Somatosensory Outcomes Following Re-Surgery in Persistent Severe Pain After Groin Hernia Repair: A Prospective Observational Study

Abstract

Purpose: After groin hernia repair (globally more than 20 million/year) 2-4% will develop persistent severe pain (PSPG). Pain management is challenging and may require multimodal interventions, including re-surgery. Quantitative somatosensory testing (QST) is an investigational psychophysiological tool with the potential to uncover the pathophysiological mechanisms behind the pain, ie, revealing neuropathic or inflammatory components. The primary objective was to examine and describe the underlying pathophysiological changes in the groin areas by QST before and after re-surgery with mesh removal and selective neurectomy.

Patients and methods: Sixty patients with PSPG scheduled for re-surgery and with an inflammatory "component" indicated by blunt pressure algometry were examined in median (95% CI) 7.9 (5.8-11.5) months before and 4.0 (3.5-4.6) months after re-surgery. The QST-analyses included standardized assessments of cutaneous mechanical/thermal detection and pain thresholds. Suprathreshold heat stimuli were applied. Deep tissue sensitivity was tested by pressure algometry. Testing sites were the groin areas and the lower arm. Before/after QST data were z-transformed.

Results: Re-surgery resulted in median changes in rest, average, and maximal pain intensity scores of -2.0, -2.5, and -2.0 NRS (0/10) units, respectively (P = 0.0001), and proportional increases in various standardized functional scores (P = 0.0001). Compared with the control sites, the cutaneous somatosensory detection thresholds of the painful groin were increased before re-surgery and increased further after re-surgery (median difference: 1.28 z-values; P = 0.001), indicating a successive post-surgical loss of nerve fiber function ("deafferentation"). Pressure algometry thresholds increased after re-surgery (median difference: 0.30 z-values; P = 0.001).

Conclusion: In this subset of patients with PSPG who underwent re-surgery, the procedure was associated with improved pain and functional outcomes. While the increase in somatosensory detection thresholds mirrors the surgery-induced cutaneous deafferentation, the increase in pressure algometry thresholds mirrors the removal of the deep "pain generator". The QST-analyses are useful adjuncts in mechanism-based somatosensory research.

Keywords: chronic post-surgical pain; groin; hernia repair; reoperation; sensory thresholds.

Figure 1

Detection Thresholds. The z-scores (z_DTH; y-axes) of the mean of the detection thresholds ((MDT + CDT + WDT)/3) before and after re-surgery (RS; x-axes). Reference z-scores are calculated from the contralateral (CL) groin before and after RS. The dot-line diagram (A) illustrates individual z_DTH-scores in the painful groin (P-groin) before and after re-surgery (Δz_DTH = 1.28; P = 0.0001; effect size = 0.49). The box-whisker plot (B) illustrates z_DTH-scores in the lower arm, in the CL-groin, and in the P-groin before and after re-surgery. Whiskers indicate the 2.5 and 97.5 percentiles. Outliers are indicated. The comparisons of before/after re-surgery data were by Wilcoxon matched-pairs signed-rank tests.

Figure 2

Mechanical and Thermal Pain Thresholds. The z-scores before and after re-surgery (RS) pertaining to blunt Pressure Pain Threshold (PPT; z_PPT). Reference z-scores are calculated from the contralateral (CL) groin before and after re-surgery. The dot-line diagram (A) illustrates individual z_PPT-scores in the painful groin (P-groin) before and after re-surgery (Δz_PPT = 0.30 P = 0.001; effect size = 0.40). The box-whisker plots (B) illustrate z_PPT-scores in the lower arm, in the contralateral groin, and in the painful groin, respectively, before and after re-surgery. Please, note in (B), the distribution of PPT scores from the lower arm is skewed since the cut-off limit was reached in most subjects. The dot-line diagram (C) illustrates individual z_HPT-scores in the painful groin before and after re-surgery (Δ_HPT = 0.26 P = 0.042; effect size = 0.28). The box-whisker plots (D) z_HPT-values in the lower arm, in the contralateral groin and in the painful groin, respectively, before and after re-surgery. Whiskers indicate the 2.5 and 97.5 percentiles. Outliers are indicated by dots. The comparisons of before/after re-surgery data were by Wilcoxon matched-pairs signed-rank tests.

Figure 3

Composite Outcome Analysis. Scatter plot depicting the relationship between the fractional changes in physical function scores (summed AAS scores) and pain intensity scores (summed NRS scores; cf. “Analyses of Fractional Changes”” in main text) (n = 56). A fractional change > 0 indicates an improvement in ADL-outcome or a reduction in NRS, whereas a fractional change < 0 indicates a deterioration in ADL-outcome or an increase in NRS. The linear regression model had an R² of 0.427 (95% CI: 0.24 to 0.61 P = 0.0001) and a slope (α) of 0.602 (95% CI: 0.41 to 0.79; P = 0.0001) indicating an evident relationship. Long-term efficacy has previously been presented and compared to a non-interventional control-group.

Figure 4

Pathophysiological Mechanisms. A schematic presentation of putative pathophysiological mechanisms in persistent severe pain after groin hernia repair. (A) Depicts the normal conditions of the nociceptive (red) and non-nociceptive system (blue) circuitry from the primary afferents to the dorsal horn neurons with ensuing central projections. The “mirrored” arrows indicate the crosstalk between the circuitries in the peripheral and central nervous systems. Following the primary surgery (B), an inflammatory reaction surrounding the polypropylene mesh implant, may develop into a peripheral “pain generator”, ie, a “meshoma”. The “pain-generator”, including neo-innervation and reinnervation of nerve fibers, induces increased nociceptive-signaling from the fascial layers, eg, a lowered blunt pressure pain threshold (PPT). In contrast, the surgery causes a long-term “deafferentation”: a reduction in intraepidermal nerve fiber density (IENFD), leading to increased cutaneous thermal and mechanical detection and pain thresholds. Following re-surgery (C), the mesh explant leads to removal of the functional “pain generator”: the spontaneous and evoked pain diminishes (increase in PPT). However, the re-surgery also increases the cutaneous deafferentation (reduction of IENFD) leading to an additional increase of cutaneous thermal and mechanical thresholds.