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MYASTHENIA GRAVIS: DIAGNOSTIC TESTS

Antibodies   Anti-AChR   MuSK   Striational   Other General principles Other diagnostic tests Repetitive nerve stimulation Single fiber EMG Tensilon testing

Decremental response to RNS in Myasthenia Gravis

General Principles of Diagnostic Testing for MG

• Rationale
  • As thymectomy or long term immunotherapy may be necessary to treat MG, it is essential to establish a firm diagnosis
  • A firm diagnosis avoids inappropriate treatments, and their side effects, in patients who do not have the disease.
  
  
• The mainstays of diagnostic testing have traditionally been
  • Pharmacological
  • Serological
  • Electrodiagnostic
  
  
• Strategy of disgnostic testing
  • As a general rule, a firm diagnosis is based upon
    • A characteristic history and physical examination, and
    • Two positive diagnostic tests, preferably serological and electrodiagnostic.
  • Diagnostic investigations of MG should usually include both
    • Testing for serum anti-AChR antibodies
    • Repetitive nerve stimulation studies
  
  
• Tensilon tests
  • May be readily performed at the bedside
  • Are not as sensitive, or specific, as the serological and electrophysiological studies
  
  
• Single fiber EMG: Reserved for selected patients in whom other tests have been negative or equivocal.
  
  
• Other diagnostic tests: Generally not required or indicated in majority of MG cases
  • Motor point muscle biopsies
    • Count AChRs at neuromuscular junctions
    • Evaluate neuromuscular transmission by in vitro electrophysiologic methods
    • Immunocytochemical staining of muscle endplates for immunoglobulin and complement
  • Tests of ocular movement
  • Genetic evaluation for defects in AChR subunits

Edrophonium (Tensilon) Testing

• Drug properties

  Cogan

  Tensilon test: Before (left); After (right)

  • Action
    • Inhibits acetylcholinesterase
    • Prolongs presence of neurotransmitter, acetylcholine, in the NMJ
    • Results in enhanced muscle strength
      • Duration: Lasts for a few minutes
      • Response
        • In patients with NMJ dysfunction
        • Not specific for MG
  • Time course: Minutes; Rapid-onset; Short-acting

• Initially
  • Dosing: 2 mg of edrophonium is administered intravenously as a test dose
  • Monitoring heart rate: Bradycardia or ventricular fibrillation may develop
• Follow-up
  • After observing for about 2 minutes, if no clear response develops
  • Up to 8 additional mg of edrophonium is injected
• A double-blind protocol with a saline injection as placebo has been advocated
• Testing should be performed with patient free of all cholinesterase-inhibitor medications
• Cholinergic side effects of edrophonium
  • May include increased salivation and lacrimation, mild sweating, flushing, urgency & perioral fasciculations.
  • Atropine should be readily available to reverse effects of edrophonium in case of hemodynamic instability
  • Extra precautions are especially important in elderly patients

• Most myasthenic muscles respond in 30 to 45 seconds after injection
• Improvement in strength that may persist for up to 5 minutes
• Requires objective improvement in muscle strength.
• Subjective or minor responses, such as reduction of a sense of fatigue, should not be over interpreted

• Only useful in patients with objective, preferably measurable, findings on physical examination
• Rarely helpful in the diagnostic evaluation of equivocal cases of MG
• Sensitivity for MG is relatively low (60%) compared to other diagnostic tests
• Tensilon testing should not be used to determine adjustments in the dose of pyridostigmine
• False positive results
  • Can occur in patients with LES, ALS or even localized, intracranial mass lesions
  • Positive testing does not necessarily predict respose to a longer-acting anti-AChE drug

Serum antibodies vs Acetylcholine Receptors

Muscle AChR (Adult)

• General testing method
  • Measurement using immunoprecipitation methodology
  • Method: Human nicotinic AChRs from skeletal muscle labeled with ¹²⁵I-α-Bungarotoxin
• Antibody (IgG) targets in MG: Nicotinic AChRs
  • α1-subunit of AChR
  • Typical myasthenia gravis
  • Epitope location: Main immunogenic region on extracellular tip of α1-subunit
  • Main immunogenic region (MIR) features
    • Cluster of overlapping epitopes
    • Conformation dependent
    • Also commonly a target of anti-AChR antibodies in Experimental autoimmune MG (EAMG)
    • Cross-linking
      • Single bivalent antibody cannot bind to both MIRs on same AChR
      • Antibodies can cross-link adjacent AChRs
  • e-subunit
    • Acquired slow channel syndrome
    • Binding to Adult AChRs (containing e-subunit)¹
      • 13% of serums with no IgG binding to fetal AChRs
      • Increases finding (sensitivity) of anti-AChR antibodies in MG by ~ 3%
      • Occur in patients with either ocular or generalized MG syndromes
  • γ-subunit: Neonatal MG (Transient); Recurrent arthrogryposis
• Measurement of serum IgG & IgM antibodies that bind to AChRs
  • Antigenic target
    • AChRs from human skeletal muscle: Mixed innervated & denervated, or
    • Myogenic cell line expressing both adult & fetal AChRs
  • Relatively specific and sensitive test for MG
  • Anti-AChR antibodies occur in
    • Adults with generalized MG: 85 to 90%
    • Childhood MG: 50%
    • Ocular MG: 50% to 70%
      • Lower titers
        
      • Bind best to adult AChRs with e subunit
    • MG and thymoma: Nearly 100%
    • Some patients taking penicillamine with or without MG
    • "False" positives
      • Thymoma without MG
        
      • Immune liver disorders
        
      • Lambert-Eaton syndrome (13%)
        
      • Primary lung cancer: 3%
        
      • Older patients (> 70 years): 1% to 3%
      • Neuromyotonia
• Antibody effects on AChRs
  • Modulation
    • Mechanism
      • Anti-AChR antibodies cross-link AChRs on the post-synaptic membrane
      • Endocytosis & degradation of AChRs are accelerated
    • Test results
      • May occur in a rare patient when anti-AChR antibody binding is negative.
      • MG: Usual loss = 20% to 90% of AChRs (Normal < 20%)
      • MG with thymoma: > 90% loss
      • False positives: Hemolysis; Muscle relaxant drugs; Serum heating
  • Blocking binding site for ACh on AChR (positive = 26% to 100% blocking).
    • Prevalence: Repeated arthrogryposis; 52% of generalized MG; 30% of ocular
    • 1% of patients with no binding or blocking antibodies
  • Complement binding
    • Damage to postsynaptic membrane: Simplification of postsynaptic folds
    • Widening of synaptic cleft
• Clinical correlations of MG & anti-AChR antibodies
  • Absolute titer of antibodies
    • No relation with severity of MG
  • Antibody blocking & modulation of AChRs
    • Some correlation with disease severity
  • Change in titer of antibodies in individual patient
    • Improvement often seen with reduction of > 50%
  • Neonatal MG: Transient
    • High anti-fetal/anti-adult muscle anti-AChR antibody ratio
  • Recurrent arthrogryposis
    • IgG vs γ subunit of fetal AChR: Blocks AChR function
  • Slow channel syndrome: Acquired
    • IgG vs adult AChR
  • Neuronal AChR: α3 subunit
    • Autonomic PN (42%)
    • Isaacs (25%)
    • LEMS (12%)
  • Also see: AChR disorders
• Patients with MG but no anti-AChR antibodies
  • Rule out hereditary MG
  • Low frequency of thymic pathology & thymoma
  • May have antibodies to other neuromuscular junction antigens

Repetitive Nerve Stimulation From: M Al-Lozi

Repetitive Nerve Stimulation (RNS): 2 to 3 Hz

• Most frequently used electrodiagnostic test for MG
• Nerve to be studied is electrically stimulated six to ten times at 2 or 3 Hertz
• Compound muscle action potential (CMAP) is recorded with surface electrodes over muscle

• Normal muscles: No change in CMAP amplitude with repetitive nerve stimulation
• Myasthenia gravis
  • Progressive decline in CMAP amplitudes with the first 4 to 5 stimuli.
  • Positive RNS test features
    • Decrement in CMAP amplitude
      • Size: More than 10% in reduction in CMAP amplitude
        • Measure from 1st to 4th or 5th potential in train
      • Smallest CMAP is often 2nd or 3rd potential in train
    • Post-exercise exhaustion
      • Exercising muscle briefly before testing exacerbates decremental response
      • Occurs rapidly after initial stimulation
    • Post-tetanic potentiation
      • Reduction in decrement in minutes after exercise
      • Occurs after post-exercise exhaustion
  • RNS is positive in about 75% of patients with generalized MG, if
    • Proximal & Clinically involved muscles are tested
    • Muscle is warm: Cooling reduces size of decrement
    • More than one muscle is tested: Strong muscles often have less decrement
  • Diagnostic issues
    • Sensitivity of RNS for MG: Greatly reduced when distal muscles are tested
    • RNS is positive in only 50% of patients with ocular MG.
    • Diagnostic specificity
      • A decremental response to RNS is not specific for MG
      • Decrements may also be seen in
        • Presynaptic disorders: Such as LEMS
        • Motor neuron diseases: Including ALS

Single fiber electromyography (SFEMG)

Normal SFEMG

Increased jitter: MG patient

From: M Al-Lozi

• Principle: Muscle fibers innervated by a single axon
  • Normal: Activated with consistent latencies
  • NMJ disorders
    • Increased variability of latencies among muscle fibers in single motor unit
    • Muscle fiber potential may be blocked if transmission at its NMJ fails completely.
• SFEMG
  • Simultaneously records potentials of two muscle fibers innervated by an individual axon
  • Measures this variability = "Jitter"
  • SFEMG is the most sensitive test for MG
    • Sensitivity: > 95% positive in generalized & ocular MG
      • When the test site includes facial muscles
  • Abnormal jitter is not specific for MG
    • May occur in other neuromuscular disorders, including ALS, polymyositis or LEMS
    • More specific for MG if large degree of jitter occurs with mild or no other changes on EMG