Bacterial meningitis: imaging features
In uncomplicated bacterial meningitis, neuroimaging methods often show no abnormalities and are not mandatory for establishing a diagnosis. The gold standard remains the analysis of cerebrospinal fluid obtained via lumbar puncture (LP).
The question of whether a CT scan is necessary prior to lumbar puncture has long been controversial. Tomography is indicated when there is suspicion of a midline shift, effacement of the basal cisterns, or cerebellar tonsillar herniation. According to clinical guidelines, CT is strictly mandatory prior to LP in the presence of focal neurological deficits, papilledema, continuous seizures, or a depressed level of consciousness (less than 12 points on the Glasgow Coma Scale).
If MRI abnormalities are nevertheless present, they include:
- Leptomeningeal contrast enhancement: contrast accumulates along the sulci and basal cisterns. This sign is best visualized on post-contrast FLAIR MRI, which reduces the artifactual hyperintensity of normal blood vessels.
- Non-contrast FLAIR: may demonstrate a hyperintense signal in the subarachnoid spaces (especially in the Sylvian fissures). This is due to a high protein concentration in the cerebrospinal fluid. It is important to remember that this is a non-specific symptom: it also occurs in subarachnoid hemorrhage, carcinomatous meningitis, and even in patients breathing high concentrations of oxygen.
- DWI sequence: in less than 10% of cases, it reveals punctate foci of high signal intensity within the sulci, which is associated with an extremely poor prognosis for the patient.
Complications of meningitis
Progression of the infection can lead to severe intracranial consequences. Typical complications include communicating hydrocephalus, ventriculitis, subdural empyema, and brain abscess.
Due to the accumulation of inflammatory exudate around arteries, secondary vasculopathy develops, leading to ischemic infarctions of the parenchyma (which are well visualized on DWI). Four patterns of such lesions are distinguished:
- Infarctions within major vessel territories.
- Infarctions within the supply territories of perforating arteries.
- Multiple bilateral lesions in the cortex and subcortical white matter.
- Isolated multiple lesions exclusively involving the cerebral cortex.
Cerebritis and brain abscess
In most cases (especially in immunocompetent individuals), abscesses are bacterial in origin, with streptococci being the most frequently isolated organisms. The infection reaches the brain tissue via hematogenous dissemination, penetrating trauma, or spreads from adjacent ENT organs.
The localization of the purulent focus is closely related to the primary source:
- Frontal sinusitis leads to a frontal lobe abscess.
- Mastoiditis is the cause of temporal lobe or cerebellar involvement.
- The hematogenous route most commonly leads to subcortical abscesses in the middle cerebral artery territory (frontoparietal region). When searching for the source, occult cardiac pathologies—such as unrecognised endocarditis or cardiac shunt defects—must not be overlooked.
The process of abscess formation is divided into four stages: early cerebritis, late cerebritis, early capsule formation, and late capsule formation. On CT, the cerebritis stage appears as a hypodense area with indistinct margins; early on, there is no contrast enhancement, but it later appears at the periphery. When the capsule is formed, a central necrotic zone (pus) surrounded by a dense rim is clearly visible on scans. Following contrast administration, this rim demonstrates typical ring enhancement, surrounded by an area of vasogenic edema.
Briefly: answers to topical questions
What MRI features are characteristic of subdural empyema?
Subdural empyema is characterized on MRI by specific signal characteristics and contrast enhancement patterns.
- T1-weighted images — intermediate signal intensity relative to cerebrospinal fluid.
- T2-weighted images — hyperintense signal.
- Contrast enhancement — pachymeningeal (dural) and leptomeningeal contrast accumulation is identified.
Which pathogens most frequently cause bacterial meningitis in newborns?
The most common causative agents of bacterial meningitis in neonates are Escherichia coli and streptococci.
- Escherichia coli (Escherichia coli) — accounts for approximately 80% of cases, with infection often occurring during passage through the birth canal.
- Group B streptococci (Streptococcus) — along with E. coli, these are most relevant during the first week of life.
Staphylococci, Klebsiella, and other microorganisms can also play a significant role in the development of the infection.
How does ventriculitis appear on contrast-enhanced MRI?
On contrast-enhanced MRI, ventriculitis manifests as specific alterations of the ventricular system.
- Contrast enhancement — enhancement is detected along the margins of the ventricles (ependymal enhancement).
- Signal characteristics — periventricular and subependymal high signal intensity is visualized.
- Morphological changes — the affected ventricles are typically enlarged (dilated), and debris may be identified within the ventricular system.
What cerebrospinal fluid changes are characteristic of bacterial meningitis?
Bacterial (purulent) meningitis is characterized by marked macroscopic and laboratory changes in the cerebrospinal fluid.
- Macroscopy — the fluid emerges under markedly increased pressure, loses transparency, becomes turbid or opalescent, and acquires a greenish (purulent) tint.
- Cytology — a pronounced neutrophilic pleocytosis is observed (cell counts elevated into the thousands).
- Biochemistry — marked proteinorrhagia (elevated protein levels), hypoglycorrhachia (decreased glucose levels), and elevated lactate levels are identified.
For the pathways of abscess spread, the "neighbors and blood" rule applies: frontal sinusitis affects the frontal lobe, mastoiditis affects the temporal lobe or cerebellum, and hematogenous seeding (via the bloodstream) most frequently settles along the middle cerebral artery territory.
Class and exam questions
Is a CT scan mandatory to confirm bacterial meningitis?
No. In uncomplicated meningitis, neuroimaging results often remain within normal limits. The primary diagnostic method is cerebrospinal fluid analysis.
In which cases is a CT scan strictly necessary prior to lumbar puncture?
CT is performed when there is suspicion of the risk of brain herniation: if there are focal neurological deficits, papilledema, seizures, or a level of consciousness dropping below 12 on the Glasgow Coma Scale.
How to differentiate the cerebritis stage from a formed abscess on CT?
Cerebritis appears as an ill-defined hypodense area without clear contrast enhancement in the early stages. A fully formed abscess features a necrotic center, peripheral vasogenic edema, and bright ring-like contrast enhancement around the capsule.
What else to study on this topic
- Causes of false-positive hyperintensity of the subarachnoid spaces on FLAIR
- Pathogenesis of vasculopathy and secondary infarctions in meningitis
- Differential diagnosis of ring-like contrast enhancement on MRI
- The role of diffusion-weighted imaging (DWI) in predicting infection outcomes
Master the entire topic in the "Diagnostic Radiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Intracranial infections" and 2,199 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 10 video lessons on the subject
Other topics in the Neuroimaging section
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.