Pelvic Floor Neurophysiology and Defecatory Disorders
Neurophysiology
The pudendal nerve (S2–S4) innervates EAS and levator ani; the pelvic splanchnic nerves (S2–S4) provide parasympathetic supply to the detrusor and rectum. Sacral reflexes coordinate defecation and micturition. Neurogenic dysfunction (e.g., cauda equina, spinal cord injury, diabetes) can cause retention, incontinence, or dyssynergia.
Defecatory disorders
Dyssynergic defecation (paradoxical contraction or failure to relax pelvic floor during straining) causes chronic constipation. Diagnosis: anorectal manometry with balloon expulsion test, defecography. Treatment: biofeedback, pelvic floor relaxation training, dietary fiber. Slow-transit constipation may require motility testing.
Traps
- Confusing dyssynergic defecation with slow-transit constipation; both can coexist.
- Missing neurogenic causes in patients with new bowel/bladder dysfunction (cauda equina red flags: saddle anesthesia, bilateral leg weakness, retention).
- Assuming all constipation is functional; consider obstruction, medications, and endocrine causes.
- Overlooking the role of biofeedback in dyssynergia.
Rule of thumb
New urinary retention with fecal incontinence and saddle anesthesia is a surgical emergency (cauda equina); do not delay imaging. For chronic constipation with straining, consider dyssynergic defecation and refer for biofeedback.