Nerve Conduction Study

بسم الله الرحمن الرحيم
السلام عليكم ورحمة الله وبركاته

A nerve conduction study NCS
is a test commonly used to evaluate the function
especially the ability of electrical conduction
of the motor and sensory nerves of the human body

Nerve conduction velocity NCV
is a common measurement made during this test
The term NCV often is used to mean the actual test
but this may be misleading since velocity
is only one measurement in the test suite


Purposes
Nerve conduction studies are used mainly for evaluation
of paresthesias (numbness, tingling, burning) and/or
weakness of the arms and legs
The type of study required is dependent in part
by the symptoms presented
A physical exam and thorough history also help
to direct the investigation
Some of the common disorders which can
be diagnosed by nerve conduction studies are

Peripheral neuropathy
Carpal tunnel syndrome
Ulnar neuropathy
Guillain-Barré syndrome
Facioscapulohumeral muscular dystrophy
Spinal disc herniation


Description

The nerve conduction study consists
of the following components

Motor NCS
Sensory NCS
F-wave study
H-reflex study
Motor NCS


Motor NCS
are performed by electrical stimulation of
a peripheral nerve and recording from
a muscle supplied by this nerve

The time it takes for the electrical impulse to travel
from the stimulation to the recording site is measured
This value is called the latency and is
(measured in milliseconds (ms
The size of the response - called the amplitude - is
also measured (Motor amplitudes are measured
in millivolts (ms
By stimulating in two or more different locations
along the same nerve, the NCV across different
segments can be determined
Calculations are performed using the distance between
the different stimulating electrodes and
the difference in latencies





Sensory NCS
Sensory NCS are performed by electrical stimulation
of a peripheral nerve and recording from a purely-sensory portion
of the nerve, such as on a finger
The recording electrode is the more proximal of the two
Like the motor studies, sensory latencies are
on the scale of milliseconds

Sensory amplitudes are much smaller than the motor
amplitudes, usually in the microvolt (μV) range
The sensory NCV is calculated based upon the latency
and the distance between the stimulating
and recording electrode





F-wave study
F-wave study uses supramaximal stimulation
of a motor nerve and recording of action potentials
from a muscle supplied by the nerve
This is not a reflex, per se, in that the action potential
travels from the site of the stimulating electrode
in the limb to the spinal cord's anterior horn cell
and back to the limb in the same nerve that was stimulated
The F-wave latency can be used to derive
the conduction velocity of nerve between the limb and spine
whereas the motor and sensory nerve conduction studies
evaluate conduction in the segment of the limb
F waves vary in latency and an abnormal variance
"is called "chrono dispersion

Conduction velocity is derived by measuring the limb
length in millimeters from the stimulation site to
the corresponding spinal segment
(C7 spinous process to wrist crease for median nerve)

This is multiplied by 2 as it goes to the cord and
(returns to the muscle (2D

2D is divided by the latency difference between
mean F and M and 1 millisecond
subtracted (F-M-1). The formula is 2D/(F-M-1)





H-reflex study
H-reflex study uses stimulation of a nerve and recording
the reflex electrical discharge from a muscle in the limb
This also evaluates conduction between the limb
and the spinal cord, but in this case, the afferent impulses
those going towards the spinal cord are in sensory nerves
while the efferent impulses
(those coming from the spinal cord)
are in motor nerves. This process cannot be changed


Small-pain-fibers method
In 1998 a small-pain-fibers (spf-NCS) method
was cleared by the FDA
This method uses an electrical stimulus with a neuroselective
frequency to determine the minimum
voltage causing conduction
Rather than comparing the data with population
averages on a bell-shaped curve, which at best
has about 65% sensitivity, the patient is his own control
In a three year LSU Pain Center study it was
found that the nerve requiring the greatest voltage
to cause conduction of the A-delta (Fast Pain) fibers
identified nerve root pathology with 95% sensitivity
Besides being painless, the test is fast
A new version, uses a potentiometer to objectively
measure the amplitude of the action potential
at a distant site along the nerve being tested
The previous version relied on the patient reporting
a sensation when the nerve fired
The spf-NCS does not require myelin loss to detect
function change, so velocity is not measured

Interpretation of nerve conductions
The interpretation of nerve conduction studies is complex
but in general, different pathological processes result in changes
in latencies, motor and/or sensory amplitudes,
or slowing of the conduction velocities to differing degrees
For example, slowing of the NCV usually indicates
there is damage to the myelin
Another example, slowing across the wrist for the motor
and sensory latencies of the median nerve indicates
focal compression of the median nerve at the wrist
called carpal tunnel syndrome
On the other hand, slowing of all nerve conductions
in more than one limb indicates generalized diseased nerves
or generalized peripheral neuropathy
People with diabetes mellitus often develop generalized
peripheral neuropathy

Patient risk
Nerve conduction studies are very helpful to diagnose
certain diseases of the nerves of the body
The test is not invasive, but can be a little painful
due to the electrical shocks
The shocks are associated with a low amount of
electrical current so they are not dangerous to anyone
Patients with a permanent pacemaker or other such
implanted stimulators such as deep brain stimulators
or spinal cord stimulators must tell
the examiner prior to the study
This does not prevent the study
but special precautions are taken

The nerve conduction study is sometimes
combined with electromyography

Other special nerve conduction studies that are
occasionally performed include double stimuli
and repetitive stimulation

:D

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