A pictorial essay of the most atypical variants of the vermiform appendix position in computed tomography with their possible clinical implications

Introduction

Appendicitis is one of the most frequent causes of acute abdomen and usually requires urgent surgical intervention in order to avoid severe complications. The vermiform appendix located in the atypical abdominal region may lead to a delay in proper treatment due to symptoms mimicking other diseases. The aim of this study was to present unusual locations of the tip of the appendix and to discuss their clinical implications.
The vermiform appendix is a diverticulum of the caecum, which usually lies in the right lower quadrant of the abdomen [1]. However, the appendix is believed to be the most variable abdominal organ because there are many possible configurations of its location [2-4]. The base of the appendix is usually located in the posteromedial aspect of the caecum (about 2 cm under the ileocecal valve, in the confluence of three taenias, in McBurney’s point) [1], while the appendix tip can be found in various sites [2-5]. The following positions were distinguished [3,6]:
retrocaecal (or retrocolic when long) – appendix situated posteriorly and superiorly to the caecum or ascending colon,
paracaecal – appendix located laterally to the caecum and ascending colon,
subcaecal – under the caecum,
pelvic or descending intraperitoneal – appendix directed inferiorly, ‘pointing’ the pelvis,
pre-ileal and post-ileal – located anteriorly and superiorly or posteriorly and superiorly to the ileum.
The most frequent location of the appendix has been a controversial issue – in several studies conducted on cadavers or during laparoscopic surgery, retrocaecal [2,3,6] or pelvic [4,7] position were the most common findings. It has been suggested that the location and length of the vermiform appendix may depend on age, gender, race, or other demographic factors [2,4,7]. The length of the vermiform appendix ranges between 1.0 cm and 27 cm, with an average length of about 7-11 cm [1-5,7]. Moreover, the appendix is one of the most mobile viscera because of its mesentery, which is called the mesoappendix. The position of the appendix may change during life, particularly after surgical intervention into the abdomen or pelvis [3,7].
Congenital atypical location of the vermiform appendix may result from developmental disturbances leading to the abnormally situated caecum due to nonrotation, malrotation, sessile caecum, or others [1]. We present the most unusual locations of the tip of the appendix found by chance on computed tomography (CT) scans in 13 patients who have been examined in our hospital between December 2014 and November 2017 for other reasons and in whom appendiceal position was reported as a CT finding.

Imaging findings

In three patients the appendix was located in the left inferior abdominal quadrant due to midgut nonrotation with left-sided caecum (Figure 1) or situs inversus (Figure 2). In five patients the subhepatic location of the appendix along the lower edge of the liver and/or next to the right kidney was observed; these patients had a long retrocaecal and consequently retrocolic appendix (Figure 3) or a high position of the caecum (Figures 4 and 5). In two cases the tip of the long retro-ileal appendix was visualised next to the descending part of the duodenum (Figure 6) or even the horizontal part of duodenum in the midline (Figure 7). We also found a long appendix crossing the right ureter (Figure 8). In such cases it may continue to encompass appendages in women (Figure 9). Two patients were found with a pelvic-located appendix reaching the wall of the rectum (Figures 10 and 11). There was one case of a long pelvic appendix ending in the scrotum as the content of an inguinal hernia (Figure 12). We noted that any surgery involving the abdomen or pelvis may change the position of the vermiform appendix, such as a case after a prostatectomy, as presented in Figure 13.

Discussion

Embryonic development

The variations of the appendix position usually result from a complicated and changeable embryonic development of the caecum [1,8,9] and/or the variable length of the appendix [5,10].
The development of the caecum and the vermiform appendix is related to the midgut rotation between the 6th and 10th gestational week. As a result of the rotation the caecum bud is translocated to the right side of the upper abdominal cavity beneath the liver. Afterwards it grows out inferiorly (a so-called descensus of the caecum). Finally, the caecum and the appendix are positioned in the right iliac fossa. Due to asymmetrical growth of the lateral part of the caecum, the vermiform appendix is usually located medially, upwardly, and dorsally; in this position it remains free after fixation of the caecum [1]. The retrocaecal position of the vermiform appendix seems to be connected with descensus of the caecum, when the appendix can bend behind the caecum, and it consequently remains fixed in this position [6].
Importantly, there are a lot of developmental disturbances during midgut rotation, leading to atypical positions of the caecum, including inter alia nonrotation with the left-sided colon and caecum, malrotation with caecum below the pylorus, subhepatic caecum due to lack of descensus, and mobile caecum resulting from lack of fixation in the right iliac fossa [1]. Subhepatic position of the vermiform appendix may be caused not only by arrested caecal descent but also by long, retrocolic appendix, whose base is situated in a normal position in the right iliac fossa [5].

Imaging techniques for visualisation of the vermiform appendix

The normal vermiform appendix and its variants can be visualised by various imaging modalities noninvasively. Ultrasound is characterised by relatively poor accuracy in finding the unchanged vermiform appendix for a few reasons – this method is highly dependent on the experience of the operator and the patient’s constitutional factors like body mass index, abdominal wall thickness, and amount of intraluminal gas in bowel loops. According to data of Yabunaka et al. an assessment of the unchanged vermiform appendix positions in the ultrasound may be underestimated, particularly regarding retrocaecal position, which was sonographically found only in 9.5% of cases [11]. Otherwise, computer tomography is an excellent imaging technique in the evaluation of the normal vermiform appendix because of its very good spatial resolution in any desired plane, avoiding most of the ultrasound-dependant limitations [12]. In magnetic resonance imaging the normal vermiform appendix may be recognisable as hypointense in T1-weighted and T2-weighted, thin-walled tubular structure [13].

Diagnostic imaging of appendicitis

Inflammation of the typically located appendix usually begins with acute pain around the umbilicus. The pain migrates to the right iliac fossa in the course of the disease [7]. Symptoms like fever, vomiting, and elevated inflammation laboratory markers are usually observed subsequently to the pain [5]. In cases of atypically located tip of the appendix the pain is localised in areas of the abdomen typical for other diseases and leads to a clinical dilemma [3,4,8,14,15]. A possible misdiagnosis may lead to complications including perforation, abscess, and peritonitis [3,7,16].
Ultrasound is usually the first-line imaging method in evaluation of patients with acute abdominal pain to confirm or exclude different conditions like cholecystitis and renal colic with hydronephrosis. However, it often may be helpful in establishing diagnosis of acute appendicitis, particularly in the typical pelvic position in the right iliac fossa, especially in children and during pregnancy [13,15,17]. It should be stressed that using ultrasound first instead of CT allows unnecessary exposure to radiation and iodine-based contrast media to be avoided [17,18].
Computed tomography is proposed only when ultrasonography is inconclusive; it is a very reliable modality in the assessment of the inflamed appendix in any location, and it allows exclusion of appendicitis by identification of the normal appendix [12,15,19,20]. Currently, it is being discussed whether non-enhanced CT (NECT) may suffice in as much as 75% of patients suspected of appendicitis and whether contrast-enhanced CT may be reserved for indecisive NECT, which may significantly reduce radiation dose and risk related to contrast media usage as well [21-23]. The acknowledged features of appendicitis in CT include an appendiceal diameter more than 6.5 mm, inflammation of periappendiceal fat, surrounding fluid, and the presence of appendicolith [19]. MRI is reserved for children and pregnant women as a second-line method [13].

Atypical appendicitis

The knowledge of anatomical variants of the vermiform appendix has particular meaning for surgeons and may lead to a proper extension of a typical muscle-splitting incision [1,2,5]; therefore, in our opinion it seems to be important to report the position of an atypically located vermiform appendix as a CT finding. In the literature there are a lot of case reports concerning clinically atypical appendicitis induced by anatomical variants of the length and/or position the vermiform appendix. Left-sided appendicitis in patients with midgut malrotation or situs inversus may be misinterpreted as acute diverticulitis, and both of these entities are followed by a raised white cell count and C reactive protein concentration [15,24]. Subhepatically located appendicitis can be clinically indistinguishable from cholecystitis, liver abscess, right renal colic, or pyelonephritis [5,8,9,20]. Inflammation of the tip of the appendix situated near the wall of the duodenum may produce symptoms of duodenitis or a duodenal ulcer.
A long appendix crossing the right ureter can cause hydronephrosis with renal colic symptoms due to inflammation or the development of an abscess pressing the ureter [16]. When the appendix crosses the ileum or is situated in the midline, appendicitis may be indistinguishable from enteritis [3]. Appendicitis in women should be also differentiated with gynaecological pathologies like inflammation of the adnexa, torsion of the ovary, or extrauterine pregnancy [15]. Sometimes a long pelvic vermiform appendix is found in the contents of a scrotal hernia; therefore, in the case of appendicitis it may produce symptoms similar to orchitis or torsion of the testis [25]. A summary of possible misdiagnoses depending on the atypical location of the appendix is depicted on Table 1.

Conclusions

The location of the appendix is variable due to the changeable length of the appendix and caecum position. Computed tomography is a very good method in depicting very unusual locations even of the unaffected vermiform appendix.
Clinicians and radiologists should be aware of atypical symptoms of appendicitis that result from the variable length and location of the appendix, which may help to avoid misdiagnosis and minimise the delay in applying the proper treatment.

Compliance with ethical standards

Ethical approval: All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. For this type of study formal consent is not required.

Conflict of interest

The authors report no conflict of interest.

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