A 30 year old male presented to the neurological services with complaints of fever, headache and blurred vision, left focal motor seizures with secondary generalization (3–4 episodes) for 9 days. Initially he was evaluated at another hospital and received anticonvulsants for 5 days as the multiple lesions seen cranial CT scan were interpreted as cerebral metastasis. He had recurrence of left motor seizures and persistent headache and was referred to our center. On admission, he was found to have left upper motor neuron facial palsy and left sided hemiplegia. An MR scan revealed multiple ring enhancing lesions with prominent perilesional edema involving basal ganglia, frontal and parietal lobes. The lesions in right frontal and parietal lobes revealed a ring enhancing eccentric target sign on post contrast T1 weighted spin echo imaging (). T2W fast spin echo image highlighted the hypointense ring like lesion in the frontal and parietal areas with the central area remaining hypointense and perilesional edema (). Routine hematological and biochemical parameters were within normal limits. Serum was reactive for HIV-1 and serum and brain tissue were positive for HIV-1 Subtype C by tat specific PCR. Antitoxoplasma antibody (IgG) was detected in serum. CD4/CD8 counts and viral load could not be estimated, because of logistic problems. Based on the MRI features and positive toxoplasma serology, diagnosis of cerebral toxoplasmosis was made and antitoxoplasma therapy was initiated. The patient succumbed, fourteen days followed hospital admission. A partial autopsy confined to examination of the brain alone was conducted, with informed consent of the close relatives. The brain revealed bilateral mild frontal atrophy, and diffuse cerebral edema, with cerebellar tonsillar herniation. Multiple discrete toxoplasma lesions, mostly in necrotizing encephalitis stage or as organizing abscesses were noted in cerebral cortex, diencephalic nuclei and cerebellum bilaterally. The lesions corresponding to the eccentric target sign on MRI were seen as organizing abscesses at grey white junction encircling the depth of the sulci in right superior frontal gyrus, and superior parietal lobule, with perilesional edema ().
Fig.1 Post-contrast T1-weighted coronal image shows characteristic eccentric target sign of cerebral toxoplasmosis in right superior frontal gyrus (a, arrow). T2 weighted image highlights alternating hyper and hypointense zones (b, arrow) with marked perilesional (more ...)
MRI imaging findings were correlated with whole mount histological preparation stained with Haematoxylin-eosin, Luxol Fast Blue stain for demyelination and Masson’s trichrome for collagen with CD68 immunostaining for histiocytes. The eccentric target sign noted on MRI in right frontal and parietal cortex corresponded histologically to large necrotizing abscess (). Concentrically thickened vessels traversing the sulcus produced the curved eccentric target sign ( arrow, ). The surrounding zone of necrosis produced the intermediate zone of hypointensity () while the enhancing rim histologically corresponded to a dense band of histiocytic response () with inflamed () and proliferating vessels delineated by immunostaining to Factor VIII related antigen of vascular endothelium (). The perilesional white matter edema with demyelination was reflected as intense hyperintensities on T2W images.
Fig.2 Lesion in right superior frontal gyrus (a) on histology shows an organizing abscess surrounding the depth of sulcus with concentric zones (b). The center has a vessel traversing the sulcus (asterix) bordered by zone of necrosis with surrounding edematous (more ...)
The periphery of the ring enhancing lesion had occasional tissue cysts with bradyzoite of Toxoplasma gondii, while the inflamed vascular zone bordering necrosis had diffusely dispersed tachyzoite forms detected by immunohistochemistry using antibodies to tachyzoite specific antigen (p30) (). The varying host response to the temporal evolution of toxoplasma lesions, anatomical location and local vascularity appears to determine the patterns observed on MR imaging. Multinucleate giant cells suggestive of HIV encephalitis were not found.