What is the purpose and interpretation of the straight leg test (SLT) in diagnosing lumbar disc herniation?

Straight Leg Test for Diagnosing Lumbar Disc Herniation

The straight leg raise (SLR) test is a highly sensitive (91%) but modestly specific (26%) physical examination maneuver for diagnosing lumbar disc herniation, particularly useful when reproduction of the patient's sciatica occurs between 30-70 degrees of leg elevation. 1

Purpose and Technique

The SLR test is designed to:

• Assess for nerve root compression/irritation
• Evaluate sciatic nerve tension
• Help diagnose lumbar disc herniation, particularly at L4/L5 and L5/S1 levels (where 90% of symptomatic herniations occur)

How to Perform:

1. Patient lies supine
2. Examiner passively raises the patient's straight leg
3. Test is positive when the patient's sciatic pain is reproduced between 30-70 degrees of elevation
4. The crossed SLR (pain produced when raising the contralateral leg) can also be performed

Diagnostic Performance

Standard SLR:

• Sensitivity: 91% (95% CI: 82% to 94%) 1
• Specificity: 26% (95% CI: 16% to 38%) 1
• Highly sensitive but limited specificity for lumbar disc herniation

Crossed SLR:

• Sensitivity: 29% (95% CI: 24% to 34%) 1
• Specificity: 88% (95% CI: 86% to 90%) 1
• More specific but less sensitive than standard SLR

Age Considerations:

• SLR positivity decreases with age 2
• Patients under 60 years have 5.4 times higher chance of positive SLR than those over 60 2
• Test is nearly 100% positive in 10-19 year age group with confirmed disc herniation 2

Clinical Application

When to Use:

• In patients with back and leg pain in a typical lumbar nerve root distribution
• As part of a focused neurological examination that includes:
  • Knee strength and reflexes (L4 nerve root)
  • Great toe and foot dorsiflexion strength (L5 nerve root)
  • Foot plantarflexion and ankle reflexes (S1 nerve root)
  • Distribution of sensory symptoms

Interpretation:

• Positive test suggests nerve root irritation, commonly from disc herniation
• Negative test, especially in older patients, does not rule out disc herniation
• The crossed SLR test, when positive, is highly specific for disc herniation

Enhanced Testing Methods

Extended SLR (ESLR):

• Adds location-specific structural differentiation movements (hip internal rotation or ankle dorsiflexion)
• Shows higher validity in detecting neural symptoms 3
• Strongly associated with pathology seen on MRI (OR 8.0 for disc herniation) 3

Slump Test:

• A variant of SLR performed in seated position
• More sensitive (84%) than standard SLR (52%) for lumbar disc herniation 4
• Slightly less specific (83%) than SLR (89%) 4
• Particularly useful when standard SLR is negative but clinical suspicion remains high

Limitations and Caveats

• SLR has poor diagnostic performance when used in isolation 5
• Better performance may be obtained when combined with other physical tests 5
• Most validation studies were conducted in surgical populations with high prevalence of disc herniation 5
• Performance characteristics may differ in primary care settings (typically higher specificity, lower sensitivity) 5
• False negatives increase with age 2
• Cannot reliably distinguish between contained and non-contained disc herniations without additional testing 6

Recommended Approach

1. Perform standard SLR test as part of a complete neurological examination
2. If SLR is negative but clinical suspicion remains high, consider the Slump test
3. When SLR is positive, perform crossed SLR for increased specificity
4. Consider patient's age when interpreting results (higher false negatives in older patients)
5. Combine with other neurological findings for improved diagnostic accuracy
6. Confirm with appropriate imaging (MRI) when clinical findings suggest significant pathology

Remember that physical examination findings should be correlated with the patient's history and, when appropriate, imaging studies for accurate diagnosis and treatment planning of lumbar disc herniation.