Focal myositis is a rare, localized inflammatory muscle disorder first described in the 1970s.¹ It is characterized by circumscribed involvement of a single skeletal muscle or muscle group, often presenting as a focal swelling that may mimic neoplastic, infectious or systemic inflammatory conditions.^2,3 Due to its nonspecific features and the absence of standardized diagnostic criteria^4,5 focal myositis remains a diagnosis of exclusion, despite its generally benign and self-limiting course.^4,6

Most reported cases follow a monophasic course with spontaneous resolution or favourable response to short-term anti-inflammatory therapy.^4,6 In contrast, recurrent forms are rare,⁷ and migratory involvement of different muscle groups has been only sporadically described.⁷ Such atypical presentations further increase diagnostic uncertainty and may lead to prolonged diagnostic delay.

In this report, we describe an unusual case of recurrent focal myositis with a migratory pattern and an approximately ten-year diagnostic delay, evolving with spontaneous clinical and biological remission and a persistently negative etiological workup. This case highlights the clinical heterogeneity of focal myositis and underscores the importance of considering this rare entity in patients presenting with relapsing migratory muscle inflammation.^4,7

A 45-year-old man was admitted to the internal medicine department for a painful swelling of the left calf associated with fever. The pain got progressively worse, and the patient was unable to walk correctly on admission.

His medical history was notable for epilepsy diagnosed in adolescence, with antiepileptic treatment discontinued and loss of neurological follow-up approximately 14 years prior to the current admission. There was no personal or family history of autoimmune disease, systemic inflammatory myopathy, malignancy, or hereditary neuromuscular disorder. He denied recent trauma, strenuous physical activity, intramuscular injections, toxic exposure, or illicit drug use.

Since 2015, the patient experienced multiple hospitalizations for recurrent episodes of focal muscle inflammation involving different peripheral skeletal muscles, with a migratory pattern. Each episode was characterized by isolated tumefaction of a single muscle, associated with local inflammatory signs, pain leading to functional impairment of the affected limb, fever, and a marked biological inflammatory syndrome. Several episodes were complicated by rhabdomyolysis with significant elevation of muscle enzymes. During one previous hospitalization, the muscle swelling evolved toward abscess formation and responded favorably to intravenous antibiotic therapy with amoxicillin–clavulanate. Between episodes, the patient consistently achieved complete clinical recovery, without residual functional deficit.

The last episode was marked by acute onset of painful swelling of the medial aspect of the left calf, evolving over a few days. On admission, the patient was febrile, with a temperature of 39 °C. Physical examination revealed a localized, firm, tender, and warm tumefaction of the medial gastrocnemius muscle, with overlying inflammatory signs and impaired mobilization of the lower limb. No associated articular involvement, cutaneous manifestations, lymphadenopathy, or neurological deficits were observed. The rest of the physical examination was normal.

Laboratory investigations demonstrated a marked inflammatory syndrome, with a C-reactive protein level of 145 mg/L and leukocytosis of 18200 elements/mm³ with neutrophil predominance. Muscle enzyme levels were only mildly elevated, with a creatine phosphokinase (CPK) level of 220 IU/L, while lactate dehydrogenase levels remained within the normal range. Blood cultures were negative.

Soft tissue ultrasonography of the left calf revealed an aspect consistent with focal myositis of the medial gastrocnemius muscle, showing focal muscular disorganization with loss of normal fibrillar architecture, bulging muscle contours, and increased vascularity on color Doppler imaging. There was associated infiltration of the subaponeurotic soft tissues, without evidence of fluid collection or abscess formation.

A muscle biopsy was performed and showed fibrotic remodeling associated with a moderate chronic inflammatory infiltrate composed predominantly of lymphohistiocytic cells, arranged mainly in a perivascular distribution and extending to peri- and intrafascicular muscle bundles. Necrotic and regenerating muscle fiberswere also identified. Overall, the histopathological findings were consistent with a nonspecific chronic inflammatory myositis ( Figure 1).

An extensive etiological workup was conducted, including investigations for systemic inflammatory myopathies, autoimmune diseases, viral and bacterial infections, and neoplastic processes, all of which were negative. In the absence of clinical, biological, or immunological features suggestive of a systemic disorder, and given the recurrent, migratory, and self-limited nature of the episodes, the diagnosis of recurrent focal myositis of idiopathic origin was retained.

Management during the current hospitalization was primarily supportive, consisting of analgesics and antipyretics, with close clinical and biological monitoring. In contrast to previous episodes, antibiotic therapy was not initiated due to the absence of abscess formation or microbiological evidence of infection. No anti-inflammatory or immunosuppressive treatment was prescribed.

The clinical course was favorable, with a gradual spontaneous regression of muscle swelling, pain, and fever after a week of evolution along with the normalization of the inflammatory markers. The patient recovered full function of the affected limb and was discharged without complications. No adverse events occurred, and short-term follow-up confirmed sustained clinical remission.

The case described a ten years history of recurrent episodes of migratory focal myositis with a negative etiological assessment spontaneously resolving within a 45-year-old man.

Focal myositis is a rare inflammatory muscle disorder that remains diagnostically challenging because of its nonspecific clinical, biological, radiological, and its histopathological features.^2,4 The present case illustrates both the strengths and limitations inherent to case reports dealing with uncommon and poorly characterized entities.

From a diagnostic standpoint, the main challenge was to differentiate recurrent focal myositis from infectious myositis, soft tissue malignancy, and systemic inflammatory myopathies. Neoplastic conditions were also a concern because focal myositis may mimic a pseudo-tumoral lesion clinically and radiologically⁸; in this context, muscle biopsy played a pivotal role in excluding malignancy and confirming a benign inflammatory process.² Systemic inflammatory myopathies, including polymyositis and dermatomyositis, were ruled out based on the strictly focal involvement and negative immunological workup.⁹

The pathophysiology of focal myositis remains incompletely understood. Several mechanisms have been proposed in the literature, including a localized immune-mediated inflammatory reaction,⁴ neurogenic inflammation,⁵ post infectious immune dysregulation, or a response to repeated microtrauma. Histopathological findings of nonspecific chronic inflammatory myositis, commonly reported in published cases,² further suggest a localized immune process within the broader spectrum of inflammatory myopathies.⁹

Most cases reported in the literature describe a monophasic and self-limiting course, with spontaneous resolution or response to short term anti-inflammatory or corticosteroid therapy.^4,6 Recurrent forms are distinctly rare,⁷ and migratory involvement has been only sporadically documented.^3,7 Compared with previously published reports, our case is remarkable for the prolonged disease duration and multiplicity of relapses.

Importantly, although focal myositis is generally regarded as a benign condition, rare cases of progression to generalized inflammatory myopathies, including polymyositis, have been reported,^3,10 suggesting that focal myositis may, in some instances, represent an early or localized manifestation within the spectrum of idiopathic inflammatory myopathies.⁹ This possibility provides a strong scientific rationale for sustained clinical vigilance.

In conclusion, this case underscores that recurrent focal myositis with a migratory pattern represents a rare but clinically significant diagnostic entity. Recognition of atypical presentations is essential to avoid unnecessary interventions, while long-term follow-up remains crucial to ensure early detection of potential progression toward systemic inflammatory myopathy.^3,10

Written informed consent for publication of the clinical details was obtained from the patient.