Aka
- NCV (nerve conduction velocity) study
- NCS (nerve conduction studies)
CI
- Pacemaker
- Glasgow doesn’t care about Pacemakers and ICD
- More for ICD as stimulation can theoretically cause the ICD to sense an abnormal signal and to cause it to be activated
- Since needles are almost never used, NCS can be done in patients on anticoagulants or antiplatelet drugs
Evaluate
- Amplitude
- Latency
- Conduction velocity
- Measure 2 different latency and then minus them → divide distance/time difference
- Cannot use latency directly as it contains muscle and NMJ time
Technique
- An electrical stimulation is applied through surface electrodes at specified locations and electrical responses are recorded in receiving electrodes.
Components of the study include
Motor component (M wave)
- Aka compound motor action potential (CMAP)
- CMAPs from muscles, where amplitude reflects the number of axons/ muscle fibres activated.
- Technique
- Stimulate
- A peripheral nerve
- Recording
- From a muscle innervated by that nerve (e.g. abductor pollicis brevis for the median nerve).
- Recording electrodes are placed in a belly–tendon arrangement, and the nerve is activated electrically by percutaneous stimulation to achieve the maximal CMAP
- Measurement
- Only assess the FASTEST conducting (large/myelinated) fibres
- Amplitude (axons)
- Normal number of axons intact
- Reduced
- Axonal loss/degeneration
- Muscle loss
- Velocity (myelin)/Latency (time from stimulation to recording)
- Normal no myelin pathology/injury
- Reduced
- Demyelination
- Carpal tunnel syndrome
- Guillain Barre
- Reinnervating fibres after nerve section
Sensory component
- Aka: Sensory nerve action potential SNAP
- Measurements
- Stimulate
- Distally: finger
- Recording
- From another point on the nerve
- Rather than from muscle.
- Latency
- Sensory velocity slowing is the most sensitive finding, with motor latency only increased in more severe disease
- Amplitude
- Sensory amplitudes are smaller than motor amplitudes.
- Technique
For the median and ulnar nerves, ring electrodes are placed over the 4th digit and the nerve is stimulated 13 cm proximal to the closest recording electrode. This image shows the stimulating site for the median nerve.
Pathology
- Cannot do SNAP in lower limbs (Peroneal nerve)
- Due to the presence of the NMJ, the time it takes for Wallerian degeneration to cause axonal injury is 5-7 days
- Shorter than sensory nerve Wallerian degeneration
- Takes 2 weeks for the Wallerian degeneration to affect SNAP
- Lesions in the
- Sensory Preganglionic nerve
- Injury proximal to the ganglia
- Sensory action potential: normal as there is no wallerian degeneration in the distal rootlet
- Motor action potential: Abnormal as there is wallerian degeneration in the distal rootlet
- Explanation
- EG
- Anterior horn cell body
- Nerve root
- Post-ganglionic
- Injury distal to the ganglia
- Sensory action potential: Abnormal as there is wallerian degeneration in the distal rootlet
- Motor action potential: Abnormal as there is wallerian degeneration in the distal rootlet
- Distal nerve
- Axon
- Schwann cells
- Neuromuscular junction
- Differentiating diseases
Common peroneal nerve | L5 radiculopathy | |
Superficial peroneal SAP | Abnormal | Normal |
Motor Conduction block | At the fibular neck | No Conduction block |
SNAP | Cannot perform at the fibular neck | N/A |
ㅤ | Ulnar neuropathy | C8 radiculopathy |
Ulnar SAP (5th digit) | Abnormal | Normal |
- It should be noted that the distal part of a severed nerve still functions for about a week before Wallerian degeneration occurs.
- Responses recorded the day after injury must not be misinterpreted.
- However: Well-preserved CMAP amplitude from a clinically weak muscle at least 7 days after the injury suggests a neurapraxic lesion.
- Conduction block will occur with pressure on a nerve (e.g. Ulnar or peroneal neuropathy).
- Initially: Changes with increasing pressure will give conduction block
- Neuropraxia
- Later: then localized slowing
- Myelin injury
- Eventually: as axons die, amplitudes reduce with sensory axons affected before motor.
- Axonal injury
- Repetitive stimulation stresses the neuromuscular junction and can be used to look for myasthenia and related conditions
- Pathology which may affect both myelin and axons equally include diabetes, uremia and paraproteinemia.
- Radiculopathies (root lesions) and neuropathies (e.g., MND, herpes zoster) are not included in this table, although they may mimic peripheral neuropathy.
EMG/NCS | Focal (Mononeuropathy) | Multifocal (Mononeuritis Multiplex) | Generalized (Polyneuropathy) |
Demyelination | Nerve entrapment | - Paraproteinemia - Diphtheria - Leprosy | - Guillain-Barre syndrome - CIPD - Lymphoma - Multiple myeloma - Amiodarone - Hereditary |
Axonal degeneration | Severe nerve entrapment | - Diabetes - Vasculitis - Neoplastic - HIV - Sarcoidosis - Amyloidosis - Lyme disease | - Diabetes - Alcohol - Drugs/toxin - Critical illness - Multiple myeloma - Hereditary |
F- wave (foot wave)
- An antidromically conducted action potential in the motor axon which, on arrival at the spinal cord, causes the anterior horn cells to fire and cause a late muscle contraction.
- And both afferent and efferent action potentials are carried in the motor axon.
- A late response wave that follows the motor response (M)
- Stimulate
- Elicited by supramaximal electrical stimulation of a mixed or a motor nerve via stimulation of
- The arm and the leg and the related motor neurons in the cervical and lumbosacral cord
- F wave requires a more potent stimulus than the H reflex.
- Measurement
- Latency measures
- Conduction between limb and spine,
- Rather than within a segment of the limb
- Delay in the F-wave indicates some slowing of conduction of the motor axon.
- Useful in investigating proximal nerve pathology
- Since the F-response traverses more proximal portions of the motor axons (twice) it may be useful in the investigation of proximal nerve pathology.
- Not useful in localizing a single nerve root problem
- Since the antidromic impulse in motor axons in a single peripheral nerve will test the multiple nerve roots forming it, F-wave is not useful for isolated radiculopathy but is valuable where multiple roots may be involved (e.g., Guillain-Barre syndrome, or chronic inflammatory demyelinating polyradiculopathy).
- DO NOT MEASURE AMPLITUDE
- Because the number of motor neurons that are re-activated is somewhat unpredictable, the amplitude of this signal is variable and, therefore, amplitude measurements are usually not used.
- A depolarizing wave travels through the alpha motor neuron.
- Which is present in both the afferent and efferent pathway
- Orthodromic conduction
- The recording electrodes measure the action potential traveling in the physiologic direction.
- Travels down the motor nerve producing a direct response
- Antidromic conduction
- The recording electrodes measure the action potential traveling opposite the physiologic direction
- Activating anterior horn cells → The resulting action potential propagates down the motor nerve, producing a potential several milliseconds after the direct response.
- F-wave latency may be prolonged in multilevel radiculopathy (not sensitive).
- Most helpful in
- Evaluating proximal root slowing, e.g. GBS.
- Multi root problems
H- reflex study (Hoffman reflex)
- A muscle reaction in response to electrical stimulation of innervating sensory fibers
- Is another form of late response that effectively measures the muscle stretch reflex.
- Takes a long time
- For example
- Practical ≈ only in S1 nerve root
- Similar information to the ankle jerk.
- By stimulating the tibial nerve in the popliteal fossa and recording over soleus, can be used to distinguish L5 (H- reflex present) and S1 (H- reflex absent) radiculopathies
- Stimulation of Ia-afferent fibers in tibial nerve → passes through a monosynaptic connection causing an orthodromic alpha-motor action potential that can be measured in the triceps surae (soleus + two-headed (medial & lateral) gastrocnemius)
- Once the reflex arc has been damaged, it often does not return to normal (making the test less useful in investigating the question of recurrent radiculopathy
- Use in masseter reflex
- New H reflex
- Theoretically, this reflex can be elicited from virtually any muscle but only the triceps surae muscle produces H-reflexes that are reliable enough to be clinically useful
- What is the most reliable electrodiagnostic measurement to quantitate spasticity ?
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