What does a positive Kernig sign indicate and how is it treated?

Kernig Sign: Indication and Treatment

A positive Kernig sign indicates possible meningeal irritation, most commonly associated with meningitis, but should not be relied upon for diagnosis due to its poor sensitivity (5-30%) despite moderate specificity (85-95%). 1, 2, 3

What is a Kernig Sign?

Kernig's sign is elicited when:

• The patient's hip is flexed 90°
• The knee is then extended
• Pain during knee extension constitutes a positive sign 1

Diagnostic Value

The diagnostic accuracy of Kernig's sign is limited:

• Sensitivity: Only 5-30% (very poor) 2, 3
• Specificity: 85-95% (moderate) 2
• Likelihood ratio for positive test: 0.97-1.84 3, 4

The UK Joint Specialist Societies explicitly recommend that "Kernig's sign and Brudzinski's sign should not be relied upon for diagnosis" (2B recommendation) 1. This is supported by multiple studies showing that these classic signs have poor discriminatory value 3, 4, 5.

Clinical Context

A positive Kernig sign may suggest:

1. Bacterial meningitis (most concerning)
2. Viral meningitis
3. Subarachnoid hemorrhage
4. Other causes of meningeal irritation

However, the "classic triad" of meningitis (fever, neck stiffness, and altered mental status) is present in less than 50% of confirmed bacterial meningitis cases 1, 2.

More Reliable Clinical Indicators

More sensitive indicators of meningitis include:

• Headache
• Fever
• Photophobia
• Vomiting
• Altered mental status 2

The absence of fever, neck stiffness, AND altered mental status effectively rules out meningitis (sensitivity 99-100% for presence of at least one of these findings) 2, 6.

Treatment Approach

When meningitis is suspected based on clinical presentation (regardless of Kernig sign):

1. Immediate hospital referral is mandatory due to risk of rapid deterioration 1

   • Transport via emergency ambulance when possible

2. Do not delay antibiotics if lumbar puncture is delayed for any reason 2

   • Early antibiotic administration is critical to reduce mortality

3. Perform lumbar puncture for definitive diagnosis 1, 2

   • CT scan before lumbar puncture if focal neurological signs, papilledema, uncontrolled seizures, or GCS ≤12 2

4. Empiric antimicrobial therapy based on likely pathogens:

   • Adults: Third-generation cephalosporin plus coverage for Listeria in older adults
   • Children: Age-appropriate antibiotics covering common pathogens

5. Adjunctive dexamethasone in suspected bacterial meningitis

   • Ideally administered before or with first antibiotic dose

Common Pitfalls

• Relying solely on Kernig's sign or other meningeal signs to rule out meningitis
• Delaying antibiotics while waiting for diagnostic confirmation
• Failing to recognize that presentation may be atypical in elderly patients (more likely to have altered mental status, less likely to have neck stiffness or fever) 1
• Missing meningococcal disease in patients without a rash (37% of meningococcal meningitis patients do not have a rash) 1

Key Takeaway

While a positive Kernig sign may raise suspicion for meningitis, its absence does not rule out the condition. Clinical suspicion should prompt immediate referral to hospital, diagnostic testing, and early antibiotic administration to reduce morbidity and mortality.