The rectoanal inhibitory reflex (RAIR) was initially described by Gowers1 in 1877. This reflex is a normal response to rectal distention by 10 to 30 cc of air; the external sphincter transiently contracts and there is a relaxation of the internal anal sphincter, enabling rectal contents to be "sampled" by the sensory area of the anal canal. Thus, the RAlR is generally a common finding during anal manometry. In the constipated patient, the RAIR represents an important marker for Hirschsprung's disease, as the agan-glionic segment impairs the relaxation of the internal sphincter secondary to rectal distention (Fig. 8.13). Additionally, in patients with megarectum, regardless of the cause of the rectal distention, the RAIR is not always seen; the large rectum requires larger volumes to elucidate the sphincteric response. Therefore, while testing for the RAIR in constipated patients, larger volumes of air should be utilized, usually greater than 50 to 80 cc. When the RAIR cannot be demonstrated utilizing much larger volumes of air, it is advisable to change the radial catheter to a spiral manometry catheter in order to demonstrate a positive reflex in a more proximal situation.
Although demonstration of a sphincteric relaxation is the anticipated response when
Figure 8.13. Rectoanal inhibitory reflex. Impairment of the relaxation of the internal anal sphincter, suggesting congenital megacolon.
testing for the RAIR, recent concepts on the amplitude of relaxation and duration have been associated with constipation (J. Netinho, personal communication, 2002).
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