13.00 Opening by the Moderator Carsten Dahl Mørch
13.05 PhD lecture by Aida Hejlskov Poulsen
14.00 Questions and comments from the Committee
Questions and comments from the audience at the Moderator’s discretion
16.00 Conclusion of the session by the Moderator
The Faculty Council has appointed the following adjudication committee to evaluate the thesis and the associated lecture:
Dr. Ulf Baumgärtner, Medical School Hamburg (MSH), Faculty of Medicine, Hamburg, Germany
Director Thierry Keller, TECNALIA, Health Division, Donostia-San Sebastián, Spain
Dr. Johannes J. Struijk, HST, Aalborg Universitet
Dr. Carsten Dahl Mørch, HST, Aalborg Universitet
In recent years, small fiber neuropathy has become a well-recognized and increasing health care problem. Small fiber neuropathy is relatively complex to diagnose and treatment options are insufficient in most patients. Accordingly, there is a need for clinical tools that can detect and assess small fiber dysfunction for diagnostic purposes and to facilitate the development of new treatment options. Electrical stimulation could potentially be an option, as it is widely used for large fiber assessment. However, conventional electrical stimulation targets large fibers, and nerve conduction studies display normal values in pure small fiber neuropathy. Consequently, recent research has been focusing on the development of specialized electrodes that will enable preferential activation of small fibers. Six different electrodes have been developed and show promise in preferential small fiber activation. However, the electrodes are limited to low stimulation intensities and suffer from poor perception threshold reproducibility.
The present Ph.D. work aimed to investigate the influence of electrode design features and to design and test a novel electrode design for small fiber activation.
The thesis is based on three separate studies, involving the development of a computational model and comparison of existing electrode designs, optimization, and experimental evaluation of a novel electrode design. In the first study compared existing electrode designs and showed that the intra-epidermal needle design was most preferential towards small fibers as it displayed higher current densities in the vicinity of small fibers and a more limited current to deeper lying large fibers than the surface electrodes. Nevertheless, the effective area for which the electrode was small fiber preferential was smaller for the intra-epidermal design, wherefore practical considerations come into play, as this electrode needs to be placed in close proximity to a small nerve fiber to achieve preferentiality. The second study was an optimization study in which the validated model from the first study was used to optimize electrode dimensions for small fiber stimulation. Minimization of the electrode dimensions led to a substantial improvement of small fiber preferentiality. Accordingly, a small cathode made it possible to activate small fibers at lower stimulation intensities than large fibers, while a small anode area and a small anode-cathode distance increased the difference in small and large fiber activation threshold, by limiting the current spread. From the optimization results, a novel electrode was designed and in the third and final study, the new design was tested experimentally. The novel electrode design was compared to a regular patch electrode setup. Evoked potential predictions did display different patterns and latencies for the novel electrode and the patch, which could be due to differences in the activated fiber populations for the two electrodes, however, this was not quantified statistically. Thereby, some co-activation of large fibers may occur for stimulation with the novel electrode design. Nevertheless, the subjects rated the intensity of the stimuli to be higher and the sensation to be sharper for the new electrode design, which together with a lower perception threshold and longer reaction times, suggests small fiber activation. The sharpness and intensity of the sensation increased with increasing stimulus intensity, which might imply that the electrode is small fiber preferential even at relatively high intensities.
Throughout the work of this Ph.D., a systematic framework utilizing the benefits of computational modeling within the design phase of a novel stimulation electrode for small fiber activation was introduced. A novel planar single-use electrode was proposed as a further step towards a promising tool for assessing small fiber dysfunction.