Kernig’s sign is pain in the lower back (and also sometimes the neck) and resistance to movement with passive extension of the knee on the flexed thigh in a recumbent patient. It is indicative of meningeal mechanosensitivity due to inflammation, either infective (meningitis) or chemical (subarachnoid hemorrhage), in which case it may coexist with nuchal rigidity and Brudzinski’s (neck) sign. If unilateral it may indicate irritation of the lumbosacral nerve roots from a ruptured intervertebral disc (in which case Lasègue’s sign may also be present).
Pearce JMS. Kernig and Brudzinski. In: Pearce JMS. Fragments ofneurological history. London: Imperial College Press, 2003: 365-366