Mucocele of the appendix

Discussion

Longitudinal US scan [Figure - 1] of the right lower quadrant shows an anechoic, cystic, blind ending mass measuring 38 mm x 23 mm. CT scan [Figure - 2] reveals a tubular, cystic mass with thin walls in the peri cecal area.

Mucocele of the appendix is a descriptive term that implies a dilated appendiceal lumen caused by abnormal accumulation of mucus. It is a rare clinical entity and the incidence of mucocele in appendectomy specimens is 0.2 -0.3% [1] . The mean age of presentation is 55 years with a female preponderance of 4:1. Clinical features include pain in the right lower quadrant, a palpable abdominal mass and intermittent colicky pain provoked by intussusception of the mucocele into the cecum [2]. Approximately 25% of mucoceles are asymptomatic and are discovered as an incidental finding on surgery [3].

Mucoceles were earlier thought to be the result of obstruction with distal accumulation of mucus. Obstruction was commonly post inflammatory but could also be due to fecoliths, congenital obstruction of Gerlach's valve or extramural compression. Currently, evidence from pathological examinations suggests that mucosal atypia or neoplasia is the basis of mucocele formation. Histologically Higa et al. have classified these lesions into three groups: 1) focal or diffuse mucosal hyperplasia without atypia, 2) mucinous cystadenoma and 3) mucinous cystadenocarcinoma [4] . Of these, the commonest type (60%) is mucinous cystadenoma, which exhibits focal or diffuse conversion of the mucosa into neoplastic epithelium. Most patients are asymptomatic and have an excellent prognosis. Mucinous cystadenocarcinomas though less common (10-15%) are often symptomatic. On histopathology, there is evidence of glandular invasion into the stroma. Myxoglobulosis, a rare variant of mucocele, is characterized by an appendix filled with "translucent globules" or a cluster of "frog eggs". These spheres represent a derivative of granulation tissue found in the wall that breaks off, undergoes necrosis, and subsequently calcifies [1]. Calcified spheres may be seen on plain films, US and CT.

Complications of mucocele include pseudomyxoma peritonei and intussusception. Pseudomyxoma peritonei is characterized by implants of mucinous epithelium on the peritoneal surfaces and mucus accumulation within the peritoneal cavity [5]. It usually follows rupture of a benign or malignant mucocele. Adhesions and intestinal obstruction are the most frequent complications of this condition. If pseudomyxoma peritonei is found in the setting of mucinous cystadenocarcinoma, the prognosis is poor with a 20% five-year survival rate [1].

Plain films of the abdomen may demonstrate a mucocele as a soft tissue mass in the right lower quadrant with or without calcification. An extrinsic impression on the cecum, terminal ileum, or sigmoid colon with non filling of the appendix may be seen on barium examination. A classic appearance is the "vortical fold" pattern described by Vorhaus; this represents a concentric ring appearance of the cecal mucosal folds directed towards the obstructed appendiceal orifice [1]. Typical US findings include a cystic mass with variable internal echogenicities and layered wall in the right iliac fossa. Posterior acoustic shadowing due to calcification, internal septations, intussusception and pseudomyxoma peritonei may also be seen [1],[3],[6]. A well encapsulated cystic mass with thick or thin walls is seen on CT [6}. Absence of periappendiceal inflammation helps in excluding acute appendicitis or appendicular abscess. The presence of punctate or curvilinear calcification in a right lower quadrant cystic mass strongly suggests a mucocele and is best demonstrated on CT. Complex or irregular mucoceles with focal, nodular enhancing components, amorphous calcification and infiltration into adjacent organs are more likely to be mucinous cystadenocarcinoma.

Characteristic US and CT findings combined with non visualization of the normal appendix help differentiate an appendicular mucocele from mimics like enteric duplication cyst, ovarian cyst, mesenteric cyst, hydrosalphinx, lymphocele, abscess and hematoma [7],[8]. Additionally imaging may help in pre-operative identification of malignant transformation in a mucocele. Accurate preoperative diagnosis is also essential to reduce the risk of iatrogenic rupture and subsequent development of pseudomyxoma peritonei.