What are the clinical characteristics, CPK (Creatine Phosphokinase) levels, electromyography (EMG) results, biopsy findings, magnetic resonance imaging (MRI) results, and antibody profiles for lupus myositis?

Lupus Myositis: Clinical Characteristics and Diagnostic Features

Clinical Presentation

Lupus myositis presents with symmetric proximal muscle weakness that is often as severe as idiopathic inflammatory myositis, requiring equally aggressive treatment. 1

Muscle Weakness Pattern

• Symmetric proximal muscle weakness affecting upper and lower extremities, manifesting as difficulty standing from a seated position, climbing stairs, and lifting arms overhead 2, 3
• Proximal muscles are relatively weaker than distal muscles in the legs 4
• Neck flexors are relatively weaker than neck extensors 4
• Quadriceps strength is typically reduced to approximately 48.9% of expected values at presentation 1
• Distal muscle weakness may occur in approximately 33% of cases 5

Associated Symptoms

• Myalgia is present in approximately 50% of patients at initial evaluation 5
• Dysphagia or esophageal dysmotility occurs in 66.6% of cases due to involvement of pharyngeal and upper esophageal muscles 5
• Progressive muscular atrophy may develop in severe cases 6

CPK (Creatine Phosphokinase) Levels

Serum CPK is significantly elevated in lupus myositis, typically reaching 10-11 times the upper limit of normal. 1

• Mean CPK elevation: 11.2 times the upper limit of normal (95% CI 5.3-29.1) 1
• Alternative reports show mean CK levels of 2153.5 IU/L in affected patients 5
• CPK levels may be markedly elevated up to 1,700 U/L in severe cases 6
• Important caveat: Slightly elevated CK levels in SLE patients on chloroquine may indicate drug-induced myopathy rather than lupus myositis, requiring muscle biopsy for differentiation 6

Electromyography (EMG) Findings

EMG reveals characteristic myopathic changes in approximately 83% of lupus myositis cases. 5

• Short-duration, low-amplitude, polyphasic motor unit potentials with increased spontaneous activity 2
• Fibrillation potentials, sharp waves, or repetitive discharges with increased insertional activity 4
• Muscle fibrillations indicative of myopathy 4
• EMG abnormalities help target the appropriate muscle for biopsy 4

Muscle Biopsy Results

Muscle biopsy reveals inflammatory myopathy changes in approximately 80% of cases when performed. 5

Histopathologic Features

• Endomysial infiltration of mononuclear cells surrounding but not invading myofibers 4
• Perimysial and/or perivascular infiltration of mononuclear cells 4
• Inflammatory changes consistent with myositis 5
• Critical distinction: Chloroquine-induced myopathy shows vacuolar myopathy without inflammatory infiltrates, differentiating it from true lupus myositis 6

Biopsy Strategy

• Target a weak muscle demonstrated by EMG abnormalities, choosing the same muscle on the opposite side for biopsy 4
• Muscle biopsy is the gold standard for confirming inflammatory myopathy diagnosis 4

Magnetic Resonance Imaging (MRI) Results

MRI demonstrates muscle edema and inflammation on T2-weighted sequences, serving as a non-invasive diagnostic tool. 4

MRI Findings

• Increased signal intensity on T2-weighted images indicating muscle edema and inflammation 4
• Fat suppression techniques and short tau inversion recovery (STIR) sequences are most useful for detecting active inflammation 4
• T1-weighted images may show muscle atrophy in chronic cases 4
• MRI helps identify appropriate biopsy sites and monitor treatment response 4
• Increased intensity and edema in affected muscles on imaging 4

Antibody Profiles

Antinuclear antibodies (ANA) are positive in 100% of lupus myositis cases, distinguishing it from idiopathic inflammatory myositis. 5

Lupus-Specific Antibodies

• ANA positive in all cases 5, 7
• Anti-dsDNA antibodies are frequently positive 8, 7
• Anti-Smith (anti-Sm) antibodies show significant association with myositis 8, 7
• Anti-U1-RNP antibodies are commonly present 8, 7

Myositis-Specific Antibodies

• Anti-Jo-1 (anti-histidyl-tRNA synthetase) carries significant weight in classification criteria (3.9 points) 4
• Anti-Jo-1 is found in approximately 20% of adult patients with inflammatory myopathy 2
• Other antisynthetase antibodies (anti-PL-7, anti-PL-12) are less common, each found in less than 5% of cases 2
• Anti-Mi-2 indicates classic dermatomyositis features, found in less than 10% of adults 2

Associated Laboratory Findings

• Thrombocytopenia and leukopenia are significantly associated with lupus myositis 8, 7
• Low C3 and C4 complement levels 8
• Elevated ESR and CRP 5, 7
• Elevated D-dimer 8

Associated Clinical Features

Lupus myositis patients demonstrate higher disease activity with multiple organ involvement. 8

• Alopecia shows significant positive association with myositis 8, 7
• Skin rash including malar rash and discoid lupus 8, 7
• Vasculitis with purpuric rash and ulcerations 8, 7
• Pericarditis 8
• Active disease (high SLEDAI scores) strongly correlates with myositis presence 8

Disease Course and Prognosis

Lupus myositis follows either a relapsing-remitting or chronic persistent course over a mean follow-up of 7.4 years, with mortality risk comparable to idiopathic myositis. 1

• Quadriceps muscle strength remains significantly depressed at long-term follow-up 1
• Point prevalence of myositis in SLE is 2.6% 8
• Direct disease complications cause mortality in approximately 9% of cases 1
• Patients are typically younger at diagnosis and more likely to be female compared to idiopathic myositis 1