REPETITIVE NERVE STIMULATION (RNS)

RNS, MYASTHENIA GRAVIS, LEMS

1. Repetitive Nerve Stimulation

2. •Physiology of Neuromuscular junction
•Procedure & technical aspects
•Interpretation
•Application in various conditions

3. ANATOMY AND PHYSIOLOGY OF
NEUROMUSCULAR JUNCTION
The NMJ essentially forms an
electrical-chemical- electrical link between the nerve and
muscle.
The chemical neurotransmitter at the NMJ is Acetylcholine
(ACH).
ACH molecules are packaged as vesicles in the presynaptic
terminal in discrete units known as Quanta (Bags of
neurotransmitter (ACH).)

4. definitions
• Quantum. A quantum is the amount of Ach
packaged in a single vesicle.
• Each quantum (vesicle) 1 mV change of
postsynaptic membrane potential.
•The number of quanta released after a nerve
action potential depends on the number of
quanta in the immediately available (primary)
store and calcium stores
•Normally 50-300(60) vesicles

5. 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

6. Calcium and quanta dynamics
•calcium :diffuses slowly out of the presynaptic
terminal in 100–200 msec.
• Ach stores: immediately available (primary)
store and secondary (or mobilization) store
•rapid RNS (more than every 100 msec, or
stimulation rate >10 Hz), calcium influx is
greatly enhanced and the probability of
release of Ach quanta increases.

7. PHYSIOLOGY
•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
released.
• 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
ACHRs.

10. Myasthenia gravis

11. definitions
•End plate potential -EPP is the potential
generated at the postsynaptic membrane
following a nerve action potential and
neuromuscular transmission.
•60 mV change in the amplitude of the
membrane potential.
• Safety factor: The safety factor of
neuromuscular transmission is simply defined
as the difference between the EPP and the
threshold potential for initiating an action
potential.

12. RNS EFFECT
During repetitive nerve stimulation in normal
subjects, ACH quanta are progressively
depleted from the primary store and fewer
quanta are released with each stimulation.
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 each stimulation.
After few seconds(1-2sec) the secondary store
begins to replace the depleted quanta with a
subsequent rise in the EPP.

13. PHYSIOLOGY OF RAPID RNS
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
potential.

14. SLOW AND RAPID RNS
•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.
•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.

15. Potentiation
• voluntary activation or high frequency
stimulation
•CMAP amp increases
•Facilitation:-recruitment
•Pseudo facilitation:-synchronisation of muscle
activity

16. Relationship -EPP,AP,CMAP

17. Neuromuscular junction disorders
•Post synaptic
•Myasthenia gravis
•Organophosphorus poisoning
•Curare induced paralysis
•Congenital Myasthenic syndromes
•Presynaptic
•Botulism
•LEMS
•Magnesium induced paralysis
•Combined defect
•Gallamine, amino glycoside antibiotics,
quinine, suxamethonium.

18. Repetitive nerve stimulation
( RNS)
•Jolly in 1895 first described progressive
reduction in visible muscle contraction in MG
(Myaesthenic reaction)
•Harvey and Masland(1941) reported electrical
decremental muscle response on repetitive
motor nerve stimulation.
•Ekstedt in 1964 described SFEMG

19. RNS- technique
•RNS is technically demanding procedure.
•Poor electrode placement, sub maximal
stimulation, movement artifacts, causes false
positive results
•Minimise artifacts
•Immobilisation

20. RNS-technique
•RNS is performed on selected motor nerves
with recording by surface electrodes.
•G1-motor point,G2-tendon
•Supramaximal stimulus
•Initial sharp negative deflection

21. Muscle selection
•Clinically weak muscles should be selected.
•Usually facial and proximal limb muscles shows
greater abnormality than distal muscles.
•Nerves involved in other diseases should be
avoided.
•Cholinesterase inhibitors should be stopped 12-
24 hrs before.

22. RNS PROTOCOL
•Slow Repetitive Nerve Stimulation (RNS) is performed in following sequence
• One Distal and one proximal motor nerves(preferable most involved muscles)
•One Sensory nerve
•RNS protocol
–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 facilitation
•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.

23. Protocol For Evaluating Disorder Of NMJ
•Warm the extremity (33 degree centigrade)
•Immobilize the muscle as best as possible
•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
response.

24. Cont. Protocol
If more than 10% decrement occurs and is
consistently reproducible:
Have patient perform maximal voluntary exercise &
immediately repeat 3 Hz RNS post exercise.
If a less than 10% decrement or no decrement:
Have patient perform maximal voluntary exercise for
1 min & perform 3 Hz RNS immediately and at
1,2,3 and 4 mins.
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.

25. Cont. Protocol
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 used.

28. Slow RNS
• supra maximal CMAP
• 3–5 stimuli to a mixed or motor nerve at a rate
of 2–3 Hz.
•slow enough to prevent calcium accumulation,
high enough to deplete the quanta
•maximal decrease in Ach release occur after the
first four stimuli
•reproducible decrement
•exercises for 10 seconds to demonstrate repair of
the decrement (‘‘post-exercise facilitation’’)
•If no decrement occurs -1 minute max voluntary
exercise –”post exercise exhaustion”

29. Slow RNS

30. Rapid RNS
•optimal frequency is 20–50 Hz,for 2–10
seconds
•brief (10-second) period of maximal voluntary
isometric exercise has the same effect as rapid
RNS
•Depletion of quanta vs calcium accumulation
•Incremental response in LEMS

31. Rapid RNS

32. Patterns of response to slow RNS

33. RNS in pre and post synaptic disorders
ParameterParameter Pre-synapticPre-synaptic Post-synapticPost-synaptic
CMAP amplitudeCMAP amplitude Small Small Normal Normal
Low rate RNSLow rate RNS
 RestingResting
 Post exercise Post exercise
facilitationfacilitation
 Post exercise Post exercise
exhaustionexhaustion
Decrement Decrement
Present Present
AbsentAbsent
DecrementDecrement
PresentPresent
Present Present
High rate RNSHigh rate RNS Increment Increment Decrement or Decrement or
normalnormal

34. Electrophysiological investigation
•Nerve conduction studies-usually normal
(low CMAP in LEMS)
•Concentric needle EMG-usually normal
•Repetitive nerve stimulation
•Single fiber EMG

35. RNS in MYASTHENIA GRAVIS
•Most commonly used test, easy.
•RNS is relatively insensitive,10-50% in ocular
myastenia,75% in generalised MG
•RNS is relatively specific(90%)
•SFEMG is Most sensitive.(90% in ocular,95% in
MG)
•Normal baseline CMAP
•Greater than 10% decremental response at rest
and post exercise
•No role for high frequency stimulation

37. Baseline and 10sec exercise

38. 60sec exercise-exhaustion

39. Congenital Myasthenic Syndromes
•Newborns of non-Myasthenic mothers.
•No Ach R antibodies.
•Respiratory distress, feeding difficulty, Ptosis
are common.
•Decremental response on 2 Hz RNS, abnormal
SF-EMG.
•End plate acetyl cholinesterase deficiency and
slow channel syndrome , a repetitive CMAP is
elicited by a single supramaximal stimulus.

40. Repetitive CMAP

41. Lambert Eaton Myasthenic Syndrome (LEMS )
•Weakness and fatigability of proximal
muscles.
•Relative sparing of EOM, bulbar muscles.
•Hyporeflexia
•Dry mouth
•Associated with SCC lung
•Antibodies against VGCC (voltage gated
calcium channel)

42. LEMS

43. LEMS
•Distal muscles RNS preferred
•3 pattern recognized
•Low normal CMAP amplitude, decremental
response at low rate RNS, normal at high rate.
•Low CMAP amplitude, decremental response
at low rate, and incremental response at high
rate RNS (>100%)—classical triad.
•Low CMAP amplitude, decremental low rate
RNS, initial decrement at high rate RNS.

44. Incrementing response
after brief exercise (10-15
sec) in LEMS. Increment
is 10-fold, with CMAP of
3.2 mV.
CP CMAP amplitude is
0.35 mV (normal >1
mV).

45. Incremental response in LEMS

46. 50hz RNS-increments

47. Botulism
•Defective release of Ach from nerve terminals.
•It cleaves synaptic vesicle protein.
•Extra ocular and bulbar weakness  limb and
respiratory weakness.
•Blurred vision, dilated pupil, constipation,
urinary retention.
•Electro physiologically resemble LEMS

48. Botulism
•Reduced CMAP in at least two muscles
•At least 20 percent CMAP amplitude
facilitation on tetanic stimulation
•Persistance of facilitation atleast 2 minutes
after activation
•No postactivation exhaustion