A 32-year-old man is evaluated for bowel dysfunction. The patient suffered multiple injuries during a motor vehicle collision 6 months ago. He was hospitalized for several weeks and is currently undergoing treatment with physical therapy. Since the injury, the patient has developed severe constipation with occasional fecal incontinence. Evaluation reveals a large amount of stool in the distal colon due to impaired motor function. Anal sphincter tone is decreased. Injury to which of the following structures best explains this patient's bowel dysfunction?

Question

A 32-year-old man is evaluated for bowel dysfunction.  The patient suffered multiple injuries during a motor vehicle collision 6 months ago.  He was hospitalized for several weeks and is currently undergoing treatment with physical therapy.  Since the injury, the patient has developed severe constipation with occasional fecal incontinence.  Evaluation reveals a large amount of stool in the distal colon due to impaired motor function.  Anal sphincter tone is decreased.  Injury to which of the following structures best explains this patient's bowel dysfunction?

Quizplus · Accepted Answer

11ec160b_35a4_b243_9249_6b6812cbd795_MD0001_00 This patient has developed both constipation and fecal incontinence with decreased tone of the anal sphincter after traumatic injury. A deficiency of parasympathetic input or excess of sympathetic input can lead to constipation by impairing peristalsis. Above the splenic flexure, peristalsis is controlled by parasympathetic innervation from the vagus nerve. Beyond the splenic flexure, it is controlled by parasympathetic innervation from the S2-S4 nerve roots. The innervation of the anal sphincter is complex and under voluntary, parasympathetic, and sympathetic control; therefore, fecal incontinence can occur due to lesions at multiple levels of the nervous system (eg, frontal cortex, spinal cord, peripheral nerves). The anal sphincter consists of an: External sphincter, which includes skeletal muscle and is under voluntary control innervated by the pudendal nerve (S2-S4 branches) Internal sphincter, which includes smooth muscle that is contiguous with the rectum. Sympathetic innervation normally maintains it in a state of tonic contraction. Pressure in the rectal vault (due to the presence of stool) stimulates parasympathetic input from the pelvic splanchnic nerves to relax the sphincter. Therefore, this patient likely has impairment of both parasympathetic and voluntary motor output from the S2-S4 nerve roots because of sacral spinal cord (ie, conus medularis) damage. This is leading to impaired peristalsis beyond the splenic flexure (ie, constipation) and flaccid weakness of the external anal sphincter (fecal incontinence). (Choices A and D) Cervical or thoracic spinal cord injury could affect the upper motor neuron pathways involved in voluntary external anal sphincter function. However, this would lead to increased sphincter tone. (Choice B) The pudendal nerve is involved in external anal sphincter innervation; dysfunction can lead to fecal incontinence. However, it does not provide parasympathetic innervation to the colon, so dysfunction would not lead to stool retention in the distal colon. (Choice E) The vagus nerve (CN X) provides parasympathetic innervation to the GI tract above the colon's splenic flexure; injury classically causes delayed gastric emptying. However, distal colon stool retention is more likely related to impaired parasympathetic activity of the S2-S4 nerve roots. Also, vagal injury would not affect anal sphincter tone. Educational objective: Sacral spinal cord injury can lead to lower motor neuron injury of the S2-S4 nerve rootlets; these contribute parasympathetic innervation to the distal colon to stimulate peristalsis and voluntary motor innervation to the external anal sphincter. Lesions in this area can lead to distal colon stool retention, fecal incontinence, and a weak external anal sphincter. __________ References: Central nervous system control of gastrointestinal motility and secretion and modulation of gastrointestinal functions. (http://www.ncbi.nlm.nih.gov/pubmed/25428846) Control of motility in the internal anal sphincter. (http://www.ncbi.nlm.nih.gov/pubmed/30827084)

A 32-Year-Old Man Is Evaluated for Bowel Dysfunction