Reason: decrease functioning of the Acetylcolineasterase when cold, making more ACH available to bind at the ACHRs. Clinically patients with MG note worsening of their symptoms in warm weather, perhaps because the acetyl cholinesterase is more active. RNS should be done with temperature of at least 33 degree centigrade.
Introduction to RNS
Normal neuromuscular junction physiology.
Lambert Eaton Myasthenic Syndrome (LEMS)
Repetitive nerve stimulation (Types)
Technical Factors that affect the RNS.
Calculations of Decremental and incremental
Protocol of RNS in EMG lab
Protocol for evaluating NMJ disorder.
INTRODUCTION TO RNS
Also called Jolly test described first by Dr
Friedrichson Jolly in 1895.
RNS has been validated as one of most useful
electrophysiological tests in the evaluation of the:
Patients with suspected Neuromuscular Junction
Disorder which includes:
Myasthenia Gravis Syndrome.
Lambert Eaton Myasthenic Syndrome.
CLINICAL PRESENTATION INCLUDES:
Proximal muscle weakness.
All of these are suggestive of possible NMJ
ANATOMY ANDPHYSIOLOGY OF
The NMJ essentially forms an
electrical-chemical- electrical link between the nerve and
The chemical neurotransmitter at the NMJ is Acetylcholine
ACH molecules are packaged as vesicles in the presynaptic
terminal in discrete units known as Quanta (Bags of
The quanta are located in three separate stores:
Primary or immediately available store
1000 quanta- beneath presynaptic nerve terminal membrane.
Secondary or mobilization store
10,000 quanta- supplies the primary stores after few seconds.
Tertiary or reserve store.
More than 10,000 quanta –in the axon and cell body
When an nerve action potential invades and depolarizes the
presynaptic junction, voltage dependent calcium channels are
activated, allowing an influx of calcium.
Results in release of ACH from the presynaptic terminals
The greater the calcium inside the greater the more quanta (ACH) are
ACH then diffuses across the synaptic cleft and binds to ACH
receptors (ACHRs) on the post synaptic membrane
In the post synaptic membrane – numerous junction are found with
ACH dependent gated channels and receptors.
Thus the binding of ACH to ACHRs clustered opens ion channels…
resulting in a local depolarization, the End Plate Potential (EPP).
The size of EPP depends on the amount of ACH that binds to the
During repetitive nerve stimulation in normal subjects,
ACH quanta are progressively depleted from the
primary store and fewer quanta are released with each
The corresponding EPP falls in amplitude but b/c of
normal safety factor it remains above the threshold to
ensure generation of a muscle action potential with
After few seconds(1-2sec) the secondary store begins to
replace the depleted quanta with a subsequent rise in
PHYSIOLOGY OF RAPIDRNS
In rapid RNS (10-50 Hz), depletion of quanta from the presynaptic
terminals is counterbalanced not only by the mobilization or
secondary stores but also by accumulation of calcium.
Normally it takes 100 msec for ca2+ to diffuse back out of the
presynaptic terminals. If RNS is rapid enough so that new ca2+
influx occurs before previously infused ca2+ had diffused back
out, ca2+ continues to accumulate in the presynaptic terminals,
causing an increased release of quanta.
This combination of factors usually leads to an increased number of
quanta released and a corresponding higher EPP. However, the result
in normal subject is same ,i.e the generation of a muscle action
The effect of rapid and slow RNS is the same to generate the Muscle
action potential in normal subjects.
In Pathological Conditions:
Where the safety factors is reduced, i.e the baseline EPP is reduced
but still above the threshold) the slow RNS will cause depletion of
quanta and may drop the EPP below threshold, resulting on the
absence of muscle action potential.
In pathological conditions, where the baseline EPP is below the
threshold and a muscle action potential is not generated, rapid RNS
may increase the number of quanta released, resulting in a larger EPP
so that threshold is reached.
This is the concept of increment with rapid RNS that are seen in
neuromuscular Junction Disorders.
Physiology of Myasthenia
In patients with MG, the number of ACHRs is
reduced, lowering the safety factor. During
RNS, some EPPs may not reach threshold and
no action potential is generated. This results in
the decrement in the amplitude of the CMAP
and basis for the Decremental response in
Repetitive Nerve Stimulation in Myasthenia
The decrement is usually calculated by comparing
the lowest CMAP amplitude or area to the baseline
(lowest CMAP divided by baseline CMAP).
With 3 Hz stimulation the lowest CMAP is usually
safety factor =15, n=quantas available, m= quantas released.
Stimulus n m EPP MFAP CMAP
1 1000 200 20 + Normal
2 800 160 16 + Normal
3 640 128 13 - Dec
4 512 102 10 - Dec
5 640 128 13 - dec
Stimulus n m EPP MFA
1 1000 200 20 + Normal
2 800 160 16 + Normal
3 640 128 13 - Dec
4 512 102 10 - Dec
5 640 128 13 - dec
LAMBERT-EATON MYASTHENIC SYNDROME (LEMS)
LEMS is a disorder of NMJ transmission characterized by
reduced release of ACH from the presynaptic terminal.
There is now clear evidence that this disorder, like MG, is an
immune-mediated disorder. The pathogenesis of
LEMS is fairly well understood and involves the production of IgG
antibodies directed at the presynaptic voltage gated calcium
channel (VGCC). These antibodies interfere with the calcium-
dependent release of ACH
quanta from the presyn
aptic membrane and subsequently
cause a reduced endplate potential on the postsynaptic
membrane, resulting in NMJ transmission failure.
LEMS is quite rare. It affects adults, generally those
older than 20 years and usually older than 40
years, of whom 70% are male and 30% are female.
Clinically, these patients present with proximal
muscle weakness (especially the lower extremities)
The distinctive clinical finding is that of muscle
After a brief period (10 seconds) of intense exercise
of a muscle, the power and the deep tendon reflex
to that muscle are transiently increased.
The increment is usually calculated by comparing the highest CMAP
amplitude orarea with the baseline CMAP
(highest CMAPdivided by baseline CMAP).
Why we take 4th
In patients with MG, this decremental response usually has a
maximumdecrement at the fourth orfifth response and then a
tendency toward repair, by reaching of the next stimulus.
PROTCOL FORRNS IN EMG
I. Record one uppermotorNCS
II. Record one uppersensory NCS
III. Record RNS frommotornerves at 3 Hz. If trapezius
response is not satisfactory, record fromthe deltoid.
IV. EMG of proximal Muscle is performed, if RNS study is
RNS can be performed by using any of the motornerve. The
most commonly used are:
Facial (Orbicularis Oculi)
Slow Repetitive Nerve Stimulation (RNS) is performed in following sequence
One Distal and one proximal motor nerves(preferable most involved muscles)
One Sensory nerve
Resting or base line trace 6 trains at-least (10 trains are preferred)
Post 10 second exercise 6 trains
Post 1 minute exercise 6 trains
1 minute post 1 minute exercise 6 trains
2 minute post 1 minute exercise 6 trains
3 minute post 1 minute exercise 6 trains
4 minute post 1 minute exercise (optional) 6 trains
If decrement is noted, perform Post 10 second exercise stimulation 6 trains, for
In Myasthenia gravis persistent Decremental Response > 10% is abnormal. The
maximum Decremental response is noted 2 or 3 minute post 1 minute exercise.
If patient is unable to perform exercise, fast RNS at 30Hz or 50Hz may be used.
Exercise play an important role in the
electromyography evaluation of patients with
suspected NMJ disorders.
Brief maximal voluntary exercise can be used
instead of rapid RNS in cooperative patients.
Exercise testing has distinct advantage of
being painless,where as rapid RNS is quite
painful and often difficult to tolerate.
POST EXERCISE EFFECT
In normal subjects with a normal safety factor slow
RNS is performed immediately after exercise and then
1,2,3,4 minutes later, the EPP never falls below the
threshold and the CMAP and area will remain the
In patients with impaired NMJ transmission, however,
the decrement in CMAP amplitude and area in
response to slow RNS becomes more marked in 2-4
minutes after the exercise.
Any decrement of more than 10% is defined as
abnormal. Normal subjects should have no
decrement. The 10% cut off allows for
inherent technical factor that are often
encountered. However any reproducible
decremental response of more than 10% is
TECHNICAL FACTORS THAT
Immobilization of Electrodes position.
Administration of ACh inhibitors . (use of
Number of Stimulations.
Lower temperature increases the amplitude of the
CMAPs. Patients with MG may report clinically
significant improvement in cold temperatures. Thus
ice bag test can be very helpful in MG.
Typically they report worsening of ptosis in bright
sunlight or on a warm day. Therefore maintaining a
constant and perhaps higher-than-ambient
temperature during RNS testing is important to bring
out abnormalities of NMJ function. Temperature of
skin overlying the tested muscle should be at least
It is best to advise patients to refrain from
taking acetylcolineasterase inhibitors
(e.g., Pyridostigmine Mestinon) for 6-8 hours
before the study, unless medically
contraindicated. These agents make more
ACH available to bind at the ACHRs and may
potentially diminished CMAP decrement,
resulting in a normal study.
If the electrode is immobilized ---- result is the change in the
CMAP amplitude which lead to misinterpretation.
So the recording electrodes should be secured well with tape.
Stimulator secured with Velcro Strap
Entire Hand with pad or board.
The Goal is to minimize the movement of the limb.
Sub maximal stimulation can give art factual decrement or Increment
in the CMAP amplitude.
So always Check to ensure that the stimulus is supra maximal before
beginning of RNS
The optimal frequency for RNS is 2 or 3Hz
RNS must be kept low to avoid accumulation of calcium at
A train of 5-10 impulses is preferable for slow RNS. The
number should be kept minimum for the patients comfort.
As when the mobilization or secondary stores begins to re supply
the immediately available store , the decrement begins to
improve. The result is so called U Shaped Decrement. Which is
a characteristic of NMJdisorders.
Warm the extremity (33 degree centigrade)
Immobilize the muscle as best as possible
Perform Routine NCS first to ensure that the
nerve is normal
Perform RNS at rest. After making sure that the
stimulus is supramaximal, perform at 3 Hz
RNS, normally there is a less than 10%
decrement b/w the first and the fourth
If more than 10% decrement occurs and is consistently
Has patient perform maximal voluntary exercise.
Immediately repeat 3 Hz RNS post exercise
If a less than 10% decrement or no decrement:
Has patient perform maximal voluntary exercise for 1 min
and perform 3 Hz RNS immediately and at 1,2,3 and 4
If a significant decrement occurs after 1 min exercise (post
exercise Exhaustion), have patient perform maximal
voluntary exercise again for 10 sec and immediately repeat
RNS at 3 Hz to demonstrate repair of the decrement.
Perform RNS on one distal and one proximal
muscles especially the weak muscles.
If no decrement is found with a proximal limb
muscle, a facial muscle can be tested.
If the compound muscle action potential is low at
baseline, have patient perform 10 sec exercise, then
stimulate the nerve supramaximally immediately
post exercise, looking for an abnormal increment
response ( greater than 140% of the baseline). If the
patient cannot exercises, rapid RNS should be