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Myositis diagnostics

VIEWS: 272 PAGES: 18

									Myositis Diagnostics Laboratory and Imaging
Paul Lambie

Laboratory Creatine Kinase
• Most common lab used for Dx and monitoring • Normally 99% MM isoform in skeletal muscle • MB fraction in skeletal muscle increases in IIM, muscular dystrophy, well trained athletes, and after severe exertion
– May be confused with myocardial infarction – Troponin I more specific for cardiac muscle

Creatine Kinase
• Increased in > 90% PM, 70-90% DM, 80% IBM at onset; 95% hCK at some time • Normal CK may be due to inhibitor • ? Normal CK associated with poor prognosis • CK levels generally correlate with disease activity but poorly with strength • Increase in CK may predict flare – up to 6 weeks in advance • Disease can remain active despite normal CK

Creatine Kinase
• Other causes of elevation
– Trauma – injury, needle stick, EMG, surgery, – Muscle disease – infection, metabolic myopathies, MD, MI, ALS – Drugs – statins, ETOH, zidovudine, colchicine, DPCM – Endocrine – hypothyroid, iK, renal failure, DKA – Increased CK-BB – CNS disease – Prolonged or strenuous exercise – Ethnic group – Increased muscle mass

Creatine Kinase
• Asymptomatic elevation (Hyper-CK-emia)
– “Presymptomatic” myopathies – Predisposition to malignant hyperthermia; 11/30 patients with unexplained hCK had abnormal halothane and/or caffeine contracture test – Manual labor occupations – Idiopathic – no explanation despite Bx, no weakness after prolonged follow up

Hyper-CK-emia
• Joy, Oh - Asymptomatic 19 pts.
– CK > 2x normal – Normal neuromuscular exam – EMG abnormal 14/19 (74%) – Bx (+) 15/19 (79%), 14/15 had abnormal EMG – 1 year follow-up – 5 pts. with mild proximal muscle weakness (4 PM, 1 mitochondrial myopathy)

Hyper-CK-emia
• Prelle, A, et.al. – 114 a- or minimally symptomatic pts. with increased CK • Lab, EMG, Bx performed • Neuromuscular disorder 21 (18.4%) • Pathologic, inconclusive results 57 (50%) • Normal 36 (31.6%)

Hyper-CK-emia Evaluation
• Rule out other causes – drugs, exercise, trauma, TSH, etc. • How extensively should pts. with normal strength be evaluated?
– ?EMG – ?Bx – ?MRI – ?Follow strength serially, further w/u if weakness

Other Muscle Enzymes
• Consider testing if CK normal • Aldolase
– May be increased in patients with normal CK – Less sensitive and specific than CK

• LDH
– Isolated hLDH, consider malignancy, hemolysis

• ALT, AST
– May correlate with disease activity – May confuse MTX liver monitoring

• Myoglobin
– Correlates with disease activity – Not readily available

Autoantibodies
• ANA
– (+) in 50-80% of IIM – Non specific but may help differentiate from dystrophies or nonautoimmune myopathies – Frequency highest in overlap syndromes, lowest in malignancy associated

• Myositis specific antibodies (MSA’s)
– Antibodies to RNA-protein complexes involved in protein synthesis – Seen in 30-50% of IIM – Rarely (+) in IBM

• Myositis associated antibodies (MAA’s)

MSA’s
• Antisynthetase Ab’s
– Jo-1 (hystidyl transferase) most common – up to 20% – Antisynthetase syndrome - arthritis, ILD, Raynauds, fever, mechanic’s hands – Myositis may be more persistent

• Anti-Signal Recognition Particle
– ? Increased cardiac involvement – Possibly more severe disease, more resistant to treatment

• Anti-Mi-2
– Associated with skin manifestations in DM – Gottron’s papules, heliotrope, “V” sign, shawl sign – Good response to treatment

MAA’s
• Anti-PM-Scl
– Associated with overlap of scleroderma (usually limited) and PM or DM – Myositis often mild, responds well to Rx – Arthritis often prominent

• U1RNP associated with MCTD • SSB 5 – 6% • SSA 4 – 17%

Use of Antibody Tests
• Not usually useful for disease monitoring • Some have suggested using MSA’s for Dx, substituting for other criteria • PM-Scl, U1RNP may help in assesment of individual patients

Imaging
• MRI
– Can show areas of inflammation, edema, fibrosis, calcification – Assess large amounts of muscle, less sampling error – May guide muscle Bx – Bx (-) in 10-25% – Can follow serially – Non specific – changes also seen in injury, infection, radiation therapy, subacute denervation, rhabdomyolysis, sickle cell crisis, compartment syndrome

MRI
• Fraser, et.al
– 40 patients with IIM (17 PM, 10 DM, 13 IBM) – MRI sensitivity for active disease 89% vs, 66% for Bx – Specificity – MRI 88%, Bx 100% – Positive predictive value – MRI 97%, Bx 100% – Negative predictive value – MRI 64%, Bx 38%

MRI
• Fraser, et.al.
– Disease duration correlated with atrophy by MRI – 8/13 IBM had predominately anterior muscle compartment involvement vs. 3/27 other IIM p<0.0001 – Focal involvement 12/13 IBM, 14/27 others – T2 and STIR intensity correlated with disease activity – 10 control patients – no inflammation by MRI (6 SLE, 1 steroid myopathy, 1 Emery-Dreifuss MD, 2 postpolio syndrome)

Discussion
• Should MRI be used to direct muscle biopsy site and/or to follow disease activity?


								
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