Tuberculous brain involvement
CNS tuberculosis most commonly manifests in three forms: meningitis, tuberculomas, and abscesses.
Tuberculous meningitis is characterized by inflammation of the meninges predominantly at the base of the brain. Post-contrast T1-weighted MRI clearly demonstrates basal meningeal enhancement (basal meningeal enhancement). Classic complications of tuberculous meningitis include:
- Hydrocephalus: occurs due to obstruction of the fourth ventricle outlets;
- Infarctions: specific arteritis of penetrating arteries leads to ischemia of the basal ganglia, internal capsules, and brainstem (ADC maps reveal areas with restricted diffusion);
- Calcification: a rare outcome during the healing process of affected meninges.
Tuberculomas (parenchymal granulomas) typically locate at the gray-white matter junction. On CT, these are small lesions with vasogenic edema. If the lesion is solid, it enhances homogeneously. When central caseous necrosis develops, ring enhancement (rim enhancement) appears. On MRI, non-caseating lesions show low signal on T1 and high signal on T2, whereas with caseous necrosis, the T2 signal becomes low.
Tuberculous abscesses are rare. They differ from tuberculomas by a larger size and a thin lobulated capsule (whereas tuberculomas have a smooth one). The central zone demonstrates restricted diffusion, making them similar to pyogenic abscesses.
Pyogenic infections and ventriculitis
In uncomplicated bacterial meningitis, neuroimaging findings often remain normal. Specific changes appear when purulent complications develop.
The main MRI marker of pyogenic infections is restricted diffusion within the pus collection.
Main variants of purulent lesions:
- Epidural abscess: MRI visualizes a fluid collection surrounded by a contrast-enhancing rim of the inflamed dura mater (dura).
- Subdural empyema: fluid hyperintense relative to CSF on FLAIR sequence is identified, with clear restricted diffusion.
- Purulent ventriculitis: CT shows dilation of the lateral ventricles, intraventricular debris (purulent matter of intermediate density), and a rim of interstitial edema around the ventricles. On MRI, debris also shows persistent restricted diffusion.
Neurosyphilis
Nervous system involvement can occur at any stage of syphilis. Two radiological patterns are distinguished:
- Meningovascular syphilis: the pathogen causes endarteritis of small vessels. On angiography, this manifests as characteristic segmental vessel narrowing — the «beading» sign (beading). This process is closely associated with the development of infarctions in the basal ganglia.
- Cerebral gummas: a rare form representing space-occupying lesions originating from the meninges. Their MRI signal and contrast enhancement characteristics are highly variable and require careful differential diagnosis with tumors.
Differential diagnosis with other pathogens
Bacterial CNS infections must be differentiated from fungal and viral lesions.
Fungal infections develop in patients with severe immunodeficiency. Small fungi (Candida, Cryptococcus) provoke meningitis, while molds (Aspergillus) cause cerebritis and abscesses. A crucial distinction: fungal abscesses do not demonstrate restricted diffusion in the non-enhancing core and often have intracavity projections.
Herpes simplex encephalitis (HSV-1 virus) occurs upon reactivation of the infection in the trigeminal ganglion or via the olfactory tract. On modern CT equipment, changes are visible within the first 3 days: edema and decreased density in the anteromedial temporal lobe and inferior frontal lobes. Subsequently, the process often becomes bilateral, and contrast enhancement takes on a gyriform character (along the gyri).
Briefly: answers to topical questions
Which pathogens most commonly cause purulent meningitis in adults?
In adults, purulent meningitis is most commonly caused by three main bacterial pathogens.
- Meningococcus (Neisseria meningitidis) is a Gram-negative diplococcus causing primary meningitis transmitted via airborne droplets.
- Pneumococcus (Streptococcus pneumoniae) is the causative agent of secondary meningitis arising as a complication of otitis media, mastoiditis, sinusitis, or upper respiratory tract infections.
- Haemophilus influenzae (Haemophilus influenzae type b) causes purulent meningitis in older children and adults.
In patients over 60 years of age, Streptococcus agalactiae is also among the frequent pathogens.
The triad of complications in tuberculous meningitis: HIC (Hydrocephalus, Infarctions, Calcification).
Class and exam questions
Are imaging changes always visible in bacterial meningitis?
No. In uncomplicated bacterial meningitis, CT and MRI findings often remain within normal limits. Imaging becomes informative upon the development of complications (hydrocephalus, empyema).
How to differentiate a tuberculoid abscess from a tuberculoma?
Tuberculous abscesses are larger, and their contrast-enhancing rim has a lobulated structure, whereas in tuberculomas it is smooth. Furthermore, the core of the abscess restricts diffusion.
What is the main differential feature of a fungal abscess?
Unlike pyogenic (bacterial) and tuberculous abscesses, the center of a fungal abscess does not show restricted diffusion on DWI.
What is characteristic of meningovascular syphilis on angiography?
Segmental «bead-like» vessel narrowing (beading) is visualized, which is a consequence of small artery endarteritis.
What else to study on this topic
- Pathogenesis of penetrating vessel arteritis in tuberculosis
- Differential diagnosis of basal meningitis (neurosarcoidosis, carcinomatosis)
- Physical principles of restricted diffusion formation in purulent lesions
- Neuroimaging specifics in opportunistic infections and AIDS
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