RNST.pptx

RNST case based mcq

1. Repetitive Nerve Stimulation Dr Vaibhav kr Somvanshi SR Neurology GMC Kota

2. 34/F with chronic fatigue and various types of somatic pain is referred for evaluation of myasthenia gravis (MG ). On examination, there is no clear weakness. Her slow repetitive stimulation studies performed in deltoid at baseline and immediately after 10s, 1minute exercise and after 4 minute are shown below. What is the interpretation? Baseline After 10s exercise

3. Just after 1 minute exercise A fter 4 minute There is no significant decrement that is defined as more than a 10% decrease in amplitude. Diagnosis: Normal Study

4. 22/F was referred for fatigue and generalized weakness . Two months ago, she developed mild fatigue after exercising at the gym. Recently, she noted a change in her voice after talking for several minutes. During the last several days, she complained of intermittent double vision and drooping of the left eye late in the day. Her slow repetitive stimulation studies performed in trapezius at baseline and immediately after 10s exercise are shown below . What is the interpretation? The first slow RNST performed at baseline reveals a signficant decrement with 19% reduction in amplitude. Postexercise (only after 10 seconds), there is a repair of the decrement with the maximum decrement less than 10%. These features suggest both significant decrement and repair of the decrement after exercise. Diagnosis: NMJ defect - MG

5. RNS at 50 Hz performed at the hypothenar muscles on stimulation of the ulnar nerve showed incremental response of 58%. Overall findings are suggestive of a pre-synaptic neuromuscular junction disorder  Diagnosis: Lambert-Eaton myasthenic syndrome (LEMS) . 59/F was referred with an 8-month history of weakness with weight loss. The onset of her symptoms was abrupt, with slowly worsening weakness over 2–3 months with numbness of feet, followed by stabilization . She complained of difficulty going up and down stairs and getting out of low chairs. Overall, she felt very fatigued. There was no difficulty swallowing or speaking . Baseline NCV suggest decreased amplitude. RNS done in hypothenar muscle post 10sec exercise suggest-

6. 16/M presented with difficulty in getting up and walking since age of 1year with dropping of both eye lids as the day pass, which improve with pyridostigmine . Her slow repetitive stimulation studies performed in deltoid at baseline and immediately after 10s exercise are shown below. What is the interpretation? Low-frequency RNST of the axillary nerve. ( A) A 50% amplitude decrement is obtained between the first and fifth stimuli ( B) After a 10-second isometric contraction of the deltoid, the amplitude decrement has improved to 13 % s/o partial repair Diagnosis- Postsynaptic congenital myasthenia confirmed by biopsy of the anconeus muscle.

7. Primary store- 1000 Quanta (1Quanta= 10000Ach molecule) Secondary store- 10000 Quanta Tertiary store- 100000 Quanta Approximately 100 ms is required to pump calcium out of the presynaptic terminal. If stimulation occurs again sooner than 100 ms (i.e., stimulation rate >10 Hz ), calcium concentration increases, the probability of release of ACH quanta increases, and more quanta are released. NMJ Physiology

8. Normal Person findings- 3-Hz Repetitive Nerve Stimulation: Stimulus N= Primary store M= released quanta EPP MFAP CMAP 1 1000 200 40 + Normal 2 800 160 32 + No Change 3 640 128 26 + No Change 4 512 102 20 + No Change 5 640 128 26 + No Change

9. 3-Hz Repetitive Nerve Stimulation: Postsynaptic Disorder (Myasthenia Gravis ) Stimulus N= Primary store M= released quanta EPP MFAP CMAP 1 1000 200 20 + Normal 2 800 160 16 + Normal 3 640 128 13 - Decrement 4 512 102 10 - Decrement 5 640 128 13 - Decrement (Repair)

10. 3-Hz Repetitive Nerve Stimulation: Presynaptic Disorder (Lambert-Eaton Myasthenic Syndrome) Stimulus N= Primary store M= released quanta EPP MFAP CMAP 1 1000 20 4 - Low 2 980 19.6 3.9 - Decrement 3 960 19.2 3.8 - Decrement 4 940 18.8 3.7 - Decrement 5 920 19.2 3.8 - Decrement (Repair)

11. RNST technical precautions Temperature Must Be >33 C Immobilization : - limb, electrode, stimulator Acetylcholinesterase Inhibitors Should Be Withheld 4 hours Prior to the Study Nerve Selection Get nerve conduction study done before RNS for patency of nerve and to look for amplitude– if low with normal latency, conduction velocity then directly go for 10s exercise to look for LEMS. Get EMG done to r/o mimickers – MND, Myotonia , polymyositis

12. Clinical Application All patients who come with fatigue, proximal weakness, diplopia, dysphagia should undergo RNS. In diagnosing various NMJ disorders – MG, LEMS, botulism, CMS For every patient – detailed clinical history, directed neurologic examination with nerve conduction studies and needle EMG– must so any decremental response during RNS can be interpreted correctly.

13. Thank You

14. References Clinical Neurophysiology Board Review by Puneet K. Gupta, Pradeep N. Modur , Srikanth Muppidi David Preston, Barbara Shapiro - Electromyography and Neuromuscular Disorders_ Clinical- Electrophysiologic -Ultrasound Correlations.