What is the diagnosis, appropriate serologic and imaging work‑up, and initial treatment plan for an adult patient (30‑70 years, slight female predominance) presenting with unexplained proximal muscle weakness, non‑erosive polyarthritis, and respiratory symptoms suggestive of interstitial lung disease?

Antisynthetase Syndrome: Diagnosis, Workup, and Initial Treatment

For an adult presenting with proximal muscle weakness, non-erosive polyarthritis, and respiratory symptoms, immediately obtain antisynthetase antibody testing (particularly anti-Jo-1, anti-PL-7, anti-PL-12), baseline pulmonary function tests with DLCO, and high-resolution chest CT, as antisynthetase syndrome with interstitial lung disease carries a 40% excess mortality rate and requires urgent immunosuppressive therapy. 1, 2

Clinical Diagnosis

Antisynthetase syndrome is characterized by the presence of anti-aminoacyl-tRNA synthetase antibodies plus one or more of the following clinical features:

• Interstitial lung disease (present in up to 71% of cases and the most prognostically significant manifestation) 2
• Inflammatory myositis with proximal muscle weakness affecting shoulders, hips, and hand grip 3, 4
• Non-erosive polyarthritis (often symmetric) 1, 5
• Raynaud's phenomenon 1, 4
• "Mechanic's hands" (hyperkeratotic, cracked skin on lateral fingers) 1, 5
• Fever 1, 5

The interstitial lung disease can precede other manifestations by months to years in 13-38% of patients, making early recognition challenging. 2

Essential Serologic Workup

Obtain the following antibody panel immediately:

• Anti-Jo-1 (anti-histidyl-tRNA synthetase) – most common, present in 20-30% of polymyositis/dermatomyositis patients 4
• Anti-PL-7 (anti-threonyl) and anti-PL-12 (anti-alanyl) – second most common antisynthetase antibodies 4, 6
• Anti-OJ, anti-EJ, anti-KS – less common antisynthetase antibodies 4
• Anti-Ro-52 – frequently co-occurs with antisynthetase antibodies 6
• Comprehensive myositis panel to exclude other inflammatory myopathies 2
• ANA, anti-dsDNA, rheumatoid factor to exclude overlapping connective tissue diseases 6

Muscle enzyme levels:

• Creatine kinase (CK) – typically elevated, sometimes dramatically 3, 5
• Aldolase and LDH – supportive findings 3

Imaging and Functional Assessment

Pulmonary evaluation (highest priority given mortality impact):

• High-resolution chest CT (HRCT) immediately – required at baseline for all patients with suspected antisynthetase syndrome, as ILD is the primary determinant of prognosis 2
• Pulmonary function tests with spirometry and DLCO – establish baseline values; FVC <70% predicted and DLCO reduction indicate worse prognosis 2
• Repeat PFTs every 3-6 months in high-risk patients (those with anti-Jo-1 or anti-MDA-5 antibodies) 2

Muscle evaluation:

• MRI of proximal muscles (T2-weighted with fat suppression and STIR sequences) – detects inflammation and guides biopsy site selection 3
• Electromyography – shows myopathic pattern with polyphasic motor unit potentials and fibrillation potentials 3
• Muscle biopsy – demonstrates endomysial mononuclear inflammatory infiltrate; confirms diagnosis when ≥4 of 5 Bohan and Peter criteria are met 3

Risk Stratification for Progressive Disease

Predictors of progressive pulmonary fibrosis and mortality include:

• Male sex 2
• Older age at disease onset 2
• FVC <70% predicted 2
• >20% disease extent on HRCT 2
• Honeycombing or UIP pattern on HRCT 2
• Reduced DLCO 2

Initial Treatment Strategy

Initiate immunosuppressive therapy immediately upon diagnosis, as early treatment may prevent irreversible lung damage:

First-line therapy:

• High-dose oral prednisone (approximately 1 mg/kg/day) combined with a steroid-sparing agent 3
• Methotrexate 15-25 mg weekly, azathioprine 2 mg/kg/day, or mycophenolate mofetil 2-3 g/day as steroid-sparing agents 3
• Mycophenolate is the preferred first-line therapy across all CTD-ILD subtypes according to recent guidelines 7
• Gradual glucocorticoid taper over months once disease control is achieved 3

Severe or refractory disease:

• High-dose intravenous methylprednisolone plus intravenous immunoglobulin (total 2 g/kg divided over 2-5 days) 3
• Escalation to cyclophosphamide, rituximab, or cyclosporine for progressive or treatment-resistant cases 3, 6

The interstitial lung disease responds to glucocorticoids in some patients but requires additional immunosuppressant drugs in others; treatment must be individualized based on disease severity and progression. 1, 8

Multidisciplinary Approach

Establish care coordination between:

• Rheumatology for systemic disease management 2
• Pulmonology for ILD monitoring and treatment 2
• Radiology for serial HRCT interpretation 2

A standardized multidisciplinary risk-based approach is essential to reduce underdiagnosis and ensure optimal outcomes. 2

Critical Monitoring Strategy

Follow-up assessments:

• PFTs every 6 months for the first 1-2 years to identify progressive disease 7
• >5% absolute or 10% relative decrease in FVC over 6 months indicates disease progression requiring treatment adjustment 7
• Repeat HRCT if symptoms develop or PFTs show decline 2
• Close monitoring for the first 3-5 years as this is the highest-risk period for progression 2

Common Pitfalls to Avoid

• Do not delay HRCT – ILD can be asymptomatic initially but causes irreversible damage; early lung function loss occurs before symptoms develop 2
• Do not attribute dyspnea solely to muscle weakness – respiratory symptoms may represent life-threatening ILD rather than myopathy-related deconditioning 2
• Do not miss overlapping connective tissue diseases – antisynthetase syndrome can coexist with SLE, Sjögren's syndrome, or rheumatoid arthritis 6, 8
• Do not undertreat based on mild symptoms – the 10-year mortality approaches 40% when ILD is present, demanding aggressive early intervention 1