Carpal tunnel syndrome (CTS) is the most common entrapment neuropathy and is caused by compression of the median nerve at the wrist. The classical neurological textbook will tell you that the symptoms caused by carpal tunnel syndrome are localised to the typical median nerve innervation territory in the hand, which involves the first three and half of the forth finger. Fact is however, that up to 70% of patients with CTS have symptoms outside the typical median nerve territory (Caliandro, La Torre et al. 2006). How do these extraterritorial symptoms fit with the common belief that CTS is driven by peripheral mechanisms?
There is a growing body of research demonstrating that patients with CTS have a generalised disturbance of somatosensory function (de la Llave-Rincon, Fernandez-de-Las-Penas et al. 2009; Zanette, Cacciatori et al. 2010). This theory is based on the finding that patients with CTS experience pain earlier than healthy people when thermal or mechanical stimuli are applied in areas that are not related to the median nerve (e.g. over the arm and neck). A generalised disturbance of somatosensory function in CTS would indeed explain the presence of widespread symptoms. Unfortunately though, most studies so far have not considered an important point. Many patients with CTS namely also have coexisting neck and/or arm pain. It therefore remains unclear whether the identified altered pain thresholds are truly caused by CTS, or are just a result of the coexisting neck or arm disorders.
To address this issue, we investigated whether the presence of abnormal extraterritorial pain thresholds can be substantiated in patients with CTS if coexisting neck and arm disorders are strictly excluded (Schmid, Soon et al. 2012). We could not replicate previous findings of abnormal pain thresholds in our patients who had symptoms localised to their affected hand. This suggests that CTS alone does not account for sensory changes outside the median nerve territory upon traditional quantitative sensory testing. Interestingly though, our patients rated the pain elicited during pain threshold testing significantly higher than the healthy participants. This was not only apparent in the median nerve area of the affected hand, but also over the neck and the lower leg. Even though traditional quantitative sensory testing did not reveal any differences in pain thresholds, the elevated pain ratings in patients with CTS may still be an early indication of a generalised somatosensory disturbance. Our data further confirms that it is about time to start thinking outside the wrist in patients with CTS!
About Annina Schmid
Annina Schmid has recently completed a PhD in Neuroscience at The University of Queensland in Brisbane, Australia and works currently as a postdoctoral fellow at the same Institution. Her thesis is entitled “Implications of mild peripheral nerve compression beyond the lesion site. Mechanisms and Interventions”. The physiology of neuropathic pain is her main fascination and she endeavours to further unravel the mechanisms underlying peripheral compression neuropathies by using the animal model that she developed during her PhD as well as by translating findings directly to patients with compression neuropathies.
References
Caliandro P, La Torre G, Aprile I, Pazzaglia C, Commodari I, Tonali P, & Padua L (2006). Distribution of paresthesias in Carpal Tunnel Syndrome reflects the degree of nerve damage at wrist. Clinical neurophysiology, 117 (1), 228-31 PMID: 16325467
Llave-Rincón, A., Fernández-de-las-Peñas, C., Fernández-Carnero, J., Padua, L., Arendt-Nielsen, L., & Pareja, J. (2009). Bilateral hand/wrist heat and cold hyperalgesia, but not hypoesthesia, in unilateral carpal tunnel syndrome Experimental Brain Research, 198 (4), 455-463 DOI: 10.1007/s00221-009-1941-z
Schmid, A., Soon, B., Wasner, G., & Coppieters, M. (2012). Can widespread hypersensitivity in carpal tunnel syndrome be substantiated if neck and arm pain are absent? European Journal of Pain, 16 (2), 217-228 DOI: 10.1016/j.ejpain.2011.06.003
Zanette, G., Cacciatori, C., & Tamburin, S. (2010). Central sensitization in carpal tunnel syndrome with extraterritorial spread of sensory symptoms PAIN, 148 (2), 227-236 DOI: 10.1016/j.pain.2009.10.025