Table 3 Investigations received by participants in group 3 (INVEST)

a. Anorectal manometry using high-resolution methods [80–82] will be used to determine defined abnormalities of recto-anal pressure gradient (see above for definition of functional defaecation disorder (FDD)) during simulated evacuation [37, 83, 84]. A standard high-resolution anal manometry (HRaM) system will be used with a standard laptop. Calibration, validation and maintenance of the equipment will follow the built-in programme and training manuals provided and will be recorded on the system at each session. Training on the system will be performed and documented in the investigator site file prior to sites commencing treatment

b. Balloon sensory testing using standardised methods [85, 86] (2 mL air per second to maximum 360 mL) will be used to determine volume inflated to first constant sensation, defaecatory desire and maximum tolerated volumes. Rectal hyposensation and hypersensation will be defined in accord to gender-specific normative data on 91 healthy adults [87]. The recto-anal inhibitory reflex will also be elicited by 50 mL rapid inflation (if necessary in 50-mL aliquots up to 150 mL)

c. A fixed-volume (50 mL) water-filled rectal balloon expulsion test [37, 70, 83] will be conducted in the seated position on a commode. Abnormal expulsion is defined as failure to expel with a 1-min effort for men and 1.5 min for women [88]

d. Whole gut transit study will use serial (different shaped) radio-opaque markers over 3 days with single plain radiograph at 120 h [89–91]

e. Fluoroscopic evacuation proctography will use rectal installation of barium porridge to defaecatory desire threshold (or maximum 300 mL) and evacuation on a radiolucent commode [90, 92–95] with preopacification of the small bowel (for enterocoele). Radiation dose, proportion of contrast evacuated and time taken will be recorded, as well as ‘functional’ (i.e. pelvic floor dyssynergia) and ‘structural’ features deemed obstructive to defaecation (e.g. rectocoele, enterocoele and intussusception) [37, 87, 96]. There is a maximum fluoroscopic screening time of 3 min