Pathology of idiopathic megarectum and megacolon

Background—The aetiology and pathology of bothidiopathic megarectum and idiopathic megacolon are unknown. Inparticular, it is unknown whether there are abnormalities involvingenteric nerves or smooth muscle.
Methods—Resected tissue was examined from 24 patients who underwent surgery for idiopathic megarectum, from sixpatients who had tissue resected for idiopathic megacolon, and 17 control patients who had surgery for non-obstructing large bowelcancer. Qualitative and quantitative histological examination wasperformed after staining with haematoxylin and eosin, periodic acidSchiff (PAS), Martius scarlet blue (MSB), and phosphotungstic acidhaematoxylin (PTAH). Neural and glial tissue were examined afterimmunostaining with S100 and PGP9.5.
Results—Compared with controls, patients withidiopathic megarectum had significant thickening of their muscularismucosae (median 78 v 33 µm, p<0.005), circular muscle(1000 v 633 µm, p<0.005), and longitudinal muscle (1083 v 303 µm, p<0.005), despite rectal dilatation. Thisthickening was relatively greater in the longitudinal than in thecircular muscle. Fibrosis of the longitudinal muscle was seen, usingMSB staining, in 58%, of circular muscle in 38%, and of muscularismucosae in 29% of patients. The relation between muscle thickening andfibrosis was variable. The density of neural tissue in the longitudinalmuscle seemed to be reduced in patients with idiopathic megarectum.There was no thickening of enteric muscle or alteration in the densityof innervation in patients with idiopathic megacolon.
Conclusion—There is notable thickening of theenteric smooth muscle in patients with idiopathic megarectum, but thearchitecture of the enteric innervation seems to be intact. Functionalabnormalities of the latter remain a possible cause of the smoothmuscle hypertrophy.

Keywords:idiopathic megarectum; idiopathic megacolon