Aim: The aetiology of idiopathic megarectum and idiopathic megacolon is unknown. We postulated that biochemical or ultrastructural abnormalities may be pathologically important, as has been observed in patients with chronic idiopathic intestinal pseudo-obstruction.
Methods and results: Resection specimens from five patients with idiopathic megarectum or megacolon were processed for paraffin wax-embedded tissue histology (haematoxylin and eosin, Gomori trichrome and picrosirius stains, phosphatase activity and periodic acid-Schiff staining), and frozen tissue for histochemistry and electron microscopy. The antibodies used in the immunohistochemistry were to myosin light chain kinase, smooth muscle myosin, alpha and beta actins, filamin, tropomyosin, phosphorylated and dephosphorylated neurofilaments and N-CAM. Variable hypertrophy of the muscularis mucosae and externa, no atrophy, and a variable nerve density decrease in longitudinal muscle and increase in the lamina propria was seen. In one patient beta actin and myosin light chain kinase immunoreactivity was reduced.
Conclusion: Variable changes in innervation, and an abnormal contractile protein immunoreactivity pattern in one patient, may be of pathogenic importance. These clinically homogeneous conditions are likely to be due to a range of underlying pathogenic abnormalities. A search for further specific biochemical abnormalities is justified.