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Questions and Answers
What is the size of the response in nerve studies measured in?
What is the size of the response in nerve studies measured in?
Millivolts.
What does CMAP stand for in nerve conduction studies?
What does CMAP stand for in nerve conduction studies?
Compound Motor Action Potential.
Which type of nerve fibers are recorded during mixed nerve conduction studies (MNAP)?
Which type of nerve fibers are recorded during mixed nerve conduction studies (MNAP)?
Both sensory and motor fibers.
Why is the H reflex tested using submaximal stimulation?
Why is the H reflex tested using submaximal stimulation?
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What happens during the F wave response when a supramaximal stimulus is applied?
What happens during the F wave response when a supramaximal stimulus is applied?
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What causes temporal dispersion and phase cancellation in SNAP or CMAP?
What causes temporal dispersion and phase cancellation in SNAP or CMAP?
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Where is the H reflex typically tested in the body?
Where is the H reflex typically tested in the body?
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What is the typical latency range for the H reflex?
What is the typical latency range for the H reflex?
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What role does myelin play in the conduction of nerve impulses?
What role does myelin play in the conduction of nerve impulses?
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Describe the function of the nodes of Ranvier in nerve conduction.
Describe the function of the nodes of Ranvier in nerve conduction.
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What are the two types of electrical responses measured during a nerve conduction study?
What are the two types of electrical responses measured during a nerve conduction study?
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What do surface electrodes in nerve conduction studies measure?
What do surface electrodes in nerve conduction studies measure?
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What is the main aim of performing a nerve conduction study?
What is the main aim of performing a nerve conduction study?
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Why is the conduction velocity of nerves assessed during nerve conduction studies?
Why is the conduction velocity of nerves assessed during nerve conduction studies?
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How does the location of nerves affect their conduction velocity?
How does the location of nerves affect their conduction velocity?
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What measurements are taken during a nerve conduction study?
What measurements are taken during a nerve conduction study?
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Study Notes
Scientific Basis of Neurophysiology
- Neurophysiology studies the nervous system's functions.
- Nerve cells (neurons) are fundamental units.
Anatomy of Nerve Cell
- Neurons have a cell body, dendrites, and an axon.
- Different neuron types exist: sensory, motor, and interneurons.
- Sensory neurons receive signals from receptors.
- Motor neurons transmit signals to muscles.
- Interneurons connect other neurons.
- Axons are insulated by myelin, a fatty substance.
- Myelin is produced by Schwann cells (PNS) and oligodendrocytes (CNS).
- The myelin sheath increases conduction speed of impulses.
- Nodes of Ranvier are gaps in the myelin sheath.
- Saltatory conduction occurs, where impulses jump between nodes.
Reflex Arc
- A reflex arc is a neural pathway for automatic responses.
- Sensory input travels through the afferent pathway.
- Interneurons process the information in the spinal cord.
- Motor output travels through the efferent pathway, stimulating muscles.
Nerve Action Potential
- A nerve action potential is a rapid change in membrane potential.
- Depolarization: membrane potential becomes more positive.
- Repolarization: membrane potential returns to its resting state.
- Hyperpolarization: membrane potential becomes more negative.
- Threshold potential: critical value to initiate an action potential.
- Voltage-gated ion channels are crucial to triggering and propagation.
Nerve Conduction Study (NCS)
- NCS measures nerve conduction velocity and latency.
- Large myelinated fibers are primarily tested.
- Measures the speed of impulse conduction of nerve fibers.
- Compound muscle action potential (CMAP) and nerve action potential (NAP) are observed.
- NCS uses surface or needle electrodes.
- Important for diagnosing neuropathies.
Points to Remember
- Routine measurements focus on the largest and fastest fibers in the nerve.
- Small myelinated (Aδ, B) and unmyelinated (C) fibers carry autonomic, sensory, and pain signals, but are seldom assessed by NCS.
- Neuropathies affecting smaller fibers might not be detected by standard NCS.
Principles of Nerve Conduction Study
- NCS applies electrical pulses to the peripheral nerves.
- A propagated nerve action potential (NAP) is recorded.
- This results in compound muscle action potential (CMAP) activation.
- Needle or surface electrodes are employed to record these signals.
Surface Electrodes
- Providing information about the conduction time from the fastest axons and the size of the response.
Volume Conduction and Waveform Morphology
- In triphasic potential: Initially positive, then negative, then positive again, depending on the stimulation location.
- Biphasic waveforms, where the initial positive phase is absent and the waveform is initially negative, can also occur, depending on the stimulation location.
- It is important that a researcher observes the shape of the trace correctly.
Aim of Nerve Conduction Studies (NCS)
- NCS aims to diagnose and characterize nerve damage.
- Assessing the fastest 20% of nerve fibers, in terms of conduction velocity and determining if the observed values fall within normal ranges.
- Assessing the conduction velocity gradient.
- Evaluating the compound muscle action potential (CMAP) in terms of its amplitude and shape.
- Evaluating whether the CMAP alters in size, shape, or duration between stimulation points.
Waves of Nerve Conduction Study
- Latency measures the time taken for an impulse to travel to the recording site.
- Amplitude measures the size of the response.
- Nerve conduction velocity across different segments can be evaluated by stimulating the nerve at multiple locations.
Nerves Studied
- Both motor and sensory nerves.
- Specific nerves studied, like the median, ulnar, radial, axillary, musculocutaneous, etc.
- Motor and sensory nerves of the arms, legs, and hands.
Compound Motor Action Potential (CMAP)
- The first test in an NCS.
- Indicating nerve anatomy, stimulation degree, and underlying pathology.
- Motor response in mV (sensory and mixed nerves in μV).
- Gain of 2-5mV/division.
- Electrical impulses duration of 200ms.
- Current increases gradually (20-50mA) up to supramaximal stimulation of target muscle
- Testing may continue with further 20% increases up to ensure supramaximal stimulation.
CMAP (contd...)
- Method to record from APB (e.g. finger) by stimulating the median nerve at the wrist.
- Active electrode is on the motor endplate, and the reference electrode is on tendon of muscle.
CMAP (contd...)
- Measures latency, from the stimulus to the first CMAP deflection.
- Represents three processes:
- Nerve conduction time from the stimulus to the neuromuscular junction (NMJ).
- Time delay across the NMJ.
- Depolarisation time of the muscle.
- Only assesses the fastest conducting motor nerve fibers.
CMAP (contd...)
- Low CMAP amplitude causes:
- Axonal loss.
- Conduction block (demyelination).
- Neuromuscular junction (NMJ) disorders.
- Myopathies.
CMAP Conduction Velocity (CV)
- CV involves stimulating the nerve at proximal and distal sites.
- Nerve conduction time is calculated.
- CV (m/sec): distance between stim sites divided by the conduction time.
- Reflects speed of the fastest conducting fibres in the nerve.
Sensory Nerve Conduction Study
- Assessing nerve fibers and not muscle/neuromuscular junction.
- More demanding because sensory responses are small(1-50µV).
- Gain: 10- 20µV/div.
- Duration of impulse is 100 or 200ms.
- Current is 5-30 mA at increments of 3-5 mA to reach supramaximal.
- Sensory fibers have lower threshold compared to motor fibers.
Median Sensory Study
- Recording finger by stimulating at wrist.
- The closest electrode to stimulator is the active electrode, whereas the electrode furthest from the stimulator is the reference electrode.
- Only cutaneous sensory nerve fibers are recorded at the finger.
SNAP
- Compound potential that represents the summation of individual sensory fiber action potentials.
- Usually biphasic or triphasic potentials.
- Obtained by stimulating sensory fibers and recording along the nerve.
- Stimulus needs to be supramaximal.
SNAP (contd...)
- Onset latency: time from the stimulus to the initial deflection of the SNAP from baseline.
- Peak latency: occurs at the midpoint of the initial negative peak.
- Amplitude: measured from baseline to negative peak or from peak-to-peak.
- Duration: from initial deflection to the first baseline crossing after the negative peak.
- In antidromic studies, conduction velocity can be calculated using a single stimulation site.
Mixed Nerve Conduction Studies (MNAP)
- Assess integrity of sensory nerves and dorsal root ganglia (DRG).
- NCS only records the largest and fastest fibers.
- Sensory nerve afferents (A-fibers) are recorded during mixed nerve stimulation.
- Fastest conducting fibers (A fibers) are affected earlier by demyelinating lesions.
- Recorded action potential is summation of all individual motor and sensory fiber action potentials.
CMAP/SNAP Comparison
- CMAP/SNAP represent compound potentials, but are inherently different.
- CMAP amplitude is measured in millivolts, while SNAP uses microvolts.
- CMAP negative peak duration is typically 5–6 ms, and SNAP duration is often 1–2 ms.
- Sensory and motor fiber stimulation, during antidromic or mixed studies, often demonstrates these differences and aid in differentiating between nerve and muscle potentials.
Top/Bottom Trace
- "Top" traces: stimulation from wrist, recording at the finger.
- "Bottom" traces: stimulation from finger, recording at the wrist.
Disadvantages of Antidromic Sensory Studies
- The entire nerve is stimulated, including sensory and motor fibers, resulting in a volume-conducted motor potential along with the sensory nerve action potential.
- The overlap of motor potentials can lead to errors in interpreting SNAPs.
Can NCS Show Normal Reading While There is a Lesion?
- Root lesions may not affect all the nerve fibres, and may not be detected by routine NCS.
- Distal sensory conduction normally unaffected by root lesions.
Proximal Sensory Studies
- Usually performed by stimulating at the elbow and recording at the fingers.
- Resulting SNAPs are characterized by longer duration and lower amplitude.
- Temporal dispersion and phase cancellation are involved.
Temporal Dispersion and Phase Cancellation
- Individual sensory/motor fiber potentials combine to produce compound potentials.
- Conduction velocities vary based on fiber conduction speed in each response.
- Slower fibers lag behind the faster fibers.
- At proximal sites, slower fibers' responses cancel out the positive trailing phase of faster fibers due to temporal dispersion and phase cancellation.
Who Has Larger Temporal Dispersion and Phase Cancellation?
- Sensory nerves experience greater temporal dispersion and phase cancellation compared to motor nerves due to the differences in the individual fiber conduction speeds.
Late Response Waves
- H Reflex.
- A Wave.
- F Wave.
H Reflex
- Submaximal sensory stimulus evokes an evoked H response in the muscle and is mediated by the spinal cord.
- Latency values vary in different muscle groups, and age.
- Includes testing for soleus, flexor carpi radialis and vastus medialis.
F Wave
- Supramaximal stimulus.
- Consists of antidromic motor response to the spinal cord.
- A recording electrode can also receive an orthodromic motor response.
- F wave is usually 5% of the CMAP height.
- Latency/configuration changes with stimulus.
- The changes are due to Renshaw cells that prevent impulses traveling in the same path.
A Wave
- "Axon reflex," It is a smaller motor potential, and indistinguishable in latency and configuration from successive stimulus in comparison to H and F wave response.
- A wave is mostly seen in submaximal stimulus.
- It is an indication of nerve damage.
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Description
Test your knowledge on nerve conduction studies with this quiz. Explore topics like CMAP, H reflex, and the roles of myelin and nerve fibers. Gain a deeper understanding of how nerve impulses are measured and analyzed in clinical practice.
