A 20-year-old white man presented with a localized unilateral swelling in the popliteal fossa. Ultrasound (US) showed the presence of an accessory muscle, the tensor fasciae suralis. The muscle was located in the proximal portion of the popliteal fossa, superficial to the medial head of the gastrocnemius. Its long tendon extended inferiorly to join the Achilles tendon. Magnetic resonance images correlated well with the US findings, confirming the diagnosis. Tensor fasciae suralis muscle is a rare cause of popliteal swelling and must be differentiated from other masses. Both US and magnetic resonance imaging can diagnose it but we suggest US as the first-line technique in its evaluation.

Pseudovasculitis, vasculitis-like syndromes, vasculitis look-alikes, or mimics of vasculitis represent a heterogeneous collection of disorders that are capable of simulating vasculitis. Inappropriate diagnosis leads to delay or absence of proper management and exposure to potentially deleterious treatment modalities such as corticosteroids and cytotoxic agents. We report the case of fibromuscular dysplasia suspected to be a polyarteritis nodosa. The progression of the lesions visualized by the ultrasonographic study and computed tomography (CT) scan after 10 days of treatment led to an emergency laparotomy. The possible deleterious role of steroids given to treat the suspected vasculitis is discussed.

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