Uncommon, unrelenting, and under recognized – This is fibro-adipose vascular anomaly

INTRODUCTION

Fibro-adipose vascular anomaly (FAVA) is a rare, benign but clinically significant vascular malformation that has only recently been recognized as a distinct entity. First described within the past decade as provisionally unclassified vascular malformation by the International Society for the Study of Vascular Anomalies, it is characterized by fibro-fatty replacement of skeletal muscle, venous ectasia, and thrombosis. It most commonly affects adults with slight female predilection (4:1). The condition is strongly associated with somatic PIK3CA gene mutations, placing it within the spectrum of PIK3CA-related overgrowth disorders. Clinically, patients typically present with persistent pain, progressive swelling, and limitation of joint movement, often mimicking more common vascular malformations or soft-tissue tumors. The lower extremities, particularly the calf and thigh muscles, are the most frequently affected sites. Awareness of this entity and recognition of its imaging hallmarks are essential, as timely diagnosis guides targeted therapy and prevents unnecessary or inappropriate interventions.

CASE SERIES

Case 1

A 57-year-old male presented with progressive swelling of the left distal thigh and leg for 1 year, associated with discomfort and mild restriction of knee flexion. There was no history of trauma, prior surgery, or systemic illness. The swelling gradually increased, raising suspicion of a soft-tissue tumor.

Initial ultrasound revealed an ill-defined, predominantly fat-containing lesion with interspersed hyperechoic soft-tissue components and intralesional vascular channels [Figure 1a]. Magnetic resonance imaging (MRI) showed a large, moderately delineated mass along the posterior distal thigh, predominantly hyperintense on T1 [Figure 1b] and T2 [Figure 1c], with suppression on fat-saturated sequences [Figure 1d]. Intervening ill-defined soft-tissue components and flow voids suggestive of venous channels were noted. Computed tomography (CT) demonstrated multiple punctate calcific foci [Figure 1e]. Dynamic post-contrast MRI demonstrated progressive heterogeneous enhancement, particularly within the soft-tissue elements [Figure 1f]. These findings indicated a fibro-fatty lesion with vascular components. The combination of fibrofatty replacement, intralesional venous channels, calcification, and progressive enhancement strongly favored FAVA over venous malformation or lipomatous tumor.^[1-3]

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Figure 1:

(a-f) A 57-year-old male with progressive swelling and discomfort of the left distal thigh and leg for 1 year. (a) Ultrasound shows an ill-defined, predominantly hyperechoic lesion with interspersed soft-tissue strands and few internal vascular channels. (b) Axial T1-weighted magnetic resonance imaging demonstrates a large, moderately delineated hyperintense lesion along the posterior distal thigh with internal fibrofatty components (orange arrow). (c) Axial T2-weighted image shows a hyperintense lesion with interspersed serpiginous flow voids representing venous channels. (d) Fat-suppressed T1-weighted image demonstrates signal suppression within the fatty components with residual enhancing soft-tissue elements (green arrow). (e) Axial computed tomography image reveals multiple punctate calcific foci within the lesion (green arrow). (f) Post-contrast T1-weighted image shows heterogeneous enhancement of the soft-tissue elements, confirming the vascular component consistent with fibro-adipose vascular anomaly.

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Case 2

An 11-year-old boy presented with acute onset of left leg pain persisting for 2 weeks. The pain was constant, with no history of trauma, infection, or systemic illness. Clinical examination revealed localized tenderness over the calf with mild restriction of ankle movements. No visible swelling was noted.

MRI demonstrated an ill-defined lesion within the posterior compartment muscles of the right leg, predominantly affecting the medial soleus, flexor digitorum longus (FDL), and flexor hallucis longus. The lesion was hyperintense on T1 [Figure 2a] and showed signal suppression on T1 fat-saturated sequences [Figure 2b]. Tortuous hyperintense vessels were seen interspersed between muscle fibers on T2 fat-sat images [Figure 2c]. Post-contrast imaging revealed progressive, heterogeneous enhancement [Figure 2d]. CT confirmed the presence of a phlebolith within the lesion [Figure 2e], supporting a vascular component. Ultrasound, although not the primary modality, can depict a heterogeneous hyperechoic lesion with minimal venous flow on Doppler. The constellation of findings – fatty infiltration, vascular channels, and phlebolith – favored the diagnosis of FAVA.^[1-3]

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Figure 2:

(a-e) An 11-year-old boy with complaints of persistent left leg pain for 2 weeks. (a) Axial T1-weighted magnetic resonance imaging shows an ill-defined, fat-containing lesion involving the posterior compartment muscles of the right leg, predominantly the medial soleus, flexor digitorum longus, and flexor hallucis longus (yellow bracket). (b) T1 fat-suppressed image shows signal suppression of fatty elements with interspersed soft-tissue strands. (c) Axial T2 fat-saturated image demonstrates hyperintense tortuous vessels interspersed between muscle fibers. (d) Post-contrast T1-weighted image reveals progressive, heterogeneous enhancement of the lesion confirming its vascular component. (e) Axial computed tomography image shows a discrete intralesional calcific focus (yellow arrow) consistent with a phlebolith, features characteristic of fibro-adipose vascular anomaly.

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Case 3

An 11-year-old boy presented with persistent pain in the middle third of the left leg for 2 weeks. There was no preceding trauma, infection, or systemic illness. On examination, there was focal tenderness along the lateral compartment with mild limitation of ankle eversion. No gross swelling was appreciated.

Ultrasound demonstrated an ill-defined heteroechoic lesion with fatty components in the lateral compartment [Figure 3a]. MRI revealed a soft-tissue lesion within the inter- and intramuscular planes, involving the peroneus brevis and longus muscles. The lesion appeared hyperintense on T1 [Figure 3b] and showed fat suppression on T1 fat-saturated sequences [Figure 3c]. Post-contrast images demonstrated progressive heterogeneous enhancement with prominent venous channels within the lesion [Figure 3d]. The imaging constellation – fibrofatty infiltration, venous channels, and enhancement – strongly supported a diagnosis of FAVA. CT imaging, although less contributory, excluded bony involvement and confirmed absence of significant calcification. Differentials included venous malformation, lipomatous lesion, and hamartomatous overgrowth, but absence of muscle contracture and constant pain favored FAVA.^[1-3]

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Figure 3:

An 11-year-old boy with persistent pain in the middle third of the left leg for two weeks. (a) Ultrasound image shows an ill-defined heteroechoic lesion with surrounding fatty component in the lateral compartment of the leg (green arrow). (b) Axial T1-weighted MRI demonstrates a soft-tissue lesion involving the inter- and intramuscular planes of the peroneus brevis and longus muscles with hyperintense fibro-fatty components(green arrow). (c) Axial T1 fat-suppressed image shows suppression of fatty signal with residual soft-tissue strands and small intralesional vascular channels(green arrow). (d) Post-contrast T1-weighted image demonstrates progressive heterogeneous enhancement with prominent venous channels within the lesion, consistent with fibro-adipose vascular anomaly (FAVA).

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DISCUSSION

FAVA is a distinct vascular malformation recognized within the spectrum of PIK3CA-related disorders. It is characterized by fibro-fatty infiltration of muscle, venous ectasia, and thrombosis. Although rare, FAVA is increasingly recognized in both adults and children due to advances in MRI. Clinically, patients present with persistent pain, swelling, and progressive functional impairment, often with joint stiffness or contracture. Unlike venous malformations, which are typically compressible and less painful, FAVA is associated with constant pain and disability.

Imaging is critical for diagnosis. Ultrasound may reveal hyperechoic lesions with minimal venous flow, while CT can show calcifications or phleboliths. MRI is the gold standard, demonstrating fibro-fatty infiltration (hyperintense on T1/T2, suppressed on fat-sat sequences), venous channels as flow voids, and progressive heterogeneous enhancement post-contrast. These features differentiate FAVA from venous malformations, lipomatous tumors, and hamartomatous lesions. Recognition of these imaging patterns prevents misdiagnosis and inappropriate treatment.

Management depends on the extent and severity of symptoms. Minimally invasive procedures such as image-guided cryoablation, sclerotherapy, or intralesional therapy are often first-line. Surgical approaches including excision, tendon lengthening, and nerve decompression are reserved for severe or refractory cases. Early diagnosis is crucial, as it guides appropriate therapy, prevents unnecessary interventions, and improves long-term function.

DIFFERENTIAL DIAGNOSIS

CONCLUSION

FAVA is a rare vascular malformation causing persistent pain and functional limitation. MRI is key for diagnosis, showing fibro-fatty infiltration, intralesional venous channels, and progressive enhancement. Early recognition prevents misdiagnosis and unnecessary interventions. Treatment ranges from minimally invasive therapy to surgery based on severity. Awareness among clinicians and radiologists ensures timely and appropriate management.

TEACHING POINTS

• FAVA is a rare but distinct vascular anomaly associated with persistent pain and functional impairment with MRI findings of fibro-fatty infiltration with venous channels and progressive enhancement are diagnostic hallmarks.

• Differentiation from venous malformation is crucial, as management strategies differ significantly.

MCQs

1. Which gene mutation is commonly associated with FAVA?

   1. BRCA1

   2. PIK3CA

   3. PTEN

   4. KRAS

   Answer Key: b

2. Which imaging modality best characterizes FAVA?

   1. X-ray

   2. Ultrasound

   3. CT

   4. MRI

   Answer Key: d

3. Which combination of clinical and imaging features most reliably differentiates fava from common venous malformations?

   1. Compressible, T2 hyperintense lesion with phleboliths, minimal pain.

   2. Painless fatty lesion without vascular channels, no venous ectasia.

   3. Persistent pain, fibro-fatty infiltration of muscle on MRI, intralesional venous channels, and progressive enhancement.

   4. Syndromic fatty hematoma with variable fatty replacement and no muscle contracture.

   Answer Key: c