the anorectum plays an important role in regulation of defecation and in the maintenance of continence.1 the most widely used test for anorectal function is anorectal manometry . a comprehensive assessment of anorectal function consists of measuring at a minimum each of the following parameters : ( 1 ) anal sphincter function , ( 2 ) rectoanal reflex activity , ( 3 ) rectal sensation , ( 4 ) changes in anal and rectal pressures during attempted defecation , ( 5 ) rectal compliance and ( 6 ) performance of a balloon expulsion test.2
anal sphincter function is assessed by measurement of resting sphincter pressure , squeeze sphincter pressure , and the functional length of the anal canal .
maximum resting anal canal tone predominantly reflects internal anal sphincter function , while voluntary anal squeeze pressure reflects external anal sphincter ( eas ) function .
functional anal canal length is defined as the length of the anal canal over which resting pressure exceeds that of the rectum by greater than 5 mmhg or , alternatively , as the length of the anal canal over which pressures are greater than half of the maximal pressure at rest ( fig .
maximal resting anal pressure is defined as the difference between intrarectal pressure and the highest recorded anal sphincter pressure at rest , and is generally recorded 1 - 2 cm from the anal verge .
maximum squeeze pressure is defined as the difference between the intrarectal pressure and the highest pressure that is recorded at any level within the anal canal during the squeeze maneuver ( fig .
rapid distention of the rectum induces a transient increase in rectal pressure , followed by a transient increase in anal pressure associated with eas contraction ( the rectoanal contractile reflex ) , and in turn , a more prolonged reduction in anal pressure due to relaxation of the internal anal sphincter ( the rectoanal inhibitory reflex , fig .
the rectoanal contractile reflex is a compensatory guarding mechanism that allows a positive anorectal pressure gradient to be maintained during transient increases in intra - abdominal pressure ( such as coughing ) , which is essential for preserving continence . in fecal incontinence patients , anal sphincter pressure
the lowest volume of air that evokes sensation and a desire to defecate , and the maximum tolerable volume are recorded.4 assessment of rectal sensation is useful in patients with fecal incontinence or rectal hyposensitivity .
when an individual is requested to ' bear down , ' as if attempting to defecate , the normal response consists of an increase in rectal pressure that is coordinated with a relaxation of the eas ( fig .
inability to perform this coordinated maneuver suggests a diagnosis of dyssynergic or obstructive defecation , a common cause of constipation.5 this response can be quantified using the defecation index = maximum rectal pressure during attempted defecation / minimum anal residual pressure during attempted defecation .
a normal defecation index is > 1.5.5 three types of dyssynergic defecation are recognized.6 most patients show paradoxical increase in anal sphincter pressure during attempted defecation with normal adequate pushing force ( type 1 , fig .
some patients are unable to generate an adequate pushing force , and exhibit a paradoxical anal contraction ( type 2 ) . in type 3 , the patient can generate an adequate pushing force , but has absent or incomplete ( < 20% ) sphincter relaxation .
rectal compliance is calculated by plotting the relationship between balloon volume ( dv ) and steady state intrarectal pressure ( dp ) .
the balloon expulsion test is used to assess rectoanal co - ordination during defecatory maneuvers.7 the test evaluates a patient 's ability to expel a filled balloon from the rectum , providing a simple and more physiologic assessment of defecation dynamics .
most normal subjects can expel the balloon within 1 minute.5 if the patient is unable to expel the balloon within 3 minutes , dyssynergic defecation should be suspected .
 anorectal manometry is the most well established and widely available tool for investigating anorectal function . 
 anal sphincter tone can be quantified by anorectal manometry . the anorectal sensory response , 
 anorectal reflexes , rectal compliance , and defecatory function are also assessed by anorectal manometry . 
 this report will focus on defining parameters for measurement and interpretation of anorectal manometry tests . 
Introduction
Anal Sphincter Function
Rectoanal Reflex Activity
Rectal Sensation
Changes in Anal and Rectal Pressures During Attempted Defecation
Rectal Compliance
Balloon Expulsion Test
Conclusion