In our case, the presence of a true epithelial lining immediately discounted the possibility of a pancreatic pseudocyst. The ciliated nature of the cells suggested either bronchial type lining or, because of the absence of mucus glands and cartilage, a possible fallopian tube remnant. However, the presence of a dual layered muscular wall with ganglion cells indicated that this was a foregut cyst.
Embryologically, foregut cysts arise from pinched off buds of the endodermal foregut.7–11
Cysts that form and remain in the mediastinum almost always show either bronchogenic or esophageal differentiation. Subdiaphragmatic cysts arise from gastric or small intestinal epithelium. In all of these instances, the cysts most often lie close to and recapitulate the structure of the native organ. Sometimes buds pinched off the supradiaphragmatic foregut migrate into the abdominal cavity via the pericardio-peritoneal canal before fusion of the diaphragm in the sixth week of embryogenesis.7–11
They are commonly located to the left of the midline, within a triangle behind the stomach as defined by the midline, splenic vein, and spleen/diaphragm ()
as in our case. Pancreatic and peri-pancreatic foregut cysts are rare lesions, with about 50 cases reported in the English literature to date.12
Triangle of subdiaphragmatic foregut cysts as indicated by the midline, splenic vein, and diaphragm/spleen. From Gray's Anatomy (1918).
Clinically, patients with subdiaphragmatic foregut cysts may present with abdominal or flank pain, with or without accompanying nausea and vomiting. More frequently, however, these lesions are incidentally identified, occurring in the absence of laboratory aberrations. Radiologically, these cysts have been described as unilocular, fluid-filled masses with rare calcifications of the wall, ranging in size from 0.3 cm up to 18.9 cm with a mean of 6.8 cm.12–14
Their malignant potential is virtually nonexistent, and if a precise preoperative diagnosis can be made, these lesions can be left alone.
However, surgical resection is performed because of difficulties in definitive exclusion of a cystic neoplasm.
The histologic appearance of these cystic lesions follows their embryological derivations. Therefore, esophageal cysts are lined by squamous mucosa, with a wall composed of well-defined muscular layers and neural tissue. Bronchogenic cysts are lined by a ciliated respiratory epithelium, with interspersed glandular cells and areas of cartilage. Enterogenic cysts are often composed of gastric, duodenal, or pancreatic epithelium, with or without an associated muscular wall. Foregut cysts of an indeterminate nature may contain one or more components from any of the above-listed cysts, thereby making a specific diagnosis difficult. Our case demonstrated a ciliated, columnar/pseudostratified lining epithelium, suggesting a bronchial origin, with 2 well-formed muscular layers and associated ganglion cells, suggesting esophageal or enteric differentiation. Other authors12
have referred to this combination as being of the esophageal type. We suggest that the label “indeterminate” would be more appropriate because of the rather hybrid combination of components.
Unexpectedly, our cyst contained 16,161 U/L of amylase, which is difficult to explain in an epithelial-lined cyst. We sought hard, but did not find, pancreatic elements in the cyst wall. The cyst was separate from the pancreas, seemingly too far for amylase to diffuse into it. In the event that preoperative fluid analysis had been performed, a diagnosis of pancreatic pseudocyst would likely have been made, leading to a markedly different therapeutic approach. Other authors have noted that amylase is occasionally present within subdiaphragmatic foregut cysts,15–18
which indicates that amylase analysis cannot wholly distinguish a pseudocyst from a cystic neoplasm. It would be worth investigating whether measurement of other pancreatic enzymes would be more accurate in this regard.