Introduction
Morel-Lavallée lesions (MLLs) were originally described by the French surgeon Victor Auguste François Morel-Lavallée in 1863.1 This type of lesion is associated with a closed degloving injury in which the deep fascia gets separated from the skin with rupture of lymphatic and blood vessels, resulting in the accumulation of hemolymphatic fluid within the newly created space.2–4 In humans, MLLs are usually associated with underlying fractures of the pelvis, acetabulum, or proximal femur. These injuries are often not immediately recognized due to the distraction by concomitant bony injuries. Morel-Lavallée lesions have been described in people as posttraumatic cysts, pseudocysts, or chronic expanding hematomas.2,5 There are 2 case reports6,7 describing the ultrasonographic features of MLLs in cats. The purpose of this study was to describe the CT features of MLLs and their association with trauma in cats.
We hypothesized that the MLL fluid would (1) accumulate between the dermis and deep fascia in the proximity of superficial trauma, (2) have a vertical attenuation gradient, and (3) have no or only mild contrast enhancement.
Methods
This was a retrospective, multicenter, observational case series conducted at the Royal (Dick) School of Veterinary Studies and the Evidensia Specialist Animal Hospital in Helsingborg, Sweden. Formal ethical approval was obtained from 1 institution where it was required (institutional reference, 221.23_SF). Medical record archives were searched for the feline patients that sustained trauma, and from these records, imaging archives were searched for CT studies with recorded degloving injury, bone trauma, subcutaneous edema, and fluid collection affecting the pelvic region. Inclusion into the study required a history of subdermal fluid accumulation within the region of trauma involving the skin and a diagnostic-quality CT study with pre- and postcontrast series. Signalment and medical records were reviewed by a diagnostic imaging intern (ND). For review of CT studies, a DICOM viewer software (Horos, version 3.3.6; Purview) on a computer workstation (iMac 27-inch; Apple Inc) with an LCD calibrated flat-screen monitor (retina display) was used and the review was done by a diagnostic imaging intern (ND) and board-certified veterinary radiologist (TS) in consensus. Both bone and soft tissue kernel images were reviewed. Images were reviewed for evidence of fluid-attenuating material between the dermis and the surface of the muscles consistent with the location of the deeper fascia, the presence of the fat-attenuating globules within the fluid-filled space, and evidence of a contrast-enhancing capsule. The lesions were evaluated for their location (affected body part, proximity to visible bone or soft tissue trauma) and lesion extension.
To assess the vertical attenuation gradient and contrast enhancement of MLL fluid, measurements were taken from sagittal plane images with the sagittal orientation tool in the imaging software. The mean attenuation of the MLL fluid was recorded from 5 round regions of interest (ROIs) on both pre- and postcontrast images. The ROIs had a diameter of 20 mm and were positioned at 5 different levels of the fluid column, which were determined as follows. In the sagittal plane, the top and bottom levels of the lesion were identified and marked. The most nondependent level was designated as top and the most dependent level as bottom, irrespective of ventral or dorsal recumbency of the patient. These 2 established levels were labeled as 100% for the top and 0% for the bottom. We ensured that hypodermis was not included in the measurements.
The height of the fluid column between the bottom and top levels was measured and divided by 3 equidistant horizontal lines. These 3 levels were labeled as 75%, 50%, and 25% to reflect their relation to the top (100%) and bottom (0%) of the MLL. The length tool was propagated throughout the entire imaging series to establish orthogonal planes for all 5 levels because the MLL fluid column heights were not all contained within 1 sagittal image (Figure 1).
Sagittal multiplanar reconstruction of case 3 in a soft tissue algorithm illustrating the region of interest (ROI) placement for the investigation of a potential vertical attenuation gradient. The top (100%), mid (50%), and bottom (0%) levels of the Morel-Lavallée lesion (MLL) fluid are annotated by full horizonal lines and the 75% and 25% levels by dashed horizontal lines. All lines are propagated throughout the entire sagittal imaging series. A—Sagittal plane image with the location of the top-level MLL with ROI placement. B—Different sagittal plane image with the location of the 75% and 50% level MLL with ROI placement. C—Different sagittal plane image with the location of the 25% and 0% level MLL with ROI placement.
Citation: Journal of the American Veterinary Medical Association 2024; 10.2460/javma.24.06.0429
The 5 mean attenuation values from precontrast images for each patient were explored for the presence of a vertical gradient. The corresponding mean values were also recorded on postcontrast images. The obtained precontrast values were subtracted from those postcontrast values, and the net enhancement was reported as a percentage change from the precontrast values. An average and minimum and maximum values were reported as descriptive statistics of the calculated net enhancement values.
Statistical methods
Statistical analyses were performed by a senior data scientist using commercial statistics software (Excel Analysis ToolPak; Microsoft Corp). Linear regression parameters for the vertical gradient were estimated with the least-squares method. R2 coefficients of determination, 95% CIs, and P values were reported. Given the low number of samples leading to an overfitting risk, obtained statistics should be treated as descriptive for the trend in vertical attenuation gradient and not as a statistical proof of its existence.
Results
Case 1
A 1-year-old neutered male domestic shorthair cat presented with recurrent swelling and fluid accumulation in the right pelvic limb following a traumatic event. This patient also suffered from an anal tear, which was surgically repaired by means of eversion of the mucosa and placement of a Penrose drain in the right perineum. Despite fluid drainage, the swelling in the right pelvic limb markedly progressed within 48 hours. Hematology performed 3 days after the trauma showed moderate neutropenia with a marked left shift, severe toxic changes in neutrophils, moderate hypoalbuminemia, lymphocytopenia, and thrombocytopenia. Bacterial culture from the right ischiorectal fossa revealed moderate growth of Escherichia coli and Enterococcus spp. A CT examination performed 3 days after the trauma showed a fluid-filled space deep to the skin and superficial subcutaneous layer but superficial to the deep fascial plane, extending from the stifle joint to the tarsal joint in the right pelvic limb, and mildly in the left pelvic limb (Figure 2). The cross-sectional area of the MLL was crescent shaped at the proximal aspect with caudal and medial distribution of accumulated fluid and ring shaped distally in the right pelvic limb, while in the left pelvic limb, it was crescent shaped throughout its length. The patient was euthanized 7 days after the trauma due to ongoing fecal contamination secondary to extensive rectal and skin necrosis. Recorded mean attenuation values in precontrast images from 5 ROIs, starting from the top 100% level, were as follows: 1.1, 1.8, 4.6, 9.7, and 21.4 HU. Recorded mean attenuation values from corresponding ROIs on postcontrast images were as follows: 2.3, 3.7, 8.6, 14.0, and 28.2 HU.
Transverse CT images of an MLL affecting pelvic limbs. A—The distal crura of a cat in ventral recumbency that suffered from pelvic limb trauma 3 days earlier (case 1). There is fluid accumulation between the skin and crural muscles consistent with MLL. On the right crus, there is marked circumferential fluid accumulation (arrows). On the left crus, the effusion is less extensive and crescent shaped (arrowhead). B—The left crus of a cat in dorsal recumbency that suffered from pelvic limb trauma 38 days earlier (case 2). There is a marked, low-attenuation (20-HU) effusion between the skin and muscles in the caudal aspect of the crus (arrowhead) consistent with MLL. In the most dependent portion of the effusion, there are areas of higher attenuation (60 HU), consistent with sedimentation or hematoma (arrows). C—The distal crura of a cat in ventral recumbency that suffered from pelvic limb trauma 53 days earlier (case 3). There is a marked, crescent-shaped effusion between the skin and muscles of the left crus consistent with MLL (arrowheads).
Citation: Journal of the American Veterinary Medical Association 2024; 10.2460/javma.24.06.0429
Case 2
A 2-year-old neutered male domestic shorthair cat presented with recurrent swelling and fluid accumulation in the left pelvic limb following a traumatic event. This patient suffered a left sacroiliac luxation, which was surgically repaired. Despite multiple drainages over a period of 38 days, a large amount of hemolymphatic fluid was repeatedly removed. Hematology performed 35 days after the initial trauma revealed marked anemia and moderate hypoalbuminemia. A CT examination performed 38 days after the trauma showed a fluid-filled space deep to the skin and superficial subcutaneous layer but superficial to the deep fascial plane, extending from the distal femoral metaphysis to the tarsal joint in the left pelvic limb (Figure 2). The cross-sectional area of the MLL was crescent shaped throughout its entire length, with fluid accumulation predominantly on the caudal aspect and to a lesser extent within the medial aspect. Irregularly marginated, well-defined areas of higher attenuation were observed within the most dependent portion of the effusion. The patient was euthanized 39 days after the initial trauma due to persistent anemia despite 2 whole blood transfusions. Recorded mean attenuation values in precontrast images from 5 ROIs, starting from the top 100% level, were as follows: 25.6, 28.7, 29.5, 31.0, and 33.8 HU. Recorded mean attenuation values from corresponding ROIs on postcontrast images were as follows: 26.7, 28.3, 28.4, 33.3, and 33.4 HU.
Case 3
A 3-year-old neutered female British Shorthair cat presented with recurrent swelling and fluid accumulation of the left pelvic limb, 40 days after initial trauma. The swelling persisted despite regular fluid drainage (Figure 3). Hematology performed 52 days after the trauma revealed moderate neutropenia with the presence of segmented neutrophils. Cytology of the drained serosanguineous fluid showed a dominance of small lymphocytes and erythrocytes. A CT examination performed 53 days after the initial trauma revealed a fluid-filled space deep to the skin and superficial subcutaneous layer but superficial to the deep fascial plane, extending from the stifle joint to the distal tibial metaphysis. The cross-sectional area of the MLL was crescent shaped, with effusion on the medial, caudal, and, to a lesser extent, lateral aspects of the crus (Figure 2). The patient underwent surgical exploration of the effusion cavity, during which a closed active suction drain was placed, and tacking sutures were applied between the subcutis and deep fascia. The cat was discharged after the drain was removed, and a small fluid pocket resolved shortly after. Four months later, the owner reported the cat had normal activity levels with no recurrence of swelling. Recorded mean attenuation values in precontrast images from 5 ROIs, starting from the top 100% level, were as follows: 4.1, 7.2, 6.6, 11.0, and 15.7 HU. Recorded mean attenuation values from corresponding ROIs on postcontrast images were as follows: 14.0, 11.8, 5.5, 11.8, and 13.8 HU.
Photograph of the left pelvic limb of case 3 taken 53 days after trauma prior to surgery. There is a marked, well-marginated swelling of the crus laterally and caudally that was soft on palpation.
Citation: Journal of the American Veterinary Medical Association 2024; 10.2460/javma.24.06.0429
Statistical analysis results
The attenuation gradient determination using linear regressions revealed R2 coefficients of determination of 0.84, 0.96, and 0.89 for cases 1, 2, and 3, respectively, indicating that the linear regression model approximated the precontrast measurements very well (this estimation has a purely descriptive character due to the low sample size). Obtained gradient estimates (regression coefficients) with their 95% CIs and P values showed a negative attenuation gradient in all cases (case 1 gradient coefficient, –0.194; 95% CI, –0.349 to –0.039; P = .028; case 2 gradient coefficient, –0.075; 95% CI, –0.102 to –0.048; P = .003; case 3 gradient coefficient, –0.108; 95% CI, –0.178 to –0.038; P = .016; Figure 4).
Vertical attenuation gradient of MLL fluid on precontrast CT examination. The MLL fluid has significantly lower attenuation in the top (100%) compared to the bottom (0%) of the fluid column, likely due to the lipid content rising to the top.
Citation: Journal of the American Veterinary Medical Association 2024; 10.2460/javma.24.06.0429
The values of the net enhancement in MLL attenuation were as follows. For case 1, 109%, 106%, 87%, 44%, and 32% (mean value, 76%; minimum, 32%; maximum, 109%). For case 2, 4%, –1%, –4%, 7%, and –1% (mean value, 1%; minimum, –4%; maximum, 7%). For case 3, 241%, 64%, –17%, 7%, and –12% (mean value, 57%; minimum, –17%; maximum, 241%). The mean percentage of postcontrast attenuation increase of all 3 cases was 44%.
Discussion
This study was the first to describe the CT features of MLLs in cats. Our hypotheses, that the MLL fluid would (1) accumulate between the dermis and deep fascia in the proximity of a superficial trauma, (2) have a vertical attenuation gradient, and (3) have no to mild contrast enhancement, were all confirmed. Morel-Lavallée lesions are part of posttraumatic, closed, soft tissue degloving injuries described infrequently in people and, as a result, are often not diagnosed in the acute phase of development.3 This type of lesion occurs in patients with a high-energy mechanism of injury with enough force to create a shearing-type disruption, creating a potential space between the skin and deep fascial plane.2,8,9 Perforating lymphatic and blood vessels, together with diffusion through the damaged tissue, contribute to fill this dead space with a combination of blood, lymph, and serous fluid.10,11
The subdermal plexus is the main vascular supply to the skin. Humans have perforating musculocutaneous and direct cutaneous vessels that run perpendicularly to the skin, whereas in dogs and cats the direct cutaneous vessels travel parallel to the skin.12 Compared to dogs, cats have no panniculus muscle, less subcutaneous tissue, and a higher percentage of perforating arteries in the inguinal region.7,13 Moreover, cats have much more mobile skin. It is therefore not surprising that MLLs are more rarely reported in cats than in humans and even more rarely in dogs.7,13
All cases in this series had MLLs predominantly in the area of the crus of the pelvic limb. In humans, MLLs are most commonly seen in the proximal thigh, where the overlying skin is mobile and the underlying fasciae are particularly tough, such as the quadriceps fascia.2 Potentially, there may be a similar predisposition for the crus in the cat. Alternatively, the mechanical forces in a typical trauma may be strongest in that area. It is interesting to note that MLLs have been seen in people following motorbike accidents in which shearing forces on the unprotected skin may be particularly strong.14 Regardless of the species, MLLs are often initially overlooked, likely due to distraction by concomitant significant bone injuries. In other circumstances, they are confused with hematomas, seromas, subcutaneous edema, contusion, or cellulitis and are not treated aggressively. Left untreated, MLLs pose a considerable risk for extensive skin necrosis compared with other types of subcutaneous swelling. In humans, spontaneous resolution of MLLs is uncommon, and surgical debridement, followed by compression bandaging, is often performed.15 Treatment of MLL solely with percutaneous drainage shows a high recurrence of fluid reaccumulation.16 In all 3 cases of our study, multiple percutaneous drainages were placed with reaccumulation of fluid. Only in case 3, due to the suspicion of the attending clinicians that the swelling represented an MLL, a more aggressive treatment with continuous drainage of the effusion and closure of the dead space was applied 53 days after trauma. The MLL resolved, and the patient survived. This emphasizes the importance of recognition of an MLL.
Lipids have very low CT attenuation values, typically ranging between –80 and –30 HU, whereas concentrated RBCs, such as in hematomas, due to the high-density iron content, typically have relatively high attenuation values (43 to 70 HU).17,18 Human MLL fluid typically has low attenuation values ranging between 6 and 30 HU.19 Fat-rich chylous pleural effusion has relatively low attenuation values (dogs, –0.9 to 15.37 HU; cats, 1.4 to 10.8 HU),20,21 whereas hemorrhagic pleural or peritoneal effusion typically has higher values in dogs (pleural, 18.10 to 54.97 HU; peritoneal, 20 to 45 HU).18,21 The attenuation values in the MLL fluid of our cases were in the same range as those of human MLLs and feline and canine chylous pleural effusion but were lower than those of canine hemorrhagic effusion. This is likely related to the relatively high fat-rich lymphatic and low hemorrhagic component of MLL effusion and can help to differentiate MLL from other fluid types.
Lipids are low in physical density and hydrophobic. In most bodily fluids containing lipids, a perfect suspension is not achieved. Therefore, lipid droplets have a propensity to rise to the top of an aqueous fluid column, leading to a measurable vertical density gradient, translatable to a vertical CT attenuation gradient. This has been described in the urine of cats with physiological lipiduria and in canine chylous pleural effusion.20,22 This is consistent with our cases, in which a significant vertical attenuation gradient was present. Computed tomographic assessment of fluid type in effusions can be difficult, due to the mixed nature of most effusions. Measuring vertical attenuation distribution in effusion can be useful in predicting its lipid content.
In humans, the blood components within the MLL fluid gradually reabsorb with time, leading to a remaining serosanguineous fluid surrounded by a hemosiderin layer.2,23 This hemosiderin layer induces a cascade of inflammation in the surrounding peripheral tissues and results in the formation of a fibrous capsule preventing further reabsorption of fluid and leading to the development of a chronic MLL.2,24 Interestingly, in case 3, a thin capsule surrounding the serosanguineous fluid was found intraoperatively.
Limitations of this study were due to the retrospective and multi-institutional nature of the study and the low case number due to the rarity of the disease. Image acquisition protocols and equipment were not standardized.
In conclusion, MLL is an effusion in a newly created space between the dermis and the muscle layers that can be recognized on CT examination. It should be differentiated from other causes of limb swelling. Early recognition and prompt adequate treatment of MLLs is important for a good clinical outcome in the management of often polytraumatic cases.
Acknowledgments
The authors acknowledge Mateusz Dziedzic for contributing to the statistical analysis.
Disclosures
The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.
Funding
The authors have nothing to disclose.
ORCID
N. Dziedzic-Nyrka https://orcid.org/0009-0007-8382-3924
S. Dancer https://orcid.org/0000-0001-9051-6649
L. Nowak https://orcid.org/0009-0000-4094-6405
M. Pinilla https://orcid.org/0000-0002-4957-1539
I. Carrera https://orcid.org/0009-0002-6294-8262
S. Garcia-Pertierra https://orcid.org/0000-0003-2685-3012
D. Clements https://orcid.org/0000-0002-0596-1885
T. Schwarz https://orcid.org/0000-0001-8412-573X
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