What is the purpose and procedure of the straight leg test (SLT) for diagnosing sciatica and lumbar disc herniation?

The Straight Leg Raise Test: Purpose and Procedure for Diagnosing Sciatica and Lumbar Disc Herniation

The straight leg raise (SLR) test is a highly sensitive diagnostic procedure used to detect lumbar disc herniation, with 91% sensitivity but modest specificity (26%) for diagnosing herniated discs causing nerve root compression. 1

Purpose of the Test

• The SLR test is primarily used to assess for nerve root tension or compression, particularly in patients with suspected lumbar disc herniation 1
• It helps identify patients with sciatica (pain radiating down the leg below the knee in the sciatic nerve distribution) suggesting nerve root compromise 1
• The test serves as part of a focused examination for patients with back and leg pain to evaluate the presence and severity of nerve root dysfunction 1

Proper Testing Procedure

• The patient lies supine with legs extended 1
• The examiner slowly raises the patient's leg with the knee extended, passively flexing the hip 1
• The angle at which pain is reproduced is noted (typically between 30-70 degrees of leg elevation) 1
• A positive test is defined as reproduction of the patient's sciatica (radiating leg pain) during leg elevation 1
• The test should be performed bilaterally to assess for a crossed SLR sign 1

Diagnostic Value

• Standard SLR test:
  • High sensitivity: 91% (95% CI, 82% to 94%) 1
  • Limited specificity: 26% (95% CI, 16% to 38%) 1
• Crossed SLR test (pain produced when raising the unaffected leg):
  • Lower sensitivity: 29% (95% CI, 24% to 34%) 1
  • Higher specificity: 88% (95% CI, 86% to 90%) 1
• A positive crossed SLR test strongly indicates disc herniation 2

Clinical Context and Interpretation

• The SLR test should be performed as part of a comprehensive neurological examination that includes:
  • Evaluation of knee strength and reflexes (L4 nerve root) 1
  • Assessment of great toe and foot dorsiflexion strength (L5 nerve root) 1
  • Testing of foot plantarflexion and ankle reflexes (S1 nerve root) 1
  • Mapping the distribution of sensory symptoms 1
• Age affects test results - younger patients (under 40) show higher rates of positive SLR tests with disc herniation than older patients 3
• Male patients are 1.3 times more likely to have a positive SLR test than females with similar pathology 3

Limitations and Considerations

• The test has modest specificity, meaning false positives can occur 1
• The SLR test's sensitivity decreases with age, with patients over 60 having significantly lower rates of positive tests despite having disc herniation 3
• The test should be interpreted in conjunction with other clinical findings and imaging when making treatment decisions 2
• Extended or modified versions of the SLR test (adding ankle dorsiflexion or hip internal rotation) may improve diagnostic accuracy 4

When to Consider Advanced Imaging

• Immediate MRI is indicated for patients with severe or progressive neurological deficits 1
• Immediate MRI is warranted for suspected cauda equina syndrome (urinary retention, saddle anesthesia) 1
• Delayed imaging (after 4-6 weeks) is appropriate for persistent sciatica without improvement despite conservative management 2

The SLR test remains a valuable clinical tool for initial assessment of patients with suspected lumbar disc herniation, though its results should be considered alongside the patient's history, other physical examination findings, and when necessary, imaging studies.