Updated: 2/13/2018

Myasthenia Gravis

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Snapshot
  • A 42-year-old woman presents to her primary care physician for fatigue. She reports that her fatigue is worse towards the end of the day. She also notes that while bathing her nephew in the shower her head would "drop." At times when she is watching television or reading a book she sees double. Lastly, she would see her left or right eyelid droop after returning from work. On physical exam, there is right-sided ptosis after sustaining upward gaze for a few minutes. A tensilon test is performed, which is demonstrated in the clinical image below. Serologic studies return positive for anti-acetylcholine receptor antibodies. Preparations are made to have a computerized tomography scan of the chest.
Introduction
  • Clinical definition
    • autoimmune disorder of the neuromuscular junction
  • Epidemiology
    • incidence
      • has a bimodal distribution
        • more common in younger women (< 40 years of age) and older men (> 50 years of age)
    • risk factors
      • HLA-B8
      • medications
        • penicillamine
        • aminoglycosides
  • Pathogenesis
    • autoantibodies directed against a protein of the neuromuscular junction
      • autoantibodies can be directed against
        • nicotinic acetlycholine receptor (AChR)
          • more common
        • muscle-specific receptor tyrosine kinase (MuSK)
      • categorized as a type II hypersensitivity reaction
    • T-cells play a role as well
      • thought to stimulate B-cell antibody production
  • Associated conditions
    • thymoma
    • thymic hyperplasia
  • Prognosis
    • most patients with ocular involvement progress to generalized myasthenia gravis
Presentation
  • Symptoms
    • general feature
      • fluctuating muscle weakness
        • commonly weakness is worse with continued use
          • e.g., worse at the end of the day
      • true muscle fatigue
        • secondary to decreasing contractile muscle force
    • ocular symptoms
      • most common presenting symptoms
        • ptosis
        • diplopia
    • bulbar symptoms
      • dysphagia
      • dysarthria
      • fatigable chewing
    • proximal muscle weakness
  • Physical exam
    • ice-pack test
      • place ice on the patient's ptosis → ptosis improves
        • low temperatures change the kinetics of acetylcholinesterase, decreasing its activity
          • this increases the amount of acetylcholine in the synaptic cleft
    • edrophonium chloride (Tensilon test)
      • only used in patients with ptosis or ophthalmoparesis
        • this allows for improvement in muscle strength to be observed
      • edrophonium is an acetylcholinesterase inhibitor that has a short duration of action
        • this increases the amount of acetylcholine in the synaptic cleft
Imaging
  • Computerized tomography (CT) scan
    • indication
      • to rule out a thymoma
    • view
      • chest
Studies
  • Labs
    • serologic testing for autoantibodies
      • anti-AChR
        • iinitial laboratory test to confirm the diagnosis
      • anti-MuSK
  • Electromyogram (EMG)
    • decremental decrease in the compound muscle action potential (CMAP)
Differential
  • Lambert-Eaton myasthenic syndrome (LEMS)
    • differentiating factor
      • muscle weakness that improves with use
      • autonomic manifestations
  • Botulism
    • differentiating factor
      • poor or impaired pupillary response to light
  • Thyroid ophthalmopathy
Treatment
  • Medical
    • corticosteroids
      • indication
        • a chronic immunotherapy agent
      • drugs
        • oral prednisone
    • acetylcholinesterase inhibitors
      • indication
        • considered first-line for symptomatic management
      • drugs
        • pyridostigmine
      • adverse effects
        • abdominal cramping and diarrhea
        • bradycardia
        • sweating
        • bronchial secretion
  • Procedural
    • intravenous immunoglobulins (IVIg) or plasmapharesis
      • indication
        • rapid immunotherapy for myasthenic crisis
  • Surgical
    • thymectomy
      • indication
        • in patients with a thymoma, irrespective if the patient has myasthenia gravis or not
Complications
  • Myasthenic crisis
    • respiratory weakness secondary to myasthenia gravis
 
 

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Questions (1)
Lab Values
Blood, Plasma, Serum Reference Range
ALT 8-20 U/L
Amylase, serum 25-125 U/L
AST 8-20 U/L
Bilirubin, serum (adult) Total // Direct 0.1-1.0 mg/dL // 0.0-0.3 mg/dL
Calcium, serum (Ca2+) 8.4-10.2 mg/dL
Cholesterol, serum Rec: < 200 mg/dL
Cortisol, serum 0800 h: 5-23 μg/dL //1600 h:
3-15 μg/dL
2000 h: ≤ 50% of 0800 h
Creatine kinase, serum Male: 25-90 U/L
Female: 10-70 U/L
Creatinine, serum 0.6-1.2 mg/dL
Electrolytes, serum  
Sodium (Na+) 136-145 mEq/L
Chloride (Cl-) 95-105 mEq/L
Potassium (K+) 3.5-5.0 mEq/L
Bicarbonate (HCO3-) 22-28 mEq/L
Magnesium (Mg2+) 1.5-2.0 mEq/L
Estriol, total, serum (in pregnancy)  
24-28 wks // 32-36 wks 30-170 ng/mL // 60-280 ng/mL
28-32 wk // 36-40 wks 40-220 ng/mL // 80-350 ng/mL
Ferritin, serum Male: 15-200 ng/mL
Female: 12-150 ng/mL
Follicle-stimulating hormone, serum/plasma Male: 4-25 mIU/mL
Female: premenopause: 4-30 mIU/mL
midcycle peak: 10-90 mIU/mL
postmenopause: 40-250
pH 7.35-7.45
PCO2 33-45 mmHg
PO2 75-105 mmHg
Glucose, serum Fasting: 70-110 mg/dL
2-h postprandial:<120 mg/dL
Growth hormone - arginine stimulation Fasting: <5 ng/mL
Provocative stimuli: > 7ng/mL
Immunoglobulins, serum  
IgA 76-390 mg/dL
IgE 0-380 IU/mL
IgG 650-1500 mg/dL
IgM 40-345 mg/dL
Iron 50-170 μg/dL
Lactate dehydrogenase, serum 45-90 U/L
Luteinizing hormone, serum/plasma Male: 6-23 mIU/mL
Female: follicular phase: 5-30 mIU/mL
midcycle: 75-150 mIU/mL
postmenopause 30-200 mIU/mL
Osmolality, serum 275-295 mOsmol/kd H2O
Parathyroid hormone, serume, N-terminal 230-630 pg/mL
Phosphatase (alkaline), serum (p-NPP at 30° C) 20-70 U/L
Phosphorus (inorganic), serum 3.0-4.5 mg/dL
Prolactin, serum (hPRL) < 20 ng/mL
Proteins, serum  
Total (recumbent) 6.0-7.8 g/dL
Albumin 3.5-5.5 g/dL
Globulin 2.3-3.5 g/dL
Thyroid-stimulating hormone, serum or plasma .5-5.0 μU/mL
Thyroidal iodine (123I) uptake 8%-30% of administered dose/24h
Thyroxine (T4), serum 5-12 μg/dL
Triglycerides, serum 35-160 mg/dL
Triiodothyronine (T3), serum (RIA) 115-190 ng/dL
Triiodothyronine (T3) resin uptake 25%-35%
Urea nitrogen, serum 7-18 mg/dL
Uric acid, serum 3.0-8.2 mg/dL
Hematologic Reference Range
Bleeding time 2-7 minutes
Erythrocyte count Male: 4.3-5.9 million/mm3
Female: 3.5-5.5 million mm3
Erythrocyte sedimentation rate (Westergren) Male: 0-15 mm/h
Female: 0-20 mm/h
Hematocrit Male: 41%-53%
Female: 36%-46%
Hemoglobin A1c ≤ 6 %
Hemoglobin, blood Male: 13.5-17.5 g/dL
Female: 12.0-16.0 g/dL
Hemoglobin, plasma 1-4 mg/dL
Leukocyte count and differential  
Leukocyte count 4,500-11,000/mm3
Segmented neutrophils 54%-62%
Bands 3%-5%
Eosinophils 1%-3%
Basophils 0%-0.75%
Lymphocytes 25%-33%
Monocytes 3%-7%
Mean corpuscular hemoglobin 25.4-34.6 pg/cell
Mean corpuscular hemoglobin concentration 31%-36% Hb/cell
Mean corpuscular volume 80-100 μm3
Partial thromboplastin time (activated) 25-40 seconds
Platelet count 150,000-400,000/mm3
Prothrombin time 11-15 seconds
Reticulocyte count 0.5%-1.5% of red cells
Thrombin time < 2 seconds deviation from control
Volume  
Plasma Male: 25-43 mL/kg
Female: 28-45 mL/kg
Red cell Male: 20-36 mL/kg
Female: 19-31 mL/kg
Cerebrospinal Fluid Reference Range
Cell count 0-5/mm3
Chloride 118-132 mEq/L
Gamma globulin 3%-12% total proteins
Glucose 40-70 mg/dL
Pressure 70-180 mm H2O
Proteins, total < 40 mg/dL
Sweat Reference Range
Chloride 0-35 mmol/L
Urine  
Calcium 100-300 mg/24 h
Chloride Varies with intake
Creatinine clearance Male: 97-137 mL/min
Female: 88-128 mL/min
Estriol, total (in pregnancy)  
30 wks 6-18 mg/24 h
35 wks 9-28 mg/24 h
40 wks 13-42 mg/24 h
17-Hydroxycorticosteroids Male: 3.0-10.0 mg/24 h
Female: 2.0-8.0 mg/24 h
17-Ketosteroids, total Male: 8-20 mg/24 h
Female: 6-15 mg/24 h
Osmolality 50-1400 mOsmol/kg H2O
Oxalate 8-40 μg/mL
Potassium Varies with diet
Proteins, total < 150 mg/24 h
Sodium Varies with diet
Uric acid Varies with diet
Body Mass Index (BMI) Adult: 19-25 kg/m2
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