Analysis of adverse effects of stimulation during DBS surgery by patient-specific FEM simulations.

Deep brain stimulation (DBS) represents today a well-established treatment for movement disorders. Nevertheless the exact mechanism of action of DBS remains incompletely known. During surgery, numerous stimulation tests are frequently performed in order to evaluate therapeutic and adverse effects before choosing the optimal implantation site for the DBS lead. Anatomical structures responsible for the induced adverse effects have been investigated previously, but only based on stimulation data obtained with the implanted DBS lead. The present study introduces a methodology to identify these anatomical structures during intraoperative stimulation tests based on patient-specific electric field simulations and visualization on the patient specific anatomy. The application to 4 patients undergoing DBS surgery and presenting dysarthria, paresthesia or pyramidal effects shows the different anatomical structures, which might be responsible for the adverse effects. Several of the identified structures have been previously described in the literature. To draw any statistically significant conclusions, the methodology has to be applied to further patients. Together with the visualization of the therapeutic effects, this new approach could assist the neurosurgeons in the future in choosing the optimal implant position.

Birefringence of the central cornea in children assessed with scanning laser polarimetry.

Corneal birefringence is a well-known confounding factor with all polarization-sensitive technology used for retinal scanning and other intraocular assessment. It has been studied extensively in adults, but little is known regarding age-related differences. Specifically, no information is available concerning corneal birefringence in children. For applications that are geared towards children, such as retinal birefringence scanning for strabismus screening purposes, it is important to know the expected range of both corneal retardance and azimuth in pediatric populations. This study investigated central corneal birefringence in children (ages three and above), by means of scanning laser polarimetry (GDx-VCC™, Carl Zeiss Meditec, Inc.). Children's measures of corneal retardance and azimuth were compared with those obtained in adults. As with previous studies in adults, corneal birefringence was found to vary widely in children, with corneal retardance ranging from 10 to 77 nm, and azimuth (slow axis) ranging from -11° to 71° (measured nasally downward). No significant differences in central corneal birefringence were found between children and adults, nor were significant age-related differences found in general. In conclusion, establishing knowledge of the polarization properties of the central cornea in children allows better understanding, exploitation, or bypassing of these effects in new polarization-sensitive pediatric ophthalmic applications.