This study is unique as it comprehensively appraised anorectal and pelvic floor structure and functions maintaining continence, risk factors for disordered anorectal functions, and the relationship between symptoms and disordered functions in women with idiopathic FI. We demonstrated: (i) structural and functional disturbances not only in the anal sphincter but also in the puborectalis in FI; (ii) significant reduction in rectal capacity in 25% of FI patients; reduced rectal capacity was associated with rectal urgency and increased perception of rectal balloon distension; and (iii) that predictive factors (age, BMI, symptoms, obstetric history, and anal sphincter appearance) explained significant portions of the interindividual variation in anal squeeze pressures and rectal capacity in idiopathic FI. The data in this study substantially extend previous observations that idiopathic FI is a multifactorial disorder.8
Only nine FI patients (17%) had normal anal resting and squeeze pressures. Conversely, imaging infrequently revealed significant abnormalities of the anal sphincter and puborectalis in asymptomatic subjects. Consistent with previous studies, internal and external sphincter appearance by MR and ultrasound were generally concordant. Ultrasound was more sensitive in the detection of internal sphincter pathology while MRI was more sensitive for visualising abnormal external sphincter morphology.23,30,31
Anal sphincter atrophy was observed almost exclusively in FI; approximately 25% of FI patients had external sphincter atrophy, demonstrated by MRI only, confirming previous uncontrolled studies with MRI.30–32
It may be important to identify external sphincter atrophy because patients with atrophy do not fare as well as patients without external sphincter atrophy after repair of external sphincter defects.30
One third of FI patients had reduced upward anorectal motion during squeeze, indicating puborectalis dysfunction and supporting a recent study in which puborectalis force was measured by an intrarectal dynamometer.9
Our study is the first to demonstrate puborectalis atrophy in FI and an association between atrophy and puborectalis dysfunction. The aetiology of puborectalis atrophy is unknown but all women with puborectalis atrophy had four or more vaginal deliveries and/or a forceps delivery, supporting previous studies suggesting possible muscle damage during vaginal delivery.33,34
Further studies are necessary to ascertain whether puborectalis atrophy can predict the effect of biofeedback therapy on puborectalis function. Dynamic MRI also revealed paradoxical puborectalis contraction during evacuation in nine patients. It is necessary to identify and address impaired evacuation as retention of stool secondary to impaired evacuation may increase a tendency for incontinence.
Rectal compliance and capacity were reduced in 20% of patients with “idiopathic” FI, extending previous studies in ulcerative colitis,35
and idiopathic FI.12
Reduced rectal compliance was not associated with the symptom of urge FI. In the stepwise logistic regression model, reduced rectal capacity was useful for discriminating between controls and FI, underscoring the importance of reduced rectal capacity to the pathophysiology of FI. Moreover, reduced rectal capacity was associated with the symptom of urgency, and with increased rectal perception. In the linear regression model, anal sphincter atrophy predicted reduced rectal capacity. It seems unlikely that reduced capacity would cause severe anal sphincter injury or vice versa. Shared mechanisms may be responsible for both disturbances and these require further elucidation. Further studies are also necessary to ascertain if reduced rectal capacity is attributable to “active” (for example, increased rectal tone) or “passive” (for example, fibrosis) mechanisms. Rectal capacity and hypersensitivity may improve after combined rectal augmentation using a segment of distal ileum and stimulated gracilis and neosphincter.37
Lastly, the pressure threshold for the desire to defecate was lower in FI, even after correcting for differences in rectal compliance and capacity, suggesting that rectal hypersensitivity cannot be entirely explained by disturbances in biomechanical properties of the rectum.
Previous studies suggesting a relatively high prevalence of pudendal neuropathy in FI were based on delayed pudendal nerve terminal motor latencies, which are subject to methodological limitations.10
In this study, a combined assessment of external sphincter, puborectalis, and ischiocavernosus was used to localise the level of neuromuscular injury in FI. Neurogenic changes isolated to the external sphincter may be caused by injury at any level from motor neurones in the sacral spinal cord to the nerve fascicles entering the anal sphincter. Local trauma, (for example, during vaginal delivery) may damage the nerve fascicles entering the sphincter and/or result in myogenic changes affecting the external sphincter. We inferred that EMG findings suggested pudendal neuropathy only when neurogenic changes affected the anal sphincter and ischiocavernosus muscle. It is unlikely that a neurogenic injury pattern in the external sphincter and ischiocavernosus would reflect selective injury of the pudendal nerve branches that innervate these muscles.
The predictive variables evaluated in this study were reasonably useful as they explained
30% of the interindividual variation in anal squeeze pressure, rectal compliance, and capacity in FI. However, these risk factors explained <30% of the interindividual variation in anal resting pressure and rectal sensation. With the exception of rectal compliance and sensation, obstetric risk factors were not particularly useful in explaining variance in anorectal functions in FI. This may reflect methodological limitations (for example, the need to collapse obstetric risk factors into two categories (mild and severe) given sample size constraints). Alternatively, it is conceivable that while vaginal delivery is a risk factor for anal sphincter injury, its contribution to FI is exceeded by other risk factors (for example, aging), as has been reported for urinary incontinence.38,39
We believe that our group of consecutive patients constitutes a representative sample of patients with “idiopathic” FI seen at a tertiary referral centre; 65% had risk factors which were associated with weakness of the anal sphincters.40
In addition, 67% of patients had one or more functional gastrointestinal disorders. Patients who were recruited had similar demographic and clinical features as those who declined to participate in the study (data not shown). Hence we perceive that the conclusions of our study are applicable to patients in a consultative practice and future studies will need to evaluate the same questions in community FI patients.