Numbness in clinical and experimental pain – A cross-sectional study exploring the mechanisms of reduced tactile function Christian Geber a,b, * , Walter Magerl b,d , Ricarda Fondel c , Marcel Fechir a , Roman Rolke a , Thomas Vogt a , Rolf-Detlef Treede d , Frank Birklein a a Neurologische Universita ¨ tsklinik Mainz, Germany b Institut fu ¨ r Physiologie und Pathophysiologie, Mainz, Germany c Klinik fu ¨r Ana ¨ sthesiologie der Universita ¨ t Mainz, Germany d Lehrstuhl fu ¨ r Neurophysiologie, CBTM, Medizinische Fakulta ¨ t Mannheim der Universita ¨ t Heidelberg, Germany Received 11 January 2008; received in revised form 2 March 2008; accepted 5 March 2008 Abstract Pain patients often report distinct numbness of the painful skin although no structural peripheral or central nerve lesion is obvi- ous. In this cross-sectional study we assessed the reduction of tactile function and studied underlying mechanisms in patients with chronic pain and in healthy participants exposed to phasic and tonic experimental nociceptive stimulation. Mechanical detection (MDT) and pain thresholds (MPT) were assessed in the painful area and the non-painful contralateral side in 10 patients with uni- lateral musculoskeletal pain. Additionally, 10 healthy participants were exposed to nociceptive stimulation applied to the volar fore- arms (capsaicin; electrical stimulation, twice each). Areas of tactile hypaesthesia and mechanical hyperalgesia were assessed. MDT and MPT were quantified adjacent to the stimulation site. Tactile hypaesthesia in pain patients and in experimental pain (MDT-z- scores: 0.66 ± 0.30 and 0.42 ± 0.15, respectively, both p < 0.01) was paralleled by mechanical hyperalgesia (MPT-z-scores: +0.51 ± 0.27, p < 0.05; and +0.48 ± 0.10, p < 0.001). However, hypaesthesia and hyperalgesia were not correlated. Although 9 patients reported numbness, only 3 of them were able to delineate circumscript areas of tactile hypaesthesia. In experimental pain, the area of tactile hypaesthesia could be mapped in 31/40 experiments (78%). Irrespective of the mode of nociceptive stimulation (phasic vs. tonic) tactile hypaesthesia and hyperalgesia developed with a similar time course and disappeared within approximately 1 day. Hypaesthesia (numbness) often encountered in clinical pain can be reproduced by experimental nociceptive stimulation. The time course of effects suggests a mechanism involving central plasticity. Ó 2008 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved. Keywords: Tactile hypaesthesia; Experimental pain; Neuropathic pain; Myofascial pain; Central plasticity; Mechanical hyperalgesia 1. Introduction Patients with chronic pain often describe a subtle numbness, i.e. reduction of tactile function in painful body regions [24,27,33]. This has been described for dif- ferent chronic pain conditions and was even more prom- inent in referred pain, e.g. in low back pain [11,27]. In clinical practice this phenomenon is important because the coincidence of pain together with patients’ reports of numbness in the corresponding skin area mimics fea- tures of neuropathic pain [51] and therefore might be interpreted as an indication of nerve or nerve root lesion. Furthermore, experimental pain studies in healthy subjects also described inhibition of tactile pro- cessing by nociceptive stimuli [1,24,29,30]. The present study addresses the origin of pain-associ- ated numbness. For this reason the inhibition of tactile 0304-3959/$34.00 Ó 2008 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.pain.2008.03.006 * Corresponding author. Address: Klinik und Poliklinik fu ¨r Neuro- logie, Johannes-Gutenberg Universita ¨t, Langenbeckstrasse 1, 55131 Mainz, Germany. Tel.: +49 6131 17 4588; fax: +49 6131 17 3271. E-mail address: geber@uni-mainz.de (C. Geber). www.elsevier.com/locate/pain Pain 139 (2008) 73–81