Differential diagnosis

There are many diseases similar to myositis and should be considered in the medical investigation. Also, when treatment does not work as expected, similar conditions should be carefully considered.

• ALS (amyotrophic lateral injury)
  • Distal affection, hyper-reflex, muscle fibrillation, spasms, slightly increased CK

• Amyloid myopathy
  • Subcutaneous changes
  • Proteinuria
  • Typical biopsy (dye with Congo)

• Carnitine palmitoyl transferase II deficiency
  • Muscle pain

• Diabetic amyotrophy and diabetic muscle infarction
  • Diabetes Type 2
  • Acute, asymmetric onset of focal pain and weakness in thighs.
  • Often weight loss
  • EMG differs from myositis

• Electrolyte shortage
  • Hypo-kalaemia
  • Hypo-kalsemi
  • Hypo-magnetite semi

• Eosinophilic myositis
  • Differential diagnoses of eosinophil myositis
    • Hypereosinophilic Syndrome: IgE, eosinophilic leukocytes> 1.5 × 10⁹/ L, skin reaction. Multiorgan involvement, Pain and weakness in proximal muscles, Raynaud's , vasculitis in small blood vessels, CK may be elevated
• Eosinophilic fasciitis (CK usually normal, typical findings in skin and subcutaneous tissue)

• Fibromyalgia
  • Very common among otherwise healthy women
  • General pain (chronic in both upper and lower extremities, neck or back)
  • Tenderpoints
  • Normal samples
• Glycogen storage disease type V: McArdle's disease / phosphorylase deficiency
  • Pain / cramps after activity
• Glycogen storage disease type II, Adult acid maltase deficiency (Pompe disease)
  • Infantile, very severe form and adult, “late-onset” form with a mean age of 36 years at diagnosis
  • Progressive loss of strength, with difficulty breathing (weak respiratory muscles).
• GVH (graft versus host) Myositis after transplantation
  • Mostly more than one year after transplant
  • Findings as in myositis

• HIV infection
  • Chronic weight and power loss
  • Clinical picture with CK increase (inflammatory myopathy) as in myositis
  • HIV test
  • CD4 T cells do not correlate well with myopathy symptoms

• Hypothyroidism
  • High TSH, Low f-T4
• Inclusion body myositis
  • Lower creatine kinase (CK) in blood, very slow progression, higher age, distal affection, no antibody, typical biopsy
• IMNM (Immunmediated Necrotising Myositis)
  • May be related to statins (cholesterol lowering drugs)
  • HMGCR antibody is present in some
• Calcinosis (columns under the skin and in soft tissue) occurs in various conditions
• Cataplexy
  • Sudden loss of muscle power in several seizures
  • By narkolepisi 70% also has cataplexy

• Kennedy disease (Spinal and bulbar muscular atrophy, SBMA)
  • Weakened force and symmetrical muscle atrophy in proximal muscles. Creatine kinase (CK) is moderately elevated. Men in 40-50 years of age. Reduced androgen sensitivity with gynecomastia, atrophy tested and decreased libido. Fasciculations in facial muscles. Speech and swallowing difficulties
  • Separated from myositis by clinical picture (facial muscles, gynecomastia, EMG and muscle biopsy)
• Lambert-Eaton syndrome
  • Myasthenia-like
  • Paraneoplastic Syndrome
  • Not eye muscle affection
  • Antibody against voltage-gated calcium channel (VGCC)
  • EMG discovery
• Macro-CK
  • Macro Enzymes are normal enzymes (or isoenzymes) that bind either to immunoglobulins (IgG) = Type 1 or lipoproteins and other substances (Type 2) and thus accumulate in serum. They are not free enzymes, but cause falsely elevated CK measurements.
  • Macro-CK is most commonly seen in people over 60 years
  • Macro-CK may be present with or without associated disease, including chronic liver disease and malignancy

• Drug induced myopathy
  • Statins
    • Pain with or without elevated creatine kinase (CK) in blood. Drug history is important
  • High corticosteroid doses (Prednisolone) over a long period of time: Steroid myopathy:
    • CK normal
  • Amiodarone hydrochloride (Cordarone) against heart rhythm disorders
  • Plaquenil
  • Cocaine
  • Alcohol
  • Colchicine
• Mitochondrial myopathy, encephalomyopathy, lactate acidosis, stroke-like symptoms (MELAS)
• Mitochondrial Disease

• Muscular Dystrophy (Hereditary progressive muscle diseases)
  • Should always be considered by chronic muscular disease
• Myastenia gravis
  • Increased physical muscular fatigue
  • Often in facial muscles, including eyes
  • Normal CK
  • EMG is characteristic
  • Anti-acetylcholine antibodies
• Myoclonic Epilepsy with Ragged Red Fibers»(MERRF)
  • "Ragged Red Fibers" are congested diseased mitochondria in tissue samples (biopsy)
  • Low body height, hearing loss, lactate acidosis, intolerance to physical exercise
• Parkinson's disease
  • Tremor

• Periodic paralysis
• Polymyalgia rheumatica
  • Quick start
  • Pronounced morning stiffness
  • Pain and stiffness, not primarily muscle weakness
  • Higher age
  • High ESR and CRP
  • Normal CK
• Post-polio syndrome
  • 15-30 years after poliomyelitis (35-60 years of age)
  • Muscle pain, weakness, Exhaustion
• Rhabdomyolysis
  • CK is very high above 10.000 (Note: kidney damage risk)
  • After injury, intoxication or critical illness
• Rhabdomyolysis and myalgia syndrome associated with RY1 mutation
  • Increased risk of hyperthermia
  • May have moderately elevated CK and muscle manifestations
• Refsum disease
  • Retinitis pigmentosa
  • Chronic polynevropathy with pares
  • Muscular atrophy distal
  • Cerebellar symptoms with ataxia
• Sarkoid myopathy
  • Most often asymptomatic
  • Sarcoidosis in other organs
  • Granulomas in biopsy
• Scleromyositis (disease with overlapping symptoms against systemic sclerosis)
• Spinal muscle atrophy
  • Hypertrophy of legs (Ørstavik K, 2020)
• Statin-induced myopathy
• Steroid-induced myopathy
  • Muscular weakness in prolonged corticosteroids
  • Normal CK

• Subaracnoidal hemorrhage and stroke
• Trichionose
  • Parasitic infection (tricin) from the intake of rat (pig) meat.
  • Strong muscle pain and enzyme (CK) increases
  • Eosinophilia
  • Muscle biopsy confirms the diagnosis
• Viral myositis (rare) in: Influenza A and B, H1N1 virus, Coxsackie virus, Epstein-Barr virus, Herpes simplex virus, Parainfluenza, Adeno virus, Echovirus, Cytomegalovirus (CMV), Measles virus, Varicella-Zoster, Human Immunodeficiency Virus (HIV), Dengue fever
  • Rapid disease development may be limited to few muscle groups. Effect of antiviral treatment

Diagnosis of elevated serum CK

• If creatine kinase (CK) is below 1000, the sample is repeated without prior physical activity
  • Elevated CK levels are expected to be halved by approx. every 24-36 hours if the triggering cause is removed. Normalization within 3-5 days is expected
• Trauma, physical overload, cramps, infections, injections, metabolic myopathy. At very high CK consider: Rhabdomyolysis
• Troponin T (CTnT) is not specific to the heart muscle and often exceeds the skeletal muscle involvement. Troponin I (cTnI), on the other hand, is more myocardial specific.
  • Troponin T can be increased by
    • Myocardial damage (heart attack, heart failure, heart contraction, Heart amyloidosis, Heart sarcoidosis
    • Acute aortic dissection
    • Acute respiratory distress syndrome (ARDS)
    • Hypotension and shock
    • Muscle trauma
    • Pulmonary embolism
    • Kidney failure
    • Overtraining
• Among ethnic black Africans, CK is up to 70% higher than in whites (reference: Brewster CM, 2012)

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