S4/5 Diminished Sensation Symmetry
S4/5 diminished sensation is NOT always symmetrical bilaterally—asymmetric patterns are common and clinically significant, particularly in spinal cord injury, sacral nerve root pathology, and unilateral lesions.
Asymmetric Patterns in Spinal Cord Injury
The presence and pattern of S4-S5 sensation varies considerably based on injury characteristics:
Unilateral S4-S5 sensation alone (without bilateral preservation) has poor prognostic value: None of 5 motor-complete patients with only unilateral sacral sharp/dull touch sensation recovered any lower extremity motor function 1
Bilateral S4-S5 sensation indicates better prognosis: In contrast, 8 motor-complete subjects with bilateral sacral sensation present showed mean lower extremity motor score increases to 12.1 ± 7.8 at 1 year, with 3 of 8 achieving functional recovery 1
Asymmetric sacral preservation occurs frequently: Studies evaluating sacral sparing demonstrate that asymmetric patterns between left and right S4-S5 dermatomes are well-documented, particularly in incomplete spinal cord injuries classified as AIS-B and AIS-C 2
Unilateral Sacral Nerve Root Pathology
Asymmetric S4-S5 sensation is the expected finding in unilateral sacral pathology:
Unilateral sacrectomy preserves contralateral function: In patients undergoing unilateral sacral resection with preserved contralateral sacral nerves, normal bowel and bladder function was retained in 87% and 89% respectively, demonstrating clear unilateral nerve root function 3
Asymmetric sacral resections show lateralized deficits: Patients with asymmetric sacral resections preserving at least one S3 nerve root retained normal bowel and bladder function in 67% and 60% respectively, confirming that unilateral preservation produces asymmetric sensory patterns 3
Surgical Considerations for S4-S5 Pathology
The clinical significance of S4-S5 laterality is emphasized in surgical planning:
Sacral chordoma surgery below S4 is recommended as first-line treatment because neurological sequelae are more acceptable at this level 4
Surgery above S3 requires careful discussion given the significant neurological consequences, with bilateral S4-S5 nerve root sacrifice resulting in 100% abnormal bowel and bladder function 3
Lower sacral segments (below S3/S4 junction) are most commonly affected in pelvic osteomyelitis associated with pressure injuries, and surgeons must preserve S2-S4 nerve roots in non-paraplegic patients to avoid incontinence 4
Bilateral Sensory Dysfunction Patterns
While asymmetry is common, some conditions produce bilateral impairment:
- Unilateral stroke can cause bilateral discriminative sensory dysfunction: Point localization was bilaterally impaired in 17 of 39 patients (44%) and stereognosis in 7 of 38 patients (18%) regardless of lesion laterality, though this applies to discriminative sensation rather than primary sensation 5
Clinical Assessment Pitfalls
Key considerations when evaluating S4-S5 sensation:
Self-report has limitations: The likelihood ratio for a positive self-report of S4-S5 sensory function is only 1.92 (95% CI: 1.0-3.6), indicating modest accuracy, with high false-positive rates particularly for motor function in those with motor levels below T10 6
Physical examination remains essential: Given the poor predictive value of self-report and the clinical significance of asymmetric patterns, direct physical examination of S4-S5 dermatomes bilaterally is mandatory 2, 6
Rostral sacral segments predict caudal function: Preserved sensation at S1 provides 90.5% prediction of caudal sacral sensory sparing (S4-S5 sensation), which can be useful when complete examination is not feasible 2