Low-frequency attenuation of motor unit potentials may reveal abnormal complexity and instability of the motor unit. An ordinary concentric needle electrode is used, so the method bridges the gap between single-fiber and conventional electromyography. MUSCLE 8 NERVE 1:423-426 1978
THE BLANKET PRINCIPLE: A TECHNICAL NOTE J. PAYAN, MRCP (Lond)
T h e cornerstone of clinical electromyography, established in Copenhagen many years ago, is still the study of single, visually separable, voluntary motor unit action potentials. We leave this proven path at our own risk, since promising shortcuts are apt to prove illusory-but we need not be deterred from asking whether the motor unit potential can yield even more information than has already been gained by measurement of its duration, amplitude, and number of phases. When we contemplate a statuary group in silhouette (fig. l ) , we observe prominences formed by heads, shoulders, and other anatomic features according to the way in which the sculptor has chosen to arrange his subjects. A blanket placed over the group will have the effect of smoothing its outline and concealing irregularities (fig. 2). A metaphorical blanket determines the smooth outline of the motor unit potential: low-frequency components, preferentially conducted through muscle tissue from distant fibers belonging to the motor unit, help to fill in the interstices between spikes attributable to fibers lying near the electrode (fig. 3). This blanket can be removed. The familiar forms of motor unit potentials in health and disease are, in a sense, arbitrarily determined in that
Figure 1 . Three old men (provenance unknown).
From the EEG-EMG Departments at Guys Hospital Kings College Hospital, and the Hospital for Sick Children, London, England Acknowledgment It is a pleasure to acknowledge my debt to Professor Fritz Buchthal under whose tutelage I spent two happy and fruitful years in Copenhagen, and to Dr Erik Stblberg, whose generosity knows no bounds Address reprint requests to Dr Payan at the EEG-EMG Department, Kings College Hospital, Denmark Hill, London SE5 9RS, England 0148-639X/0105/0423$00 OO/O 1978 Houghton Mifflin Professional Publishers
The Blanket Principle
Figure 2. A blanket covers the group.
MUSCLE & NERVE
Sept/Oct 1978
423
h
I
\
Figure 3. Below: a normal motor unit potential 12 msec in duration and 300 pV in amplitude. Above: the same potential, with the high-pass filter setting changed from 32 Hz to 3.2 kHz. Ten consecutive sweeps in each case. Gain increased X10 above. The number of spikes has been doubled by the severe attenuation of low frequencies.
they depend not only on the number and arrangement of endplates and the conducting properties of nerve and muscle fiber, but also on consciously selected physical conditions, among the most important of which are the dimensions of the leading-off surface of the electrode and the frequency response of the amplifier. The comprehensive collection of data concerning normal potential duration for which Professor Buchthal's laboratory has been responsible was obtained with a standard commercial concentric needle electrode having a leading-off surface area of 0.07 mm2 and an amplifier with a frequency response of 2 Hz-10 kHz. In the accompanying illustrations, the potentials shown were recorded with the same needle (Disa 13L31 or Medelec MCD50), and the desired degree of low-frequency attenuation was brought about by the introduction of a x 100 multiplier so that the maximal high-pass filter settings normally available on the Medelec AA6 MkII amplifier (16 and 32 Hz) could be instantly switched to 1.6 or 3.2 kHz. The theoretical shortening of potential duration at 16 or 32 Hz (which is, in practice, negligible) was outweighed, for the present purpose, by the convenience of rapid selection. With the high-pass filter set at 3.2 kHz, the recorded spike approaches the derivative of the original signal, the rise time of which determines its amplitude. It then becomes necessary to increase the amplifier gain
424
The Blanket Principle
"up"' Figure 4. Potential from the tibialis anterior muscle of a 40-year-old man with scapuloperoneal dystrophy. From above downward: at low gain; gain increased X5; gain further increased and low frequencies attenuated; gain increased still further. Ten sweeps are superposed in each frame; "stepladder" = 1 msec. The small, unstable component seen at the bottom had been completely concealed.
MUSCLE & NERVE
Sept/Oct 1978
Figure 5. Potential from the deltoid muscle of a 70-yearold woman with chronic polymyositis. Above: the potential has a normal duration and form. Middle: the potential at a faster sweep speed. Bottom: low frequencies attenuated and gain increased. Ten superpositions in each frame. Abnormal complexity and instability are revealed.
Figure 6. Left: potential 200 ,uV and 7 5 msec in duration from the flexor carpi ulnaris of a 16-year-old boy six months after a severe elbow injury. Right: above, the same potential superposed XI0 and recorded at faster sweep speed; below, the potential with low frequencies attenuated. The point on the potential used for triggering the sweep had to be changed. A pronounced increase ;n instability and complexity is revealed.
The Blanket Principle
MUSCLE & NERVE
SepffOct 1978
425
32 HZ-8 KHZ
+ 10 ms
3.2 KHz-8 KHZ
k XI0
+
i
5 ms Figure 7. Potential from the tibialis anterior muscle of a 70-year-old man with a chronic polyneuropathy of unknown cause. Above: at low gain, notching appears on the main positive deflection and, possibly, some small late components that bring the duration up to about 30 msec. Below: left, six consecutive appearances of the potential at faster sweep speed and increased gain, and with low-frequency attenuation; right, the same superposed X10. The notching seen above is shown to be of great significance. and the presence of many small, late, stable components is confirmed.
by a factor of x5 to x 10 to obtain a potential of amplitude comparable to the original, and an increase in oscilloscope-sweep speed to 0.5-1 mseckm must be made to allow one to see clearly the numerous small spikes that are sometimes revealed by low-frequency filtering. When the low-frequency “blanket” has been stripped away, poientials of seemingly acceptable form may show unsuspected complexity and instability not encountered in the normal muscle (figs. 4-7); no more should be claimed for the method. It is emphatically not single-fiber electromyography, despite the suggestive appearance of certain potentials (e.g., fig. 7). T h e relatively low
426
The Blanket Principle
resolving power of the larger leading-off surface means that the spikes seen are rarely true single-fiber potentials but are the result of summation; fiber density cannot be measured, and the amount ofjitter may be considerably underestimated (see E Sdlberg, J V Trontelj: Single Fibre Electromyography). This method lacks the precision of single-fiber EMG but, if used in the course of an ordinary electromyographic investigation, may tell us something we would not otherwise have appreciated about the structure and function of the motor unit at no additional discomfort to the patient. It may, perhaps, be looked upon as a bridge between two complementary disciplines.
MUSCLE & NERVE
SepVOct 1978
THE BLANKET PRINCIPLE: A TECHNICAL NOTE J. PAYAN, MRCP (Lond)
T h e cornerstone of clinical electromyography, established in Copenhagen many years ago, is still the study of single, visually separable, voluntary motor unit action potentials. We leave this proven path at our own risk, since promising shortcuts are apt to prove illusory-but we need not be deterred from asking whether the motor unit potential can yield even more information than has already been gained by measurement of its duration, amplitude, and number of phases. When we contemplate a statuary group in silhouette (fig. l ) , we observe prominences formed by heads, shoulders, and other anatomic features according to the way in which the sculptor has chosen to arrange his subjects. A blanket placed over the group will have the effect of smoothing its outline and concealing irregularities (fig. 2). A metaphorical blanket determines the smooth outline of the motor unit potential: low-frequency components, preferentially conducted through muscle tissue from distant fibers belonging to the motor unit, help to fill in the interstices between spikes attributable to fibers lying near the electrode (fig. 3). This blanket can be removed. The familiar forms of motor unit potentials in health and disease are, in a sense, arbitrarily determined in that
Figure 1 . Three old men (provenance unknown).
From the EEG-EMG Departments at Guys Hospital Kings College Hospital, and the Hospital for Sick Children, London, England Acknowledgment It is a pleasure to acknowledge my debt to Professor Fritz Buchthal under whose tutelage I spent two happy and fruitful years in Copenhagen, and to Dr Erik Stblberg, whose generosity knows no bounds Address reprint requests to Dr Payan at the EEG-EMG Department, Kings College Hospital, Denmark Hill, London SE5 9RS, England 0148-639X/0105/0423$00 OO/O 1978 Houghton Mifflin Professional Publishers
The Blanket Principle
Figure 2. A blanket covers the group.
MUSCLE & NERVE
Sept/Oct 1978
423
h
I
\
Figure 3. Below: a normal motor unit potential 12 msec in duration and 300 pV in amplitude. Above: the same potential, with the high-pass filter setting changed from 32 Hz to 3.2 kHz. Ten consecutive sweeps in each case. Gain increased X10 above. The number of spikes has been doubled by the severe attenuation of low frequencies.
they depend not only on the number and arrangement of endplates and the conducting properties of nerve and muscle fiber, but also on consciously selected physical conditions, among the most important of which are the dimensions of the leading-off surface of the electrode and the frequency response of the amplifier. The comprehensive collection of data concerning normal potential duration for which Professor Buchthal's laboratory has been responsible was obtained with a standard commercial concentric needle electrode having a leading-off surface area of 0.07 mm2 and an amplifier with a frequency response of 2 Hz-10 kHz. In the accompanying illustrations, the potentials shown were recorded with the same needle (Disa 13L31 or Medelec MCD50), and the desired degree of low-frequency attenuation was brought about by the introduction of a x 100 multiplier so that the maximal high-pass filter settings normally available on the Medelec AA6 MkII amplifier (16 and 32 Hz) could be instantly switched to 1.6 or 3.2 kHz. The theoretical shortening of potential duration at 16 or 32 Hz (which is, in practice, negligible) was outweighed, for the present purpose, by the convenience of rapid selection. With the high-pass filter set at 3.2 kHz, the recorded spike approaches the derivative of the original signal, the rise time of which determines its amplitude. It then becomes necessary to increase the amplifier gain
424
The Blanket Principle
"up"' Figure 4. Potential from the tibialis anterior muscle of a 40-year-old man with scapuloperoneal dystrophy. From above downward: at low gain; gain increased X5; gain further increased and low frequencies attenuated; gain increased still further. Ten sweeps are superposed in each frame; "stepladder" = 1 msec. The small, unstable component seen at the bottom had been completely concealed.
MUSCLE & NERVE
Sept/Oct 1978
Figure 5. Potential from the deltoid muscle of a 70-yearold woman with chronic polymyositis. Above: the potential has a normal duration and form. Middle: the potential at a faster sweep speed. Bottom: low frequencies attenuated and gain increased. Ten superpositions in each frame. Abnormal complexity and instability are revealed.
Figure 6. Left: potential 200 ,uV and 7 5 msec in duration from the flexor carpi ulnaris of a 16-year-old boy six months after a severe elbow injury. Right: above, the same potential superposed XI0 and recorded at faster sweep speed; below, the potential with low frequencies attenuated. The point on the potential used for triggering the sweep had to be changed. A pronounced increase ;n instability and complexity is revealed.
The Blanket Principle
MUSCLE & NERVE
SepffOct 1978
425
32 HZ-8 KHZ
+ 10 ms
3.2 KHz-8 KHZ
k XI0
+
i
5 ms Figure 7. Potential from the tibialis anterior muscle of a 70-year-old man with a chronic polyneuropathy of unknown cause. Above: at low gain, notching appears on the main positive deflection and, possibly, some small late components that bring the duration up to about 30 msec. Below: left, six consecutive appearances of the potential at faster sweep speed and increased gain, and with low-frequency attenuation; right, the same superposed X10. The notching seen above is shown to be of great significance. and the presence of many small, late, stable components is confirmed.
by a factor of x5 to x 10 to obtain a potential of amplitude comparable to the original, and an increase in oscilloscope-sweep speed to 0.5-1 mseckm must be made to allow one to see clearly the numerous small spikes that are sometimes revealed by low-frequency filtering. When the low-frequency “blanket” has been stripped away, poientials of seemingly acceptable form may show unsuspected complexity and instability not encountered in the normal muscle (figs. 4-7); no more should be claimed for the method. It is emphatically not single-fiber electromyography, despite the suggestive appearance of certain potentials (e.g., fig. 7). T h e relatively low
426
The Blanket Principle
resolving power of the larger leading-off surface means that the spikes seen are rarely true single-fiber potentials but are the result of summation; fiber density cannot be measured, and the amount ofjitter may be considerably underestimated (see E Sdlberg, J V Trontelj: Single Fibre Electromyography). This method lacks the precision of single-fiber EMG but, if used in the course of an ordinary electromyographic investigation, may tell us something we would not otherwise have appreciated about the structure and function of the motor unit at no additional discomfort to the patient. It may, perhaps, be looked upon as a bridge between two complementary disciplines.
MUSCLE & NERVE
SepVOct 1978
