S d curve

DR.SANYAL KUMAR
JR
DEPT OF PMR

 Strength duration curve is a graph between electrical stimuli of
different intensities and recording the time needed by each
stimulus to start the response.
 S-D curve should be plotted after 20th day of injury/lesion.
 After 21st/22nd day, regeneration of nerve will start, generally it
take about 270 days to regenerate.
 The purpose of S-D curve plotting is to know whether the
stimulated muscle is innervated, denervated or partially
denervated.
 There are also other method for this purpose like EMG and
NCV.

 The apparatus used for plotting S-D Curve supplies rectangular
impulses of different duration.
 Impulse with duration of 0.01, 0.03, 0.1, 0.3, 10, 30, 100, 300 ms
are required.
 The stimulator may be of either the constant current or constant
voltage type.
 The constant current stimulator was thought to produce the
more accurate result but constant voltage stimulator is rather
more comfortable for patient.

 The patient must be warm, fully supported and in sufficient light.
 The indifferent electrode may be applied over some convenient area usually on
the midline of the body or the origin of the muscle group.
 Active electrode placed over fleshy part of muscle. (Sometime two small
electrode may be used, one over each end of muscle belly).
 Current is applied using the longest stimulation first and increased until a
minimal contraction is obtained.
 Intensity of current (or voltage) is noted and impulse is shortened. This
procedure is repeated with each stimulation in turn, the intensity of current
being increased as required.
 A minimal contraction is used, as this make it easy to detect any change in
strength, and electrode should be placed in same point over the muscle
throughout the test.
 The S-D Curve is plotted from the result of the test, and although it will be
further to the left with constant voltage than with the constant current
stimulator.
 The shape of the curve is the essential feature.

 The S-D Curve is of this typical shape because the impulses of
longer duration all produce a response with same strength of
stimulus, irrespective of their duration, while those of shorter
duration, require an increase in the strength of the stimulus each
time the duration is reduced.
 The point at which the curve begin to rise is variable, but is
usually at a duration of impulse of 1 ms with constant current
and 0.1 ms with constant voltage stimulator.

 S-D Curve of complete denervation is when duration of impulse
is 100 ms or less, the strength of the stimulus must be increased
each time the duration the duration is reduced and no response
is obtained to the impulse of very short duration.
 So the curve rises steeply and is further to the right than of
normally innervated muscle.

 S-D Curve of partial denervation is the impulses of longer duration can
stimulate both innervated and denervated muscle fibers, so a contraction is
obtained with a stimulus of low intensity.
 As impulse are shortened, the denervation fibers responds less readily, a
stronger stimulus is required to produce a perceptible contraction and the
curve rises steeply like that of denervated muscle.
 With the impulses of shorter durations, the innervated fibers responds to a
weaker stimulus than that required for the denervated fibers.
 Kink in S-D Curve is seen at the point where two section meet.
 The shape of curve indicates the proportion of denervation.
 A kink appears in the curve and as reinnervation progresses.
 Progressive denervation is indicated by the appearance of a kink, increase
in the slope and shift of the curve to the right.

 When stimulus is given using the maximum pulse width
available on the stimulator, the intensity of the current required
to produce a twitch is called rheobase of the muscle.
 Mainly 100 to 300 ms duration are used to record rheobase.
 The pulse is always rectangular measured in miliamperes or
volts.
 Rheobase is measured using the cathode on the motor point of
the nerve or by using bipolar technique.
 Normal values of rheobase are 2 to 18 mA or 5 to 35 volts.

Deltoid 14 volts, 5mA
Triceps 18 volts, 5mA
Abductor digiti minimi 30volts, 8mA
Frontalis 14volts,4mA

 Resistance of skin and subcutaneous tissue
 Edema and inflamation
 Ischemia and underlying pain
 Temperature variation
 Position of electrode
 Amount of subcutaneous tissue
 Degeneration
 Deneravtion
 Partial denervation generally produce no changes in rheobase.
 Re-innervation can show a sharp rise in rheobase which herald
clinical recovery.

 At the double intensity of rheobase, the minimal pulse width
required to produce the twitch is called chronaxie of muscle.
 Chronaxie is an index of excitability and is time in millisecond,
necessary to induce minimal visible contraction with a stimulus
of twice the strength of the rheobase.
 Normal values of chronaxie are less than 1ms (0.05 to 0.5 ms).
 Variation in chronaxie depending on whether a constant current
machine or a constant voltage machine is used.
 At birth chronaxie is 10 times higher than normal and 18 to 20th
month, the chronaxie falls to normal values.

Muscle Constant voltage Constant current
Deltoid 0.01ms 0.1ms
Abductor digiti minimi 0.04ms 0.2ms
Tibialis anterior 0.04ms 0.1ms

 Texture of skin
 Ischemia
 Oedema
 Fatigue
 Position of stimulating electrode
 Denervation
 Partial denervation
 Re-inervation
 Nerve root lesion
 Peripheral neuropathy
 Myopathy (No significant change)

 Low frequency generator with varying pulses from 0.02 to
1000ms.
 Moist saline pad
 Electrodes
 Leads
 Bandage
 Plastic protactors

 Accommodation is the property of nerve or muscle membrane to
react less strongly to a slowly increasing current intensity by
accommodating the electrical impulse.
 Measure of the constant of accommodation is lambda.
 Accommodability are calculated from ratio of rectangular wave
rheobase and the value of the progressive current rheobase.

Normal 3 to 6
Denervated Below 3
No accommodation 1 or below

 When repeated stimulation are given to a muscle after sometime
it accommodates. If the muscle is stimulated by triangular
impulses and rectangular pulses at different time frames, the
triangular impulse requires more intensity to produce
contraction of the same strength as that of rectangular ones. The
intensity of current used by triangular impulse divided by
rectangular is called A.Q.
 A.Q. values show you the state of innervation, denervation, and
partial innervation.

Value of A.Q Outcome
1 Dysfunction of muscle persists for more than 6 months then there is difficult to
regenerate the nerve by electrotherapy, and no further plotting required.
2-4 The nerve has degenerated but it will respond to electrotherapy.
4-6 The neuromuscular system is unimpaired and it will respond to electrotherapy and
muscle get back to almost within 3 weeks.

 It is simple, reliable and cheaper.
 Indicate proportion of denervation.
 Less time consuming.

 In large muscles, only proportion of fibers may respond hence
picture is not clearly shown.
 It’s a qualitative rather than quantitative method of testing
innervation.
 It won’t point out the site of lesion.

S d curve

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S d curve