Rationale: Chronic abdominal pain is a frequently occurring condition. Although hardly ever considered, the abdominal wall is the primary cause in 10-30% of cases. Most often it is caused by entrapment of an intercostal nerve in the anterior rectus sheath, the Anterior Cutaneous Nerve Entrapment Syndrome (ACNES). Treatment consists of local anaesthetic injections combined with methyl-prednisolon. When ineffective, a neurectomy at the site of penetration out of the ventral rectus sheet should be considered. This neurectomy however is effective in 73% of cases, leaving some 25% of patients in pain. Whether these refractory ACNES patients suffer from underlying pathologic pain disorders is subject to investigation, by using quantitative sensory testing (QST).

Objective: To investigate nociceptive processing and possible underlying pathological pain processing mechanisms in ACNES patients.

Study design: An observational case-control study.

Study population: Patients treated for ACNES (n = 50) compared to healthy controls from an existing database.

Measurements: Quantitative sensory testing (QST) of nociception, performed after treatment of ACNES for both successfully treated and refractory patients in comparison to healthy controls. Visual Analogue Scores (VAS) measured before, during and after testing procedures. Pain Anxiety Symptom Scale (PASS) and Pain Catastrophizing Scale (PCS) questionnaires.

Main study parameters: Pressure pain and electrical pain thresholds as investigated by QST. Secondary study parameters are VAS-scores and results of PASS and PCS questionnaires.

Inclusion Criteria:

1. Patient has been diagnosed with abdominal complaints, matching ACNES:

   • Patient had a constant site of tenderness that is superficially located with a small (<2cm2) area of maximal tenderness.
   • The most intense pain could be localized with the tip of one finger.
   • Tenderness increased by abdominal muscle tensing (Carnett's test).

2. has been treated (successfully and unsuccessfully) for ACNES.

3. Patient is at least 18 years old on the day the informed consent form will be signed.

4. Patient is willing and able to comply with the trial protocol.

5. Patient is able to speak, read and understand the local language of the investigational site, is familiar with the procedures of the study, and agrees to participate in the study program by giving oral and written informed consent prior to screening evaluations.

Exclusion Criteria:

1. Abdominal complaints were due to a condition other than ACNES (e.g. pain related to scar tissue).
2. Patient has (a history of) another (chronic) pain syndrome that interferes with the interpretation of QST results.
3. Patient has (a history of) Raynaud syndrome or fenomenon, or a medical disorder that interferes with the study measurements or may pose a risk for the patient.
4. Patient does not feel a pinprick test to the lower extremities, due to affected sensory input (e.g. neuropathy as a result of diabetes mellitus).
5. Female patient is pregnant during the course of the study.

Study population will consist of ACNES patients that have been treated both successfully and refractory. The control group will consist of demographically equal healthy controls from an existing database