What is RAIR (Rectoanal Inhibitory Reflex)?

What is RAIR (Rectoanal Inhibitory Reflex)?

RAIR is a physiological reflex characterized by relaxation of the internal anal sphincter in response to rectal distension, serving as a critical component of normal defecation and continence mechanisms. 1

Physiological Mechanism

• RAIR occurs when rectal distension triggers automatic relaxation of the internal anal sphincter, allowing rectal contents to contact the anal canal sensory epithelium for sampling and discrimination between gas, liquid, and solid stool 2

• The reflex depends on intact intramural autonomic ganglion cells within the rectal wall and is modulated by the autonomic nervous system 3

• The reflex exhibits differential characteristics along the anal canal: maximal relaxation occurs at the middle and upper portions (1.2-3.2 cm from anal verge) and is more prominent posteriorly 2

Clinical Assessment During Anorectal Manometry

• RAIR is routinely assessed during anorectal manometry (ARM) as part of the standardized IAPWG protocol, typically using rectal balloon distension with volumes ranging from 15-177 cc 1

• The reflex demonstrates volume-dependent characteristics: relaxation pressure progressively increases with balloon volumes up to approximately 71 cc, then plateaus 2

• RAIR duration increases with larger distension volumes: in normal patients, average duration ranges from 8.89 seconds at 20 cc to 14.15 seconds at 60 cc 4

Diagnostic Significance

• Absence of RAIR is pathognomonic for Hirschsprung disease, making it the single most important diagnostic finding for this condition 5

• Prolonged RAIR duration may indicate spinal cord pathology: patients with myelomeningocele demonstrate significantly longer RAIR duration (15.21 seconds at 20 cc, 26.02 seconds at 60 cc) compared to functional constipation patients 4

• Abnormal RAIR modulation in amplitude or duration can occur in multiple sclerosis (33.3% and 42.9% respectively), reflecting autonomic nervous system dysfunction 3

Common Pitfalls in Assessment

• Sensor location and orientation critically affect RAIR detection: anterior and more distal catheter positioning may miss the reflex entirely, as maximal relaxation occurs posteriorly and in the mid-to-upper anal canal 2

• Standard manometry catheters may distort the anal canal during measurement, potentially affecting accuracy 6

• In patients with refractory functional constipation and prolonged RAIR duration, occult spinal cord pathology (tethered cord, lipoma) should be ruled out rather than assuming a purely functional etiology 4

Role in Biofeedback Therapy

• RAIR assessment is integral to biofeedback therapy planning for patients with fecal incontinence and dyssynergic defecation 7

• The reflex provides baseline information about internal anal sphincter function and helps identify patients who may benefit from sensorimotor coordination training 7