Heterogeneous sensory processing in persistent postherniotomy pain

Previous studies on sensory function in persistent postherniotomy pain (PPP) have only identified pressure pain threshold to be significantly different from pain-free patients despite several patients reporting cutaneous pain and wind-up phenomena. However the limited number of patients studied hinders evaluation of potential subgroups for further investigation and/or treatment allocation. Thus we used a standardized QST protocol to evaluate sensory functions in PPP and pain-free control patients, to allow individual sensory characterization of pain patients from calculated Z-values. Seventy PPP patients with pain related impairment of everyday activities were compared with normative data from 40 pain-free postherniotomy patients operated > 1 year previously. Z-values showed a large variation in sensory disturbances ranging from pronounced detection hypoesthesia (Z = 6, cold) to pain hyperalgesia (Z = -8, pressure). Hyperalgesia for various modalities were found in 80% of patients, with pressure hyperalgesia in similar to 65%, and cutaneous (mechanical or thermal) hyperalgesia in similar to 35% of patients. The paradoxical combination of tactile hypoesthesia and hyperalgesia was seen in similar to 25% of patients. Increased pain from repetitive tactile and/or brush stimulation was found in 51%, suggesting a role of altered central nociceptive function in this subpopulation. A high incidence (26%) of pressure hyperalgesia was found in the contralateral groin, with a significant correlation (rho = 0.58, p = 0.002) to the hyperalgesic level on the painful side, again suggesting central nervous mechanisms in PPP. In conclusion, this study shows that a standardized trauma results in heterogeneous combinations of hypo- and hyperalgesia. Z-score evaluation of sensory function identifies subpopulations in PPP, which may be used in selecting surgical and/or pharmacological treatment strategies. (C) 2010 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.