What is the classification system for Myasthenia Gravis (MG)?

Classification of Myasthenia Gravis

MGFA Clinical Classification System

The Myasthenia Gravis Foundation of America (MGFA) clinical classification is the standardized system used to classify MG severity and guide treatment decisions. 1, 2

MGFA Classification Classes

• Class I (Ocular MG): Weakness limited to extraocular muscles only, causing ptosis and/or diplopia, with all other muscle strength normal 3, 1

• Class II (Mild Generalized MG): Mild weakness affecting muscles other than ocular muscles, subdivided into:

  • Class IIa: Predominantly affecting limb and/or axial muscles 1
  • Class IIb: Predominantly affecting oropharyngeal and/or respiratory muscles 1

• Class III (Moderate Generalized MG): Moderate weakness affecting muscles other than ocular muscles, subdivided into:

  • Class IIIa: Predominantly affecting limb and/or axial muscles 1
  • Class IIIb: Predominantly affecting oropharyngeal and/or respiratory muscles 1

• Class IV (Severe Generalized MG): Severe weakness affecting muscles other than ocular muscles, subdivided into:

  • Class IVa: Predominantly affecting limb and/or axial muscles 1
  • Class IVb: Predominantly affecting oropharyngeal and/or respiratory muscles, may require feeding tube but not intubation 1

• Class V (Myasthenic Crisis): Intubation required, with or without mechanical ventilation, except when used during routine postoperative management 4, 1

Serologic Classification System

MG should also be classified by antibody profile, as this determines treatment responsiveness and prognosis. 5, 6

Antibody-Based Subgroups

• AChR-antibody positive MG: Found in 80-85% of generalized MG patients and 40-77% of ocular MG patients 3, 5

• MuSK-antibody positive MG: Present in 5-8% of patients, often with prominent bulbar and respiratory involvement 5, 6

• LRP4-antibody positive MG: Found in <1% of patients 5, 6

• Seronegative MG: Approximately 10% of patients test negative for all known antibodies 5, 6

Clinical Subgroup Classification

Beyond MGFA classes and antibody status, MG is further classified by clinical features that impact treatment and prognosis. 5, 7

Key Clinical Subgroups

• Early-onset MG: Typically affects younger patients, more common in females 5

• Late-onset MG: Debuts at older age, more common in males 5, 6

• Thymoma-associated MG: Present in 10-20% of AChR-positive patients, detected at disease onset 5, 6

• Ocular MG: 50% of patients initially present with purely ocular symptoms, though 50-80% progress to generalized disease within a few years 3, 7

• Generalized MG: Affects 85% of patients within the first three years, involving head, neck, and extremity muscles 1, 7

Practical Application for Treatment Decisions

• Classes III-V require frequent pulmonary function monitoring with NIF and VC measurements using the "20/30/40 rule" (vital capacity <20 ml/kg, maximum inspiratory pressure <30 cmH₂O, or maximum expiratory pressure <40 cmH₂O indicates respiratory failure risk) 4

• Class V (myasthenic crisis) mandates immediate ICU admission, high-dose corticosteroids (methylprednisolone 1-2 mg/kg/day IV or prednisone 1-1.5 mg/kg/day), and either IVIG (2 g/kg over 5 days) or plasmapheresis (5 sessions over 5 days) 4

• MuSK-positive patients may respond differently to standard treatments compared to AChR-positive patients, requiring tailored immunosuppressive approaches 5, 6