Vasculitis with Fibrinoid Necrosis

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.

What is the most likely direct mechanism of muscle injury?

A. T-cell mediated inflammation of myofibers

B. B-cell mediated inflammation of myofibers

C. Metabolic myopathy

D. Ischemia

Answer:

D. Ischemia

Vasculitis

Frank muscle involvement in medium and small vessel vasculitis is uncommon, but well described. Histological findings of vasculitis include:

• Fibrinoid necrosis – pathognomonic for active vasculitis
• Mural injury
• Full thickness invasion of vessel wall
• Although fibrinoid necrosis is not congophilic, there is a characteristic appearance shown on dark field florescence of H&E-stained section.

Common etiologies of vasculitis in muscle:

• Polyarteritis nodosa is the most common followed by ANCA-associated vasculitis or rheumatoid arthritis associated vasculitis.

Mechanism of injury to muscle: ischemia

• Although concurrent inflammation of muscle fibers is may be seen in the setting of vasculitis; the injury to muscle is most often due to arterial compromise and subsequent ischemia.

Additional pathology:

• Other organ involvement is common
• Associated autoimmune disease is common
• Eosinophils (when present) suggest ANCA-associated vasculitis

Reference(s) / Additional Reading:

• Conticini E. Inflammatory muscle involvement in systemic vasculitis. Autoimmun Rev. 2022;21(3):103029.