Background

Liver lobe anatomic variations are uncommon and may often be the source of clinical difficulties [1]. There may be ectopic liver lobes which are completely independent, sharing no anatomical continuation with the main organ, and there may also be supernumerary liver lobes which are in fixed or pedunculated anatomical continuation with the main organ, displaying normal parenchymal structures (i.e., vessels, biliary ducts) [2]. We present a trauma patient with an anatomic variation of the left liver lobe, which was initially misinterpreted as a possible perisplenic hematoma, leading to unnecessary further investigations. To our knowledge, the anatomic variation we encountered in the present case has not been previously reported. Additionally, its ability to represent a diagnostic pitfall further enhances its educational value. As a result, we propose a specific name for its characterization that is inspired by existing literature.

Case presentation

A 32-year-old male patient was brought to our hospital’s emergency department following a low-energy motor vehicle accident. At the time of presentation, the patient was ambulatory, had a GCS of 15/15, and appeared hemodynamically stable with normal Hct/Hb blood counts. No clinical signs or symptoms of bleeding were noted, and no significant complaints were documented, besides minor tenderness over the left flank and overlying abrasions detected on physical examination. The patient's past medical history was insignificant. As per protocol, a FAST (Focused Assessment with Sonography for Trauma) scan was ordered. At the time of examination of the left upper quadrant, a crescent-shaped hypoechoic lesion/collection was documented medially to the spleen, raising suspicion of a perisplenic hematoma (Fig. 1). No other significant findings were noted in the remaining regions being scanned. The positive FAST scan mandated further investigation and, since the patient remained stable, he was immediately transferred to the CT room. A contrast-enhanced abdominopelvic CT scan was performed, which did not identify the presence of intraperitoneal free fluid, solid organ injury, or any other significant or trauma-related findings. However, variant left hepatic lobe morphology was demonstrated, as shown and described in Fig. 2. This modified appearance was recognized to correspond to the erroneously suspected perisplenic hematoma depicted in the initially performed false-positive FAST scan. Given the absence of any significant clinical, laboratory, or trauma-related imaging findings, the minor left flank tenderness was attributed to the overlying skin abrasions, and the patient was finally discharged the next day, following an uneventful observation.

Fig. 1
figure 1

Left upper quadrant view during a FAST (Focused Assessment with Sonography for Trauma) examination. A mildly hypoechoic crescent-shaped region is showcased (indicated by the outlined dashed oval area) medially to the upper half of the spleen (S), raising suspicion of a possible perisplenic hematoma

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Fig. 2
figure 2

CT scan of the abdomen and pelvis. Axial CT image in the arterial (a) and portal (b) scanning phase demonstrate extension of the lateral part of the left liver lobe (LLL) across the midline. Following an acute angulation (a), the left liver lobe parenchyma is seen to assume a crescent shape (asterisk), lying juxtaposed to the spleen (S) medially to its visceral surface

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Discussion

Although anatomic variations of the liver lobes are uncommon, their incidence may often present clinical difficulties [1]. As previously mentioned, there may be ectopic and accessory liver lobes. The most usually encountered accessory lobe is the Riedel’s lobe, which resembles a segmental hypertrophy of the right liver lobe [2]. Additionally, the size of the left hepatic lobe, as well as its shape, may also demonstrate variations. A well-known variation of hepatic morphology is demonstrated when part of the left liver lobe extends through the midline into the left upper quadrant, partly encasing the spleen from its anterior aspect [3]. Due to its resemblance to a beaver’s tail, this appearance has been documented in the literature as the “beaver tail liver” [1, 4, 5]. Liver parenchyma in this variation is composed of normal tissue [1], and hence, it does not present an added risk of developing intrinsic pathology [6]. However, although this has not been specifically proven, the “beaver tail liver” has been considered as more susceptible to injury in the setting of upper abdominal or lower chest trauma [4, 6, 7], probably due to its superficial location with respect to the spleen. Familiarity with this variant is of outmost importance in the context of abdominal trauma since the unexpected proximity of the liver and spleen can be misconstrued as a perisplenic or splenic subcapsular hematoma on imaging examinations [1, 8,9,10,11], leading to diagnostic pitfalls.

In the present case, we describe a trauma patient with an analogous pitfall owing to a previously unreported left liver lobe morphology that was initially misdiagnosed as a perisplenic hematoma on the FAST (Focused Assessment with Sonography for Trauma) scan. In our patient, the lateral part of the left liver lobe was found lying in juxtaposition to the spleen, medially to its visceral surface, after extending across the midline, angulating acutely, and assuming a crescent-shaped configuration. Inspired by the aforementioned well-known “beaver tail liver” variant, we perceived that in our case, the crescent-shaped liver parenchyma also resembled a beaver’s tail, which, however, due to the acute angulation and its medial juxtaposition to the spleen’s visceral surface, appeared to be “in hiding” (Fig. 3). Hence, we propose the use of the term “hiding beaver tail liver” (HBTL) for the description of this unique appearance. It should be emphasized that the HBTL morphology should not be confused with findings such as the “kissing sign” of liver and spleen, encountered on imaging when the aforementioned organs contact each other in patients with hepatomegaly and/or splenomegaly [12], nor with the above-mentioned “beaver tail liver” variant where the lateral part of the left liver lobe extends and surrounds the anterior and lateral aspect of the spleen [1]. Discriminating features of the HBTL morphology when compared to these appearances are the medial juxtaposition of the lateral part of the left liver lobe to the visceral surface of the spleen as well as the acute angulation between the lateral part of the left liver lobe and the remainder of the liver.

Fig. 3
figure 3

“Hiding beaver tail liver” illustration. The crescent-shaped lateral part of the left liver lobe parenchyma resembles a beaver’s tail, which, due to the acute angulation and its medial juxtaposition to the visceral surface of the spleen, appears to be “in hiding.” The remainder of the liver represents the beaver’s body, from which the tail arises (illustration inspired by [5])

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Similar to the “beaver tail liver,” and as demonstrated by this case, the HBTL also poses the risk of being misinterpreted on imaging examinations as perisplenic or splenic subcapsular fluid/hemorrhage. This further highlights the educational value of this unique case and the need to familiarize radiologists and ultrasound-performing clinicians with this possible pitfall that may lead to redundant investigations. Whenever such a perisplenic/splenic “lesion” is suspected, the authors suggest the use of the same techniques that are suggested for the differentiation of true pathology from the “beaver tail liver” variant; those being angulations of the ultrasound transducer in an attempt to exclude or verify its continuity with the remainder of the liver [7], and the use of Color Doppler imaging in order to exclude or verify the presence of portal veins and/or hepatic vessels [1, 6, 8]. The authors speculate that contrast-enhanced ultrasound could also prove helpful in such a differential diagnosis conundrum by displaying contrast-enhancement of the liver parenchyma or absence of contrast-enhancement in the case of a hematoma. If, however, the ultrasound findings remain unclear, then further examinations such as CT should be obtained. In our case, the patient’s left flank tenderness upon presentation further guided interpretation toward an initial false-positive diagnosis on FAST examination. However, the findings were eventually vindicated by the subsequent CT examination, and the patient’s symptoms were attributed to the overlying abrasions.

Conclusions

The “hiding beaver tail liver” (HBTL) is a hepatic morphology variant encountered when the lateral part of the left liver lobe (beaver’s tail) extends across the midline and lies medially to the visceral surface of the spleen, following an acute angulation (in hiding). This characteristic morphology distinguishes it from other similar appearances such as the well-known “beaver tail liver” variant and the “kissing sign” of liver and spleen. Radiologists and ultrasound-performing clinicians must be aware of this variant morphology and the techniques with which it can be differentiated from true pathology, as it can mimic a perisplenic hematoma, leading to false diagnosis and unnecessary further investigations.