Meningitis is the most common form of CNS infection. It is divided into three general categories: acute bacterial meningitis, mostly bacterial infections, lymphocytic meningitis, usually viral, and chronic meningitis, the classic example is tuberculosis.
Acute bacterial meningitis.
Etiology. Usually it is caused by a bacterium. Neonates: group B streptococci and Escherichia coli, children under 7 years: Haemophilus influenzae; older children: Neisseria meningitidis; adults: Streptococcus pneumoniae (pneumococcus).
It is more common in infants and children early in life. Are observed around 1.5 cases per 1000 births, associated with neonatal sepsis. Meningitis epidemic can occur at any age. The diagnosis of meningitis is established by history, clinical examination and laboratory evaluation. Neuroimaging studies are typically only used to monitor complications.
TC. In acute bacterial meningitis, is usually normal. There may be mild ventricular dilatation and enlargement of the subarachnoid space. In some cases there may be blurring of the tank or the base of the convexity subarachnoid space by the exudate. Less than 50% of cases with meningitis has documented clinically abnormal meningeal enhancement on CT with contrast.
MR. The image is superior to CT in the evaluation of meningitis. In T1, we can observe obliterated tanks, which are reinforced with contrast, and the extension of the convexity subarachnoid space, with strengthening of grooves (in severe cases). The complications to be surveyed are: hydrocephalus, ventriculitis, subdural empyema and collection, cerebritis, abscess, edema, hernia and heart attacks.
Hydrocephalus and ventriculitis. The abundant fibrinous purulent exudate may block the subarachnoid space, resulting in communicating hydrocephalus. Or it may obstruct the cerebral aqueduct and foramen of exit of the fourth ventricle, causing non-communicating hydrocephalus (intraventricular). It occurs in 30% of patients with acute bacterial meningitis, and more than 90% of newborn patients with meningitis. The ependyma is reinforced strongly in contrast with ventriculitis due to the collapse of the barrier hemoencefálica by the inflammatory process. Sometimes there is inflammation of the choroid plexus (plexitis choroid).
The collection or subdural hygroma is common in acute meningitis, occurring in 25% of cases under the age of 1 year. These collections are the most sterile, but 2% are infected, giving the subdural empyema. The hygromas are probably caused by inflammation of the subdural veins, leading to passage of fluid to the subdural space. The differential diagnosis is with prominent subarachnoid space, which is normal in children. In imaging studies, the hygromas appear as collections extraaxiais crescent-shaped, with density or signal similar to CSF, which does not penetrate and resolve spontaneously.
The epidural or subdural empyema are complications from 20 to 33% of intracranial infections. Can be caused by sinusitis, particularly frontal sinus infection, post-craniotomy, or in 10-15% of cases, can be complications of meningitis. Have a high mortality rate. In imaging studies, appear as collections of crescent or lenticular, with low density on CT. MRI T2 are slightly hyperintense relative to CSF. Are observed in the convexity or the interhemispheric region, with a surrounding membrane that is reinforced and strong-monotonically with contrast. Are often associated with thrombosis of cortical veins with venous infarction.
'Cerebritis' or abscess are due to complications in 10% of cases of meningitis. The so-called 'cerebritis' may or may not develop into an abscess (see table below).
Infections of the CNS are the most common cause of cerebrovascular disease acquired in children. Infarcts occur in up to 25% of children with meningitis, especially in severe cases, and in tuberculous meningitis. In adults, this complication is 37% of the cases. Occurs by vessel lesion or arterial and venous sinus thrombosis.
Lymphocytic meningitis are mostly of viral origin. The imaging findings are normal, unless it coexists encephalitis.
Chronic meningitis. The most common example is tuberculosis, by hematogenous dissemination to the leptomeninges. Exudate is characterized by thick, gelatinous in the basal cisterns. In children, tuberculous meningitis is accompanied by a concomitant miliary brain infection and is associated with pulmonary tuberculosis. Another important cause of chronic meningitis is cryptococcosis. The chronic meningitis have a predilection for the tank base, although they may occur anywhere. Mortality is 25 to 30%. Sequels: pachymeningitis, atrophy, infarcts, calcification, permanent damage to nerves and hydrocephalus.
Unenhanced CT: meningeal thickening plate, especially at the base. Sometimes, dural calcifications may occur in the form of popcorn. In contrast, no meningeal enhancement.
MRI without contrast, is not detected. In contrast, basal meningeal enhancement. The calcified nodules are hypointense on all sequences.