A 10 month old child presents with two weeks history off fever, vomiting and alteration of sensorium Cranial CT scan reveals basal exudates and hydrocephalus, the most likely etiological agent is –
Insiduous /gradual onset of fever over a duration of two weeks, with features of raised intracranial tension in the form of vomiting and altered sensorium with characteristic basal exudates and hydrocephalus on cranial CT scan are almost characteristic of tuberculous meningitis.
TUBERCULOUS MENINGITIS 

Tuberculous meningitis usually arises from the formation of metastatic caseous lesions in the cerebral cortex or meningitis that develops during the lymphohaematogenous spread of primary infection. This initial lesion increases in size and discharges tubercle bacilli in the subarachnoid space.
This results in gelatinous basal exudate which interferes in the normal flow of CSF, in and out of the ventricular system at the level of basal cisterns leading to a communicating hydrocephalus.

Clinical manifestations of tuberculous meriins

The clinical course of TB meningitis has three stages -


Prodromal stage or stage of invasion


Insidious onset                                                     o Headache and vomiting
Low grade fever, loss of appetite                    o Photophobia
Disturbed sleep                                                    o Constipation

2. Stage of meningitis
Signs of meningeal irritation develops - Neck rigidity, Kernig's sign
3. Stage of coma
o Loss of consiousness                           o Bradycardia

Pupils are dilated & fixed                 ❑ Chyne-strokes or Biot type of respiration
Episodic decerebration

CT findings in TBM :

Basal cisterns are obliterated by isodense / hyperdense exudate
Post contrast there is a avid enhancement of the basal meninges extending into the ambient, sylvian, pontine and chiasmatic cisterns.

Meningeal enhancement may continue over the cerebral and cerebellar hemispheres.
3. Hydrocephalus is present in 45-87% of patients at time of diagnosis.
Mycobacterium tuberculosis
Cryptococcus neoformans
Listera monocytogenes
Streptococcus pneumoniae