Assessment of brain damage and plasticity in the visual system due to early occipital lesion: comparison of FDG-PET with diffusion MRI tractography.

STURGE-WEBER SYNDROME (SWS) is a rare disorder, affecting one in 20,000–50,000 people, and is associated with a somatic gene mutation (1) and characterized by a facial port-wine birthmark and leptomeningeal vascular malformation (2). Children with SWS can have a variety of neurologic problems, including motor deficit, visual field impairment, cognitive decline, and seizures. The first neurological symptoms (most commonly seizures) often start during the first year of life and show a highly variable clinical course (3). Cerebral abnormalities in SWS affect only one hemisphere in 85% of the cases, with the most common location of the leptomeningeal angioma in the posterior cerebral cortex involving the occipital lobe (2,3). Clinical magnetic resonance imaging (MRI) can detect brain venous vascular abnormalities as well as tissue damage (atrophy, calcification) underlying the angioma, while 2-deoxy-2[18F]fluoro-D-glucose positron emission tomography (FDG-PET, typically reserved for evaluation for epilepsy surgery) often shows cortical glucose hypo-metabolism extending beyond the structural brain abnormalities (4–6). SWS with unilateral hemispheric involvement allows for the study of early focal brain damage and injury-induced brain plasticity in an etiologically homogeneous patient population. Early postnatal brain injury can induce effective reorganizational processes in the developing human brain, which attempts to compensate for the functional effects of injury (7–9). Imaging can be used to study remote functional effects of focal brain lesions in children. For example, in an activation 15O-water PET study of children with SWS affecting one hemisphere, interhemispheric reorganization of language functions was found despite some retained functions in the affected hemisphere (10). Also, some children with early-onset severe unihemispheric damage due to SWS show relatively preserved cognitive functions (11,12). These observations suggest that early deterioration in the affected hemisphere may induce effective reorganizational processes, which can lead to a transfer of many functions to nonaffected cortical regions either within the ipsilateral or in the contralateral hemisphere. Increased glucose metabolism detected by PET, remote from an early cerebral lesion, may be a hallmark of such functional reorganization (7,13). In our previous PET study, we observed increased glucose metabolism in the unaffected occipital cortex contralateral to the severely damaged occipital lobe of SWS children (14). However, the mechanisms and functional correlates of this observation remained uncertain. Diffusion-weighted imaging (DWI) streamline tractography is a noninvasive tool to detect structural and architectural change in specific cerebral fibers by measuring the dephasing of spins of protons in the presence of a spatially varying magnetic field (15–18). In the present study, we combined DWI streamline tractography with FDG-PET in children with unilateral SWS. We hypothesized that decreased occipital glucose metabolism will be associated with decreased visual streamline volume on the affected side. We also hypothesized that high glucose metabolism in the contralateral occipital (visual) cortex may be associated with high DWI streamline volume of the corresponding visual pathway. Finally, we evaluated the clinical correlates of the imaging abnormalities in the visual system, both ipsi- and contralateral to the lesion. The overall purpose of the study was to determine the relation between glucose metabolic changes of the primary visual cortex, structural abnormalities of the corresponding visual tracts, and visual symptoms in these children.