Etiology

In most children, no etiologic factors can be found and the term "functional constipation" is often used.

Childhood constipation frequently results from a combination of painful defecation (from any cause) and a rectum of sufficient capacity to allow stools to be withheld.[14] The likely sequence of events follows initial hard stools, often due to low fiber, poor nutrient, and/or insufficient water intake or, less commonly, due to delayed colonic motility. The excessive colonic time for stool transport leads to high levels of colonic reabsorption of water and further hardening of the stool. This often starts as an acute problem, which can then progress to:

  • Pain on passing the stool, which leads to involuntary and later learned sphincter contraction (withholding behavior) to avoid discomfort

  • Increasing delays between episodes of defecation, leading to enlargement of residual stool volume in the rectum

  • Further increase in the size of the loaded rectum and more effective withholding tactics

  • Increase in pain and fear as delayed stool is incrementally larger and thus even more painful

  • Eventually, overflow fecal incontinence occurs when the stool stimulates rectal contractions and the internal sphincter temporarily relaxes, with passage of softer stool going around retained hardened fragments.

The tendency to develop constipation seems to be familial, with a positive family history in nearly half of all severe cases.[15]

Although the etiology of constipation in 90% to 95% of children is functional, a number of organic causes have been identified.​[1]​ Perianal group A streptococcal infection causing painful defecation has been found to be a strong precipitator in a small, but easily treated, subgroup of children with constipation.[16] There are cases where food intolerance (particularly cows’ milk) can lead to proctitis and subsequent discomfort on defecation, causing constipation.[17][18][19]​​ Lichen sclerosus and trauma to the anus may also cause pain on defecation leading to constipation. Accidental direct trauma to the anus is unusual, and any evidence of anal trauma must lead to the suspicion of child sexual abuse.[20] Refusal to defecate may be a presentation of complex psychological problems, such as communication disorders that fall within the autistic spectrum.[2] Additionally, ADHD has been found to be associated with constipation.[3]​​ Rarer causes of constipation include those presenting in the first weeks of life, such as Hirschsprung disease or anorectal anomalies. Spinal cord anomalies including tethered cord can lead to constipation and typically present in early childhood. Voiding dysfunction may also be present in these cases.

Excessive volitional stool retention (withholding) and the distressing nature of fecal incontinence may lead to psychological dissociation and denial of symptoms or their importance.[21]

Pathophysiology

Painful defecation can lead to fecal retention. As with other hollow organs where emptying is ineffective (i.e., obstructed bladders, congenital obstructive pyloric stenosis), with protracted childhood constipation there is considerable dilation of the rectum and its muscle layers. This in turn leads to the rectum developing a larger and larger capacity (megarectum) to the point where the stool is too large to be passed through the anal canal, leaving only the continuous overflow of fecal material and stool incontinence. The social and psychological consequences of the incontinence exacerbate the problem. The retained fecal mass seems to be in constant, nonpropulsive motion. This tends to mold the stool into a ball that is difficult to pass and easier to withhold. All these factors lead to a cycle of fear and further retention. Additionally, slow transit constipation may ensue due to poor colonic propulsive function, and outlet obstruction constipation, often due to abnormal pelvic floor musculature coordination, can contribute to the symptoms.

Emerging research also suggests a role for dysbiosis of the gut microbiome. Studies evaluating the gut microbiome of children with constipation have found lower levels of microorganisms including Lactobacillus compared to controls. It is hypothesized that these disturbances may disrupt intestinal physiology and motility, contributing to the development of constipation, although there is still much to be understood about the role of the microbiome.[22]

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