Decreased perception of the rectum has been described in about one-third of patients with ODS [7]. The tonic response of the rectum to evoked urge to defecate seems to be absent or significantly blunted in ODS patients with or without STC [8]. The most likely cause is impairment of afferent sensory pathways due to pelvic nerve injury such as during childbirth, pelvic surgery (hysterectomy), or excessive perineal descent and prolapse.

Several terms (anismus, dyssynergia, spastic pelvic floor, and puborectalis syndrome) are used to reflect the inability to relax the pelvic floor muscles in order to open the anal canal and facilitate defecation. This might be related to subconscious cortical inhibition, as it is more common after sexual abuse. The currently popular “safe toilet syndrome” is a more subtle variant of anismus, demonstrating the influence of various psychological components [9].

Rectocele is a result of weakening of the rectovaginal septum, leading to “herniation” into the posterior vagina. The bulging at straining, together with the entrapment of stool in the rectocele, prohibits efficient evacuation. Isolated rectocele can occur, but often they are a sign of overall pelvic weakness. The complex of rectocele, rectal intussusception, enterocele and anterior compartment prolapse is known as ‘descending perineal syndrome’.

In enteroceles, small bowel or sigmoid (sigmoidocele) becomes trapped between the vagina and rectum in a deepened pouch of Douglas. The role of enterocele in ODS is debatable, as two studies found no influence of adequate enterocele repair on symptoms of ODS [10, 11].

External rectal prolapse can be accompanied by symptoms of ODS, but fecal incontinence is often more pronounced. Bowel mucosa and pudendal nerves can be damaged as a result of stretch. Recurrent or persistent full-thickness prolapse dilates the sphincters and anal canal. Repetitive stimulation of the rectoanal inhibitory reflex can result in insensitivity to the reflex. In addition to this, the intussusseptum occludes the lumen of the rectum causing mechanical obstruction. Internal prolapse or intussusception seems to be the precursor of external prolapse, and is increasingly recognized as a cause of ODS [12]. Early studies demonstrating internal intussusception in asymptomatic volunteers [13] have been contradicted by others [14] showing that high-grade full-thickness intussusception is confined to symptomatic patients. The good results of laparoscopic ventral rectopexy in both external and internal intussusception, where anatomical correction is achieved without resection of the hyposensitive mucosa, argues in favor of the important role of prolapse in ODS.

Anamnesis should identify patients who are mainly worried about underlying (malignant) pathology and are satisfied by reassurance only, instead of further treatment. A subgroup of patients has unrealistic views of “normal” bowel patterns. A common, but easily negated, misbelief seems to be that if defecation fails to occur for a few days, toxins arise that “poison the body”.

Inability to relax, and especially paradoxical contractions on straining, can raise suspicion of anismus, although digital examination alone gives a high rate of false-positives results [15]. Perineal descent and rectocele have to be assessed. Sometimes internal prolapse can be felt touching the fingertip on straining.

Flexible sigmoidoscopy/colonoscopy should be used with a low threshold to exclude anorectal malignancies and strictures.

Colon transit studies are an easy, safe, and inexpensive way to diagnose whole-gut slow-transit constipation where the radiopaque markers are found mainly proximal in the descending and transverse colon. Left-sided retention can sometimes be seen in obstructed defecation.

Defecating proctography gives very valuable functional dynamic information. Inability to void any contrast can be indicative of anismus, but the clinician has to be aware that embarrassment can mimic the disorder and lead to a high rate of false-positive results. Defecating proctography is the preferred tool in assessing anatomical causes of obstruction such as enterocele, rectocele, and intussusception, although incomplete emptying can lead to underdiagnosis of the latter. An examination under anesthetic is used to diagnose these initially missed prolapses.

Anorectal physiology with rectal balloon distension is used to examine rectal sensory perception and rectal wall contractility, sensory threshold volume, urge to defecate volume, and maximum tolerable volume. Paradoxical contractions or pelvic dyssynergia can be detected. The rectal anal inhibitory reflex is absent in congenital Hirschsprung’s disease and systemic sclerosis.

A thickened internal sphincter can be seen in anismus, rectal intussusceptions, and solitary ulcer of the rectum syndrome. Congenital myopathy/hypertrophy of the internal anal sphincter is a rare cause of ODS. Diagnosis is made by observation of inclusion bodies on biopsy specimens. Successful treatment with nitrates, calcium-channel blockers, and strip myectomy has been reported [16]. A balloon expulsion test is mandatory when considering colectomy.