PDF
ePub
Citation
Print
INTRODUCTION
Tuberculosis is a common disease in Nepal. Approximately 40,000 people get tuberculosis and 20,000 get new sputum positive cases every year. 5000-7000 people die each year from tuberculosis (1). Central nervous system (CNS), one of the most devastating clinical manifestations of tuberculosis is noted in 5 to 10% of extrapulmonary tuberculosis cases, and accounts for approximately 1% of all TB cases (2).
CNS tuberculosis manifests as meningitis, tuberculoma or tubercular abscess. Tubercular abscess rarely occurs in the brain, and presents with various clinical features. Diagnosis is usually made by computerized tomography (CT) or magnetic resonance imaging (MRI) scans. MRI is the choice of investigation because of its multiple parameters of evaluation, e.g. conventional imaging (T1, T2 weighted imaging, fluid attenuated recovery [FLAIR]), diffusion weighted imaging (DWI), proton magnetic resonance spectroscopy (MRS), arterial spin labeling (ASL) or dynamic susceptibility contrast (DSC) perfusion study (345). Newer imaging parameters in MRI have greatly increased its sensitivity and specificity. Here we report a case of tubercular abscess presented with decreased vision. MRI has played a key role in its diagnosis, although the final diagnosis was established by acid fast bacilli (AFB) staining of abscess material and histopathology of the excised specimen.
CASE REPORT
A 17-year-old female patient presented with complaints of headache and gradually decreasing vision since one month. She gave no history of fever or recent weight loss. There was no evidence of an immuno-compromised state. Also, there was no history of loss of consciousness or weakness of limbs. On examination, she was alert, oriented with a Glasgow Coma Scale of 15/15. Except for papilledema she had a normal neurological examination.
She underwent laboratory and imaging studies. Hematological studies were within normal limits except for slightly increased ESR (24 mm in the first hour). Non-contrast CT scan (Fig. 1a) of her head revealed an irregular cystic mass with concentric, slightly thickened wall, in the left frontal lobe. Moderate perilesional edema was noted. Mass effect was evident by effacement of adjacent sulci, left lateral ventricle and slight midline shift towards left side. Differential diagnosis considered in this non-contrast CT scans were pilocytic astrocytoma, metastasis and abscess.
An MRI of her head was done in Philips Ingenia 3.0T scanner. Routine imaging studies included fast spin echo T1 (Fig. 1b) and T2 (Fig. 1c) weighted images, and FLAIR (Fig. 1d). Thereafter, DWI, MRS and DSC perfusion studies were performed. In vivo, MRS was accomplished using the water suppressed multi voxel spin echo sequences with long TE (144 ms) technique. It showed an outline of a lobulated mass in the anterior aspect of the left frontal lobe, which displayed a concentric hypo intense wall in T2 weighted images and iso-signal intensity in T1 weighted images. Alternating hypointense areas were noted between the wall in T1, and high signal intensity in T2 weighted images. Marked T2 hyperintensity was evident around this mass, representing moderate perilesional edema. In addition, a significant mass effect was also seen. FLAIR axial image displayed an irregular concentric hypointense wall having perilesional hyperintensity. Post Gadolinium study (Fig. 1e, f) showed two slightly irregular wall, markedly enhancing walls with some nodular solid enhancing component in the anterior aspect.
No restricted diffusion was seen within this mass in DWI (Fig. 1g, h). Apart from the changes noted above, rest of the brain showed normal morphology with normal parenchymal signal intensity. Posterior cranial fossa structures were normal. The most likely diagnosis considered was glioma/metastasis. However, the hypointense rings in the T2 sequence made inflammatory lesion an equally likely diagnosis.
Thereafter, newer sequences were done for further evaluation of this lesion. MRS showed increased lactate and lipid peaks in the central part of this mass (Fig. 2). Few areas at the periphery of the mass showed increased choline peak without significant decrease in NAA level.
ASL (Fig. 1i) before DSC perfusion study showed decreased relative cerebral blood volume (rCBV) in this region. DSC perfusion study also showed decreased rCBV in the center, and no increased rCBV was seen in the region of wall (Fig. 1j). These features in MRS, ASL and Perfusion imaging, combined with conventional MRI features, suggested a brain abscess, most probably of tubercular origin.
The patient underwent a craniotomy and complete excision of the lesion. Intra-operatively, an organized capsulated mass having a central necrotic and purulent area, was seen (Fig. 3a). She tolerated the surgery well and was discharged from the hospital on 7th postoperative day, in stable condition.
AFB staining of material from the mass showed abundant staining of AFB (+++). Histopathological examination of the specimen showed discrete to confluent epithelioid cell granulomas with extensive caseation necrosis and Langhans giant cells (Fig. 3b). No glial tissue was evident. Thus, the final diagnosis of tubercular abscess of left frontal lobe was confirmed, and the patient was started on anti-tubercular treatment.
DISCUSSION AND CONCLUSION
CNS tuberculosis is not an uncommon extra-pulmonary manifestation of tuberculosis. In brain, it usually manifests as meningitis and tubercular granuloma (tuberculoma). Tuberculoma often shows a ring enhancing lesion with perilesional edema in CT and MRI (67). This lesion could be treated by anti-tubercular therapy. Tubercular abscess formation can occur in brain, where it simulates a mass and cause mass effect (89). It can mimic other intraaxial masses like glioma, metastasis etc. CT scan is unable to differentiate it from other masses. MRI is the choice of investigation in this case. DWI is a good imaging sequence for differentiating abscess from other masses. Restricted diffusion, with high signal intensity in DWI and low ADC values, is seen in both tubercular and pyogenic brain abscess. However, no restricted diffusion was seen in this case, thus making the diagnosis a bit confusing (101112). The patient was on a corticosteroid regime. This medication, and an organized abscess cavity, may be the possible cause for no restricted diffusion in DWI. The MRS is another important tool in MRI which can differentiate abscess from malignant lesions. It showed lipid/lactate peaks, and loss of choline and NAA peaks in the wall and in the perilesional edema (1011). However, in our case, there was increased choline without a significant loss of NAA in some peripheral areas. This gave a doubtful impression of the possibility of malignancy.
MR perfusion scan demonstrates the degree of angiogenesis of the lesions and is thus useful in the differentiation between neoplastic and infectious lesions. It is also used to differentiate between tumoral recurrence and radionecrosis in the post treatment follow up. It can be performed in high field MRI by ASL or DSC perfusion study. ASL has emerged as a new tool in MRI, which can show the relative cerebral blood volume in the lesion without MR contrast. It had clearly shown the decreased rCBV in our case. We had to resort to MR contrast in DSC perfusion study. Only then could it delineate relative cerebral blood volume in the lesion (131415). In our patient, DSC perfusion study resulted in beautifully displaying the significantly decreased rCBV.
Imaging features in non-contrast CT suggest the differential diagnosis of cystic malignant lesion or abscess. However, considering all the features in MRS, ASL and DSC perfusion studies combined, we could conclude a diagnosis of an abscess. Furthermore, the morphological features of concentric wall and MR features of T2 hypointensity suggested the diagnosis of a tubercular abscess.
XML Download