What is the role of muscle fiber density in the performance of single-fiber electromyography (SFEMG)?

Updated: Mar 13, 2019
  • Author: Pradeep C Bollu, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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Answer

Answer

The amplitude of the AP recorded with an SFEMG electrode from an average muscle fiber decreases to 200 microvolts (µV) when the electrode is approximately 300 µm from the muscle fiber. Thus, we can infer that APs greater than 200 µV arise from muscle fibers within 300 µm of the recording surface. By measuring in many sites within a muscle the mean number of time-locked APs that have an amplitude greater than 200 µV and rise time of less than 300 microseconds (µs), we can calculate the fiber density, which quantitates the local concentration of muscle fibers within the motor unit. This provides information that is analogous to type grouping in muscle biopsies. [13, 14]

Fiber density is a sensitive means of detecting and quantitating rearrangement of the muscle fiber topography in the motor unit; it is increased in neurogenic conditions but also in some myopathies. Fiber-density measurements are made by observing the signals on the oscilloscope screen. As the patient voluntarily activates the muscle, the electrode is positioned to record with maximum amplitude the AP from one muscle fiber. This AP triggers the oscilloscope sweep and is delayed for display so that the number of synchronized APs with amplitudes over 200 µV can be counted.

APs are recorded in 20 separate sites within a muscle, usually via 3 separate insertion sites. The fiber density (FD) is the mean number of APs, including the triggering AP, counted in these 20 sites (see image below). The normal FD is different among different muscles and increases in adults older than 60 years, especially in distal muscles.

Diagram of fiber density measurements in single-fi Diagram of fiber density measurements in single-fiber electromyography (SFEMG). Semicircles represent the recording territories of the SFEMG electrode when it is positioned to record, respectively, the signals in the adjacent outer quadrants of the figure.

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