(For Brain Infections, see Brain Infections.)
Meningitis is inflammation of the meninges of the brain or spinal cord. It is often infectious and is one of the most common CNS infections. Sometimes inflammation involves both the meninges and brain parenchyma (meningoencephalitis). Meningitis may become evident over hours or days (acute) or a longer period (subacute or chronic).
The most common types of acute meningitis are acute bacterial meningitis and aseptic meningitis. Acute bacterial meningitis is a severe illness characterized by purulent CSF. It is rapidly progressive and, without treatment, fatal. Aseptic meningitis is milder and typically self-limited; it is usually caused by viruses but sometimes by bacteria, fungi, parasites, or noninfectious inflammation.
Symptoms and Signs
Many cases of infectious meningitis begin with a vague prodrome of viral symptoms. The classic meningitis triad of fever, headache, and nuchal rigidity develops over hours or days. Passive flexion of the neck is restricted and painful, but rotation and extension are typically not as painful. In severe cases, attempts at neck flexion may induce flexion of the hip or knee (Brudzinski’s sign), and there may be resistance to passive extension of the knee while the hip is flexed (Kernig’s sign). Neck stiffness and Brudzinski’s and Kernig’s signs are termed meningeal signs or meningismus; they occur because tension on nerve roots passing through inflamed meninges causes irritation.
Although brain parenchyma is not typically involved early in meningitis, lethargy, confusion, seizures, and focal deficits may develop, particularly in untreated bacterial meningitis.
Diagnosis and Treatment
Acute meningitis is a medical emergency that requires rapid diagnosis and treatment. After IV access and blood cultures are obtained, lumbar puncture is done to obtain CSF for Gram stain, culture, cell count and differential, and glucose and protein content. These tests must be done as rapidly as possible. However, patients with signs compatible with a mass lesion (eg, focal deficits, papilledema, deterioration in consciousness, seizures) require head CT before lumbar puncture because there is a small possibility that lumbar puncture can cause cerebral herniation if a brain abscess or other mass lesion is present.
CSF findings aid in the diagnosis of meningitis (see Table 1: Meningitis: Cerebrospinal Fluid Abnormalities in Various Infections). Presence of bacteria on Gram stain or growth of bacteria in culture is diagnostic of bacterial meningitis. Gram stain is positive about 80% of the time in bacterial meningitis and usually differentiates among the common causative pathogens. CSF lymphocytosis and absence of pathogens suggest aseptic meningitis but may represent partially treated bacterial meningitis