Acute and subacute effects of irreversible electroporation on nerves: experimental study in a pig model

Helmut Schoellnast, Sebastien Monette, Paula C Ezell, Ajita Deodhar, Majid Maybody, Joseph P Erinjeri, Michael D Stubblefield, Gordon W Single Jr, William C Hamilton Jr, Stephen B Solomon, Helmut Schoellnast, Sebastien Monette, Paula C Ezell, Ajita Deodhar, Majid Maybody, Joseph P Erinjeri, Michael D Stubblefield, Gordon W Single Jr, William C Hamilton Jr, Stephen B Solomon

Abstract

Purpose: To evaluate whether irreversible electroporation (IRE) has the potential to damage nerves in a porcine model and to compare histopathologic findings after IRE with histopathologic findings after radiofrequency ablation (RFA).

Materials and methods: This study was approved by the institutional animal care and use committee. Computed tomography (CT)-guided IRE of 11 porcine sciatic nerves was performed in nine pigs, and histopathologic analysis was performed on the day of ablation or 3, 6, or 14 days after ablation. In addition, acute RFA of six porcine sciatic nerves was performed in six pigs that were harvested on the day of ablation. All nerves and associated muscles and tissues were assessed for histopathologic findings consistent with athermal or thermal injury, respectively, such as axonal swelling, axonal fragmentation and loss, Wallerian degeneration, inflammatory infiltrates, Schwann cell proliferation, and coagulative necrosis. The percentage of fascicles affected was recorded.

Results: All nerves had an axonal injury. The percentage of affected nerve fascicles after IRE was 50%-100%. Axonal swelling and perineural inflammatory infiltrates were detectable at every time point after ablation. Axonal fragmentation and loss, macrophage infiltration, and Schwann cell proliferation were found 6 and 14 days after ablation. Distal Wallerian axonal degeneration was observed 14 days after ablation. The endoneurium and perineurium architecture remained intact in all cases. RFA specimens at the day of ablation revealed acute coagulative necrosis associated with intense basophilic staining of extracellular matrix, including collagen of the perineurium and epineurium consistent with thermal injury.

Conclusion: IRE has the potential to damage nerves and may result in axonal swelling, fragmentation, and distal Wallerian degeneration. However, preservation of endoneurium architecture and proliferation of Schwann cells may suggest the potential for axonal regeneration. In contrast, RFA leads to thermal nerve damage, causing protein denaturation, and suggests a much lower potential for regeneration.

© RSNA, 2011.

Figures

Figure 1a:
Figure 1a:
(a) CT image before IRE ablation shows level of planned probe entry with right sciatic nerve (long arrow) and accompanying arteria comitans nervi ischiadici (short arrow) surrounded by fat tissue. (b) CT image after placement of IRE electrodes with right sciatic nerve (long arrow) and accompanying artery (short arrow) between electrode tips.
Figure 1b:
Figure 1b:
(a) CT image before IRE ablation shows level of planned probe entry with right sciatic nerve (long arrow) and accompanying arteria comitans nervi ischiadici (short arrow) surrounded by fat tissue. (b) CT image after placement of IRE electrodes with right sciatic nerve (long arrow) and accompanying artery (short arrow) between electrode tips.
Figure 2:
Figure 2:
Histopathologic slides after IRE ablation. H&E = hematoxylin-eosin stain, S100 = S100 immunohistochemical stain. S100-stained sections are transverse, except M, which is longitudinal. Original magnification, ×600; scale bar = 20 μm. A–C, Slides of untreated nerve show normal morphology. Arrows = axons, arrowheads = Schwann cells surrounding myelinated axons and characterized by cytoplasmic and nuclear S100 staining. D–F, Axonal swelling is seen (arrows). Schwann cell morphology and number are normal (arrowheads). G–I, Swollen axons (arrow) and ellipsoids containing axonal and myelin debris (a) are seen. Increased cellularity is also seen. Most nuclei are S100 negative, but small clusters of S100-positive Schwann cells are observed on occasion (b). J–L, Swollen axons (arrow) and ellipsoids (a) that occasionally contain phagocytes with foamy cytoplasm (arrowhead). Prominent thick bands of S100-positive Schwann cells (b) are present.
Figure 3:
Figure 3:
Histopathologic slide obtained 14 days after IRE ablation. Endoneurial (arrow) and epineurial (∗) extracellular matrix has a normal architecture. Collagen is blue. (Masson trichrome stain; original magnification, ×600.) Scale bar = 20 μm.
Figure 4:
Figure 4:
Histopathologic slide obtained 4 hours after RFA. Endoneurial (arrow) and perineurial (∗) collagen shows intense basophilic staining, compatible with thermal injury, which indicates protein denaturation. Compare this image with images of the eosinophilic normal nerve (Fig 2, A) and the eosinophilic nerve 4 hours after IRE ablation (Fig 2, D). Nuclei are pyknotic. (Hematoxylin-eosin stain; original magnification, ×600.) Scale bar = 20 μm.

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