neurologic disorders
Acute Bacterial Meningitis
Diagnosis of Acute Bacterial Meningitis
Cerebrospinal fluid (CSF) analysis As soon as acute bacterial meningitis is suspected, blood cultures and Lumbar Puncture (Spinal Tap) for CSF analysis (unless contraindicated) are done. Blood should be analysed when lumbar puncture is done so that blood glucose levels can be compared with CSF glucose levels. Treatment should be started as follows: If bacterial meningitis is suspected and the patient is very ill, antibiotics and corticosteroids are given immediately, even before lumbar puncture. If bacterial meningitis is suspected and lumbar puncture will be delayed pending CT or MRI, antibiotics and corticosteroids should be started after blood cultures but before neuroimaging is done; the need for confirmation should not delay treatment. Clinicians should suspect bacterial meningitis in patients with typical symptoms and signs, usually fever, changes in mental status, and nuchal rigidity. However, clinicians must be aware that symptoms and signs are different in neonates and infants and may be absent or initially mild in older patients, patients with alcoholic use disorder, and immunocompromised patients. Diagnosis can be challenging in the following patients: Those who have had a neurosurgical procedure because such procedures can also cause changes in mental status and neck stiffness Older patients and patients with alcohol use disorder because changes in mental status may be due to metabolic encephalopathy (which may have multiple causes) or to falls and subdural hematomas Focal seizures or focal neurologic deficits may indicate a focal lesion such as a Brain Abscess . Because untreated bacterial meningitis is lethal, tests should be done if there is even a small chance of meningitis. Testing is particularly helpful in infants, older patients, patients with alcohol use disorder, immunocompromised patients, and patients who had neurosurgical procedure because symptoms may be atypical. Do a lumbar puncture even if clinical findings are not specific for meningitis, particularly in infants, older patients, patients with alcohol use disorder, immunocompromised patients, and patients who have had neurosurgery. If findings suggest acute bacterial meningitis, routine tests include CSF analysis Complete blood count and differential Metabolic panel Blood cultures plus polymerase chain reaction (PCR), if available Unless contraindicated, Lumbar Puncture (Spinal Tap) (see also Contraindicationsto immediate lumbar puncture are signs suggesting markedly increased intracranial pressure (ICP) or an intracranial mass effect (eg, due to edema, hemorrhage, or tumor). Thus, lumbar puncture should be considered high risk with any of the following: Papilledema Focal neurologic deficits Focal seizures A known central nervous system mass lesion Large stroke Suspected central nervosus system focal infection In such cases, lumbar puncture may cause Brain Herniation and thus is deferred until neuroimaging (typically CT or MRI) is done to check for increased ICP or a mass effect. When lumbar puncture is deferred, treatment is best begun immediately (after blood sampling for culture and before neuroimaging). After ICP, if increased, has been lowered or if no mass effect or obstructive hydrocephalus is detected, lumbar puncture can be done. CSF should be sent for analysis: cell count, protein, glucose, Gram staining, culture, PCR, and other tests as indicated clinically. A multiplex film-array PCR panel can provide rapid screening for multiple bacteria and viruses plusCryptococcus neoformansin a CSF sample. This test, which is not always available, is used to supplement, not replace, culture and traditional tests. Simultaneously, a blood sample should be drawn and sent to have the CSF:blood glucose ratio determined. CSF cell count should be determined as soon as possible because white blood cells (WBCs) may adhere to the walls of the collecting tube, resulting in a falsely low cell count; in extremely purulent CSF, WBCs may lyse. Typical CSF findings in bacterial meningitis include the following (see table): Increased pressure Fluid that is often turbid A high WBC count (consisting predominantly of polymorphonuclear neutrophils) Elevated protein A low CSF:blood glucose ratio A CSF:blood glucose level of < 50% suggests possible meningitis. A CSF glucose level of ≤ 18 mg/dL or a CSF:blood glucose ratio of < 0.23 strongly suggests bacterial meningitis. However, changes in CSF glucose may lag 30 to 120 minutes behind changes in blood glucose. In acute bacterial meningitis, an elevated protein level (usually 100 to 500 mg/dL) indicates blood-brain barrier injury. CSF cell count and protein and glucose levels in patients with acute bacterial meningitis are not always typical. Atypical CSF findings may include Normal in early stages except for the presence of bacteria Predominance of lymphocytes in about 14% of patients, particularly in neonates with gram-negative meningitis, patients with meningitis due toL. monocytogenes, and some patients with partially treated bacterial meningitis Normal glucose in about 9% of patients Normal WBC counts in severely immunosuppressed patients When initial CSF findings are equivocal, a repeat lumbar puncture 12 to 24 hours later can sometimes clarify which direction CSF changes are heading or whether there was a laboratory error. Identification of the causative bacteria in CSF involves Gram staining, culture, and, when available, PCR. Gram staining provides information rapidly, but the information is limited. For bacteria to be reliably detected with Gram stain, about 105bacteria/mL must be present. Results may be falsely negative if any of the following occur: CSF is handled carelessly. Bacteria are not adequately resuspended after CSF has been allowed to settle. Errors in decolorization or reading of the slide occur. If clinicians suspect an anaerobic infection or other unusual bacteria, they should tell the laboratory before samples are plated for cultures. Prior antibiotic therapy can reduce the yield from Gram staining and culture. PCR, if available, and latex agglutination tests to detect bacterial antigens may be a useful adjunctive tests, especially in patients who have already received antibiotics. Determination of antibiotic sensitivity requires bacterial culture. Until the cause of meningitis is confirmed, other tests using samples of CSF or blood may be done to check for other causes of meningitis, such as viruses (particularly herpes simplex), fungi, and cancer cells. Samples from other sites suspected of being infected (eg, urinary or respiratory tract) should also be cultured. 