Myopathies — disorders of skeletal muscle — span a vast diagnostic territory: inflammatory (dermatomyositis, polymyositis, inclusion body myositis, immune-mediated necrotizing myopathy), inherited (Duchenne, Becker, limb-girdle muscular dystrophies, myotonic dystrophy), metabolic (Pompe disease, McArdle disease), mitochondrial, toxic (statin myopathy, alcohol, glucocorticoid), and channelopathies. The pharmacotherapy is correspondingly diverse — from immunosuppression to enzyme replacement to gene therapy. This page covers the major myopathy categories and their treatments.

Inflammatory Myopathies

Dermatomyositis (DM)

  • Prednisone: 1 mg/kg/day; gradual taper.
  • Methotrexate: 7.5-25 mg weekly; steroid-sparing.
  • Mycophenolate: alternative steroid-sparing.
  • IVIG: 2 g/kg over 2-5 days monthly; particularly effective for skin.
  • Rituximab: refractory disease.
  • Hydroxychloroquine: for skin manifestations.
  • Cyclophosphamide: severe refractory.
  • JAK inhibitors: tofacitinib for selected refractory.
  • Tumor screen mandatory (paraneoplastic associations).

Polymyositis (PM)

  • Similar to DM (steroids first-line; steroid-sparing).
  • Many cases now recognized as IMNM or IBM in retrospect.

Immune-Mediated Necrotizing Myopathy (IMNM)

  • Often anti-HMGCR or anti-SRP antibody-positive.
  • Steroids + steroid-sparing (mycophenolate, methotrexate, rituximab).
  • IVIG.
  • Refractory cases may need cyclophosphamide.

Inclusion Body Myositis (IBM)

  • Less steroid-responsive than DM/PM.
  • IVIG: limited benefit; sometimes tried.
  • Exercise: important.
  • Symptomatic supportive care.
  • Investigational: arimoclomol, rapamycin in trials.

Anti-Synthetase Syndrome

  • Anti-Jo-1 most common.
  • Often ILD; mechanic’s hands; arthritis.
  • Treat aggressively (steroids + mycophenolate, rituximab).

Drug-Induced Myopathies

  • Statins: discontinue; monitor CK; HMG-CoA reductase antibody screening.
  • Glucocorticoid: prevention with exercise; minimize dose.
  • Colchicine: discontinue.
  • Antimalarials (chloroquine, hydroxychloroquine): rare.
  • Zidovudine: HIV/antiretroviral.

Duchenne Muscular Dystrophy (DMD)

Standard Care

  • Glucocorticoids: prednisone or deflazacort; slows progression; ~6 months to start.
  • Deflazacort (Emflaza): FDA-approved 2017; less weight gain.
  • Side effects: weight gain, growth suppression, fractures, behavioral, cataracts.
  • Bone protection, vitamin D, calcium.

Gene-Targeted Therapies

  • Eteplirsen (Exondys 51): ASO; exon 51 skipping; for amenable mutations (~13% of DMD); IV weekly; modest dystrophin restoration.
  • Golodirsen (Vyondys 53): ASO; exon 53 skipping; ~8% of patients.
  • Viltolarsen (Viltepso): ASO; exon 53 skipping; alternative.
  • Casimersen (Amondys 45): ASO; exon 45 skipping; ~8% of patients.
  • All FDA-approved on accelerated pathway; dystrophin restoration biomarker; clinical efficacy data emerging.

Gene Therapy

  • Delandistrogene moxeparvovec (Elevidys): AAV-based gene therapy delivering micro-dystrophin.
  • FDA-approved 2023 for ambulatory pediatric patients (4-5 yo); expanded to broader DMD population 2024.
  • One-time IV infusion.
  • Cost: very high.
  • Cardiac and liver monitoring; immunosuppression peri-infusion.

Newer Strategies

  • Vamorolone (Agamree): dissociative steroid; FDA-approved 2023; similar efficacy with less side effects.
  • Givinostat: HDAC inhibitor; FDA-approved 2024 for DMD; oral; modulates inflammation and muscle regeneration.

Becker Muscular Dystrophy

  • Milder than DMD due to partial dystrophin function.
  • No specific disease-modifying therapy currently approved (gene therapy approaches under investigation).
  • Supportive: PT, orthopedic, cardiac monitoring.

Limb-Girdle Muscular Dystrophies (LGMDs)

  • Heterogeneous; multiple genetic forms.
  • No general disease-modifying therapy.
  • Specific to subtype:
    • LGMD2I (anoctaminopathy): supportive.
    • Limb-girdle myasthenic syndromes: similar to MG.
    • Pompe disease (LGMD2V): enzyme replacement (see below).

Myotonic Dystrophy

DM1 and DM2

  • Myotonia: mexiletine (oral Na⁺ channel blocker); also useful for cramps.
  • Excessive daytime sleepiness: modafinil; address sleep apnea.
  • Cardiac monitoring: conduction abnormalities (PR prolongation, AV block, arrhythmia).
  • No disease-modifying therapy currently approved.
  • Cataract surgery for cataracts (common).
  • Avoid succinylcholine, sedatives carefully.

Metabolic Myopathies

Pompe Disease (GAA Deficiency)

  • Alglucosidase alfa (Lumizyme): enzyme replacement; IV every 2 weeks.
  • Avalglucosidase alfa (Nexviazyme): newer formulation; improved delivery.
  • Cipaglucosidase alfa with miglustat (Pombiliti, Opfolda): emerging.
  • Early treatment particularly important; pre-symptomatic in infantile form.
  • Newborn screening expanding.

McArdle Disease (Glycogen Storage Disease Type V)

  • Sugar before exercise.
  • Vitamin B6: emerging benefit.
  • Aerobic training to reduce attacks.
  • Avoid statins.

CPT II Deficiency, Other Fatty Acid Oxidation Disorders

  • Triheptanoin (UX007): emerging.
  • Avoid prolonged fasting.
  • Specific dietary management.

Mitochondrial Myopathies

Symptomatic

  • Coenzyme Q10, riboflavin, creatine, L-carnitine (empirical “mitochondrial cocktail”).
  • Evidence modest.

Specific Conditions

  • MELAS: arginine for acute stroke-like episodes; citrulline.
  • Leber hereditary optic neuropathy: idebenone in some markets; emerging gene therapy.
  • Friedreich ataxia: omaveloxolone (Skyclarys) FDA-approved 2023.

Channelopathies (Muscle)

Myotonia Congenita (Cl⁻ Channel)

  • Mexiletine: first-line.
  • Lamotrigine: alternative.
  • Acetazolamide: alternative.

Paramyotonia Congenita (Na⁺ Channel)

  • Mexiletine.
  • Acetazolamide.
  • Avoid cold, depolarizing exercise.

Periodic Paralyses

  • Hyperkalemic (Na⁺ channel): avoid K⁺-rich foods; acetazolamide; thiazide diuretics; mexiletine.
  • Hypokalemic (Ca²⁺ or Na⁺ channel): avoid Na⁺/insulin; potassium during attacks; acetazolamide; dichlorphenamide (FDA-approved); spironolactone.
  • Andersen-Tawil syndrome (Kir2.1): cardiac monitoring; acetazolamide.

Malignant Hyperthermia

  • RYR1 mutations.
  • Triggered by halogenated anesthetics, succinylcholine.
  • Treatment: dantrolene IV; cooling; supportive.
  • Genetic testing for family screening.
  • Avoid trigger anesthetics in known carriers.

Statin Myopathy

  • Discontinue statin; CK monitoring.
  • Most resolve.
  • If persists, consider anti-HMGCR-mediated IMNM (different disease; requires immunotherapy).
  • Re-challenge with different statin often possible.
  • Ezetimibe, bempedoic acid alternatives.

Steroid Myopathy

  • Common with chronic high-dose steroids.
  • Prevention: minimize dose; resistance exercise.
  • Treatment: dose reduction; PT.

Critical Illness Myopathy

  • Often coexists with critical illness polyneuropathy.
  • Prevention: minimize neuromuscular blockade, glucose control.
  • Recovery often slow.

Toxic Myopathies

  • Alcohol, cocaine, heroin: discontinuation, supportive.
  • Colchicine: discontinue.
  • Cyclosporine: rare.
  • Antimalarials: rare; discontinue.
  • Zidovudine: alternative ART.

🔍 Did You Know?

The 2023 FDA approval of delandistrogene moxeparvovec (Elevidys) — the first one-time gene therapy for Duchenne muscular dystrophy — represents a transformational moment in muscle disease therapy. Elevidys is an AAVrh74-based gene therapy delivering an engineered micro-dystrophin gene small enough to fit in AAV vector. The therapy provides a functional (though smaller) version of dystrophin, the protein deficient in DMD. The initial approval was for ambulatory DMD patients ages 4-5, expanded in 2024 to broader age ranges based on emerging data. Key implications: one-time IV infusion versus lifelong corticosteroid and supportive care; cost is high (~$3.2 million per dose) but balanced against decades of expensive DMD care; immunosuppression around the infusion to prevent immune response to AAV capsid; cardiac and liver monitoring for adverse events. The lesson generalizes: gene therapy for monogenic diseases has matured to clinical reality, and the approach is being applied across hemophilia, sickle cell, SMA, retinal dystrophies, and many other conditions. For practicing neurologists, the take-home: DMD treatment has fundamentally evolved beyond corticosteroid maintenance to include disease-modifying gene therapy, and patients should be referred to specialized MDA-affiliated centers for evaluation. The same paradigm shift is unfolding for other neurologic conditions — from spinal muscular atrophy (already mature) to Huntington disease (early trials) to gene therapy for inherited neuropathies. The era of treating rare neurogenetic disease through corrected biology has arrived.

Pitfalls and Pearls

  • Inflammatory myopathies: steroids + steroid-sparing; methotrexate, mycophenolate.
  • DM: tumor screen mandatory.
  • IBM: less steroid-responsive than DM/PM.
  • Anti-HMGCR IMNM: distinct from statin myopathy; needs immunotherapy.
  • DMD: glucocorticoids slow progression; gene therapy emerging.
  • Vamorolone: dissociative steroid for DMD; less side effects.
  • Elevidys (delandistrogene moxeparvovec): gene therapy for DMD.
  • ASOs for DMD: eteplirsen, golodirsen, viltolarsen, casimersen for exon skipping.
  • Myotonic dystrophy: mexiletine for myotonia; cardiac monitoring critical.
  • Pompe disease: enzyme replacement (alglucosidase alfa, avalglucosidase alfa).
  • McArdle disease: sugar before exercise; second-wind phenomenon.
  • Channelopathies: mexiletine for myotonia; acetazolamide for periodic paralyses.
  • Malignant hyperthermia: dantrolene; family screening.
  • Statin myopathy: discontinue; if persists, screen anti-HMGCR.
  • Mitochondrial myopathies: empirical “cocktail”; specific syndromes have specific treatments.
  • Friedreich ataxia: omaveloxolone FDA-approved 2023.
  • Newborn screening: SMA, Pompe, MCAD — pre-symptomatic identification dramatically improves outcomes.

References

  1. Lundberg IE, Tjärnlund A, Bottai M, et al. 2017 European League Against Rheumatism/American College of Rheumatology classification criteria for adult and juvenile idiopathic inflammatory myopathies. Ann Rheum Dis. 2017;76(12):1955-1964.
  2. Mendell JR, Sahenk Z, Lehman K, et al. Assessment of systemic delivery of rAAVrh74.MHCK7.micro-dystrophin in children with Duchenne muscular dystrophy: a nonrandomized controlled trial. JAMA Neurol. 2020;77(9):1122-1131.
  3. Schoser B, Stewart A, Kanters S, et al. Survival and long-term outcomes in late-onset Pompe disease following alglucosidase alfa treatment: a systematic review. J Neurol. 2017;264(4):621-630.
  4. Statland JM, Bundy BN, Wang Y, et al. Mexiletine for symptoms and signs of myotonia in nondystrophic myotonia: a randomized controlled trial. JAMA. 2012;308(13):1357-1365.
  5. Sansone VA, Burge J, McDermott MP, et al. Randomized, placebo-controlled trials of dichlorphenamide in periodic paralysis. Neurology. 2016;86(15):1408-1416.