Myasthenia Gravis

Myasthenia gravis (MG) is the most common autoimmune disorder of the neuromuscular junction, with a prevalence of 150–250 per million and an incidence of up to 30 per million person-years. Pathogenic antibodies target postsynaptic endplate proteins—most commonly the acetylcholine receptor (AChR)—producing painless, fluctuating, and fatigable skeletal muscle weakness. MG affects predominantly younger women and older men, with an increasing incidence in patients over 50 years old. Historically lethal in up to 70% of cases, contemporary immunotherapy enables 75% of patients to achieve minimal manifestation status or remission within 2 years. The past decade has witnessed a revolution in targeted biological therapies, including complement inhibitors and neonatal Fc receptor (FcRn) blockers, transforming the treatment landscape for refractory disease.

Pathophysiology

Normal Neuromuscular Transmission

At the neuromuscular junction, acetylcholine (ACh) released from the motor nerve terminal binds to AChRs on the postsynaptic muscle endplate, generating an endplate potential. When this potential reaches threshold, voltage-gated sodium channels in adjacent junctional folds amplify the signal, triggering muscle contraction. AChR clustering on the endplate depends on a signaling cascade: agrin (secreted by the motor nerve) binds LRP4, which activates MuSK phosphorylation and interactions with Dok-7 and rapsyn for AChR aggregation. Acetylcholinesterase, anchored to the basal lamina by collagen Q, terminates signaling by hydrolyzing ACh.

Antibody-Mediated Attack

In MG, pathogenic antibodies target endplate proteins, reducing the probability of the endplate potential reaching threshold. Three major antibody targets have been identified:

Seronegative MG (~10% of patients) lacks detectable antibodies by standard radioimmunoassay. Cell-based assays, which detect antibodies against clustered AChRs on cell surfaces, can identify AChR antibodies in nearly one-fifth of these patients. Some patients may seroconvert within 6–12 months of MG onset, justifying repeat testing.

Role of the Thymus

The autoimmune attack in MG is T-cell dependent, arising from loss of self-tolerance in the thymus. Impaired T regulatory cell function leads to excess activation of autoreactive CD4+ T cells, with release of proinflammatory cytokines (particularly IL-17) that stimulate B cells to produce AChR antibodies. This process drives thymic hyperplasia with enlarged germinal centers, particularly in early-onset AChR MG. The thymus role is less clear in MuSK MG and late-onset AChR MG, where the thymus is typically normal or atrophic, respectively.

Clinical Presentation

Ocular Manifestations

Two-thirds of patients present with ocular symptoms: asymmetric, variable eyelid ptosis and binocular diplopia. Three characteristic signs accompany myasthenic ptosis:

• Cogan lid twitch: After downward gaze and rest, upward gaze produces eyelid overshoot followed by rapid return to the ptotic position
• Curtain sign: Manually lifting the more ptotic lid worsens ptosis in the contralateral eye
• Frontalis sign: Compensatory frontalis contraction raises the eyebrows to keep eyes open

Patients often report a prediagnosis history of prism glasses, strabismus surgery, or blepharoplasty that failed to improve symptoms. Because extraocular weakness is dynamic, prism lenses and strabismus surgery are generally ineffective.

Bulbar and Generalized Weakness

Bulbar symptoms are the initial presentation in ~20% of patients: chewing fatigue, dysphagia, nasal regurgitation, and flaccid dysarthria with a nasal quality that worsens with prolonged speaking. Facial weakness produces a characteristic "snarl" instead of a smile. Axial and limb weakness (15–20% at onset) is commonly asymmetric but not unilateral, with preferential involvement of deltoids, triceps, and wrist/finger extensors in the arms. Neck flexion weakness is more common than neck extension weakness, although severe neck extensor weakness causes a dropped head. Isolated respiratory or laryngeal weakness is rare.

MGFA Clinical Classification

AChR-Positive vs. MuSK-Positive MG

Diagnostic Workup

Serologic Testing

A positive AChR or MuSK antibody test confirms the diagnosis in the setting of compatible clinical findings:

• AChR binding antibodies (radioimmunoassay): Present in 85–90% of generalized MG and 50–70% of ocular MG; >90% specificity; titers do not correlate with severity
• AChR modulating antibodies: Increase sensitivity by ~10%; included in standard panels or sent reflexively when binding antibodies are low/negative
• AChR blocking antibodies: Do not add diagnostic sensitivity; unnecessary
• MuSK antibodies: Test in all AChR-negative patients with generalized MG; ~7% of overall MG population
• Cell-based assays: Greater sensitivity than radioimmunoassay for detecting low-titer AChR antibodies; useful in apparently seronegative MG
• LRP4 antibodies: Reserve for double-seronegative MG with confirmed abnormal electrodiagnostics; low specificity
• Striated muscle antibodies (anti-titin, anti-ryanodine): Neither diagnostic for MG nor predictive of thymoma; removed from MG antibody panels

Electrodiagnostic Testing

Other Testing

• Chest CT (without contrast): Mandatory in all newly diagnosed MG to evaluate for thymoma; contrast is not required (comparable sensitivity without contrast); 10–20% of patients with MG have thymoma
• Ice pack test: Applying ice to a ptotic eyelid for 2 minutes; ~80% sensitivity and specificity in ocular MG
• Thyroid function tests: Autoimmune thyroid disease co-occurs with MG, especially in women with AChR MG; check in all newly diagnosed patients and in established patients with worsening symptoms
• Genetic testing: Consider congenital myasthenic syndrome in double-seronegative patients with symptom onset in infancy, childhood, or early adulthood

Thymoma and Thymectomy

Thymomatous MG

Thymoma is present in 10–20% of MG patients. Thymectomy is performed in nearly all patients with thymomatous MG to remove both the tumor and residual thymic tissue. Following thymoma resection, patients require ongoing MG treatment and surveillance imaging for tumor relapse, with oncology consultation for adjuvant chemotherapy or radiation as indicated.

Nonthymomatous MG: The MGTX Trial

The landmark MGTX trial (NEJM 2016) was a multicenter, rater-blinded, randomized controlled trial comparing extended transsternal thymectomy plus prednisone vs. prednisone alone in patients aged 18–65 with nonthymomatous AChR generalized MG of ≤5 years duration:

• At 3 years: Thymectomy group had significantly lower QMG scores (6.15 vs. 8.99) and lower prednisone requirements (44 mg vs. 60 mg alternate-day)
• Fewer patients in the thymectomy group required azathioprine (17% vs. 48%) or hospitalization for exacerbations (9% vs. 37%)
• 5-year extension (Lancet Neurol 2019): Benefits were sustained, with persistent improvement in clinical status, reduced immunotherapy requirements, and fewer hospitalizations
• Post hoc analysis: Benefits were less clear for patients with late-onset MG (>50 years)

Treatment

The goal of MG treatment, as defined by the Myasthenia Gravis Foundation of America, is achieving minimal manifestation status (subtle findings on examination but no symptoms) or remission while minimizing treatment-related adverse events. Treatment selection is individualized based on patient factors (age, sex, childbearing potential, comorbidities) and MG characteristics (distribution, severity, antibody status, thymoma status, response to prior therapies).

Cholinesterase Inhibitors

Pyridostigmine bromide is the most commonly prescribed initial therapy. It inhibits acetylcholinesterase, increasing ACh availability at the endplate. Key points:

• Provides rapid but transient symptomatic improvement; does NOT modify the disease course
• Standard dosing: 30–90 mg, 3–4 times daily; onset within 30 minutes, lasting ~4 hours
• Side effects: Dose-dependent muscarinic (GI cramping, diarrhea, salivation) and nicotinic (fasciculations, cramps) cholinergic effects
• Extended-release 180 mg tablets: Seldom used due to irregular absorption and unpredictable responses
• Cholinergic crisis: Rare but dangerous; excessive dosing causes depolarization blockade, increased weakness, and excessive secretions with aspiration risk
• Symptomatic benefit decreases as underlying MG improves with immunotherapy, signaling readiness to taper

Corticosteroids

Prednisone is the most widely used initial immunotherapy, effective in ~80% of patients with improvement within weeks to months:

• High-dose strategy: 0.75 mg/kg/day or 60 mg/day for moderate-to-severe generalized MG
• Intermediate-dose strategy: 20–30 mg/day for mild generalized MG, ocular MG, or high-risk patients
• Taper goal: <7.5 mg/day within 1 year; reduce dose by ~25% every 4–8 weeks if stable
• Corticosteroid-related exacerbation: Occurs in up to 15% within 10 days of starting or after a large dose increase; consider pretreatment IVIg/PLEX in at-risk patients (elderly, moderate-to-severe bulbar weakness)
• Most patients without thymectomy require a low maintenance prednisone dose for life

Oral Nonsteroidal Immunosuppressants

Rescue Therapies

Used for MG exacerbation or crisis, preoperative optimization, and bridging to delayed immunosuppressant effects:

• Plasma exchange (PLEX): 5–6 exchanges on alternate days; improvement after the 3rd exchange; lasts 3–12 weeks; complications related to central venous access (use peripheral access when feasible); preferred over IVIg in MuSK MG
• IVIg: 2 g/kg over 2–5 days; improvement in 5–7 days; similar efficacy to PLEX in AChR MG; more accessible (no large-bore catheter or specialized equipment); avoid in hyperviscosity or recent thrombosis
• Subcutaneous immunoglobulin: Rarely used; slow onset (not appropriate for rescue); offers patient autonomy for maintenance dosing in refractory cases

Targeted Biological Therapies

Seven monoclonal antibodies or small molecules targeting specific immune pathways have received FDA approval for MG since 2017. These agents offer rapid, robust improvement but are expensive, require regular infusions or injections, and are currently used most often as add-on therapies for refractory generalized MG or when conventional immunosuppressants are contraindicated.

B-Cell Depletion: Rituximab

Rituximab is a monoclonal antibody that depletes >95% of circulating CD20+ B cells. Although not FDA-labeled specifically for MG, it has transformed the treatment of MuSK MG:

• MuSK MG: 67% reach MMS or remission (vs. 26% controls); improvement within 3–8 weeks; durable remissions lasting years; should be considered as early or initial treatment
• AChR MG: Variable response; best evidence in recently diagnosed or late-onset MG; a phase 2 RCT (BeatMG) in chronic, treatment-resistant AChR MG showed safety but not steroid-sparing effect at 12 months
• Well tolerated; progressive multifocal leukoencephalopathy is exceedingly rare (3 reported MG cases, all on multiple immunosuppressants)
• Inebilizumab (anti-CD19): Depletes a broader range of B cells including plasmablasts and plasma cells; phase 3 trial showed significant improvements in AChR and MuSK MG at 26 weeks

Complement Inhibitors

These agents block C5 cleavage, preventing membrane attack complex formation. They are indicated for AChR generalized MG only (complement activation is central to AChR but not MuSK pathophysiology). Meningococcal vaccination (MenACWY + MenB) is mandatory at least 2 weeks before the first dose.

FcRn Inhibitors

FcRn inhibitors block neonatal Fc receptor-mediated IgG recycling, accelerating IgG degradation and reducing circulating pathogenic antibodies. Unlike plasma exchange, they selectively reduce IgG without affecting coagulation factors or non-IgG immunoglobulins. Onset of benefit is typically within 1–2 weeks.

Practical considerations for FcRn inhibitors:

• Transient IgG reduction means live-attenuated or live vaccines should be avoided during treatment cycles
• Many patients can reduce pyridostigmine and prednisone but not nonsteroidal immunosuppressants
• Rapid onset supports potential use as rescue therapy (similar to PLEX but more selective)
• Zilucoplan (complement inhibitor) can be co-administered with FcRn inhibitors because it escapes IgG recycling

Medication Pitfalls: Drugs That Worsen MG

Special Situations

Myasthenic Crisis

Myasthenic crisis is respiratory failure from severe weakness of respiratory and oropharyngeal muscles. It is most common early in the course of generalized MG and in patients with thymoma; ~20% of patients experience at least one episode. With contemporary ICU care, mortality is <5%, though mean intubation duration remains 12–13 days.

• Triggers: Infection (especially bronchopulmonary), surgery, rapid immunosuppression withdrawal, corticosteroid initiation/escalation, medications that worsen MG, pregnancy/puerperium; no trigger identified in ~30%
• Signs of impending crisis: Severe bulbar weakness with inability to handle secretions, rapid shallow breathing, orthopnea, accessory muscle use, hypercapnia; hypoxemia is a late sign
• Management: ICU admission; IVIg or PLEX as rescue therapy; initiate or optimize long-term immunotherapy; vital capacity and negative inspiratory force guide extubation readiness

Ocular MG

About 15% of patients remain purely ocular beyond 2 years. Treatment is stepwise: pyridostigmine first (often incomplete relief), then prednisone 20–30 mg/day with taper to ≤7.5 mg/day. The EPITOME trial showed nearly all prednisone-treated patients reached sustained MMS and tapered to 10 mg/day without relapse. Oxymetazoline 0.1% ophthalmic solution may provide transient ptosis relief. Early prednisone treatment was associated with a 7% generalization rate vs. 36% with pyridostigmine alone or no treatment (retrospective data).

Pregnancy

MG can manifest during pregnancy or puerperium. Exacerbations are more common in the first or third trimesters and postpartum, though myasthenic crisis is very rare. Key principles:

• Achieve MMS before conception; for AChR MG, thymectomy ideally ≥1 year before pregnancy; for MuSK MG, rituximab may be considered preconception
• Safe medications: Pyridostigmine, prednisone, azathioprine, IVIg/PLEX at minimum effective doses
• Contraindicated: Mycophenolate mofetil, methotrexate, cyclophosphamide (teratogenic); limited safety data for novel biological agents
• Deliver at a center with neonatal ICU (risk of transient neonatal MG from passive antibody transfer)
• Avoid: IV magnesium (worsens MG) and IV cholinesterase inhibitors (trigger uterine contractions)

Checkpoint Inhibitor-Associated MG

Immune checkpoint inhibitors used in cancer therapy can cause de novo MG or exacerbate preexisting disease, typically within 3 months of treatment initiation. Checkpoint inhibitor-associated MG is usually seronegative, fulminant, and monophasic, often overlapping with myositis and myocarditis (high mortality). Management involves pausing the checkpoint inhibitor, IV methylprednisolone with prednisone taper, and PLEX/IVIg or complement inhibition for severe cases.

Prognosis and Monitoring

With appropriate treatment, 75% of patients achieve MMS or remission within 2 years. However, 10–15% remain refractory to standard therapies, and diagnostic delays of approximately 1 year remain typical. Most patients require some degree of immunotherapy for life, especially those without thymectomy. The novel biological agents have provided meaningful options for refractory disease, though access may be limited by cost and administration logistics.

Emerging Therapies

The MG treatment pipeline continues to expand rapidly:

• Chimeric antigen receptor (CAR) T-cell therapy: Descartes-08 (phase 1b/2a) uses mRNA-engineered autologous T cells targeting B-cell maturation antigen; significant, sustained improvement without cytokine release syndrome or neurotoxicity; phase 2b enrolling
• Chimeric autoantibody receptor (CAAR) T cells: Express MuSK as the extracellular antigen to selectively eliminate autoreactive B cells; phase 1 in MuSK MG
• Anti-IL-6 therapy: Satralizumab (LUMINESCE trial) showed small but significant improvements in ADL and strength in AChR, MuSK, and LRP4 MG; tocilizumab has shown benefit in small series
• Novel complement inhibitors: Gefurulimab, pozelimab/cemdisiran, iptacopan in phase 3 trials
• Batoclimab: Another FcRn inhibitor (human IgG1 monoclonal antibody) in phase 3

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