Morel-Lavallée lesion of the distal thigh: Emergency department diagnosis with point-of-care ultrasound

Abstract

A Morel-Lavallée lesion (MLL) is a rare, closed traumatic soft-tissue degloving injury that is often misdiagnosed. The mechanism that results in this lesion involves a shearing force that separates the hypodermis from underlying fascia, thereby disrupting perforating vascular and lymphatic vessels and resulting in a hemolymphatic fluid collection prone to infection and pseudocyst formation. We describe a case of a 70-year-old male with an initially missed MLL despite CT imaging that was ultimately diagnosed on point-of-care ultrasound (POCUS) on a second emergency department visit. Although an MLL is often misdiagnosed on radiography and CT imaging, POCUS is an expeditious imaging modality that can facilitate diagnosis of an MLL by visualizing specific sonographic characteristics.

Introduction

A Morel-Lavallée lesion (MLL) is a rare, closed soft-tissue degloving injury caused by high-energy blunt force or low-velocity crush injuries and—more rarely—direct-blows to the knee and contact sports [1,2]. These injuries involve tangential trauma to the fascial planes whereby shearing forces separate the hypodermis from underlying fascia. This disrupts perforating vascular and lymphatic vessels and results in a hemolymphatic fluid collection that can leave patients predisposed to infection, pseudocyst formation, skin necrosis, chronic persistence, and a high risk of deep infection if surgery is required for a nearby fracture [1], [2], [3], [4].

In the emergency department (ED), patients presenting with post-traumatic extremity swelling are often diagnosed with an uncomplicated hematoma. Persistent swelling points to a wider differential diagnosis that may include pathologies such as skin infection, fat necrosis, soft tissue sarcoma, early myositis ossificans, bursitis, and deep vein thrombosis (DVT) [1]. In these settings, additional imaging modalities and work up are required. In this report, we describe a case of a patient with an initially missed MLL despite CT imaging that was diagnosed on point-of-care ultrasound (POCUS) on a return ED second visit.

Case

A 70-year-old man presented to the ED with generalized weakness as well as persistent knee pain and swelling. Two weeks ago, he sustained a mechanical fall onto his left knee while trying to pull down his pants to use the bathroom. At that time, radiographic imaging of his knee was unremarkable and a CT of the knee showed a nonspecific fluid collection (Figs. 1 and 2). This resulted in an initial diagnosis of a simple post-traumatic hematoma. The patient was discharged back to home where he has been having difficulty with daily activities of living and staying hydrated due to his large knee lesion causing pain with ambulation. This prompted a return visit to the ED.

Fig. 1.

Radiography of the left knee with an anterior-posterior view showing distal and medial left thigh soft tissue swelling without evidence of an acute fracture or joint effusion.

Fig. 2.

Computed tomography without contrast showing axial (A) and coronal (B) views of the left distal femur/medial knee showing a lesion consistent with the appearance of a subcutaneous hematoma or nonspecific fluid collection (yellow arrows) measuring approximately 4.5 × 13.3 × 15.4 cm that extends superomedially at the left medial distal thigh. There is no evidence of acute fracture or dislocation. No joint effusion is evident.

On examination, the patient was afebrile and without respiratory distress on room air. His initial blood pressure was 84/50 mmHg. His blood pressure responded quickly to intravenous hydration. Of note, he had a persistent left distal thigh swelling with ecchymosis. The skin in this location was hypermobile to palpation. Screening labs were overall unremarkable. POCUS of the left knee revealed an anechoic soft tissue region with no internal color flow measuring approximately 22.5 cm craniocaudally and 3.6 cm deep (Figs. 3 and 4). POCUS did not visualize any evidence for cellulitis or an abscess. The patient was diagnosed with an MLL and he was admitted for hydration, pain control, and ultimately discharged with compression banding for symptomatic control and orthopedic follow-up for chronic management of his persistent knee lesion.

Fig. 3.

Point-of-care ultrasound utilizing a high-frequency liner probe with proximal (A) and distal (B) transverse views of the left knee and distal medial thigh was remarkable for findings of hyperechoic fat globules (yellow arrow tips) and intralesional septations (yellow arrow) that are characteristic of a Morel-Lavallée lesion.

Fig. 4.

Ultrasound of the left knee and distal medial thigh revealed an anechoic soft tissue region with no internal color flow measuring approximately 22.52 cm craniocaudally and 3.55 cm deep with the distal portion of the lesion on the right side with medial condyle of left femur (yellow asterisk). There are characteristic hyperechoic fat globules (yellow arrow tips), intralesional septations (yellow arrow), and a lenticular or fusiform shape of the fluid collection that is formed along the contour the fascial plane between subcutaneous tissue and muscle.

Discussion

Our patient's presentation was complicated by an atypical mechanism and location. As in a third of cases, the MLL lesion was initially missed on CT imaging [1]. Unlike this case, MLLs most commonly occur in the peritrochanteric region along the proximal lateral thigh. This area's predisposition to shearing injury is attributed to the large surface area and mobility of the skin coupled with the presence of a dense capillary network in the proximal thigh and gluteal region [3]. In a review of over 200 MLLs, most lesions occurred in the greater trochanter, pelvis, and proximal thigh, whereas a minority of cases occurred at the knee [5]. Prompt diagnosis of MLL is crucial as this pathology leaves patients predisposed to infection, pseudocyst formation, skin necrosis, and chronic persistence [1,3,4]. As high-energy trauma is typically required for the development of an MLL, there may be concomitant fractures that require assessment in the ED and are complicated by a nearby MLL due to a heightened risk of deep infection [1].

A MLL is a clinical diagnosis that is supplemented with imaging [1,3]. These lesions present as fluctuant soft-tissue masses with skin hypermobility, significant ecchymosis, and decreased sensation due to sheared cutaneous nerves [2], [3], [4]. Most lesions occur acutely but may present months to years after the inciting trauma [2]. Radiography has little value in differentiating MLLs from other soft tissue masses (Fig. 1). As encountered in our case, CT imaging also has limited value outside of confirming a nonspecific fluid collection, which may contribute to misdiagnosis (Fig. 2) [1]. Although not expedient to obtain in the ED, MRI is the modality of choice [1]. On MRI, MLLs are classified into 6 types primarily based on the T1 and T2 characteristics of the fluid collection [2]. More generally, MLL will visualize characteristic edges that taper and fuse with surrounding fascial layers [1]. Chronic MLLs will be more homogenous and smoother with a fibrous capsule possessing a hypointense hemosiderin ring, whereas acute lesions are more heterogeneous and irregular [1]. As demonstrated in our case, POCUS is more expedient in the ED and can confirm the location as between muscle fascia and the hypodermis as well as exclude active flow that may point to a traumatic pseudoaneurysm [1,4,6]. POCUS can also visualize characteristic intralesional septations and hyperechoic fat globules (Figs. 3 and 4) [[2], [3], [4],7]. Furthermore, POCUS can visualize sonographic features of cellulitis, such as cobble stoning, and an abscess, such as “swirl sign,” in addition to assessing for concomitant knee effusion and DVT [6].

Management of MLLs depends on the lesion size, severity, and proximity to an intended surgical incision for a concomitant injury [2,3]. For acute lesions without an underlying fracture, an initial conservative treatment approach with compression banding may be attempted, with subsequent percutaneous drainage for refractory lesions. For MLLs refractory to the conservative approach, associated with an underlying open fracture, or near an anticipated surgical incision site, open debridement may be indicated.

Conclusion

An MLL is a rare traumatic soft-tissue degloving injury that is often misdiagnosed. Prompt diagnosis is essential as the hemolymphatic fluid collection associated with MLLs are prone to infection, pseudocyst formation, and chronic persistence. As CT imaging can easily miss MLLs and MRI may not be expedient to obtain the in the ED, POCUS can be employed as an expeditious imaging modality that can identify specific sonographic characteristics that can differentiate an MLL from simple hematomas and other pathologies.

Patient consent

The patient provided written informed consent for publication of this case report.