Description
Pseudomyxoma peritonei (PMP) is a rare clinical syndrome characterised by the accumulation of mucin in the peritoneal cavity, commonly resulting from the rupture of a mucinous tumour, most frequently of appendiceal origin. The disease is often slow-progressing and it involves the distribution of mucin or mucinous tumours throughout the peritoneal cavity, leading to compromised organ function due to pressure and mass effect.
Pathogenesis
Pseudomyxoma peritonei typically arises from a primary appendiceal neoplasm that has perforated the wall of the appendix. The rupture of a mucin-producing tumour, such as a mucinous adenoma or a mucinous adenocarcinoma, results in the leakage of mucin into the peritoneal cavity. This disseminated mucin and associated tumour cells can implant on peritoneal surfaces and produce more mucin leading to a build-up of mucinous ascites.
Epidemiology, Risk Factors & Associations
Pseudomyxoma peritonei is a rare disease, with an estimated incidence of 1-2 cases per million population per year. The disease affects women more frequently than men, with a female-to-male ratio of approximately 3:2. The peak incidence of PMP is in the 5th and 6th decades of life.
Primary mucinous tumours of the appendix are the most common association, with nearly 80-90% of PMP cases originating from these tumours. Less commonly, PMP can be associated with primary mucinous tumours of the ovary, colorectum, and rarely other sites.
Clinical Features
- Progressive increase in abdominal girth due to the build-up of mucinous ascites (“jelly belly”).
- Abdominal discomfort or pain.
- Changes in bowel habits or urinary symptoms due to pressure on the bowel or bladder.
- Palpable abdominal mass or fullness.
- Weight loss.
- In women, ovarian masses may be detected on pelvic examination.
Complications
Complications from PMP result from the progressive accumulation of mucin, leading to:
- Abdominal compartment syndrome: Increased intra-abdominal pressure can compromise venous return and respiratory function.
- Bowel obstruction: Pressure on the bowel can lead to symptoms of obstruction.
- Malnutrition: Resulting from increased metabolic demand and decreased oral intake due to early satiety or bowel obstruction.
- Renal failure: Pressure on the ureters can lead to hydronephrosis and renal failure.
Subtypes
Pseudomyxoma peritonei has been classified into two main histopathological subtypes:
- Disseminated peritoneal adenomucinosis (DPAM): Characterised by abundant mucin with few, scattered, well-differentiated mucinous epithelial cells. This subtype has a more indolent course with better prognosis.
- Peritoneal mucinous carcinomatosis (PMCA): Features more abundant mucinous epithelial cells that are more poorly differentiated, often with infiltrative growth and angiolymphatic invasion. This subtype behaves more aggressively and carries a worse prognosis.
Pathological Features
Histopathology
Examination of the mucin reveals a varying amount of mucinous epithelium. In DPAM, there is well-differentiated mucinous epithelium, while PMCA exhibits poorly-differentiated mucinous epithelium. Acellular mucin pools are common.
Serology
There are no specific serological markers for PMP, but CA 19-9 and CEA may be elevated, reflecting the underlying mucinous neoplasm.
Genetics
KRAS and GNAS mutations have been frequently identified in PMP, especially in cases associated with low-grade appendiceal mucinous neoplasms.
Radiological Features
General Features
Accumulation of mucin tends to occur in certain dependent areas of the abdomen, such as the right lower quadrant, pelvis, and the right upper quadrant beneath the right hemidiaphragm.
CT
- Scalloping of the liver and spleen due to the pressure exerted by the mucin.
- Perihepatic and perisplenic implantations.
- Omental caking
- Thickening of the peritoneum and mesentery
- Ascites with varying degrees of attenuation, often higher than simple fluid due to the mucin content.
- The primary appendiceal tumour may be visible.
MRI
- T2: Mucin appears as high signal intensity
- T1: Mucin can range from low to high signal intensity depending on its protein content.
PET
FDG-PET/CT is usually not helpful in PMP as mucinous tumours tend to be metabolically inactive.
Grading and Staging
PMP is often graded into two categories based on histological features: DPAM and PMCA. There is no universally accepted staging system for PMP, as it does not behave like other peritoneal malignancies with typical lymphatic spread. Instead, the Peritoneal Cancer Index (PCI) is often used to describe the distribution and volume of disease in the peritoneal cavity.
Differential Diagnosis
- Mucinous Carcinomatosis: Typically associated with a known primary malignancy, often colorectal or ovarian.
- Meigs’ Syndrome: Characterised by a benign ovarian tumour, ascites, and pleural effusion.
- Peritoneal Tuberculosis: Can cause ascites and peritoneal thickening, but usually associated with systemic symptoms such as fever and weight loss, and lymphadenopathy is often present.
Management
- Management of PMP is complex and should be coordinated by a multidisciplinary team experienced in the treatment of peritoneal surface malignancies.
- The cornerstone of treatment is cytoreductive surgery (CRS) to remove all visible disease, followed by hyperthermic intraperitoneal chemotherapy (HIPEC) to kill microscopic disease.
- Given the significant risk of recurrence, patients require careful follow-up with regular physical examination, imaging, and possibly tumour marker assessment.