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Background: Rectoanal inhibitory reflex (RAIR) is a physiological reflex implicated in anorectal continence. A lack of RAIR modulation is only described in spinal cord-injured patients with a lesion under L2. No quantitative data has been published concerning the normal modulation in amplitude and in duration in functional disorders. Methods: A retrospective analysis of anorectal manometry, performed in 40 safe-neurological patients, suffering from idiopathic constipation and/or faecal incontinence, has been done. RAIR were obtained by five successive rectal distensions (10-50 ml).Resting pressure, residual pressure, percentages of relaxation, slope and duration of inhibition were estimated. Four hypotheses of normal modulation in amplitude and duration were set up using these parameters. The cut-off values chosen for the hypotheses were similar to results obtained by calculating median value +/− 2SD of the parameters. Results: All the 40 patients had present RAIR. Concerning the modulation of RAIR, we tested the hypotheses with the aim of finding those applying to patient's largest number. Amplitude: 85% of the patients had a normal modulation defined by a difference >8 cm H2O between two non-consecutive residual pressure on three successive rectal distensions. Duration: 77.5% of the patients had a normal modulation defined by a time difference >2 s between two non-consecutive durations on three successive rectal distensions. Conclusion: Determination of normal values of RAIR modulation in functional disorders is interesting in clinical practise, suggesting that the patients with a lack of normal RAIR modulation (in amplitude or in duration) may have a neurological disease. [ABSTRACT FROM AUTHOR]