Kernig sign is one of the eponymous clinical signs of meningitis. This test typically is performed in patients while supine and is described as resistance (or pain) with passive extension of the knees. This resistance is thought to be due to meningeal inflammation in the setting of meningitis or other clinical entities that may irritate the meninges. Since its conception in the 19th century, the clinical value of Kernig sign has been more thoroughly explored. It remains a commonly performed maneuver in cases of suspected meningitis.
Kernig sign was initially described in the 19th century by Russian physician Vladimir Kernig. Kernig observed that patients who had bacterial or tuberculous meningitis were unable to completely extend their legs while sitting upright. He continued to investigate this finding over several decades of his career and first reported this sign at a medical congress in St. Petersburg in 1882. He presented his observations again in Copenhagen, Denmark, in 1884 and published the same year in Berliner Klinische Wochenschrift.
Kernig described a positive sign as the inability to extend the knees more than 135 degrees. In severe cases, patients were unable to extend the knees past 90 degrees. The maneuver was performed with the patient seated in the upright position with the hips flexed 90 degrees to the trunk. Kernig also described instances in which patients were unable to extend the elbows in cases of meningitis. Kernig did not use the pain as a marker for a positive test. The presence of contracture or extensor spasm at the knee was considered a positive finding. He noted, however, that the clinical severity of meningitis was not always associated with the severity of contraction of the extremities.
The brain is covered by three meningeal layers, Dura mater, Pia mater, and Arachnoid mater. Meningitis is an inflammation of these meningeal layers, especially, pia and arachnoid.  In meningitis, there is hypertonia of lower limbs and increased sensitivity of lower limb nerves. According to a hypothesis, Kernig sign is a protective response to prevent spasm and pain in lower extremities that can be triggered by the maneuver performed for the Kernig sign. The maneuver causes a stretch to the hypersensitive and inflamed nerve roots.  A positive Kernig sign patient should undergo a CT scan followed by lumbar puncture and antibiotics. Common causes of meningitis are inflammation due to infections, i.e., bacterial, viral, and fungal infections. These infections can be differentiated based on cerebrospinal fluid analysis, staining, and culture after a lumbar puncture. In bacterial and fungal infections, glucose is reduced in CSF. Lymphocytes are predominant in viral and fungal infections in the CSF. CSF opening pressure is increased in fungal and bacterial infections. 
Infants who are tense or have high muscle tone do not make good candidates for the Kernig test. The test is also not reliable for patients who are very lethargic, paraplegic, or comatose because these individuals may not have obvious meningeal signs. Further, there is no correlation between Kernig sign and the severity of meningeal inflammation. It should be avoided in patients with any sort of leg, knee, or hip injury.
No special equipment is needed. A bed is required to keep the patient supine, and the examiner can perform the maneuver.
Examiners can perform this maneuver alone without any helper.
In clinical practice, it is important to have the infant relaxed at the time of the test. Babies who are extremely irritable or tense do not make good candidates for the test.
To elicit the Kernig sign, clinicians typically perform the exam with the patient lying supine with the thighs flexed on the abdomen, and the knees flexed. The examiner then passively extends the legs. In the presence of meningeal inflammation, the patient will resist leg extension or describe pain in the lower back or posterior thighs, which indicates a positive sign.
Elicitation of Kernig sign often is performed in conjunction with other examination techniques to detect meningitis in symptomatic patients, particularly Brudzinski sign. The Brudzinski nape of the neck sign was described in 1909 by Polish pediatrician Brudzinski. A positive Brudzinski sign refers to the maneuver in which the clinician passively flexes the neck, which then results in flexion of the hips and the knees of the patient.
It is important not to rely on the Kernig test for meningitis. If meningitis is suspected, the infant must undergo a lumbar puncture.
The clinical signs associated with meningitis, including Kernig and Brudzinski signs, also correlate with other diseases. The differential diagnosis in the setting of positive signs includes spinal cord tumors, myelitis, intervertebral disc prolapse, sciatica, multiple sclerosis, trauma, subarachnoid hemorrhage, and increased intracranial pressure from multiple causes or in the setting of stroke. When seen in meningitis, these signs are usually present acutely within 24 hours of symptom onset. The thought is that these findings are due to mechanical factors in the spine. When the meninges in the spinal cord and spinal nerves are inflamed, patients will resist stretching the cord and the nerves to prevent pain resulting from this inflammation. These clinical maneuvers appear to provoke this stretching.; this is why contractures occur when performed, resulting in a positive test.
Bacterial meningitis is a significant cause of morbidity and mortality across the world. It is among the top ten infectious causes of death, with a mortality rate of 20% to 30%, responsible for 135,000 deaths each year. Physical exam findings, including nuchal rigidity, Kernig, and Brudzinski signs, are used at the bedside to help in the diagnosis of cases of suspected meningitis. Several studies have evaluated the utility and accuracy of these bedside exam findings for diagnosing meningitis. Both Kernig sign and Brudzinski sign have a limited role in ruling out meningitis in their absence, with low sensitivities of 5%. However, the signs have specificities of 95% and are considered highly predictive of bacterial meningitis when present in the appropriate clinical setting. The jolt accentuation maneuver, described more recently in 1993, is a bedside maneuver to identify cases of meningitis with exacerbation of a headache caused by rotation of the head horizontally two or three times per second. The jolt accentuation maneuver is one of the most sensitive bedside exam findings with a sensitivity of 97%.
The diagnosis and management of meningitis are with an interprofessional team that includes an emergency department physician, infectious disease specialist, neurologist, nurse practitioner, and pediatrician.
Meningitis is an important disease entity that carries high rates of morbidity and mortality, requiring early recognition and management. Kernig sign is a bedside physical exam maneuver used since its description in the 19th century to help in the diagnosis of meningitis. A positive test is the elicitation of pain or resistance with passive extension of the patient’s knees past 135 degrees in the setting of meningeal irritation. Due to its low sensitivity, the absence of Kernig sign does not rule out meningitis; however, with its high specificity, a positive Kernig sign is highly suggestive of the diagnosis.
Much of the literature on Kernig sign is many years old, and there are no randomized studies to determine its validity. The healthcare worker needs to be aware that the gold standard test for meningitis is the lumbar puncture to assess the cerebrospinal fluid. No physical sign can replace this test.
The Kernig test is not very sensitive to meningitis, and if absent, it does not mean that the patient does not have meningitis. However, if the test is positive, then the test is highly specific for meningitis. Of course, a positive test must correlate with clinical and laboratory features.
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