The S3-S4 myotome supplies the pelvic floor and the external anal and urethral sphincters. These roots, with S2, also carry the parasympathetic supply to the bladder and lower bowel through the pelvic splanchnic nerves. In ISNCSCI the motor test for the lowest sacral segments is voluntary anal contraction.
| Nerve root | S3-S4 |
|---|---|
| Area | Voluntary anal contraction and pelvic floor squeeze |
| How to test | During a rectal examination the patient squeezes the examiner's finger as if holding in a bowel movement, and anal tone at rest is noted. |
| Muscles | |
| Nerve Supply | Pudendal nerve; Nerve to levator ani; Pelvic splanchnic nerves (parasympathetic) |
Closes the anal and urethral sphincters, supports the pelvic organs and controls continence.
Preserved voluntary anal contraction makes a spinal cord injury motor incomplete, which strongly improves the outlook. Loss of anal tone or squeeze with back pain and saddle numbness is a red flag for cauda equina syndrome. Myotome charts vary slightly between sources, and most muscles receive more than one root, so a single root lesion usually weakens rather than paralyses.
Compression of the sacral roots causing loss of anal tone, urinary retention or incontinence and saddle anaesthesia.
Damage from childbirth or prolonged cycling pressure can weaken the pelvic floor and sphincters and cause incontinence or numbness.
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