What are the clinical signs of S1 radiculopathy?

Clinical Signs of S1 Radiculopathy

S1 radiculopathy classically presents with ankle plantar flexion weakness, absent or diminished Achilles reflex, and sensory loss along the lateral foot and posterior calf.

Motor Findings

The hallmark motor deficit involves:

• Gastrocnemius/soleus weakness (ankle plantar flexion) 1
• Foot eversion weakness 2
• Hamstring weakness (atypical but documented) 2

The plantar flexion weakness is the most reliable motor sign, as the gastrocnemius-soleus complex is predominantly innervated by S1. Testing should involve having the patient perform single-leg heel raises, which will be weak or impossible on the affected side.

Reflex Changes

• Absent or diminished Achilles (ankle) reflex 1, 3

This is one of the most consistent and reliable clinical findings in S1 radiculopathy. The Achilles reflex may remain absent even after symptom resolution 1.

Sensory Deficits

• Hypesthesia in the S1 dermatome: lateral aspect of the foot, posterior calf 1
• Sensory loss in the painful dermatome occurs in approximately 56% of cases 3

However, be aware that pain patterns from S1 nerve root compression only approximate sensory dermatomes with 50-80% overlap, and there is wide variability between patients 4.

Pain Distribution

• Radicular pain with leg pain worse than back pain 5
• Pain typically radiates down the posterior thigh and lateral leg to the lateral foot
• Posterior knee pain can occur, though this is less typical 2

Common Diagnostic Pitfalls

Atypical presentations exist: One case report documented S1 radiculopathy initially presenting with isolated hamstring weakness and posterior knee pain, leading to diagnostic delay 2. Don't exclude S1 radiculopathy if the classic lateral foot symptoms are absent.

Pain patterns alone are insufficient: Clinicians cannot accurately determine whether L5 or S1 is affected based solely on pain distribution, even with knowledge of typical patterns 4. You must integrate multiple clinical findings.

Electrodiagnostic testing has limitations: EMG abnormalities occur in less than 50% of mild S1 radiculopathy cases 3. F-wave duration differences between sides may be more sensitive than standard parameters 6, but normal electrodiagnostic studies do not exclude the diagnosis.

Clinical Examination Algorithm

When evaluating for S1 radiculopathy, systematically assess:

1. Strength testing: Single-leg heel raises (plantar flexion strength)
2. Reflex testing: Achilles reflex comparison bilaterally
3. Sensory examination: Light touch and pinprick along lateral foot and posterior calf
4. Pain pattern: Confirm leg pain predominates over back pain with radiation to lateral foot

The diagnosis requires congruence between clinical findings and MRI evidence of nerve root compression 3. Physical examination findings of myotomal weakness, abnormal deep reflexes, and paresthesias predict EMG abnormalities when present 3.