Limitations of functional magnetic resonance imaging in mapping function near a vascular lesion: A case study

Abstract

We present a case of functional reorganization of the somatosensory system in a 15 year-old female with a history of perinatal stroke of the middle cerebral artery. The patient presented with hemiparesis and epilepsy and underwent comprehensive pre-surgical evaluation for epilepsy surgery, including mapping of somatosensory function with functional magnetic resonance imaging (fMRI). The fMRI results indicated inter-hemispheric reorganization of somatosensory function from the left to the right hemisphere, and showed no residual somatosensory function in the peri-lesional area of the affected left hemisphere. However, following implantation of subdural electroencephalography (EEG) electrodes, recordings from left hemisphere lesional and peri-lesional areas showed evoked electrophysiological responses to tactile stimulation. Bedside and intra-operative stimulation mapping confirmed multiple somatosensory responsive sites in the left hemisphere in or near the lesion, in contradiction of the fMRI results. Since the BOLD signal is a measure of the local ratio of oxygenated to deoxygenated blood, fMRI represents only an indirect measure of neuronal activity. Modeling of fMRI activation depends on intact neurovascular coupling and adequate signal to noise ratios, which may be altered in the presence of a vascular lesion. Our results suggest that pre-surgical mapping of cortical function with fMRI can be unreliable in the presence of a vascular lesion and can lead to false-negative results. In such cases, direct measures of electrophysiological activity, such as electrocorticography, scalp EEG and MEG should be preferred.

Keywords

Functional imaging - stroke - electrical stimulation - epilepsy surgery - evoked potentials