The internal anal sphincter is morphologically derived from the circular muscle of the rectum, but marked differences have been observed in the motor activities of these two morphologically continuous structures. Immunocytochemical studies using anti-neurotransmitter antibodies (vasoactive intestinal peptide, substance P, metenkephalin, neuropeptide Y), enzyme histochemistry for acetylcholinesterase, and electron microscopy were carried out on internal sphincter specimens from 14 patients with internal sphincter achalasia, 5 normal controls, and on rectum from 4 patients with Hirschsprung's disease (HD). The various peptide-containing nerves were increased in internal sphincter achalasia compared to normal controls and patients with HD. The pathophysiology of internal sphincter achalasia appears to differ from that of HD. It is a distinct clinical entity and should be considered separate from the latter.
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