Lasègue’s sign is pain along the course of the sciatic nerve induced by stretching of the nerve, achieved by flexing the thigh at the hip while the leg is extended at the knee ("straight leg raising"). This is similar to the maneuver used in Kernig’s sign (gradual extension of knee with thigh flexed at hip). Both indicate irritation of the lower lumbosacral nerve roots and/or meninges. The test may be positive with disc protrusion, intraspinal tumor, or inflammatory radiculopathy. Pain may be aggravated or elicited sooner using Bragard’s test, dorsiflexing the foot while raising the leg thus increasing sciatic nerve stretch, or Neri’s test, flexing the neck to bring the head on to the chest, indicating dural irritation.
A positive straight leg raising test is reported to be a sensitive indicator of nerve root irritation, proving positive in 95% of those with surgically proven disc herniation. The specificity may be somewhat lower.
Various modifications of Lasègue’s sign have been described. crossed straight leg raising, when the complaint of pain on the affected side occurs with raising of the contralateral leg, is said to be less sensitive but highly specific. Femoral stretch test (q.v.) or "reverse straight leg raising" may detect L3 root or femoral nerve irritation.
Pearce JMS. JJ Forst and Lasègue’s sign. In: Pearce JMS. Fragments of neurological history. London: Imperial College Press, 2003: 362-364