How do you tell an L5 from an S1 nerve root presentation?
A practical guide to differentiating L5 and S1 radiculopathy in outpatient MSK, including history clues, neurological examination and safety-netting.
Subjective triage
- Leg pain often worse than back pain, travelling below the knee in a dermatomal pattern.
- L5: pain or pins and needles into the lateral shin, dorsum of the foot and big toe.
- S1: pain or altered sensation into the calf, heel, sole and lateral border of the foot.
- Aggravated by sitting, bending or coughing and sneezing; may ease with walking or lying.
- Ask about weakness: tripping or foot slapping (L5) versus difficulty pushing off or rising onto tiptoes (S1).
Differential dilemma: vs acute hamstring strain
| Feature | Lumbar radiculopathy | Acute hamstring strain |
|---|---|---|
| Onset | Often insidious or after bending/lifting | Sudden, during sprinting or overstretching |
| Pain distribution | Dermatomal, below the knee, with paraesthesia | Localised to the posterior thigh |
| Neural tests | Straight leg raise reproduces leg symptoms; sensitised by ankle dorsiflexion | Stretch pain without change on neural sensitising |
| Neurology | Possible myotomal weakness, reflex or sensory change | Normal neurological examination |
Red flags: recognise, screen, refer
Recognise
- Bilateral leg symptoms
- Saddle anaesthesia or bladder, bowel or sexual dysfunction
- Progressive or profound motor weakness
- Systemic features: unexplained weight loss, fever, history of cancer
Screen
Ask cauda equina questions directly and record answers at every contact. Repeat neurological examination to track change.
Refer
Suspected cauda equina syndrome needs same-day emergency assessment. Progressive neurological deficit needs urgent medical review.
The clinical reasoning chain
- 1
History
Leg-dominant pain to the dorsum of the foot, worse with sitting.
- 2
Pattern
Dermatomal distribution fits an L5 nerve root rather than referred somatic pain.
- 3
Objective finding
Positive straight leg raise, reduced great toe extension, altered dorsum sensation, ankle reflex intact.
- 4
Diagnostic decision
Probable L5 radiculopathy with no red flags: education, graded activity and monitoring of neurology.
Quick reference FAQ
How do I differentiate L5 from S1 in outpatients?
Combine distribution, myotomes and reflexes. L5 typically affects great toe extension and dorsum sensation with a preserved ankle reflex. S1 typically affects plantarflexion strength, lateral foot sensation and a reduced ankle reflex.
Does every patient with sciatica need an MRI?
No. UK guidance does not recommend routine imaging. Imaging is considered when it would change management, for example when surgery is being considered or red flags are present.
What is the usual prognosis?
Many people improve over weeks to a few months with conservative care. Explain this honestly while safety-netting for neurological change.