Kernig sign

**Technique:** The patient lies supine. The examiner flexes one hip and knee to 90°, then passively extends the knee. The sign is positive if extension is resisted or causes pain (hamstring spasm and pain in the back or posterior thigh) or if the patient can't extend the knee beyond about 135°. A positive result is usually bilateral.

**Pathophysiology:** Inflamed or irritated meninges and nerve roots (lumbosacral) are stretched when the sciatic nerve and roots are tensioned by hip flexion plus knee extension. This triggers a protective reflex spasm of the hamstrings.

**Clinical significance:** - Classic in bacterial, viral, and fungal meningitis, and in subarachnoid hemorrhage (blood in the CSF irritates the meninges). - Part of the meningeal signs: **Kernig** (knee extension resisted), **Brudzinski** (passive neck flexion causes involuntary hip and knee flexion), nuchal rigidity, photophobia, and headache. - Both Kernig and Brudzinski signs have **low sensitivity (~5–30%) but high specificity (~95%)**. Their absence does NOT rule out meningitis, especially in infants, the elderly, and immunocompromised patients. - The classic triad of meningitis is fever, nuchal rigidity, and altered mental status. Headache is the most common symptom.

**Differential for a "positive" sign or similar findings:** Lumbosacral radiculopathy or sciatica, which mimics it (compare straight leg raise), and hamstring tightness.

**Management pearls (Step 2):** - Suspected bacterial meningitis: don't delay empiric antibiotics (ceftriaxone + vancomycin, plus ampicillin if age >50 or immunocompromised for Listeria) and dexamethasone. Get blood cultures first. - Lumbar puncture: obtain a head CT first only if there are focal deficits, papilledema, altered mental status, new seizure, or immunocompromise. - CSF in bacterial meningitis: high neutrophils, high protein, low glucose, high opening pressure. - Subarachnoid hemorrhage: "worst headache of my life." Noncontrast CT head first, then LP for xanthochromia if CT is negative.

**Mnemonic:** Kernig = "Knee" (resists extension of the Knee). Brudzinski = "Brow"/neck flexion causes hip and knee flexion.

**History:** Described by Russian physician Vladimir Kernig in 1882. Brudzinski described his sign in 1909.