MxA Positive Myopathy

Clinical History:

• History: 68-year-old man with 1 week of bilateral proximal upper and lower extremity weakness and 1 month of facial, eyebrow, neck, chest, and back rash (initially diagnosed as seborrheic dermatitis)
• PMH: HTN, PVD, asthma, hyperlipidemia, GERD, type 1 DM with retinopathy, IgA deficiency, hypothyroidism, former light smoker
• FH: Stroke, diabetes, arthritis, cancer, macular degeneration, heart disease
• Meds: Atorvastatin, insulin, levothyroxine, zolpidem; treated with Solu-Medrol and prednisone
• Physical exam: Mild-moderate proximal weakness, lower extremity edema
• MRI: Diffuse heterogeneous edema of bilateral quadriceps, adductors, and sartorius, suspicious for inflammatory myopathy
• Labs: CK peaked at 1570, trending down to 680; aldolase 20; mildly elevated CRP; ANA, Lyme, and RF negative; HMGCR Ab, MG panel, extended myositis and ENA panels pending

MxA staining is not characteristic of:

A. Dermatomyositis

B. Immune-mediated, necrotizing myopathy

C. Lupus myositis

D. Viral myositis

Answer:

B. Immune-mediated, necrotizing myopathy

MxA (Myxovirus resistance protein A (MxA) staining is most characteristic of dermatomyositis where it is characterized by perifascicular sarcoplasmic pattern). However, MxA is a nonspecific surrogate for interferon-γ and may be seen in a variety of type II interferon-driven processes including lupus myositis, viral myopathy and other muscle overlap autoimmune conditions.

Perifascicular pathology with MxA positivity

Pattern classically associated with dermatomyositis

Other etiologies with perifascicular pathology:

• Lupus myositis
• Antisynthetase syndrome
• Sjögren syndrome
• Systemic sclerosis (minority of cases)
• Polymyositis (rarely)

Myxovirus resistance protein A (MxA) – surrogate for interferon-γ (type II) response; may be seen in:

• Dermatomyositis (perifascicular sarcoplasmic pattern)
• Lupus myositis (staining prevalence associated with activity)
• Viral myopathy
• Other autoimmune conditions

Reference(s) / Additional Reading:

• Waisayarat J. Sarcoplasmic Myxovirus Resistance Protein A. J Inflamm Res. 2023;16:5417-5426.
• Xing C. Myxovirus resistance protein A (MxA) expression in myositides. Muscle Nerve. 2024;69(5):548-555.