Introduction

Mucocele of the appendix (MA) is a very uncommon disease seen in 0.2–0.3 % of all of the appendectomies performed and in 8.0–10.0 % of all of the appendiceal tumors resected [1]. This condition is more common in females and in people over 50 years of age [2, 3]. The term mucocele simply refers to a cystic mass filled with mucin, as in a dilated appendix. Simple mucoceles secondary to inflammatory obstruction do occur with or without atrophic epithelial changes [4]. There are four histologic subtypes in MA: retention cyst, mucosal hyperplasia, cystadenoma, and cystadenocarcinoma.

A peritoneal inclusion cyst (PIC), also known as a multilocular inclusion cyst and an entrapped ovarian cyst, is a nonmalignant, reactive, mesothelial proliferation of peritoneal cells that results from an insult to the peritoneum. The most common peritoneal insults are endometriosis, pelvic inflammatory diseases (PID), previous abdominal or pelvic surgery, and trauma [3].

Unfortunately, we do not have any definite and specific preoperative diagnostic methods for MA and PIC. Even with imaging modalities such as ultrasonography (USG), computed tomography (CT), and magnetic resonance imaging (MRI), differential diagnosis of those from an ovarian tumor or a hydrosalpinx is still a challenge. We herein report a very rare case with MA and PIC. As far as we know, this is the first report of a case having both MA and PIC with preoperative imaging findings and postoperative histologic confirmation.

Case presentation

A 31-year-old nulliparous Japanese woman visited our hospital with a complaint of lower abdominal pain and a high fever of 39.0 °C. Physical examination revealed a rebound, guarding pain on her lower abdomen. Pelvic examination revealed that she felt tender on motion of her uterus with normal vaginal discharge. She had no associated urinary or bowel symptoms. Her past medical history included PID a year before that had been successfully treated with antibiotics. She had not had any other gynecologic diseases including endometriosis. Transvaginal USG (TV-USG) showed a cyst measuring 2.0 cm in diameter in the right ovary and a hemorrhagic cyst measuring 5.0 cm in diameter in the left ovary (Fig. 1). Chlamydia trachomatis antigen in her cervix was negative. Her laboratory tests revealed a hemoglobin concentration of 10.8 g/dl, elevated white blood cell (WBC) count of 10,170 cells/μl, and C-reactive protein (CRP) concentration of 6.2 mg/dl. Urinary tract infection was ruled out by urinalysis and urine culture. PID complicated with a hemorrhagic left ovarian cyst was considered to be the most probable diagnosis. She was admitted to our hospital and underwent antibiotics therapy (piperacillin 4 g/day for 5 days) and bed rest.

Fig. 1
figure 1

Initial transvaginal ultrasound images. a A cyst (dotted arrows) measuring 2.0 cm in diameter in the right ovary (arrows). b Hemorrhagic cyst in the left ovary

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Although her symptoms subsided and the WBC and CRP values decreased in a few days, fluid in the abdominal cavity was noted on TV-USG. MRI also demonstrated bilateral ovarian cysts and ascites (Fig. 2). Culdocentesis revealed serous ascites with no malignant cells. Examination of tumor markers in her blood indicated normal values (CA-125: 12.2 IU/ml, CA19-9: 5.2 U/ml, CEA: 0.8 ng/ml). She was discharged from the hospital on the 7th day of her admission.

Fig. 2
figure 2

MRI of the pelvis on the 3rd day after admission. a Sagittal T2-weighted MRI showing a right ovarian cyst (white arrow) and a large amount of ascites (yellow arrow). A leiomyoma (red arrow) was noted in the uterus (black arrow). b Sagittal T2-weighted MRI showing a hemorrhagic cyst in the left ovary (arrows)

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Although ascites gradually disappeared in a few weeks, it reappeared rapidly in a month (Fig. 3a). TV-USG revealed a right ovarian cyst with a suspected right hydrosalpinx (Fig. 3b) and the normal left ovary seen in the ascites (Fig. 3a). MRI also showed a large amount of ascites, a right ovarian cyst, and a suspected right hydrosalpinx (Fig. 3c, d). As her mild right lower abdominal pain persisted, laparotomy was performed. A thin-walled cystic mass covered the lower peritoneal cavity (Fig. 4a). In addition, a mono-cystic mucosal tumor of the appendix (Fig. 4b) was found near the normal right ovary, along with normal bilateral adnexa. Therefore, resection of the peritoneal cyst and appendectomy were performed. Histologic examination revealed a solitary cyst of the retroperitoneum and a retention cyst of MA (Figs. 5, 6). Thus, the large amount of ascites demonstrated by USG and MRI was ultimately found to be an enlarged PIC. And the right ovarian cyst and hydrosalpinx also detected by them turned out to be MA and a swollen appendix. The postoperative course of the patient was uneventful.

Fig. 3
figure 3

TV-USG and MRI findings of the pelvis at 1 month after discharge from the hospital. a TV-USG showing ascites (short white arrows), a right ovarian cyst (yellow arrows), and the left ovary seen in the ascites (dotted yellow arrows). b TV-USG showing the normal right ovary (short white arrow) with a cyst (long white arrow) and a suspected right hydrosalpinx (black arrow). c Sagittal MRI showing a large amount of ascites (black arrows), a right ovarian cyst (short white arrow), and a suspected right hydrosalpinx (long white arrow). d Axial MRI showing a large amount of ascites (black arrow), a right ovarian cyst (short white arrow), and a suspected right hydrosalpinx (long white arrow)

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Fig. 4
figure 4

Intraoperative findings. a Intraoperative photograph showing a thin-walled cystic mass covering the lower peritoneal cavity. b A mucocele of the appendix appeared after resection of the peritoneal cyst shown in a

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Fig. 5
figure 5

Histology of the peritoneal cyst lined with mesothelial cells (×20)

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Fig. 6
figure 6

Histology of the appendix showing a retention cyst filled with mucin, absent of the covering epithelium (×40)

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Discussion

Mucocele of the appendix can be confused radiologically with an ovarian tumor, which may prove to be a diagnostic challenge. The differential diagnosis includes adnexal and appendiceal tumors [5, 6]. MA is characterized by abnormal mucus accumulation and dilatation of the lumen of the vermiform appendix [7]. Therefore, a histopathologic microphotograph of the accumulated mucin in the appendiceal lumen is needed for its definitive diagnosis. The most common presenting signs and symptoms of patients with this entity are right lower quadrant abdominal pain (27 %), abdominal mass (16 %), weight loss (10 %), and change in bowel habits (5 %) [5]. These, however, are nonspecific, and thus diagnosis of MA can never be made based on them alone.

There are four histologic subtypes of MA: retention cyst, mucosal hyperplasia, cystadenoma, and cystadenocarcinoma. Cystadenomas and cystadenocarcinomas are mucin-secreting tumors. It is therefore desirable to identify MA preoperatively to avoid rupture of it during surgical procedures, which may lead to pseudomyxoma peritonei (PMP) [3].

Actually, an appendiceal tumor with a giant peritoneal cystic mass seen in our case made us suspect PMP at laparotomy. Most acknowledge that PMP originates in the appendix, and increasingly evidence suggests a similar site of origin in females. In women, synchronous ovarian and appendiceal diseases are common. PMP is thought to involve growth of an appendiceal adenoma progressing to occlude the appendiceal lumen with distension of the appendix by mucus and mucinous tumor cells. In most cases, appendiceal perforation is an occult event. The epithelial cells leaking to the peritoneal cavity continue to proliferate, producing large quantities of mucus [8]. However, the giant cyst in our case contained not mucinous but serous fluid with no epithelial cells. Histologic examination revealed PIC lined with mesothelial cells (Fig. 5).

Peritoneal inclusion cyst is also rare but a well-described benign tumor of unknown etiology. It is first described by Plaut in 1928 [9] and shows cystic mesothelial proliferations. PIC is thought to be due to an inflammatory reaction. This condition usually occurs in the peritoneal cavity in the abdomen or in the pelvis, and the most common predisposing factors in the clinical history are previous surgery, PID, or endometriosis. All of these conditions are believed to interfere with peritoneal reabsorption. The clinical presentation of PIC includes abdominal or pelvic pain, a mass found clinically or radiologically, or an incidental surgical finding [10]. Thus, PIC is considered to be a benign inflammatory process. Even with diagnostic modalities such as USG and CT, preoperative diagnosis is not conclusive, and there are no protocols for diagnostic imaging of this condition. Differential diagnosis includes ovarian cysts, ovarian tumors (benign or malignant), and cystic lymphangiomas. On the other hand, when presenting with acute signs of inflammation as in our case, PID complicated by an abscess would be the most common differential diagnosis as associations with appendicitis are very rare.

Although there have been many case reports of MA mimicking an adnexal mass [5, 7, 11, 12], the diagnosis could be made only at the time of surgery in all of the cases. O’connor et al. [10] reported a case with PIC complicated by appendicitis and mentioned that PIC involving appendicitis is very rare. As far as we know, however, this is the first report of a case having both MA and PIC found simultaneously at surgery. We surmise that her past medical history of PID might have led to the formation of PIC in our patient. However, we do not know the exact pathogenic mechanism of the enlarging PIC in a relatively short time period. As Zagrodnik and Rose [4] mentioned that MA may be diagnosed clinically from features of acute appendicitis, the presence of MA might have caused an inflammatory process mimicking PID, resulting in the enlarging PIC in the present case. In this way, there might be some association between the presence of MA and the enlarging PIC in our case.

Moyle et al. concluded that the different types of cystic pelvic masses may have similar imaging appearances and radiologic evaluation may be of limited diagnostic use. In the present case, a large amount of ascites demonstrated by USG and MRI was ultimately found to be an enlarged PIC, and a right ovarian cyst and a hydrosalpinx also detected by them turned out to be MA and a swollen appendix. We retrospectively reviewed the USG images and now consider the characteristic features of MA with PIC to include: (1) A movable cyst is noted near the normal right ovary, (2) bilateral normal ovaries are confirmed, and (3) intestines are not observed in PIC because it is a confined cyst in contrast to ascites. It is therefore essential to understand the relationship of a mass with its anatomic location, to identify normal ovaries, and to relate the imaging findings to the patient’s clinical history in order to avoid misdiagnosis [3]. In the present case, her past history of PID could be a diagnostic help for PIC. And if normal ovaries had been identified with TV-USG or MRI, MA might have been diagnosed correctly in the preoperative period. In particular, if a movable and isolated cyst from the normal right ovary had been confirmed by TV-USG examinations (Fig. 3b), an accurate preoperative diagnosis might have been achieved.

In conclusion, we herein reported a very rare case having simultaneously both MA and PIC. As these two conditions often mimic gynecologic diseases clinically and radiologically, patients with an adnexal mass or symptoms suggesting PID should be examined carefully considering such conditions in a daily gynecologic practice.