Multidetector computed tomography features of linea arcuata (arcuate ...

Surg Radiol Anat (2007) 29:397–403 DOI 10.1007/s00276-007-0218-0

ME DICAL IMAGING

Multidetector computed tomography features of linea arcuata (arcuate-line of Douglas) and linea arcuata hernias B. Coulier

Received: 21 February 2007 / Accepted: 21 May 2007 / Published online: 12 June 2007 © Springer-Verlag 2007

Abstract The aim of the study was to estimate the prevalence of delineation of semi circular or linea arcuata (arcuate line of Douglas) (LA) during 64-row multidetector Computed Tomography (mdCT) of the abdomen and to illustrate the so-called linea arcuata hernia (LAH), a peculiar type of interstitial parietal hernia of the anterior abdominal wall consisting of an ascending protrusion of intraperitoneal structures under the LA. The mdCT abdominal wall anatomy of a continuous series of 315 unselected patients was retrospectively reviewed and explored with multiplanar reformations (MPR). Considering a grade 1 lesion (G1) as a single delineation of the LA due to minimal bulging of intraperitoneal fat, a grade 2 hernia (G2) as a substantial herniation under the LA and grade 3 hernia (G3) as a frank prominent herniation of peritoneal structures (essentially omental fat and/or intestinal loops), lesions of all types were found in 8.57% of patients with more than 81% of lesions being G1; this prevalence reached 14.97% in men but only 1.35% in women (M:F sex ratio 12.5:1); all lesions were found in patients older than 51 years and 76% of them were older than 71 years with a great majority of men. Most patients were absolutely free of symptoms related to theses abnormalities. The anatomy of the rectus abdominis sheath and LA are reviewed and the mdCT signs of LA and LAH are described and illustrated. In conclusion, 64-row mdCT can frequently delineate the linea arcuata (LA) and more rarely linea arcuata hernias (LAH), particularly in old men, but

B. Coulier (&) Department of Diagnostic Radiology, Clinique St Luc, rue St Luc, 5004 Bouge (Namur), Belgium e-mail: [email protected]

these abnormalities mostly represent fortuitous asymptomatic Wndings. Keywords Rectus abdominis muscle · Rectus sheath · Abdomen · Computed Tomography · Hernia · Arcuate line of Douglas · Linea arcuata

Introduction During the last decade, considerable technical developments and increasing availability of multidetector Computed Tomography (mdCT) have led more and more to the greater use of CT in current abdominal clinical practice; these progresses have greatly enhanced the CT’s ability to accurately discriminate between those patients with a normal or abnormal abdomen. Moreover, 64-row mdCT with its new high performances in terms of spatial resolution, high table speed and high quality of multiplanar reconstructions, today oVers unrivalled high-quality image of the entire abdominal wall. These images can be obtained without any motion artifact, the entire wall being imaged during a short single breath hold. Our study tries to estimate the prevalence of spontaneous demonstration of the linea arcuata (LA) (arcuate line of Douglas) during clinical abdominal 64-row mdCT and illustrates the so-called linea arcuata hernia (LAH), a peculiar type of interstitial parietal hernia of the anterior abdominal wall. This interstitial hernia consists of a generally asymptomatic ascending protrusion of intraperitoneal structures—usually fat tissue and/or intestinal loops—under linea arcuata. The anatomy of the rectus abdominis sheath and LA are reviewed and the mdCT signs of LA and LAH are described and illustrated.

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Material and methods The abdominal mdCT of a continuous series of 315 unselected patients (148 women, 15–94 years, mean age 62.5 § 5.7 years and 167 men, 17–87 years, mean age 62.5 § 15.26 years) were retrospectively reviewed and analyzed for scrupulous examination of the anterior abdominal wall anatomy. These patients were scanned for various clinical abdominal reasons with a 64-row mdCT (Lightspeed Ultra, General Electric, WI, USA). The collimation was 64 £ 0.65 mm with a pitch comprised between 0.516 and 1.375. The 1.25/0.65-mm axial series were used for axial cineview analysis and dynamic MPR on a Workstation (Advantage Workstation 4.3., General Electric, WI, USA). Patients with previous surgical repair of the abdominal wall, cachectic patients or patients whose abdominal CT was altered by motion or breathing artifacts (26 patients) were systematically excluded from this study. All patients were free of symptoms related to the abdominal wall. Results were classiWed into three grades: a single delineation of the linea arcuata (LA) due to minimal bulging of

Fig. 1 A case of elementary grade 1 lesion: sagittal oblique (a) and axial (b–d) MPR views illustrating the minimal delineation (white arrow) of the left linea arcuata (LA) caused by elementary bulging of peritoneal fat

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Surg Radiol Anat (2007) 29:397–403

intraperitoneal fat was classiWed as grade 1 (G1) (Fig. 1); a minimal but substantial real herniation of fat and/or intestinal loops under the linea arcuata was classiWed as grade 2 (G2) linea arcuata hernia (LAH) (Figs. 2, 4, 5) and frankly prominent hernias of abdominal structures (essentially omental fat and/or intestinal loops) were classiWed as grade 3 (G3) LAH (Figs. 3, 5). Results were also expressed in terms of hemi-abdominal entities (HAE), both sides being frequently unequally aVected in the same patient.

Results Linea arcuata delineation itself (LA = G1) and linea arcuata hernias (LAH = G2 or G3) were visible in 8.57% of all patients and this prevalence reached 14.97% in men but only 1.35% in women (M:F sex ratio 12.5:1). All positive cases were found in patients older than 51 years and 76% of positive patients were older than 71 years with a great majority of men. Positive results were found bilaterally in 21/27 patients (77.8%) and unilaterally in 6/27 patients (22.2%). All results are expressed in Table 1.


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Fig. 2 Example of grade 2 LAH: four successive axial (a) and three sagittal oblique views (b) illustrating the delineation of the right linea arcuata (LA) (white arrows) by a small spontaneous herniation of ileal structures

Discussion Above the level of the semi circular or linea arcuata (LA), the composition of the rectus sheath is classically composed by a central splitting of the aponeurosis of the internal oblique muscle inclosing the rectus abdominis muscle. This aponeurotic complex is anteriorly covered by the aponeurosis of external oblique muscle and posteriorly by that of transverse abdominis muscle. Some variations have been found during anatomic studies [8] but in all cases at least one complete muscular aponeurosis is found separating the rectus abdominis muscle from the transversalis fascia. This aponeurotic system is frequently clearly delineable during abdominal CT in patients having a still Wrm and consistent abdominal muscular mass. Below the level of the ALD, the classical description hold in that all three musculo-aponeurotic layers pass anterior to the rectus abdominis muscle. Therefore the fascia transversalis remains the only structure covering the posterior surface of the rectus abdominis. Anteriorly, the aponeurosis of external oblique muscle tends to remain separate from the underlying two other muscles, this being more noticeable nearer the pubis. Therefore, this aponeurosis is mainly no more than a superWcial covering of internal oblique muscle and contributes little to the rectus sheath itself. The aponeurosis of

the internal oblique and transverse muscles fuse at or just medial to the lateral border of rectus abdominis muscle; this common fused aponeurosis forms an entirely aponeurotic covering of rectus abdominis muscle in the majority of cases; muscular Wbers of transverse abdominis, internal oblique or both are associated in about 50% of cases [4, 5, 8]. The LA, which thus corresponds to the inferiorly concave ending of transverse abdominis and posterior sheet of internal oblique aponeurosis, has a variable position; on some occasions its summit is situated as high as the umbilicus and on others almost at the pubic line; the medial end is usually lower than the lateral. The schematic anatomy of the rectus sheath is illustrated in Fig. 6. From this modiWcation of the rectus sheath under the LA results an area of relative parietal “weakness” more prone to develop parietal hernias. This situation is found in Homo sapiens and many mammalian quadrupeds. This anatomical arrangement presents no particular functional anatomic diYculty for quadrupeds because their inguinal canal is directed “uphill” during ambulation and therefore is not subjected to signiWcant gravitational stress. In humans however, gravitational stress necessitated by erect posture includes bearing the weight of the intra-abdominal organs directed toward the lower abdomen and considerably ampliWes this intrinsic anatomic weakness [7].

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Fig. 3 Illustrations of a right grade 3 hernia diagnosed by CT. US in a 67-year-old woman; on sagittal (a) and axial (c) CT views and on sagittal (b) and axial (d) US views, an ascending herniation of the great omentum (white star) is clearly delineated between the rectus muscle and its posterior aponeurotic system abruptly ending at the level of the

linea arcuata (white arrows); the omental nature of the herniation is recognizable thanks to its small vessels (black arrowhead); The Vasalva maneuver increases the volume of the herniated omental material during US (black star)

This situation of weakness is reinforced by the fact that precisely, also at this level, the internal oblique and transverse muscles do not constitute solid muscular bands but only fascias or thin musculo-aponeurotic bands separated by fascias. Moreover those two layers of thin musculoaponeurotic bands now run in a parallel instead of perpendicular way, a situation more prone to produce interparietal or interstitial herniations [9]. Among these interstitial hernias, the Spigelian hernia represents a well-known

entity [4, 9] seen mainly in elderly men or women with muscle wasting. For the same anatomic reasons, rectus sheath haematomas that are a frequent but sometimes misdiagnosed disease in patients under anticoagulative drugs, hemodialysis, or present simply in the elderly are also predominantly found in this lower aspect of the rectus muscle; in this place, the perforating branches of the inferior epigastric artery which run in the preperitoneal fat may more commonly rupture

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Fig. 4 Left grade 2 hernia: successive axial (a) and sagittal oblique views (b) of the left abdominal wall illustrating the typical abrupt ending of the linea arcuata (white arrows) which appears separated from the rectus abdominis muscle by a ascending herniation of intraabdominal fat (white star); a typical fatty Spiegelian hernia is simultaneously found typically producing through a defect of the common fused aponeurosis of internal oblique and transverse muscles (white arrowhead) and bulging into the preserved aponeurosis of external oblique (black arrowhead)

causing a large hematoma widely spreading in this loose space [3]. In association with this anatomic weakness, systemic connective tissue disease, related to acquired and/or heritable factors, also seems to play an important role in the development of abdominal wall hernias [9]. The parietal “weakening” of the rectus sheath is commonly clearly visible on abdominal CT but the identiWcation of the diVerent bands is generally rarely possible [4]. Similarly, in most cases, the LA itself remains invisible not only because it is extremely thin and coupled to the rectus abdominis muscle but also because it is orientated toward the axial cutting plane. It may only be imaged when suYciently decoupled from the rectus abdominis muscle by single bulging of peritoneal fat or really by herniation of abdominal structures—essentially peritoneal, omental fat or intestinal loops—under the LA and thus behind the rectus muscle itself. Today thanks to its new high performances in term of spatial resolution, high table speed and high quality of multiplanar reconstructions, the new generation of 64-row mdCT actually oVers high-quality capabilities to image the entire abdominal wall with a very high deWnition and without any motion artifact; the entire wall may be imaged during a short single breath hold. Using these new performances, our study reveals that a dynamic scrupulous analysis of the anterior abdominal wall on native millimetric mdCT axial series allows the demonstration of the LA or LAH in a signiWcant proportion of patients, especially in relatively older men, a population

precisely prone to develop other type of inferior abdominal wall hernias. Indeed all our positive patients were old men with the exception of two cases diagnosed in also rather older women. The general prevalence of LA and LAH in our patients was 8.57% but reached 14.97% in men with a M:F sex ratio of 12.5:1. In all cases our patients appeared free of symptoms related to these abnormalities or parietal hernias; these were Wnally fortuitously demonstrated during abdominal CT obtained for various other pathological conditions. This absence of clinical signiWcance also explains important limitations in our study: Wrst the lack of surgical proof or validation of our mdCT Wndings represent the most limiting factor but it was naturally out of question to surgically explore asymptomatic patients; second, the fact that most mdCT abdominal examinations are performed during suspended inspiration, which especially in men may cause an additional valsalva maneuver; this condition may artiWcially reveal or increase the grade of the lesions. Of course, because of the retrospective study design and the ethical priority of an as low as achievable radiation exposure, a relievable diVerentiation between real hernias and temporarily anterior bulging of the abdominal wall cannot be made. During our study, surgical proof was obtained in only one patient; this 67-year-old woman presented with vague abdominal pain in the right lower quadrant where a subtle parietal mass was clinically palpable. CT and Ultrasound studies revealed a grade 3 LAH containing a portion of the

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Fig. 5 Bilateral ALH (grade 2 on the right and grade 3 on the left): axial (a), sagittal (b) and coronal (c) MPR views illustrates the delineation of the linea arcuata (white arrowhead) due to the interposition of an ascending omental herniation (white star) between the rectus abdominis muscle and its posterior aponeurotic system; the omental nature of the herniation is recognizable thanks to its small vessels (black arrowhead); a very small similar omental hernia is also visible on the right (black star); the 83-year-old patient for whom the unenhanced abdominal CT for the surveillance of abdominal aortic aneurysm was asked was completely asymptomatic on axial CT views obtained 1 year before (d), the LAH (stars) bilaterally appeared much more prominent containing not only omental fat but also ileal loops

Table 1 Results

Grade

All patients

Males

Females

Hemi-abdominal entities (HAE)

Total

315

167

148

630

Positive cases

27

25

2

48

8.57%

14.97%

1.35%

7.61%

% of posit.

LA* linea arcuata = G1, LAH** linea arcuata hernia = G2 or G3

G(1 + 2 + 3) G1 (LA*)

6.19%

G2 (LAH**)

1.11%

G3 (LAH)

0.31%

greater omentum (Fig. 3). This hernia was associated with a more classical inguinal hernia containing the right ovary. During surgical repair of this inguinal hernia, a laxity of the LA was eVectively noted; attention was paid to an adequate Wxation of the propylene mesh on the rectus sheath just above the level of the LA to prevent recurrent herniation. It

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is not however proved that the vague patient’s symptoms were owed to this hernia. Previous reported cases of rectus sheath hernia are extremely rare, usually traumatic, and diagnosed surgically [1]; only four cases of spontaneous non traumatic hernias have been described [1, 5, 6]; all occurred in the supra-


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Fig. 6 Schematic anatomy of the anterior abdominal wall (modiWed from Coulier [4]). Linea semicircular = semi circular or linea arcuata

umbilical region above the linea arcuata and thus constituted diVerent entities than those described in the present study; these rare reported cases emphasize the role of other loci of reduced resistance, mainly those giving pathway to the neurovascular structures [5]. Up to now, to our knowledge, only one case of LAH has been reported [2]; the reported patient had bilateral LAH containing intestinal structures and adhesions had developed on the right side probably explaining the clinical pain. The diagnosis had also been facilitated by the use of mdCT.

Conclusion Thanks to its new high performances in terms of spatial resolution and multiplanar reformations, 64-row mdCT can frequently delineate the linea arcuata and more rarely linea arcuata hernias, particularly in old men; but these abnormalities mostly represent fortuitous asymptomatic Wndings.

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