Propafenone (Rhythmol) (see Propafenone, [[Propafenone]])
Propanolol (Inderal) (see Propanolol, [[Propanolol]])
Sotalol (Betapace, Betapace AF, Sotalex, Sotacor) (see Sotalol, [[Sotalol]]): with additional Class III antiarrhythmic properties (inhibits potassium channels)
Penicillamine (see Penicillamine, [[Penicillamine]]) [Ocular myasthenia gravis after D-penicillamine administration . Br J Ophthalmol. 1989 December; 73(12): 1015–1018]
Phenytoin (Dilantin) (see Phenytoin, [[Phenytoin]]) [A case of myasthenia gravis associated with long-term phenytoin therapy. Rinsho Shinkeigaku. 1996 Nov;36(11):1262-4]
Prednisone (see Prednisone, [[Prednisone]]): may occur with high dose, with onset days after administration
IgG Auto-Antibody Against the Motor Endplate Acetylcholine Receptor: leads to impaired neuromuscular transmission
Effects on Respiratory Muscles: Acute/Chronic Hypoventilation
Diagnosis
FVC: danger level <15 cc/kg
PFT: restrictive
Decreased VC (due to muscle weakness, decreased lung compliance, and decreased chest wall compliance)
Good test to monitor progression of disease during myasthenic crisis
Decreased TLC
Increased RV (due to inability to overcome recoil of chest wall outward at volumes below FRC)
Decreased MVV: MVV maneuvers are hazardous in MG, since repeated muscular efforts may precipitate respiratory fatigue and failure
CXR/Chest CT Pattern
Anti-Ach Receptor Ab: positive in 90% of MG cases
Blocking antibodies
Binding antibodies
Modulating antibodies
Tensilon Test: usually leads to marked improvement in symptoms (unlike in LEMS, where Tensilon results are less definitive)
Repetitive Nerve Stimulation: results in decreased compound muscle action potentials (in contrast to LEMS, where it leads to increased action potentials)
Clinical
Muscle weakness (exacerbated by muscle contraction) of ocular, bulbar, respiratory (usually, expiratory > inspiratory muscles/occasionally, respiratory involvement can be the only involved), and skeletal muscles
Acute/Chronic Hypoventilation (see [[Acute Hypoventilation]] and [[Chronic Hypoventilation]]): acute ventilatory failure may be the initial presentation of myasthenia gravis
MG tends to have more ocular/bulbar dysfunction than LEMS
Symptoms tend to worsen throughout the day (in contrast to LEMS, which improves throughout the day)
Myasthenic Crisis
Crisis due to disease progression (usually involves respiratory failure or aspiration, due to pharyngeal muscle involvement)
Precipitants of Myasthenic Crisis
Thymectomy: may precipitate crisis
Surgery: may precipitate crisis (risk factors for post-operative ventilation: bulbar involvement/ expiratory muscle weakness): days of ventilatory support required can be predicted from the equation: DAYS OF SUPPORT = (550/MEP%) – 5
Co-Existent Hypothyroidism/Hyperthyroidism (occurs in 3-8% of MG cases): excarbate MG
Anti-Cholinergic Agents/Aminoglycosides/Procainamide: all have NMJ-blocking properties
Infection (of any type)
Cholinergic Crisis
Associated with excessive use of anticholinesterase inhibitors
Most commonly seen with intercurrent infection
Characterized by deterioration, requiring increased doses of medications
Only way to diagnose is by stopping medication temporarily
Pulmonary HTN/Cor Pulmonale (see Pulmonary Hypertension, [[Pulmonary Hypertension]]): due to hypoxia and acidosis with resultant pulmonary vasoconstriction
Treatment
Anticholinesterases
Pyridostigmine
Improves respiratory muscle strength/lung volumes/inspiratory and expiratory flow rates/compliance during deflation (but not inflation pressure-volume curve)
Pyridostigmine does not affect the pattern of breathing
Prostigmine improves respiratory muscle strength, does not increase endurance (effect can be overcome by prolonged MVV maneuver)
Steroids
May be effective
Plasmapheresis
Effective when steroids fail
May require 3-5 exchanges before effect (because <50% of antibody is intravascular at any time)
Approximate 70% reduction in Ab concentration: required for remission
Avoidance of Agents Which May Exacerbate Myasthenia Gravis
