Somatosensory Phenotype of Patients Suffering From Unspecific Neck and Arm Pain

Neck pain is a common musculoskeletal problem, with up to 30-50 % of adults experiencing neck pain in any given year. Neck pain in combination with arm pain is more common than neck pain alone and is associated with higher Ievels of self-reported disability and psychological impairment. In most cases, no pathophysiological correlate can be found in various investigations including magnetic resonance imaging of the cervical spine and tests for nerve conductivity (bedside neurological examination and standard electro diagnostic tests). In such cases, the neck-arm pain is labelled as non-specific. Although clinical examinations that evaluate the conductivity of the nervous system are normal in non-specific neck-arm pain (NSNAP), neurodynamic tests (ND) have been identified as positive in a subgroup of patients. ND are clinical tools designed to elongate the nerve bed, therefore increasing the mechanical loading of neural tissues. As such, ND provide information about alterations of neural tissue mechanosensitivity, i.e. the presence of heightened nerve mechanosensitivity. It is assumed that increased neural mechanosensitivity reflects gain of function, presumably mediated by small nociceptive fibres intraneurally or within the epineurium (nervi nervorum). However, it remains unclear whether the subgroup of patients with NSNAP and positive ND have indeed increased gain of function of this small fibre population compared to those patients with normal ND.

Quantitative sensory testing (QST) is a commonly used tool to determine the function of different subsets of small (C, Ad-fibres) and large nerve fibres (Ad-fibres). Using QST, a recent study has identified a loss of small fibre function in patients with unilateral NSNAP and positive ND as apparent by reduced warm detection thresholds compared to the non-affected side. However, this difference was small and potentially not clinically rele-vant and the relatively small sample size of n=8 does not allow generalization of the results. In the same study, patients with NSNAP were also characterized by a gain of function, i.e. cold hypersensitivity. Similarly, widespread thermal and pressure hypersensitivity have been documented in patients with nonspecific arm pain (with or without neck pain). None of these studies has however investigated a potential difference between patients with NSNAP with positive versus negative ND, leaving it unclear whether the QST findings can indeed be attributed to a positive ND.

A recent study compared patients with NSNAP and neuropathic features using ND testing and a self report neuropathic pain screening tool and patients with NSNAP and no neuropathic features. Interestingly, the first group showed heightened nerve mechanosensitivity apparent by an earlier onset of symptoms during ND testing. This raises the question whether patients with NSNAP and positive ND have indeed a dysfunction of their nervous system that differentiates them from patients with normal ND.