What is the difference between Brudzinski's sign and Kernig's sign?

From the Guidelines

Brudzinski's sign and Kernig's sign should not be relied upon for the diagnosis of meningitis due to their low sensitivity and specificity. These signs are clinical tests used to detect meningeal irritation, which is often associated with meningitis. Brudzinski's sign is performed with the patient lying supine, where the examiner flexes the patient's neck, and a positive sign occurs when this neck flexion causes involuntary flexion of the hips and knees. Kernig's sign is tested with the patient lying supine with the hip and knee flexed at 90 degrees, and the examiner then attempts to extend the knee; resistance or pain with this maneuver indicates a positive Kernig's sign.

The low diagnostic accuracy of these signs is highlighted in studies, such as the one published in the Journal of Infection 1, which states that Kernig’s sign and Brudzinski’s sign should not be relied upon for diagnosis. Another study published in Clinical Microbiology and Infection 1 also concludes that the sensitivity and negative predictive value of Kernig and Brudzinski sign is low in the diagnosis of meningitis. Furthermore, a study summarized in the same journal 1 found that these clinical findings have low diagnostic accuracy for prediction of cerebrospinal fluid (CSF) pleocytosis, with sensitivity of neck stiffness at 31%, Brudzinski at 9%, and Kernig at 11%.

Key points to consider when evaluating patients for meningitis include:

• The presence or absence of headache, altered mental status, neck stiffness, fever, rash, seizures, and any signs of shock should be documented 1
• The classic triad of fever, neck stiffness, and altered mental status is reported in only 41–51% of patients with bacterial meningitis 1
• A petechial rash is identified in 20–52% of patients and is indicative of meningococcal infection in over 90% of patients 1
• Rapid admission to hospital and further evaluation, including consideration of a lumbar puncture, are crucial for patients suspected of having meningitis or meningococcal sepsis 1

From the Research

Comparison of Brudzinski's and Kernig's Signs

• Both Brudzinski's and Kernig's signs are used to diagnose meningitis, but their diagnostic accuracy is limited 2.
• A study found that Kernig's sign and Brudzinski's sign had a sensitivity of 5% and a likelihood ratio for a positive test result of 0.97, indicating poor diagnostic value 2.
• Nuchal rigidity, another meningeal sign, had a sensitivity of 30% and a likelihood ratio for a positive test result of 0.94, but only showed diagnostic value in patients with severe meningeal inflammation 2.
• Examination maneuvers such as Kernig sign or Brudzinski sign may not be useful to differentiate bacterial from aseptic meningitis due to variable sensitivity and specificity 3.
• The diagnosis of meningitis relies on the examination of cerebrospinal fluid obtained from lumbar puncture, rather than clinical findings or physical examination maneuvers 3.

Diagnostic Value

• The diagnostic accuracy of Brudzinski's and Kernig's signs was not significantly better in patients with moderate meningeal inflammation or microbiological evidence of CSF infection 2.
• The signs did not accurately discriminate between patients with meningitis and patients without meningitis 2.
• Delayed initiation of antibiotics can worsen mortality, and treatment should be started promptly in cases where transfer, imaging, or lumbar puncture may slow a definitive diagnosis 3.

Clinical Implications

• Better bedside diagnostic signs are needed to diagnose meningitis, as the classic meningeal signs have limited diagnostic value 2.
• Empiric antibiotics should be directed toward the most likely pathogens and should be adjusted by patient age and risk factors 3.
• Vaccination against the most common pathogens that cause bacterial meningitis is recommended, and chemoprophylaxis of close contacts can help prevent additional infections 3.