Muscular polyarteritis nodosa (PAN), in which involvement is confined to skeletal muscle, is a rare and often underrecognized clinical entity. Patients typically present with nonspecific features, such as fever and myalgia, without a rise in serum creatine kinase (CK) levels,1 making diagnosis challenging and often delayed.
A 10-year-old girl presented with an 18-day history of fever and limb pain. Physical examination revealed no skin rash and pain on grasping the upper and lower limbs; there was no muscle weakness. Noncontrast magnetic resonance imaging (MRI) was unremarkable. Laboratory tests showed a normal CK level (25 IU/L), an elevated leukocyte count (10.7 × 109/L), elevated serum C-reactive protein level (13 mg/dL), elevated erythrocyte sedimentation rate (140 mm/h), and negative antineutrophil cytoplasmic antibodies. Contrast-enhanced T2-weighted MRI demonstrated diffuse, symmetrical hyperintense lesions along intramuscular vessels in the limbs with cotton-wool–like, small fluffy enhancing foci (Figures 1A,B, yellow arrows). A muscle biopsy from the area of abnormal MRI enhancement in the thigh showed infiltrates of inflammatory cells in and around the arterial wall within the perimysium, as well as luminal narrowing, and fibrinoid necrosis (Figure 1C). Contrast-enhanced computed tomography revealed no vasculitis in other organs. Muscular PAN was diagnosed. Complete remission was achieved with prednisolone (1 mg/kg/day) and maintained for 24 months with azathioprine (1 mg/kg/day).
MRI and pathological findings. (A,B) Contrast-enhanced T2-weighted MRI demonstrated diffuse, symmetrical hyperintense lesions along intramuscular vessels in the limbs with cotton-wool–like, small fluffy enhancing foci (yellow arrows). (C) A muscle biopsy from the area of abnormal MRI enhancement in the thigh showed infiltrates of inflammatory cells in and around the arterial wall within the perimysium, luminal narrowing, and fibrinoid necrosis. MRI: magnetic resonance imaging.
Clinicians should maintain a high index of suspicion for muscular PAN in patients with myalgia, normal serum CK levels, and elevated inflammatory markers. The cotton-wool appearance on contrast-enhanced MRI, characteristic of muscular PAN,2 is instrumental in the diagnosis and selection of the biopsy site.
Footnotes
CONTRIBUTIONS
MH: conceptualization, investigation, writing - original draft. HI, YH: investigation, writing - review and editing. MS: conceptualization, investigation, supervision, project administration, writing - review and editing, MH, MS: writing. All authors: data collection, reading and approval of the final manuscript.
FUNDING
No specific funding was received from any bodies in the public, commercial, or not-for-profit sectors to carry out the work described in this article.
COMPETING INTERESTS
The authors declare no conflicts of interest relevant to this article, nor have any honorarium, grant, or other form of payment to produce this manuscript.
ETHICS AND PATIENT CONSENT
Institutional review board approval was not required according to the authors’ institutions. The patient and their parents provided written consent.
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