Page 10 - 2020 Spring CMTA Report

Q ASK AN EXPERT
A

I’m confused about the difference between electromyograms (EMG)
and nerve conduction studies (NCS)—can you please explain?

Dr. Richard A. Lewis answers:
EMG, which stands for electro -
myogram (“myo”= muscle), is the
term used for electrodiagnostic
tests (EDX) for neuromuscular
disorders. The total EDX includes
Dr. Lewis is the both nerve conduction studies
co-director of the (NCS) and needle EMG.
Inherited Neuropathy Depending on the clinical EMG Test (above) and NCS Testing (below)
Clinic and the director question, one or both parts of
of the EMG Labo - the test may be conducted.
ratory and of the
Clinical Specialty For CMTers, the most im -
Clinic at Cedars-Sinai portant component is the NCS,
Medical Center in which determines whether a per-
Los Angeles. He is son has a neuropathy and whether
currently on the it involves sensory nerves, motor
Steering Committee
of the Inflammatory nerves or both. If the disorder just
Neuropathy involves motor nerves, it’s Heredi-
Consortium. tary Motor Neuropathy (HMN); To determine motor nerve suggests that many have stopped.
if sensory, Hereditary Sensory conduction velocity, it is necessary The velocity determines whether
Neuropathy (HSN); and if both to stimulate the nerve at two the disorder primarily affects the
Hereditary Motor/Sensory locations: In the arm, the two myelin and Schwann cells or the
Neuropathy (HMSN). HMSN stimulation sites are the wrist and axon. The disorders that comprise
is the primary disorder that the elbow. In the leg, they are the CMT1 are characterized by very
comprises CMT. ankle and knee. The motor nerve slow velocities and are due to
Sensory nerves usually only velocity in the arm is determined mutations of genes that form
require one stimulation point— by subtracting the latency of wrist myelin. CMT2 has more normal
the wrist (sometimes the finger) stimulation from the latency on velocities but very low amplitudes
or ankle and recording over the elbow stimulation. Dividing that and are disorders of the axon.
nerve a short distance away. The nerve latency into the distance Normal nerves conduct at
sensory velocity is determined by from wrist to elbow determines ~ 50 meters/second (m/sec).
the time it takes for the signal to the motor nerve conduction CMT1A usually has nerve veloci-
reach the recording electrode velocity (see box, item B). ties around 20 m/sec. There is a
(latency) divided into the distance The amplitude of the response somewhat arbitrary cut-off of 38
between stimulating and record- indicates whether the nerve fibers m/sec for motor nerve conduction
ing site (see box, item A). are functioning; a low amplitude in the median or ulnar nerve of
the forearm that determines
CMT1 or CMT2. CMTX is con-
A FEW SAMPLE CALCULATIONS: sidered intermediate with velocities
between 30 and 40 m/sec.
A) MEDIAN SENSORY VELOCITY: The EMG portion of the test,
15 cm from wrist to finger divided by 3 millisecond (msec) latency =
50 meters/second velocity which involves the insertion of
fine needles in the muscle, can
B) MEDIAN MOTOR NERVE CONDUCTION VELOCITY
Latency from elbow (e.g. 8 msec) minus latency from wrist (3 msec) divided into the distance from elbow to wrist (30 cm) = determine if there is nerve damage
60 meters/second to the muscle not identified by the
nerve conduction tests. This can
10 THE CMTA REPORT SPRING 2020