• Thermal thresholds [ Time Frame: 2019-2020 ]
  Warmth detection threshold (WDT), cool detection threshold, heat pain threshold (HPT) and cold pain threshold (CPT) are made by a computerized contact thermode (Thermotest, Somedic AB, Sweden) with an active thermal surface of 12.5 cm^2 (2.5 x 5.0 cm^2) as previously described in detail. The thresholds are determined from a baseline temperature of 32°C with a ramp rate of + 1°C/s. Cut-offs for heat and cold are 50°C and 5°C, respectively. The assessments are made in triplicate and the mean values are used in the statistical analyses.
• Pressure algometry [ Time Frame: 2019-2020 ]
  Deep-tissue pain sensitivity is assessed using a hand-held pressure algometer with a neoprene-coated tip of area 1.0 cm2 (Somedic AB, Sweden), as previously described. The algometer is applied perpendicularly to the skin with a pressure rate of 30 kPa/s. The study subject is told to report the pressure pain threshold (PPT) by activating the button device when pain is perceived. The cut-off limit is 350 kPa. Testing is done in triplicate and the average value is used in the statistical analyses.
• Suprathreshold heat stimulation [ Time Frame: 2019-2020 ]
  A short tonic heat stimulus (heating area 12.5 cm^2; ramp rate: 1°C/s, plateau: 47°C, 5 s; STH) is delivered in order to evaluate the suprathreshold heat pain perception (NRS).
• Sensory mapping [ Time Frame: 2019-2020 ]
  Sensory mapping in the groin areas is assessed with a 25°C metal roll (Somedic AB), moved at a rate of 1 to 2 cm/s from skin with normal cool perception, using an octagonal approach, into the areas in order to indicate sensory changes. Changes in the study subject's cool perception are indicated by a marker on the skin, and subsequently transferred to a transparent sheet for later area assessment in a vector-based drawing drawing program (Canvas 12.0; ACD Systems, Seattle, WA).
• Tactile thresholds [ Time Frame: 2019-2020 ]
  Tactile detection thresholds (TDT) and tactile pain threshold (TPT) are determined by polyamide monofilaments (Stoelting, IL, USA; nominal buckling force ranging from 0.04 to 4,400 mN) using a modified Dixon up-and-down method, with 3 descending and 3 ascending stimulus intensities. The filament with the smallest buckling force leading to tactile recognition (TDT) is registered. The tactile pain threshold (TPT) is determined by the same stimulus paradigm, but registering the perception of pain (sharpness/sting; TPT).
• Temporal summation [ Time Frame: 2019-2020 ]
  Temporal summation test, i.e., the perception in response to repetitive (0.3 to 3 Hz) mechanical stimulation [i.e., wind-up like pain: WUP], indicates presence of central sensitization. The repetitive 1 Hz stimuli for 60 s are either dynamically delivered by a brush or statically delivered by a polyamide filament (one nominal rank below TPT). The study subjects are told to report the level of pain (NRS) every 15 s-1 during the stimulation. Signs of aftersensations are followed 60 s after discontinuation of the stimulation, and the intensity of discomfort or pain is rated by NRS.

• Assessments of Anxiety and Depression [ Time Frame: 2019-2020 ]
  Hospital Anxiety and Depression Scale (HADS; 14 items scale; 0-21 units)
• Assessment of Pain Catastrophizing [ Time Frame: 2019-2020 ]
  Pain Catastrophizing Scale (PCS; 13 item scale; 0-65 units)

The concept of normality is a cornerstone in medical practice and research. As an example, in clinical chemistry, a laboratory value based on a patient plasma sample, exceeding the +/- 1.96 x standard deviation (SD) range, referenced from a normative material, is per definition outside the normal range (the reference interval). Obviously, a number of reasons for this deviation may exist. The sample value could reflect a "true" pathological condition, but could just as well also be caused by error, e.g., technical measurement error, drug-interaction error, random error, or, reflect a value occurring in 5% of the healthy population. Conversely, a sample value in the normal range evidently does not exclude a pathological condition.

The reference interval is calculated from a large number of healthy subjects sampled across age, anthropometrics, ethnicity and gender. Normative reference intervals are certainly of help particularly in the screening of subjects, but may be of limited value in the detailed assessment of pathophysiological processes. Also, increasing the number of analyses in a subject expands the risk of making a type I error (acquiring "false" positive results). The likelihood of one or more type I errors in the analysis of 10 different laboratory values in one subject, is impressive 46% ([1 - 0.95^10] =0.46). It is well-known that multiple measurements are commonly performed in medical practice and research, but corrected significance levels are not always used.

Quantitative sensory testing (QST) is defined as perceptually quantifiable responses to graded chemical (capsaicin), electrical, mechanical (pressure, punctate, vibratory, and light touch) or thermal (cool, warmth, cold pain, and heat pain) stimuli. Thus, QST is a psychophysical method primarily assessing small nerve fiber function in the skin. The method is ubiquitously used for the assessment of peripheral neuropathies, e.g., chemotherapy-induced neuropathy, painful diabetic polyneuropathy, post-herpetic neuralgia, and post-surgical neuropathy. Sensory mapping and QST are essential tools in grading definite or probable neuropathic pain as stated in the definition: "Demonstration of the distinct neuroanatomically plausible distribution by at least one confirmatory test." During the last decade, the German Research Network on Neuropathic Pain (DFNS) has acquired normative sensory data from healthy individuals using a standardized testing protocol. Clearly, a deviating sensory response from a patient with a painful peripheral neuropathy could be evaluated by use of these normative data and eventually be classified as an abnormal response.

But, other alternatives exist, that hypothetically might provide an improved diagnostic specificity and sensitivity. First, in unilateral sensory deficits (mono-neuropathies), assessments at the contralateral side are possible, allowing a comparison with the pathological side. The within-subject variances (WSV) are often significantly smaller than the between-subject variances (BSV) in QST-assessments. In a normative study including thermal assessments (n = 100), the WSV and the BSV ranged between 18-23% and 77-82% of the total variance, respectively. Correspondingly, in a study of patients with the post-thoracotomy pain syndrome, the estimated WSV and BSV were 5-28% and 72-95%, respectively. Thus, scenarios using the subject as own control may reduce data variability, and improve diagnostic efficacy. However, the use of a contralateral homologous area as a control area in sensory testing has been questioned by several authors. The occurrence of mirror image sensory dysfunction (MISD) may affect contralateral assessments, requiring a neutral control area in the subject. Second, instead of using healthy controls in pain studies, use of pain-free patients (e.g. post-groin hernia repair, post-thoracotomy) as controls have been recommended in persistent post-surgical pain studies.

In spite of the importance of selecting an optimally controlled research design, the research group is only aware of one QST-study, adressing the control-group problem, i.e., a study including patients with complex regional pain syndrome type I (CRPS I) restricted to one extremity. The study examined the validity of using the contralateral side as control compared to using normative data from healthy individuals. The study recommended that the contralateral side in CPRS I patients should be used as a control.

Thus it may be inferred, that following approaches are available evaluating sensory data from an anticipated pathological site: an empirical approach (á priori set reference values); an absolute approach (comparing the subject's pathological side with normative data); and a relative approach (comparing the subject's side-to-side differences with normative data).

• P-GHR
  Patients with persistent severe pain after groin hernia repair.
• NP-GHR
  Patients without pain after groin hernia repair.
• NP
  Healthy non-operated controls

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INCLUSION CRITERIA:

All study subjects must meet all the following criteria to be eligible to enroll in the study:

• Age above 18 yrs and below 65 yrs
• Signed informed consent
• Body mass index (BMI): 18 < BMI < 35 kg/m^2
• ASA I-II

In addition, for study subjects without pain after GHR (NP-GHR):

• having undergone uncomplicated, elective, unilateral open GHR a.m. Lichtenstein
• no restriction in ADL-functions due to the GHR
• activity-related groin pain < 3 (numerical rating scale [NRS]: 0 = no pain, 10 = worst imaginable pain)

EXCLUSION CRITERIA:

Any study subject, who meets one or more of the following criteria, is not suitable for inclusion in this study:

• do not speak or understand Danish
• who cannot cooperate with the investigation
• abuse of alcohol or drugs - according to investigator's evaluation
• use of psychotropic drugs (exception of SSRI)
• neurologic or psychiatric disease
• chronic pain condition
• regular use of analgesic drugs
• skin lesions or tattoos in the assessment areas (groins, lower arm)
• nerve lesions in the assessment sites (e.g., after trauma, abdominal surgery)

In addition, for healthy non-operated study subjects (NP):

• subjects, who have had previous surgery in or near the groin region
• use of prescription drugs one week before the trial
• use of over-the-counter (OTC) drugs 48 hours before the trial
• experiencing pain at rest > 3 (NRS)
• experiencing activity-related pain in or near the groin > 3 (NRS)

In addition, for study subjects without pain after GHR (NP-GHR):

• experiencing pain at rest > 3 (NRS)
• experiencing activity-related pain in or near the groin > 3 (NRS)