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The Evaluation of Weakness in the Electromyography Lab
Anthony Chiodo, MD, MBAUniversity of Michigan Health SystemAAPMR Meeting, San Diego
38 year old right handed woman comes in with a chief complaint of weakness
Noticed it first in her inability to complete her usual circuit training program over the last month
Subsequently affected her aerobic exercise program and day to day home management activities
Case Study
She does not think there has been any numbness or tingling
She has not noticed it affecting one area of the body first but is uncertain
She has had no new pain, no fever or chills, no change in bowel or bladder control
Past medical history is remarkable for hypothyroidism for which she takes synthroid
Family history is significant for a maternal grandmother with rheumatoid arthritis, paternal grandparents with diabetes mellitus and heart disease, father with hypertension, mother with hypothyroidism
Deep tendon reflexes are 1+ Pin sensation and light touch is
symmetric Strength testing shows shoulder and hip
girdle muscles in the 3-4 range, hamstrings 3, elbows 4, hands and feet 4+ to 5
There is no tenderness and no pain with ROM
Physical Examination
Motor disorder Defect affecting the relationship between
the distal motor axon and muscle Defect can be pre-synaptic, synaptic, or
post-synaptic Acquired disorders are pre- or post-
synaptic
NMJ Disorders: Nature of the Abnormality
Post-synaptic: Normal Pre-synaptic: inability to achieve
transmission: decreased motor evoked amplitudes but normal latency and conduction
Motor Nerve Conduction Studies
Pre-synaptic: if no motor units are seen with activation, fibrillation potentials and positive waves are possible Botulism Otherwise, decreased amplitude motor units
of varying amplitude and rapid recruitment Post-synaptic: varying amplitude motor
units with rapid recruitment If myasthenia gravis: may see proximal
fibrillation potentials and positive waves
Routine Needle Examination
Molecules = Quanta X # Released(p) Typical Quanta = several thousand
acetylcholine molecules, generate MEPP, amplitude 1 mV
P typically around 60 quanta released per nerve stimulus
Muscle action potentials at 7-20 mV
Presynaptic Acetylcholine Stores
Reserve: 300,000 quanta Mobilization: 10,000 quanta Immediate Release: 1,000 quanta
Types of Acetylcholine Stores
Decreased quanta released per nerve stimulation
Synaptic vesicle fusion time(Ca++ dependent) is 100-200 msec
Maximal decrement at 2-3 Hz No decrement at 10 Hz Normal decrement < 8%
Concept of Decrement
1st stimulus: 60 quanta 2nd stimulus: 56 quanta 3rd stimulus: 53 quanta 4th stimulus: 55 quanta as reserve
quanta become available Accounts for decrease in EPP and results
in increase risk of failure (blocking) over four repetitive stimulations at 2 Hz
At 2 Hz Repetitive Stimulation
Other Disorders NMJ Transmission Myotonia Neurogenic Disorders with
Denervation/Reinnervation Rapidly Progressive ALS Polyneuropathy Mononeuropathy Radiculopathy
What Other Disorders Have a Decrement To Low Frequency Repetitive Nerve Stimulation?
Yes, temperature Increased decrement and blocking at
increased temperature due to increased acetylcholinesterase activity
May account for the fact that the effect is more pronounced in proximal muscles
Are there any physiological parameters that effect this finding?
Acetylcholine Receptor Antibodies Normal number of MEPP’s MEPP amplitude decreased by 80% Post-activation facilitation Post-activation exhaustion
Myasthenia Gravis
Increased calcium in endplate increases the quanta released
Decrement decreased, small increase in motor evoked amplitude
Post-activation Facilitation
Depletion of mobilization and immediate release stores, before reserve store becomes available
Decreased receptor excitability Characteristic of Myasthenia Gravis
Post-activation Exhaustion
Recording surface 25 mcm to pick up from single muscle fiber
Quantify the differences in time of onset of firing of two muscle fibers from the same motor unit
Jitter is the mean difference in this firing onset time
Blocking is the rate of failure of a muscle fiber from firing with it’s motor unit
SFEMG
Pre- and post- synaptic neuromuscular junction disorders
Ongoing neuropathic processes: motor neuron disease, neuropathy, radiculopathy
Diagnoses with Increased Jitter
Hallmark: Fluctuating weakness Diplopia Ophthalmoplegia Ptosis Facial Weakness Dysphagia Vocal cord weakness Respiratory muscle weakness Pelvic floor muscle weakness
Myasthenia Gravis Clinical Symptoms
I: Ocular IIA: Mild generalized IIB: Moderate generalized III: Acute severe with bulbar
symptoms IV: Late severe V: Muscle atrophy
Clinical Classifications of Myasthenia Gravis Severity: Osserman
Active: 5-7 years Inactive: 10 years Burned Out: Slow improvement seen
40-50% of ocular myasthenics will become generalized in the first 2 years
Clinical Course in Myasthenia Gravis
Antibodies: 80% generalized, 55% ocular
Seronegative: 70% with anti-MuSK Ig Rep Stim: 76% generalized, 48% ocular Limb SFEMG: 89% generalized, 60%
ocular Facial SFEMG: 92% ocular
Myasthenia Gravis Diagnosis
Evaluate/treat other autoimmune disorders: RA, thyroid, B-12
Pacing High K+ diet Avoid excessive heat>cold Watch for cyclic changes in women Avoid botox, quinamm,
aminoglycosides, tetracyclines, anesthetic agents, anticonvulsants
Preventing Exacerbations in Myasthenia Gravis
Mestinon: Acetylcholinesterase inhibitor Prednisone Imuran, Cytoxan, Cyclosporine, Mycophenolate Therapeutic Plasma Exchange Thymectomy AchR-based Immunoadsorbants Mucosal injection of AchR-recombinant
fragments IG to proinflammatory cytokine IL-18 and
costimulatory factor CD40L Create viral manipulated antigen presenting
cells that express AchR to present to AchR-specific T-cells and activate Fas ligand “guided missile”
Myasthenia Gravis Treatment
Creatine plus resistance exercise with normal treatment in mild MG shows improved strength and muscle mass
Isometric exercise effective in improving strength in mild MG
Exercise in Myasthenia Gravis
Increased acetylcholinesterase activity Decreased sensitivity of acetylcholine
receptors More rapid presynaptic acetylcholine
depletion Can explain proximal>distal weakness
Effect of Temperature in Myasthenia Gravis
Antibodies prevent pre-synaptic Ca++ influx prevents quanta release
Decreased number of MEPP’s of normal amplitude
Decrement to low frequency rep stim due to many muscle fibers activated near-threshold so decreased release is miniscule but significant
Post-exercise facilitation due to increased Ca++ in cell resulting in increased quanta release with next nerve stimulus
Myasthenic Syndrome
Immunogenic: responds to TPE, prednisone, and 2,4-DAP
Tumorogenic: responds to cancer therapy
Eaton-Lambert Types
Food, wound or infantile Markedly decreased pre-synaptic release
due to botulinum toxin binding to and entry into the nerve terminus membrane
Cleave proteins in synaptic vesicle membrane inhibiting release
Complete binding may result in no increment to exercise
Complete binding may lead to fibrillation potentials and positive waves
Botulism
LEMS: hallmark is marked incremental response with exercise
AIDP Critical illness myopathy
Botulism Differential Diagnosis
Does not include neonatal myasthenia gravis
Presynaptic: failed production, storage and mobilization of acetylcholine
Acetylcholine receptors: decreased number, decreased binding, prolonged binding/opening
Congenital absence of acetycholinesterase
Congenital Myasthenia Gravis