Which motor unit action potentials (MUAPs) findings are characteristic of sporadic inclusion body myositis (s-IBM)?

Updated: Jun 08, 2018
  • Author: Michael P Collins, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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The MUAPs typically show normal to reduced amplitude—reduced duration for simple (nonpolyphasic) MUAPs and variable increase in complexity (phases and turns). When assessing duration, only simple MUAPs should be measured so as to increase diagnostic sensitivity.

Increase in complexity (eg, increases in phases, turns, or the presence of late components or satellites) is a nonspecific finding and may be seen as an early abnormal finding in neurogenic or myopathic processes.

Occasional MUAPs in s-IBM may appear "enlarged" or high amplitude. Careful assessment shows that these are narrow spikes with minimal area. See the image below.

Top - A large, complex motor unit action potential Top - A large, complex motor unit action potential (MUAP; 5 phases, approximately 2500 microV amplitude and 3 ms duration) firing at a progressively increasing rate (ie, shifting left) at about 13 Hz in apparent isolation. In normal muscle, other motor units typically would be recruited at this threshold (calibration 500 microV/division vertical; 10 ms/division horizontal). In the bottom trace the sensitivity is increased to 100 microV/division vertical (no change in horizontal time base), showing very small motor unit action potentials (MUAPs) in the baseline on either side of the large MUAP. This phenomenon may give rise to a mistaken "neurogenic" impression of the MUAP, as these small potentials are overlooked easily or mistaken for baseline noise or fibrillation potentials. Note also that despite the large amplitude of this MUAP, the spikes include essentially no area, giving them a needle-like appearance. Copyright, Paul E Barkhaus, MD, 2000, with permission.

In s-IBM, MUAPs are generally stable. In other words, jitter typically is not increased. See the image below.

On the left are 3 motor unit action potentials (MU On the left are 3 motor unit action potentials (MUAPs) that have been "captured" from the same site and analyzed using a computer-assisted method. Note that the middle one has a satellite or "early" potential linked to it, characterized by the blackened/blurred area created by their superimposition to the left of the main portion of the MUAP. The reason for this is the increased variability in the interpotential interval on successive sweeps (ie, increased jitter). On the right, this middle MUAP is displayed in faster mode (9 sweeps). Note that on the fifth trace, the early component is absent, indicating a block. This shows the infrequent phenomenon in s-IBM of increased jitter and blocking, Copyright, Paul E Barkhaus, MD, 2000, with permission.

Table 2. MUAP Features in Myopathy (Open Table in a new window)


Changes in MUAP Features

Nonspecific abnormality

Increased complexity (ie, phases, turns, late components)

Only amplitude reduced

Specific for myopathy

Shortened duration (simple or nonpolyphasic MUAPs)

Area reduced

Recruitment of MUAPs is "early" in myopathic processes. This is interpreted as a more rapid recruitment of motor units for level of effort. Thus, discharging motor units appear to be firing faster and interference pattern (ie, pattern at full effort) appears full but reduced in amplitude. See the image below.

Interference pattern in biceps brachii. Top trace Interference pattern in biceps brachii. Top trace - Normal interference pattern at full effort (calibration - 500 microV/division vertical; 1 s/division horizontal). The middle trace is an interference pattern from a patient with severe s-IBM (calibration - 100 microV/division vertical; 1 s/division horizontal). This epoch of signal actually shows the patient going from minimal activation at the left (beginning of the sweep) to full effort on the far right. The "notch" just to the right of the second division mark shows a baseline shift from needle electrode movement. Overall, no amplitude change of "fullness" is seen going from minimal to full effort, and the amplitude of the signal epoch is less than half of what might be expected in normal muscle. The bottom trace is an expanded segment showing interference pattern from biceps brachii; this trace is from a patient with advanced s-IBM (calibration - 100 microV/division vertical; 10 ms/division horizontal), from the early or far left portion of the middle sweep (see "H" bar position between the middle and lower sweeps). This shows a relatively full baseline of small-amplitude, complex motor unit action potentials (MUAPs). Copyright, Paul E Barkhaus, MD, 2000, with permission.

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