Myasthenia Gravis

Myasthenia gravis (MG) is an autoimmune disease of the neuromuscular junction, most often driven by antibodies against the postsynaptic acetylcholine receptor (AChR), with antibodies against muscle-specific kinase (MuSK) and lipoprotein-related protein 4 (LRP4) accounting for most of the antibody-defined remainder.^[1] The hallmark clinical feature is fatigable, fluctuating weakness — weakness that worsens with sustained or repeated muscle use and characteristically improves with rest — most classically affecting the eyelids and extraocular muscles first.^[2]

MG is genuinely uncommon but not rare: a Canadian population-based study found Ontario's age- and sex-standardized prevalence rose steadily from 16.3 per 100,000 in 1996 to 26.3 per 100,000 in 2013, with a crude prevalence of 32.0 per 100,000 (3,611 diagnosed cases) by the end of that period, while the standardized incidence remained essentially stable at 2.3 to 2.7 per 100,000 per year throughout — a pattern the authors interpreted as reflecting patients living longer, likely because of improved treatment rather than more new disease.^[3] Onset follows a bimodal age distribution: younger women in their 20s to 40s, and older men, who also carry a higher rate of the thymoma sometimes found alongside MG.^[2]

Two themes run through this entire chapter and deserve equal weight to the symptomatic and immunosuppressive treatment ladder. First, an unusually long list of otherwise routine medications — aminoglycoside, fluoroquinolone, and macrolide antibiotics; intravenous magnesium; beta-blockers, including ophthalmic timolol; certain antiarrhythmics and calcium channel blockers; botulinum toxin in any formulation; and the neuromuscular blocking agents and inhalational anesthetics used in surgery — can worsen or unmask MG, sometimes precipitating a genuine myasthenic crisis, making careful medication selection and reconciliation one of the highest-yield things a pharmacist does for a patient with MG.^[4] Second, myasthenic crisis — respiratory failure requiring ventilatory support — affects an estimated 15 to 20% of MG patients at some point and is, in about one-fifth of cases, the very presentation that first brings MG to medical attention; recognizing it promptly, and distinguishing it from the much rarer but genuinely different cholinergic crisis caused by acetylcholinesterase-inhibitor excess, is this chapter's other central safety message.^[5]

Asymmetric, fatigable ptosis — here right-sided, with compensatory left-lid pseudo-retraction from Hering's law of equal innervation — a classic presenting sign of myasthenia gravis. (Kurukumbi M, Weir RL, Kalyanam J, Nasim M, Jayam-Trouth A, J Med Case Rep 2008;2:245, Wikimedia Commons, CC BY 2.0)

The core clinical pattern: fatigable, fluctuating weakness

At the neuromuscular junction, autoantibodies deplete or functionally block postsynaptic AChR directly (the majority pattern), or — in MuSK- and LRP4-antibody disease — disrupt the agrin-LRP4-MuSK signalling pathway that normally clusters AChR at the postsynaptic membrane, producing the same net effect of unreliable neuromuscular transmission through a mechanistically distinct route.^[1] Clinically, this produces localized muscle weakness in a characteristic distribution that is exacerbated by repeated activity and typically worsens later in the day.^[2] Asymmetric ptosis and variable diplopia are the presenting symptoms in about 60% of patients; in most, weakness goes on to spread, causing fluctuating dysarthria, dysphagia, difficulty chewing, and dyspnea. Limb involvement is less common as a presenting feature and, when present, is typically proximal.^[2]

Simple bedside tests for fatigable weakness

Three simple bedside maneuvers can demonstrate the fatigable pattern directly: asking the patient to look upward for one minute to watch for worsening ptosis, asking the patient to sustain lateral gaze for 30 to 60 seconds and report new double vision, and asking the patient to perform five consecutive resisted shoulder abductions to observe gradual weakening of the deltoid.^[2]

Myasthenia Gravis Foundation of America (MGFA) clinical classification

The MGFA clinical classification is the standard framework for describing MG severity and distribution at a point in time, and is used throughout the literature (including the trials in Pharmacologic Treatment Options) to describe study populations. It is presented here as a teaching reference.

Ocular versus generalized disease

Roughly half of patients present with weakness confined to the eyes (ocular MG, MGFA Class I). Left untreated, a substantial majority go on to generalize — most within the first two years after onset — and current consensus guidance favours early treatment specifically to reduce this risk, generally preferring low-dose over high-dose corticosteroids when an acetylcholinesterase inhibitor alone is insufficient.^[6]

Antibody subtypes carry real prognostic and treatment weight

AChR antibodies are highly specific and positive in about 85% of generalized MG but only about 50% of pure ocular disease — a normal AChR antibody does not exclude ocular MG.^[2] MuSK-antibody-positive MG is a clinically distinct entity: it more often produces prominent bulbar, facial, neck, and respiratory weakness, sometimes with visible facial and tongue muscle atrophy; it often responds poorly to — and can even be worsened by — pyridostigmine; thymectomy has no established benefit because the thymic pathology of AChR-positive disease is absent; and rituximab, rather than the standard steroid-sparing agents, is the treatment of choice for refractory disease (see Special Populations and Choose Initial Treatment).^[4][6] A smaller remaining group is LRP4-antibody-positive or fully seronegative, generally following a course closer to AChR-positive disease.^[1]

• Sex and age — a bimodal pattern: women in their 20s to 40s, and older men, who also carry a higher rate of associated thymoma.^[2]
• Thymoma — found on chest imaging in 10 to 15% of MG patients overall, more often in older-onset disease; every newly diagnosed patient needs chest computed tomography to look for it (see Assessment and Diagnosis).^[2]
• Thymic hyperplasia — common in early-onset, AChR-antibody-positive generalized MG, and the rationale for thymectomy in that specific subgroup (see Choose Initial Treatment).^[1]
• Other autoimmune disease — autoimmune thyroid disease in particular, but also rheumatoid arthritis, systemic lupus erythematosus, and others, occur at higher-than-expected rates alongside MG.^[1]
• Certain drugs and immunotherapies — D-penicillamine and the immune checkpoint inhibitors are established causes of genuine, de novo drug-induced MG, distinct from simply worsening pre-existing disease (see Drug Tables).^[4]

Diagnosis combines the clinical pattern with serologic and electrodiagnostic testing

A neurologist typically performs most of the confirmatory work-up.^[2] AChR antibodies are highly specific and positive in about 85% of generalized MG, but only about 50% of pure ocular disease, so a negative result does not exclude ocular MG.^[2] When AChR antibodies are negative, testing for MuSK and LRP4 antibodies helps subtype the remainder (see Recognize the Condition).^[1]

Single-fibre electromyography has the highest sensitivity of any confirmatory test, at about 90%, but requires a neuromuscular specialist to perform. Repetitive nerve stimulation testing (looking for a decremental response with repeated stimulation) is more widely available but has relatively poor sensitivity.^[2]

The edrophonium (Tensilon) test — intravenous edrophonium producing transient improvement of weakness — is now considered obsolete in North America because of drug shortages and concerns about cardiorespiratory compromise during the test itself; do not expect it to be used or available.^[2]

Chest imaging to rule out thymoma

Computed tomography of the chest is usually ordered once the diagnosis of MG has been confirmed, specifically to identify thymoma, which occurs in 10 to 15% of patients.^[2] Its presence changes management directly: surgical removal is indicated for a thymoma regardless of the patient's age or MG severity, both as MG treatment and as oncologic treatment (see Choose Initial Treatment).

Differential diagnosis

In a young patient with diplopia, consider multiple sclerosis — additional sensory symptoms are common in MS but essentially never occur in MG. In an older patient presenting with dyspnea or limb weakness, consider amyotrophic lateral sclerosis (ALS) — ALS spares eye movements (which MG frequently affects) and typically produces visible fasciculations.^[2] Against the other classic causes of acute neuromuscular weakness, MG's fluctuating, fatigable pattern is the key distinguishing feature, in contrast to Guillain-Barré syndrome's steadily ascending weakness or botulism's steadily descending weakness — see the differential table in this platform's Guillain-Barré Syndrome chapter for the full three-way comparison.

• Control fatigable weakness symptomatically with pyridostigmine, recognizing that it does not modify the underlying disease.
• Achieve minimal symptom expression, or remission, with immunosuppression appropriately tailored to disease severity, distribution, and antibody status.
• Identify and treat thymoma, and appropriately select nonthymomatous patients for thymectomy.
• Avoid iatrogenic worsening — the genuinely practical centerpiece of pharmacist involvement in MG care — by screening every new prescription, including over-the-counter and as-needed items, against the drug-avoidance list.
• Recognize and rapidly escalate myasthenic crisis, and correctly distinguish it from cholinergic crisis.
• Minimize corticosteroid-related morbidity through an early steroid-sparing strategy.
• Support safe medication use and family planning across pregnancy, breastfeeding, and the surgical/perioperative setting.

Patient and caregiver education

• Explain the fatigable pattern explicitly — symptoms genuinely worsen with sustained activity and heat (including hot showers or baths) and improve with rest — so patients can pace activity and time pyridostigmine doses before anticipated exertion or meals rather than reactively after symptoms appear.
• Set expectations for the delayed onset of steroid-sparing agents (often many months to full effect) so a patient does not lose confidence in a new medication before it has had time to work.
• Review the specific symptoms that warrant urgent contact rather than waiting for the next visit (see Red Flags and Refer) — new breathlessness, difficulty swallowing or handling secretions, and any rapid worsening after a new medication chief among them.

The pharmacist's central role: medication reconciliation against the avoidance list

Screen every new prescription, and every over-the-counter or as-needed item, against the drug-avoidance list in Drug Tables at every encounter — not only at diagnosis. This is genuinely one of the highest-value, lowest-cost interventions available for an MG patient, given how many routinely prescribed drugs can worsen the disease or precipitate crisis.

Aspiration precautions and nutrition

For patients with bulbar involvement, arrange a formal swallowing assessment, time meals to coincide with peak pyridostigmine effect, and consider texture modification; escalate to enteral nutrition support if oral intake cannot safely meet needs.

Vaccination planning

Confirm age-appropriate vaccination status, including meningococcal vaccination specifically before any complement inhibitor is started (see Drug Tables), proactively and early — ideally before significant immunosuppression begins, since live vaccines should generally be avoided during substantial immunosuppressive therapy.

Perioperative and surgical planning

Coordinate any planned surgery — including thymectomy — with anesthesia in advance; MG patients are more sensitive to neuromuscular blocking agents and inhalational anesthetics, with a block that can be prolonged and unpredictable (see Drug Tables).

Psychosocial support

MG is a chronic, often relapsing-remitting disease with a genuine fatigue burden even outside overt flares; screen for and address the psychosocial impact, and connect patients with peer-support and neuromuscular-disease resources where available.

Pregnancy

MG can flare during pregnancy, particularly in the first trimester and postpartum period, and disease activity should be optimized before conception whenever the clinical situation allows planning.^[5] Critical avoidance intravenous magnesium sulfate — commonly used for pre-eclampsia, eclampsia, or tocolysis — is a well-documented precipitant of myasthenic crisis; flag any pregnant patient with MG to the obstetric team before magnesium is given, and ensure the flag is visible in the chart (see Red Flags and Drug Tables).^[4] Pyridostigmine, prednisone, azathioprine, and IVIG are generally considered compatible with pregnancy; mycophenolate mofetil and methotrexate are teratogenic and should be discontinued with an adequate washout before a planned conception. Roughly 10 to 20% of infants born to mothers with MG develop transient neonatal myasthenia from transplacental antibody transfer — a self-limited weakness (poor feeding, hypotonia, occasionally respiratory difficulty) typically resolving within weeks.^[1] Alert the neonatal team in advance so this can be anticipated and supported rather than misattributed to another cause.

Breastfeeding

Pyridostigmine, prednisone, azathioprine, and IVIG are generally considered compatible with breastfeeding; avoid mycophenolate mofetil and methotrexate as with pregnancy.

MuSK-antibody-positive MG

Treat differently from AChR-positive disease MuSK-positive MG more often produces prominent bulbar, facial, neck, and respiratory weakness, sometimes with visible facial and tongue atrophy; it frequently responds poorly to — and can even be worsened by — pyridostigmine.^[4] Thymectomy has no established benefit, since the thymic hyperplasia that justifies it in AChR-positive disease is absent.^[6] Rituximab, rather than the standard first-line steroid-sparing agents, is the preferred escalation for refractory disease in this subgroup.^[6]

Juvenile and pediatric MG

MG in children and adolescents is genuinely rarer than adult-onset disease and more often presents as ocular-predominant disease; distinguish it clearly from transient neonatal myasthenia (above), which is passive antibody transfer rather than the child's own autoimmune disease. Management principles otherwise follow the same treatment ladder, adjusted for age and weight, with pediatric neurology involvement.

Older adults and thymoma

Late-onset MG in older men carries a higher rate of associated thymoma and warrants the same mandatory chest imaging as any newly diagnosed patient (see Assessment and Diagnosis).^[2] Polypharmacy compounds the drug-avoidance risk in this group — reconcile medication lists against the avoidance list (see Drug Tables) with particular vigilance, since older adults are more likely to be prescribed several of the higher-risk classes (beta-blockers, calcium channel blockers, fluoroquinolones) for unrelated indications.

Perioperative and anesthesia considerations

MG patients are more sensitive than unaffected individuals to nondepolarizing neuromuscular blocking agents, with a block that can be prolonged and unpredictable; nondepolarizing agents and inhalational anesthetics are best avoided where a suitable alternative exists, and when a nondepolarizing agent is required, close postoperative monitoring and sugammadex reversal reduce the risk of postoperative myasthenic crisis (see Drug Tables).^[4]

Renal impairment

Adjust azathioprine and methotrexate cautiously in reduced renal function, and note that renal impairment independently raises the risk of aminoglycoside- and magnesium-associated neuromuscular blockade (see Drug Tables) — a genuine double reason for caution when these agents cannot be avoided.^[4]

Hepatic impairment

Azathioprine and methotrexate both carry hepatotoxicity risk and need closer monitoring, or an alternative agent, in significant hepatic impairment.

Disease-activity monitoring

Track functional status at every visit using a validated instrument such as the MG Activities of Daily Living (MG-ADL) scale or the Quantitative Myasthenia Gravis (QMG) score — the same instruments used in the pivotal trials referenced in Pharmacologic Treatment Options and Drug Tables — rather than relying on antibody titres, which correlate poorly with clinical activity in individual patients.

Steroid-sparing agent monitoring

• Azathioprine: dose according to TPMT activity where available; monitor CBC and liver enzymes regularly.
• Mycophenolate mofetil: monitor CBC and for signs of infection.
• Methotrexate: routine CBC, liver enzymes, and renal function; give with folic acid.
• Cyclosporine and tacrolimus: monitor renal function, blood pressure, and (for tacrolimus) trough levels.

Corticosteroid monitoring

Standard bone- and GI-protective bundle for a prolonged corticosteroid course: vitamin D and adequate calcium intake with bone-density monitoring, regular blood pressure and glucose checks, and a proton pump inhibitor for corticosteroid-related reflux where indicated (see Drug Tables).

Biologic-specific monitoring

• Complement C5 inhibitors (eculizumab, ravulizumab, zilucoplan): confirm meningococcal vaccination status is complete and current before starting, and maintain a low threshold to investigate fever given the encapsulated-organism infection risk throughout treatment.
• FcRn antagonists (efgartigimod alfa, rozanolixizumab, nipocalimab): monitor for infection given the on-target reduction in total IgG; no meningococcal-specific requirement.
• Rituximab: monitor B-cell counts, immunoglobulin levels, and for infection per specialist-directed protocol.

Ongoing medication reconciliation

Re-screen the full medication list against the drug-avoidance list (see Drug Tables) at every follow-up, not only at diagnosis — new prescriptions from other providers are a recurring, preventable source of exacerbation.

Post-thymectomy follow-up

Continue routine disease-activity and steroid-sparing-agent monitoring after thymectomy; benefit accrues gradually over months to years rather than immediately post-surgery, matching the MGTX trial's 3- and 5-year time horizons.^[7]

1. Inadequate response to pyridostigmine alone, or generalized disease at diagnosis: add corticosteroid induction together with a steroid-sparing agent (azathioprine or mycophenolate mofetil) from the outset, rather than waiting for steroid monotherapy to fail.
2. Inadequate response to azathioprine or mycophenolate mofetil after an adequate trial (allow the many months these agents take to reach full effect): switch to or add methotrexate, cyclosporine, or tacrolimus, and confirm whether thymectomy has already been considered in an AChR-antibody-positive, nonthymomatous candidate.
3. Refractory, AChR-antibody-positive generalized MG despite an adequate trial of corticosteroids plus a non-steroid immunosuppressant: escalate to a complement C5 inhibitor (eculizumab, ravulizumab, or zilucoplan) or an FcRn antagonist (efgartigimod alfa, rozanolixizumab, or nipocalimab), selecting based on antibody status, dosing route/frequency preference, and the meningococcal-vaccination requirement for the complement inhibitors.
4. MuSK-antibody-positive disease refractory to pyridostigmine and standard immunosuppression, or any subtype refractory to the above: rituximab.
5. Rapidly worsening weakness, bulbar compromise, or any respiratory red flag at any point in this ladder: do not simply continue escalating outpatient immunosuppression — treat as possible myasthenic crisis, obtain bedside respiratory assessment, and start IVIG or plasma exchange (see Clinical Decision Algorithm).
6. Apparent treatment failure shortly after starting a new medication for an unrelated indication: reconsider a drug-avoidance-list culprit (see Drug Tables) before assuming primary disease progression.

• Prescribing an aminoglycoside, fluoroquinolone, or macrolide antibiotic for an MG patient without checking for a safer alternative first, or without at least counseling closer monitoring if no alternative exists.^[4]
• Starting a high dose of corticosteroid in an unmonitored outpatient with generalized MG, rather than titrating up gradually or arranging inpatient/IVIG-plasma-exchange bridging for patients who need rapid disease control.^[4][5]
• Giving or allowing intravenous magnesium sulfate for pre-eclampsia, eclampsia, or tocolysis in a pregnant patient with MG without flagging the risk to the obstetric team first.^[4]
• Confusing myasthenic crisis with cholinergic crisis and empirically increasing pyridostigmine when a patient's weakness worsens — this can make a true cholinergic crisis significantly worse (see Clinical Decision Algorithm).^[5]
• Offering thymectomy to a MuSK-antibody-positive patient expecting the same benefit seen in AChR-positive disease — the underlying thymic pathology, and the trial evidence, simply do not apply.^[4][6]
• Starting a complement C5 inhibitor (eculizumab, ravulizumab, or zilucoplan) without first confirming meningococcal vaccination is complete.
• Skipping chest computed tomography for thymoma once MG is diagnosed, or assuming a young patient's age makes thymoma unlikely enough to defer imaging.^[2]
• Using a nondepolarizing neuromuscular blocking agent or inhalational anesthetic for a surgical MG patient without coordinating the anesthetic plan with anesthesia and neurology in advance.^[4]
• Assuming pyridostigmine is disease-modifying, or that a patient whose symptoms are well controlled on pyridostigmine alone therefore has mild, self-limited disease rather than a condition still requiring monitoring for progression.
• Reassuring a patient with new bulbar or respiratory symptoms as "just fatigue" and deferring escalation, rather than obtaining bedside respiratory assessment promptly.^[5]
• Injecting botulinum toxin, for a cosmetic or unrelated therapeutic indication, in a patient with known MG without explicit caution and slow dose titration.^[4]
• Prescribing methotrexate for MG on the assumption it carries the same steroid-sparing evidence as azathioprine or mycophenolate mofetil — a dedicated randomized trial found no steroid-sparing benefit.^[8]

How to use this table: Doses are adult doses and require patient-specific confirmation (renal/hepatic function, weight, allergy history). Pediatric dosing is weight-based per the current product monograph unless stated. Drug pricing is intentionally omitted. Select any drug name to open its complete Pharmasist Drug Database page.

Symptomatic Acetylcholinesterase Inhibitor Therapy

First-line for essentially all newly diagnosed patients except often MuSK-antibody-positive MG; purely symptomatic and not disease-modifying.

Corticosteroid Induction Therapy

First-line immunosuppression for inadequate symptomatic control or generalized disease; start a steroid-sparing agent concomitantly rather than after steroid failure.

Steroid-Sparing / Non-Steroid Immunosuppressants

Onset of effect is delayed by months — start alongside induction corticosteroid, not after it fails.

Rapid Immunomodulation — Myasthenic Crisis and Refractory Exacerbation

Onset within days; used for crisis, rapid pre-operative optimization, or disease refractory to maintenance immunosuppression.

Complement C5 Inhibitors — Refractory AChR-Antibody-Positive Generalized MG

All three require confirmed, current meningococcal vaccination before starting because of the encapsulated-organism infection risk inherent to terminal complement blockade.

Neonatal Fc Receptor (FcRn) Antagonists — Refractory Generalized MG

Accelerate degradation of circulating IgG, including the pathogenic autoantibody; infection risk relates to the resulting drop in total IgG rather than to complement blockade, so there is no meningococcal-specific requirement.

B-Cell Depletion

The preferred escalation specifically for MuSK-antibody-positive disease, where thymectomy has no established role.

Cholinergic Crisis and Muscarinic Symptom Management

Atropine treats the muscarinic symptoms of acetylcholinesterase-inhibitor excess; it does not reverse the underlying weakness, which requires holding the offending inhibitor and, if needed, ventilatory support.

Drugs That Can Worsen or Unmask MG, or Precipitate Myasthenic Crisis

This is the single highest-yield table in this chapter for day-to-day pharmacist practice. Evidence levels reflect the strength of the published association; "Avoid" means avoid if any reasonable alternative exists and monitor closely if not, not an absolute contraindication in a genuine emergency.

Supportive Therapy Alongside Long-Term Corticosteroids

Standard bone- and GI-protective bundle for any patient on a prolonged corticosteroid course.