Muscle Weakness

a. Inflammatory:

Sub-acute, usually painful:

1) Polymyositis and dermatomyositis (DM): proximal muscle weakness, if involves the skin then its DM

» Clinical presentation:

  • Photosensitive rash
  • More in females
  • Symmetrical
  • Joint pain -> better w\ activity
  • Hair loss
  • Raynaud’s phenomenon
  • Risk of interstitial lung disease
  • Associated w\ malignancy (ovary, lung, GI, lymphoma)
  • Ask about esophageal\cardiac\respiratory muscle involvement


» Investigations:

  • Muscle enzymes: creatinine kinase (CPK) (shooting high) + aldolase
  • ESR and CRP
  • Autoantibodies: anti-Mi-2, anti-Jo-1 (means myositis w\ lung fibrosis)
  • EMG: myositis triad (muscle fibrillations, spontaneous discharges, sharp waves)
  • MRI: inflamed muscles (thigh\deltoid)Muscle weakness 6
  • Tumor markers


» Treatment:

  • Steroids (prednisolone)
  • Steroid sparing agents: MTX, AZA, cyclophosphamide, mofetil
  • IVIG
  • Hydroxychloroquine for skin lesionsMuscle weakness 5


2) Inclusion body myositis 

  • More in elderly
  • More in males
  • Chronic
  • Distal + proximal
  • Asymmetrical \ LL
  • Involves esophagus


3) Autoimmune:

  • Connective tissue disease, SLE, scleroderma

Muscle weakness 7


b. Metabolic: 

  • Acute\subacute, painless, normal muscle enzymes
  • DM, hypokalemia, hypocalcemia, hypothyroid, Cushing’s

Muscle weakness 8


c. Drugs\toxins:

  • Acute, painful (causes necrosis)
  • Statins, alcohol, antiretroviral


d. Infectious:

  • Subacute, painful
  • Viral (peds: benign reactive myositis)


e. Neuropathic:

  • Chronic
  • MND (motor neuron disease)


f. Stroke:

  • Acute



Random notes:

  • Periodic hypokalemia paralysis -> comes in attacks
  • Myasthenia gravis -> worse w\ activity, diurnal variation
  • Lambert-Eaton -> improves w\ activity


Download the PDF version: here


  • Davidson’s Principles and Practice of Medicine
  • The Johns Hopkins Internal Medicine Board Review
  • Master the boards

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