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CT Scanning and MRI

CT scans of the head and magnetic resonance imaging (MRI) of the brain generally do not aid in the diagnosis of meningitis. Some patients may show meningeal enhancement, but its absence does not rule out the condition.

The practice of obtaining CT scans of the head may lead to the unnecessary delay in the performance of diagnostic lumbar puncture and the initiation of antibiotic therapy. The delay in the institution of antimicrobial therapy may be detrimental to the total outcome in these patients. Cerebral herniation following the lumbar tap procedure is rare in individuals with no focal neurologic deficits and no evidence of increased ICP. If it occurs, it usually happens within 24 hours following the lumbar puncture and should always be considered in the differential diagnosis if the patient's neurologic status deteriorates.

Guidelines from the Infectious Diseases Society of America recommend the following as indications for performing a screening head CT scan before a lumbar puncture^[21] :

• Age of 60 years or older
• History of CNS disease (eg, mass lesion, stroke, focal infection)
• Immunocompromise
• History of seizure 1 week or less before presentation
• Abnormalities on neurologic exam

Obtain blood cultures and initiate treatment before imaging studies and lumbar puncture in patients with suspected bacterial meningitis. Results may be normal or demonstrate small ventricles, effacement of sulci, and contrast enhancement over convexities. Late findings include venous infarction and communicating hydrocephalus. Rule out brain abscess, sinus or mastoid infection, skull fracture, and congenital anomalies. Acute bacterial meningitis is shown in the CT and MRI scans (all from the same patient) below.

Acute bacterial meningitis. This axial nonenhanced computed tomography scan shows mild ventriculomegaly and sulcal effacement Acute bacterial meningitis. This axial T2-weighted magnetic resonance image shows only mild ventriculomegaly. Acute bacterial meningitis. This contrast-enhanced, axial T1-weighted magnetic resonance image shows leptomeningeal enhancement (arrows).

Neuroimaging is indicated in patients with prolonged fever, focal neurologic symptoms and signs, evidence of increased ICP, and suspected basilar fracture. It is also indicated for evaluation of the paranasal sinuses. These studies are helpful in the detection of CNS complications of bacterial meningitis, such as hydrocephalus, cerebral infarct, brain abscess, subdural empyema, and venous sinus thrombosis.

Go to Imaging in Bacterial Meningitis for complete information on this topic.

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