Clinical Focus Box 263

Hirschsprung's Disease and Incontinence: Motor Disorders of the Large Intestine and Anorectum

Hirschsprung's disease is a developmental disorder that is present at birth but may not be diagnosed until later childhood. It is characterized by defecation difficulty or failure. The disease is often called congenital megacolon, because the proximal colon may become grossly enlarged with impacted feces, or congenital agan-glionosis, because the ganglia of the ENS fail to develop in the terminal region of the large intestine. Mutations in RET or endothelin genes account for the disease in some patients.

Enteric neurons may be absent in the rectosigmoid region only, in the descending colon, or in the entire colon. The aganglionic region appears constricted as a result of continuous contractile activity of the circular muscle, whereas the normally innervated intestine proximal to the aganglionic segment is distended with feces.

The constricted terminal segment of the large intestine in Hirschsprung's disease presents a functional obstruction to the forward passage of fecal material. Constriction and narrowing of the lumen of the segment reflects uncontrolled myogenic contractile activity in the absence of inhibitory motor neurons

Incontinence is an inappropriate leakage of feces and flatus to a degree that it disables the patient by disrupting routine daily activities. As discussed earlier, the mechanisms for maintaining continence involve the coordinated interactions of several different components. Consequently, sensory malfunction, incompetence of the internal anal sphincter, or disorders of neuromuscular mechanisms of the external sphincter and pelvic floor muscles can be factors in the pathophysiology of incontinence.

Sensory malfunction renders the patient unaware of the filling of the rectum and stimulation of the anorectum, in which case he or she does not perceive the need for voluntary control over the muscular mechanisms of continence. This condition is tested clinically by distending an intrarectal balloon. The healthy subject will perceive the distension with an instilled volume of 15 mL or less, whereas the sensory-deprived patient either will not report any sensation at all or will require much larger volumes before becoming aware of the distension.

Incompetence of the internal anal sphincter is usually related to a surgical or mechanical factor or perianal disease, such as prolapsing hemorrhoids. Disorders of the neuromuscular mechanisms of the external sphincter and pelvic floor muscles may also result from surgical or mechanical trauma, such as during childbirth.

Physiological deficiencies of the skeletal motor mechanisms can be a significant factor in the common occurrence of incontinence in older adults. Whereas the resting tone of the internal anal sphincter does not seem to decrease with age, the strength of contraction of the external anal sphincter does weaken. Moreover, the striated muscles of the external anal sphincter and pelvic floor lose contractile strength with age. This condition occurs in parallel with a deterioration of nervous function, reflected by decreased conduction velocity in fibers of the pelvic nerves. Clinical examination with intra-anal manometry reveals a decreased ability of the patient with disordered voluntary muscle function to increase in-tra-anal pressure when asked to "squeeze" the intra-anal catheter.

inhibitory motor neurons that are inactivated by the ENS minibrain during each slow wave.

Control by Inhibitory Motor Neurons of the Length of Intestine Occupied by a Contraction and the Direction of Propagation of Contractions. The state of activity of inhibitory motor neurons determines the length of a con tracting segment by controlling the distance of spread of action potentials within the three-dimensional electrical geometry of the muscular syncytium (Fig. 26.17). This occurs coincidently with control of contractile strength. Contractions can only occur in segments where ongoing inhibition has been inactivated, while it is prevented in adjacent segments where the inhibitory innervation is ac-

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