RNS diagnosis of myasthenia and lambert eaton .pptx

repetitive nerve stimulation , diagnosis of myasthenia gravis

1. RNS Dr Priya

2. Definitions Quantum. -A quantum is the amount of Ach packaged in a single vesicle (5000-10000 molecules ) Miniature EPP –Presynaptic terminals spontaneously release Ach quantum Causing MEPP End plate potential -EPP is the potential generated at the postsynaptic membrane following a nerve action potential and neuromuscular transmission

3. Muscle fiber action potential (M F AP) –If EPP exceeds the threshold  CMAP –Sum of MAPs generated by n no of fibers in motor unit  Safety Factor –Amplitude of EPP above the threshold needed to generate MAP

4. Physiology of RNS Ach stores : immediately available (primary) store and secondary (or mobilization) store Primary or immediately available store 1000 quanta-beneath presynaptic nerve terminal membrane. Secondary or mobilization store 10,000quanta-suppliestheprimarystoresafterfewseconds. Tertiary or reserve store . More than 100,000 quanta–in the axon and cell body

5. Low Rate RNS –(<2- 3 Hz) –Progressive decline of Ach Quanta from Primary store  EPP falls in amplitude but –the normal safety factor- remains above the threshold to Generate muscle action potential with each stimulation. After the first few seconds, the secondary mobilization store replaces the depleted quanta --Rapid RNS( 20- 50Hz) It takes 100msec 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+ being pumped out, Causing increased release of Ach quanta

6. EPPs A-Normal B-Postsynaptic slow RNs C-Presynaptic Slow RNS D. –Presynaptic–Rapid RNS

7. Decremental response: The decrement is usually calculated by comparing the lowest CMAP amplitude or area to the baseline CMAP. With 3 Hz stimulation the lowest CMAP is usually the 4 th or 5th

8. Exercise testing or Tetanic Stimulation Maximum Voluntary contraction of Involved muscle for 10 seconds , F/b 3 Hz RNS – Post-exercise Facilitation Maximum voluntary exercise for 1 min Slow RNS at 1,2,3,4 Min – Post-Exercise Exhaustion

9. Postexercise facilitation and exhaustion

10. “Pseudo facilitation ”/Post-exercise facilitation After a brief exercise, EPPs are facilitated. Brief maximal exercise causes the muscle fibers to fire more synchronously. As a result of a faster rise time of all the EPPs, which results in more MFAPs firing simultaneously. Pseudo facilitation results in an increase in CMAP amplitude, but usually with a decrease in CMAP duration and little change in the CMAP area.

11. Pseudofacilitation may cause an increment of up to 40% in normal. Between 40-100% - equivocal >100% - presynaptic disorder

12. Decremental response in RNS Motor neuron disease Some myopathic conditions Myotonic disorders and The metabolic myopathies (e.g., McArdle’s disease) a clinical history and directed neurologic examination, as well as routine nerve conduction studies and needle EMG, must be performed so that any decremental response during RNS can be interpreted correctly.

13. 9. Always perform concentric needle EMG of proximal and distal muscles, especially of clinically weak muscles. Any muscle with denervation or myotonia on needle EMG may demonstrate a decrement on RNS.

14. Nerves used Commonly- median, ulnar, facial, spinal accessory Others – axillary, musculocutaneous, radial ( anconeus)

15. Technical factors- Anticholinesterases withheld 3-4 hrs before study Recording Electrodes placed in Belly Tendon Montage Immobilization of Electrodes and Limb Muscles –1. Deltoid Highest diagnostic yield (78%), Trapezius(65%). 2. ADM is Technically easier and less diagnostic 3. Facial nerve –Nasalis/OrbicularisOculi.( CMAP amplitudes are small. Immobilization difficult.) Stimulation Frequency for Low Rate RNS –2 to 3 Hz Number of Stimulations –Train of 5 to 10 Pulses

16. Myasthenia Gravis Classic Findings 1. Normal CMAP 2. Decremental response at low rate RNS 3. Normal or minimal post- exercise facilitation 4.Normal or decremental response at a High rate RNS 5. Post Exercise or post-tetanic Exhaustion

17. LEMS Classic Findings Low Normal CMAP Decremental Response at Low Rate RNS Post-exercise facilitation High Rate RNS 100 % increment in two muscles or 400% in single muscle.

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