**3. Results**

Figures 1-3 show the typical firing pattern of different spontaneous fibrillation categories. Random and slightly irregular fibrillations with pauses could usually be recorded for several minutes. In many cases we collected several samples of a sequence, one of which was chosen for the analysis. This was possible because these fibrillation sequences were persistent with the same firing pattern. Also a sequence of rhythmic fibrillation potentials had to proceed several seconds in order to be accepted. Rhythmic fibrillations were usually elicited by needle insertion and they showed a gradual shift of interpotential intervals during the recording time (Conrad et al. 1972). A short-lasting burst of insertion activity was not accepted. Slightly irregular fibrillations with pauses did not show a gradual shift in the basal interval, nor were they affected by needle insertion.

**2. Material and methods**

44 Electrodiagnosis in New Frontiers of Clinical Research

tential intervals and wave forms.

unit. In such a case the sequence was omitted.

taken from the tape and the procedure was repeated.

**3. Results**

interval difference (APCID) (Conrad et al. 1972) were calculated.

The different potential categories were analyzed using Student´s unpaired t-test.

Spontaneous activity was recorded with concentric needle electrodes (DISA 13L58) and a 4 channel Disa 1500 EMG machine interfaced with a 4-channel Teac R 61 D cassette recorder. Amplification was set at 50 µV/div, and high-pass and low-pass filters at 20 and 2000 Hz, respectively. 94 sequences of spontaneous activity were divided into the following categories by audiovisual analysis: random fibrillations, slightly irregular fibrillations with occasional pauses, regular fibrillations (Partanen & Danner 1982), "myokymic" fibrillations, and end plate spikes (Partanen 1999). 10- or 20-second samples of the given sequences were digitized (sampling frequency 10 kHz) and analyzed in a Hewlett Packard 340 computer for interpo‐

"Myokymic" discharges of partially denervated muscles usually exhibited short sequences and they were found to be either "fibrillation-like" or "motor unit potential-like". These sequences were occasionally studied with another EMG needle inserted a few millimeters from the primary electrode in parallel with the muscle fibres. In "fibrillation-like" myokymic discharges it was difficult to find a synchronous discharge of the same muscle fibre in the second EMG channel whereas in "motor unit potential-like" sequences a synchronous discharge was readily observed indicating a sum potential of several muscle fibres of the motor

The raw EMG data were processed with an automatic analysis system (Partanen 1999). The rise rate, computed using a simple "low pass differentiator" (Usui & Admiror 1982) with a user definable threshold level was applied for potential recognition. Each potential recognized as belonging to the given sequence was marked with a cursor. Thereafter the sequence was checked visually on a large screen, potential by potential in order to correct possible misclas‐ sifications of the automatic program. In case of uncertainty, caused, for example, by superim‐ position of potentials the data were discarded and a new sample of the given sequence was

Subsequently the analysis program computed the number of intervals, mean, standard deviation, median, minimum, maximum, and interval range of the intervals, as well as the amplitude, and the spike duration of the averaged potential. The initial positive deflection of the potential could also be measured. In order to assess the regularity of firing, also the mean consecutive difference (MCD) (Stålberg et al. 1971) and the average proportional consecutive

Figures 1-3 show the typical firing pattern of different spontaneous fibrillation categories. Random and slightly irregular fibrillations with pauses could usually be recorded for several minutes. In many cases we collected several samples of a sequence, one of which was chosen

**Figure 1.** A regular sequence of fibrillation potentials. Interruptions in line indicate interruptions in recording. On the vertical axis is the interpotential interval of fibrillation potentials. On the horizontal axis is the number of successive intervals. There are 20 intervals per division. From Partanen, J.V. & Danner, R. (1982), Author´s own work.

**Figure 2.** Randomly occurring fibrillations 33 days after muscle biopsy. Vertical axis is the interpotential interval. Hori‐ zontal axis is the number of successive intervals; there are 10 intervals per division. From Partanen, J.V. & Danner, R. (1982), Author´s own work.

muscle fibres (Fig. 4). The bursts were spontaneous, not elicited by needle insertion. End plate spikes were found when the needle insertion hit an "active spot" of the muscle (Fig. 5-6). They were most readily found at the end plate zone, but they were not confined to it (Partanen, 1999). In several instances we could simultaneously record two unsynchronous foci of end

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plate spikes at completely different sites of a normal muscle.

**Figure 5.** A sequence of end plate spikes. Observe the gradual slowing of the firing frequency.

Nousiainen, U. (1983), Author´s own work.

**Figure 6.** The firing pattern of a single sequence of end plate spikes (about 10 s, 78 intervals) recorded from the gas‐ trocnemius muscle. Horizontal axis: number of successive intervals. Vertical axis: interpotential interval in ms. Note the variability in interpotential intervals, numerous short intervals and the gradual increase of the mean interval. APCID 176.4 ms, MCD 48.0 ms and the minimum and maximum intervals 9 ms and 292 ms, respectively. From Partanen, J. &

**Figure 3.** Slightly irregular fibrillations with occasional pauses (long intervals); 51 days after muscle biopsy. Note the slight irregularity in the basal (short) intervals. From Partanen, J.V. & Danner, R. (1982), Author´s own work.

**Figure 4.** "Myokymic" fibrillations. Note the doublets and triplets and short rapid bursts of potentials.

"Myokymic" fibrillations were found in chronically partially denervated muscles, polymyo‐ sitis and after chemotherapy. The duration of the bursts was short. There were also single fibrillation potentials, doublets and triplets and independent potentials from several different muscle fibres (Fig. 4). The bursts were spontaneous, not elicited by needle insertion. End plate spikes were found when the needle insertion hit an "active spot" of the muscle (Fig. 5-6). They were most readily found at the end plate zone, but they were not confined to it (Partanen, 1999). In several instances we could simultaneously record two unsynchronous foci of end plate spikes at completely different sites of a normal muscle.

**Figure 5.** A sequence of end plate spikes. Observe the gradual slowing of the firing frequency.

**Figure 3.** Slightly irregular fibrillations with occasional pauses (long intervals); 51 days after muscle biopsy. Note the

slight irregularity in the basal (short) intervals. From Partanen, J.V. & Danner, R. (1982), Author´s own work.

46 Electrodiagnosis in New Frontiers of Clinical Research

**Figure 4.** "Myokymic" fibrillations. Note the doublets and triplets and short rapid bursts of potentials.

"Myokymic" fibrillations were found in chronically partially denervated muscles, polymyo‐ sitis and after chemotherapy. The duration of the bursts was short. There were also single fibrillation potentials, doublets and triplets and independent potentials from several different

**Figure 6.** The firing pattern of a single sequence of end plate spikes (about 10 s, 78 intervals) recorded from the gas‐ trocnemius muscle. Horizontal axis: number of successive intervals. Vertical axis: interpotential interval in ms. Note the variability in interpotential intervals, numerous short intervals and the gradual increase of the mean interval. APCID 176.4 ms, MCD 48.0 ms and the minimum and maximum intervals 9 ms and 292 ms, respectively. From Partanen, J. & Nousiainen, U. (1983), Author´s own work.

We also performed an analysis of the initial positivity on 39 different end plate spikes and 33 different fibrillation potentials. 14 out of the 39 end plate spikes had an initial positive deflection, with the mean duration 0.5 ms, SD 0.17, and range 0.3-0.9 ms. The rest had a negative onset. All 33 fibrillation potentials had an initial positive deflection, with mean duration 1.6 ms, SD 0.4, and range 0.6-2.4 ms. Thus, when an initial positive deflection was observed in end plate spikes, it was significantly (t= 9.9; p<0.001) shorter than that of fibrillation potentials. The 95 % confidence interval for difference was 0.8 to 1.2 ms.

**Random/With pauses Random/Regular With pauses/Regular**

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**Interval** Min NS NS NS Max \*\*\* \*\* NS Mean \*\*\* NS NS APCID \*\*\* \*\*\* \*\*\* MCD \*\*\* \*\*\* \*\*\* Amplitude NS NS NS Spike duration NS NS NS

**Table 2.** The significance of differences between various fibrillation categories. "With pauses": slightly irregular

Denny-Brown & Pennybacker (1938) described the periodic contractions of denervated muscle fibres as true fibrillations and differentiated fibrillation from fasciculations and myokymia. Jasper & Ballem (1949) found positive sharp waves, often in combination with fibrillation potentials and claimed that they may represent local potentials set up at the needle point by the injury. They stated that positive sharp waves do not occur in a normal muscle. Kugelberg & Petersén (1949) also described positive sharp waves, "synchronized activity" in totally denervated muscles as well as fibrillation potentials of both constant frequency (regular fibrillations) and "repetitive fibrillary activity", i.e. slightly irregular fibrillations with pauses (see Results). It was claimed that only rhythmic, regular fibrillation potentials have clinical significance (Stöhr 1977). However, also irregular fibrillations do exist, and in fact there are several types of them. Irregular fibrillations do not usually change their firing pattern during the time of an EMG recording. The incidence of irregular fibrillations reported in literature is very variable. Heckmann & Ludin (1982) pointed out that even in totally denervated muscle irregularly firing potentials may be found (in canine muscle), and Buchthal & Rosenfalck (1966) stated that half of the fibrillation potentials whose discharge pattern was examined appeared irregular. In fact, the period of time after nerve or muscle injury seems to be essential. Approximately half of fibrillation sequences were irregular 30 days after muscle injury, while in more recent injuries the sequences were mainly regular (Partanen & Danner 1982). My experience in clinical ENMG work is that irregular fibrillations are most common 1 – 3 months after axonal injury and in extreme cases only a number of irregular fibrillation sequences may be present with no regular fibrillations at all (unpublished personal observation). There may

\*\*\* p ≤ 0.001, \*\* p ≤ 0.01, \* p ≤ 0.05

fibrillation potentials with pauses.

**4.1. Firing patterns of fibrillation potentials**

**4. Discussion**

Table 1 presents the mean characteristics in different fibrillation potential categories and the significance of difference of the variables compared to end plate spikes. Table 2 presents the differences between variables of different fibrillation categories.


\* p ≤ 0.05 compared to end plate spikes

**Table 1.** Interval, regularity and potential variables of different spontaneous activity categories. "With pauses": slightly irregular fibrillation potentials with pauses.


**Table 2.** The significance of differences between various fibrillation categories. "With pauses": slightly irregular fibrillation potentials with pauses.
