRI (or CT) of abdomen and pelvis to look for pelvic floor tumour
Findings
- Include spontaneous activity (fibs & PSWs, see above).
- The earliest possible finding (within 2–3 d) is reduced recruitment with volitional activity, but this occurs only with significant compression of motor fibers.
- EMG is useful if there is a concern about possible overlapping peripheral neuropathy (e.g. carpal tunnel syndrome vs. C6 radiculopathy).
- Takes time as Wallerian degeneration needs to occur
- Reduce number of MUPs due to a partially working nerve
- Reduce recruitment can also be due to just neuropraxia that is temporary
Days after onset of radiculopathy | Electrophysiological abnormalities |
>0 | - Reduced number of MUPs (reduced interference pattern), - Fasciculations, - H-reflex prolonged latency, - Reduced F waves |
>1 week | Fibrillation potentials and positive sharp spikes in paraspinal muscles |
>2 weeks | Fibrillation potentials and positive sharp spikes in proximal limb muscles |
>3 weeks | Fibrillation potentials and positive sharp spikes in distal limb muscles |
EMG criteria for radiculopathy
- Fibrillations and/or positive sharp waves in at least 2 muscles innervated by a single nerve root in question, but by 2 different peripheral nerves
- Abnormal paraspinals: this supports the diagnosis, but is not required since paraspinals will be normal in ≈ 50%
Lumbar radiculopathy from herniated disc
- With radiculopathy, SNAP is usually normal
- Paraspinal muscle fibrillations may occur.
- Accuracy in predicting level of involvement is ≈ 84%.
- Foot drop: the short head of the biceps femoris in the LE is the first muscle innervated by the peroneal division of the sciatic nerve at or just above the popliteal fossa just after the nerve splits off from the sciatic nerve.
- In cases e.g. of foot drop it is a good muscle to test to determine if there is a peroneal neuropathy vs. a more proximal lesion (i.e., above the popliteal fossa).
- Findings with healing radiculopathy (e.g. following discectomy or spontaneous healing):
- Motor potentials return first (if nerve injury were “complete,” it would take a month to return)
- If lost, sensory potentials return last or may not return
- Following laminectomy, paraspinal muscle potentials may no longer be useful for EMG because cutting the muscles during surgery alters their electrical signals resulting in effective denervation due to muscle injury.
- Fibs and PSWs decrease in amplitude over time but may remain present indefinitely
Interpreting the report
- What the report says: e.g.
- “Chronic cervical radiculopathy” What this usually means: there are large amplitude fast firing motor units (sometimes termed “decreased recruitment”)
- Neurosurgical implications:
- This does not imply anything about ongoing compression.
- If report further qualifies it e.g. as “without evidence of ongoing denervation” this usually means no sharp waves (PSWs) or fibrillation potentials (fibs) were seen.
- This implies that there was some nerve injury at some point, but that there has been healing and unlikely that there is ongoing compression.
EMG in plexopathy
- Reduction of SNAP with no paraspinal muscle fibrillations (the dorsal rami exit proximally to innervate the paraspinals, and are involved ≈ only with root lesions).
ㅤ | Conduction velocity | MUAPs | CMAPs | SNAPs | To be aware |
Neuropraxia | Maintained | None | Normal, slight reduction proximally | Reduced | Permanent conduction slowing due to thinner internodes |
Axonotmesis | Reduced | None | Reduced proportional to axonal loss | Reduced | Positive sharp waves and fibrillations |
Neurotmesis | Nil | None | Reduced proportional to axonal loss | Absent | Positive sharp waves and fibrillations |
EMG in nerve root avulsion
- Produces muscles weakness and sensory loss with normal SNAP since the lesion is proximal to the dorsal root ganglion (where the cell bodies for the sensory nerves are located).
Made with Bullet