Nerve Conduction Studies Flashcards
Where does electric current flow?
Between two poles
What is the role of the cathode? The anode?
Cathode = depolarizes the nerve Anode = hyperpolarizes tissue in the area
What is the goal to stimulation?
- to create a sudden and rapid alteration in the resting membrane potential of the nerve being evaluated
- bring all axons of that nerve to threshold
Why is a supramaximal stimulus needed?
- provides reproducible results
- it’s representative of the capabilities of that peripheral nerve
What type of axons allow current to flow most easily?
Large-diameter axons, they have a lower resistance
Which axons reach threshold easiest?
smaller cell bodies and smaller axons
During voluntary contraction, what is the recruiting order?
small (type I/slow twitch) to large
During an externally applied current, what is the recruiting order?
Large to small
-current flows down the path of least resistance
How do you know that a supramaximal stimulation was used?
If the stimulus intensity is increased and the compound AP does not get larger with higher levels of stimulus, a supramaximal stimulation is achieved
What ways does the stimulation level vary from patient to patient?
Skin thickness, adipose tissue, connective tissue, musculature
What is a NCS used for?
use to evaluate the function of the motor and sensory nerves of the human body
What is the common measurement made during a NCS test?
nerve conduction velocity (NCV)
What are the 2 main things that NCS evaluates?
paresthesia and/or weakness of the arms and legs
What are some indications for NCS?
- symptoms indicative of nerve damage (numbness, weakness)
- differentiation between local or diffuse disease process (mononeuropathy or polyneuropathy)
- get prognostic information on the type and extent of nerve injury
Examples of peripheral neuropathy
mononeuropathy (carpal tunnel) mononeuritis multiplex (vasculitides, RA, SLE, sacroidosis, leprosy, Lyme disease, amyloidosis) Polyneuropathy (diabetic neuropathy)
Examples of myopathy
muscular dystrophies
myotonia
congenital myopathies
metabolic myopathies
What can cause radiculopathy?
Nerve damage from herniated discs
Give an example of a disease of the NMJ
Myasthenia gravis
NCS are used to determine what 2 things?
1) nerve conduction velocity
2) size of the collective AP
What can NCS evaluate the function of?
peripheral nerves, NMJ, collective muscle fibers innervated by the nerve being examined
What information is obtained during a NCS?
- excitability of PNS
- nerve conduction time
- nerve conduction velocity
- amplitude of AP following stimulation
- duration of AP
- fatigability of PNS in response to repetitive stimulation
What are the contraindications and precautions for NCS?
Same as for electrical stimulation
What is orthodromic placement?
Used in sensory testing, active is placed proximal to the point of stimulation (in the direction that sensory fibers normally conduct an AP). Afferent fibers conducting towards the CNS
What is antidromic placement?
Active recording electrode is placed along the nerve distal to point of stimulation, opposite the direction that sensory fibers normally conduct an AP
What are the filters used for sensory and motor potentials?
Motor = 10 Hz - 10,000 Hz Sensory = 2 Hz - 2,000 Hz
What are the sweep speeds for sensory and motor potentials?
Motor = 2-5 milliseconds Sensory = 1-2 milliseconds
What is the sensitivity/gain for sensory and motor potentials?
Motor = 1,000 - 5,000 Sensory = 5 - 10
Where are the 3 electrodes placed?
Active = over nerve or muscle segment being studied Reference = off of and distal to the muscle being studied Ground = placed on a bony prominence between the stimulating and active electrodes
What is the optimal temperature of the skin for a NCS? (UE, LE, room temp)
UE = 30-32 C
LE = 30 C
Room temp = 25 C
Why might a 14 year olds conduction speed differ from a 25 year olds?
Some elements of the PNS might not be fully mature until 16 years old, typically conducts slower than expected
Why might a 40 year olds conduction speed differ from a 30 year olds?
Individuals older than 40 might see a slight slowing of NCV. By age 50, 1-2 m/s per decade slowing is enough that a separate set of normative values are available. After ago 70, slowing is much more significant
What are the components of the NCS?
SNAP, CMAP, F-wave, H-reflex, A-wave, blink reflex, direct facial nerve study
What is the measurement for the time it takes for the electrical impulse to travel from the stimulation to the recording site?
Latency (measure in milliseconds)
What is the size of the response called?
Amplitude (measure in millivolts)
T/F: Motor nerve conduction can distinguish between pre and post ganglionic lesions.
False - cell body is located in the spinal cord
Equation for NCV
Conduction velocity = distance (mm)/proximal latency - distal latency (msec)
T/F: Both myasthenia gravis and Lambert-Eaton are examples of postsynaptic dysfunction.
False
T/F: A fibrillation potential is defined as the “spontaneous firing of a motor neuron and all of the muscles fibers innervated by that motor neuron.”
False
T/F: An H-reflex is physiologically equivalent to a MSR. Following stimulation, an AP is carried proximally via afferent axons to the spinal cord, where the combined potential enters the dorsal horn of the spinal cord, passes through at least one synapse, and then is carried via efferent axons to the appropriate distal muscle.
True
Which of the following best represents the role of a patient history and physical examination, in relation to electrophysiologic testing?
D. Electrophysiologic testing is based on a sound history and physical exmination
With a compression of the superficial branch of the ulnar nerve at Guyon’s Canal, which of the following would be expected?
A. prolonged distal latency to D5
What is the typical minimum NVC speed of a normal mixed nerve in the UE?
50 m/s
When comparing DSLs to DMLs, taken over the same distance, what would be expected?
DML > DSL
When examining a patient with a potential NMJ disorder, which of following repetitive stimulation findings would be most likely?
Repetitive stimulation amplitude would increase compared with single-stimulation amplitude in Eaton-Lambert
The time lapse from the stimulation to the onset of the compound motor unit action potential (CMAP) is referred to as…
DML
EMG is the recording and study of the electrical activity of the muscle. The first procedure typically performed during an EMG evaluation is the study of…
insertional activity
A polyphasic MUAP is defined as having how many phases?
> 5
Are motor or sensory amplitudes larger?
Motor
How is the sensory NCV calculated?
based upon the latency and the distance between the stimulating and recording electrode
Are SNAPs or CMAPs more sensitive in detecting incomplete peripheral nerve injuries?
SNAPs
How can the location of a lesion to the DRG be determined?
SNAPs, if the lesion is proximal to the DRG, the SNAP will be preserved despite clinical sensory abnormalities
How many stimulation sites are needed for a sensory NCS?
One
