Doctors EMG discussions

These Forums are no longer active. We have maintained some of the frequently asked questions and answers for educational purposes.

Evidence of Re-Innervation in EMG findings

Question: In terms of reinnervation findings of EMG, including increased recruitment, instability of MUP, increased Amplitude of MUP, etc, which of these features are reliable to predict the true re-innervation in morphology?

Answer 1: One of the first things to appear after denervation is fibrillations and positive waves of course. Following that (in about 2 months) the first signs of reinnervation are an increase in the number of phases of a motor unit, polyphasic potentials, and increased instability of the MUAP (high jiggle) with decreased recruitment followed by an increase in the amplitude.

Answer 2: The Fiber Density (FD) increases after reinnervation due to collateral sprouting. The increase of FD indicates that muscle fibers increased for the same motor unit. The FD therefore, is a most sensitive method to quantify reinnervation, and therefore, the local organization of motor unit (morphology!). Also, FD corresponds to type grouping in pathology.

EMG Technique (fasciculations)

Question: Is there a way to distinguish between benign and malignant fasciculations electromyographically and clinically? Is it true that rhythmic fasciculations tend to be benign while singular big thumps tend to be malignant?

Answer 1: This is a very common concern and generally no single answer is satisfactory because almost every single rule you make to define a benign versus malignant fasciculation has an exception. Generally speaking, Neurologists rely on far more than the presence (or absence) of fasciculation potentials to make the diagnosis of ALS. If fasciculations become worrisome to a patient, I generally recommend they seek a qualified neurologist’s opinion to ease their fears.

Answer 2: The best measure of benign versus malignant fasciculation potentials comes from the EMG needle exam by the company they keep. In other words, if fibrillation potentials and positive sharp waves are present with large motor unit potentials and polyphasic waveforms with poor recruitment along with fasciculations, this would tend to suggest a more ominous potential. A good history and physical exam is a priority—remember an EMG exam is only an extension of our physical examination.

Single Fiber Amplifier Settings

Question: What are the recommended settings for SFEMG? Specifically, I am concerned about the Low Frequency Filter Setting.

Answer: The recommended or typical filter settings are LFF 500 Hz and HFF 10 KHz. I may use LFF 1KHz (narrower settings). This LFF setting would reduce the disturbing distant muscle fibers. This setting, however, affects the shape of the single fiber potential but should not affect the jitter value or fiber density reading.

Thoroughness of EMG Studies

Question: Do most Electromyographers routinely follow the guidelines of testing 20-25 units for both spontaneous and voluntary activity?

Answer: I agree not all EMGers follow these guidelines, although I do catch myself sampling less frequently in follow-up studies when I am only looking for changes.

From a mathematical perspective, sampling more units greatly increases the chances of finding an involved unit:

• Sampling 20 units in a muscle with 10% involvement gives you a 90% chance of finding an involved unit
• Sampling 4 units in a muscle with 10% involvement gives you a 56% chance of finding an involved unit
• Sampling 20 units in a muscle with 25% involvement gives you a 93% chance of finding an involved unit
• Sampling 4 units in a muscle with 25% involvement gives you a 70% chance of finding an involved unit

Single Proximal Conduction Block

Question: How significant should a single or isolated motor nerve proximal conduction block be as a diagnostic sign?

Answer: No single “isolated” abnormality should be diagnostic for a specific lesion. “Inching” along the course of the nerve often can demonstrate if the CMAP diminishes abruptly as in MMN, or progressively as in chronic axonal loss or demyelination. The presence of a single proximal conduction block across common sites of entrapment is not helpful in establishing a diagnosis of MMN.

NCV Latency

Question: Does latency depend on the distance taken by the examiner? Does it depend on the amount of electric stimulation?

Answer: Yes, the latency depends on the distance—the greater the distance, the longer the impulse has to travel and the greater the latency. It also depends on the amount of electricity you use; if you are submaximal, you are not stimulating all the fibers and your measurement will be inaccurate. That’s why in nerve conduction it is advised that you use supramaximal stimulation which is maximal stimulation + 25% over that to ensure stimulation of all the nerve fibers.

Computer Assisted EMG

Question: Can computer programs analyze motor units by themselves?

Answer: Computers have come a long way since the early 80’s and can “assist” EMGers a great deal but not make diagnosis. There are still a lot of things that an experienced EMGer can pick up that can be missed by the computer. More importantly if the recording electrode is not placed in the appropriate area of the muscle, you feed poor quality data to the computer and as the saying goes “garbage in, garbage out.”

60 Hz Electric Interference

Question: During my EMG there was a very steady wavy line instead of the normal straight line. What was this?

Answer: What you describe sounds like pure 60 Hz interference from nearby devices or a poorly attached ground. None of that is pathological, just electrical artifact.

Can EMG Test Flexibility?

Question: Can EMG test muscle flexibility?

Answer: Routine EMGs cannot assess the flexibility of the muscles—it is only a diagnostic tool. Flexibility is best measured clinically.

False Negative Results in EMG

Question: Why do some studies indicate that needle EMG can give false negatives 30-40% of the time in detecting a root lesion?

Answer: The 30-40% false negatives applies only to radiculopathies. This is due to many factors, including the fact that while radiculopathies may be painful, they may actually not cause any nerve damage (which is what is picked up by the needle exam of the muscle), sampling or interpretation errors, detection error due to poor relaxation, timing of the exam, etc.

EMG is considered to have the highest yield in entrapment/compression neuropathies such as Carpal Tunnel, Ulnar, Radial or Peroneal Neuropathies, or Bell’s Palsy.