Background: Although CT scanning is used widely for making the diagnosis and detecting the complications of tuberculous meningitis (TBM) in children, the radiological features are considered non-specific. CT is particularly suggestive of the diagnosis when there is a combination of basal enhancement, hydrocephalus and infarction, and even then the diagnosis may be in doubt. In this paper we introduce a new CT feature for making the diagnosis of TBM, namely, hyperdensity in the basal cisterns on non-contrast scans, and we assess which of the recognized CT features is most sensitive and specific.
Objective: To determine the sensitivity and specificity of the presence of high-density exudates in the basal cisterns (on non-contrast CT) and basal enhancement (on contrast-enhanced CT) for the diagnosis of TBM in children, and to correlate these with the complications of infarction and hydrocephalus.
Materials and methods: Retrospective review of CT scans with readers blinded to the diagnosis, which was based on a definitive culture of cerebrospinal fluid (CSF) for TBM or other bacteria. Computer-aided conversion of hard-copy film density to Hounsfield units was employed as well as a density threshold technique for determining abnormally high densities.
Results: The most specific feature for TBM is hyperdensity in the basal cisterns prior to IV contrast medium administration (100%). The most sensitive feature of TBM is basal enhancement (89%). A combination of features (hydrocephalus, infarction and basal enhancement) is as specific as pre-contrast hyperdensity, but has a lower sensitivity (41%). There were statistically significant differences in the presence of hydrocephalus (p=0.0016), infarcts (P=0.0014), basal enhancement (P<0.0001) and pre-contrast density (P<0.0001) between the negative and positive TBM patient groups. The presence of granulomas was not statistically significant between the two groups (P=0.44).
Conclusions: The presence of high density within the basal cisterns on non-contrast CT scans is a very specific sign for TBM in children. This will enhance diagnostic confidence, allow early institution of therapy and could reduce expenditure on contrast medium, scan time and radiation exposure. With the use of threshold techniques we believe that the pre-contrast hyperdensity may be detectable by a computer program that will facilitate diagnosis, and may also be modified to detect abnormal enhancement. Basal enhancement is a sensitive sign for the diagnosis of TBM and should be sought after contrast medium administration when no hyperdensity is seen in the basal cisterns or when this finding needs to be confirmed. The CT scan feature of hyperdense exudates on pre-contrast scans should be added to the inclusion criteria for the diagnosis of TBM in children.