Specific Imaging Findings

The diagnosis of uncomplicated infectious meningitis is a clinical one and imaging is not indicated. If done, the most specific findings are high CSF signal intensity on FLAIR images and leptomeningeal/subarachnoid space contrast enhancement. Contrast enhancement may also be seen on CT, but not to the extent that it is shown by MRI. Patients suspected of having complications need to be imaged with MRI including DWI, post-contrast images, T2*-weighted images (to look for thrombosed cortical veins), MRV and MRA. Diffusion imaging is very helpful in demonstrating reduced diffusion in any local collection of pus (abscess, empyema, ventriculitis) and also in detecting acute infarctions. Ventricular debris with an irregular level is the most frequent sign of ventriculitis, while hydrocephalus and ependymal enhancement are less commonly encountered. Large subdural sterile fluid collections are typical in children with Haemophilus influenza meningitis. Tubercular meningitis has a predilection for involvement of basal cisterns, especially the interpeduncular fossa, typically with thick enhancing exudates, frequently leading to infarcts in the territories of the middle cerebral artery perforating vessels.

Pertinent Clinical Information

Infectious meningitis is more common in children and it can grossly be divided into bacterial (pyogenic), viral (lymphocytic) and tubercular. The last two may occasionally assume a chronic form. The diagnosis of meningitis is a clinical one (not an imaging one) and is based on abnormal CSF that shows: high numbers of white blood cells, high protein, and low glucose. The most important clinical symptoms are: fever, headaches, signs of increased intracranial pressure, nuchal rigidity, irritability, lethargy and altered mental status and seizures. Hyperreflexia may also be present. Complications of meningitis occur in up to 50% of patients, the most common ones being ventriculomegaly, subdural collection, and infarct.