Tactile thresholds are preserved yet complex sensory function is impaired over the lumbar spine of chronic non-specific low back pain patients: a preliminary investigation
Introduction
Chronic non-specific low back pain (CNSLBP) is a common and costly healthcare problem for which there are few effective interventions [1]. Recent evidence indicates significant structural and biochemical changes within the brains of patients with CNSLBP [2], as well as evidence of alterations in the representation of the back in the primary sensory cortex (S1) [3], [4]. Sensory cortical representation is a plastic phenomenon that is dependent on the response profiles of neurons in S1. It is considered important in representing the consciously felt body, and thus alterations in this representation may have consequences for the conscious body image [5]. One perspective that is gaining acceptance in other complex pain problems is that disruption of the cortical representation of the painful body part and the resultant body perception disturbance might contribute to the clinical condition [5], [6]. Moreover, treatment approaches aimed at normalising cortical representation and body perception seem to be effective in the management of other complex pain problems, such as phantom limb pain and complex regional pain syndrome type I [7], [8], [9], [10].
In light of these brain changes seen in the back pain population, the authors were interested to explore whether patients with CNSLBP demonstrate evidence of altered perception of their back. One approach to investigating body perception is via ‘cortical’ sensory tests, such as two-point discrimination, which are dependent in part on the integrity of the cortical representation of that body area [5]. Recent studies have explored whether patients with CNSLBP exhibit evidence of altered perception of their back. Moseley [11] demonstrated deficits in two-point discrimination over the low back area along with marked alterations in body image in a small group of CNSLBP sufferers, and more recently these deficits in tactile acuity have been found to be related to lumbo-pelvic motor control impairments in a similar patient population [12]. Importantly, simple tactile threshold was unaffected in these studies, indicating that deficits in tactile acuity may not be due to any gain or loss in the peripheral transduction and transmission of sensory information but may have its origins in central processing.
The ability of the brain to manipulate the representation of the body is critical for normal function and perception. It is currently unknown whether this ability is compromised in patients with CNSLBP. A cortical sensory task that may offer a way to investigate this ability is graphaesthesia, or recognition of symbols drawn on the skin. It is a task that requires not only good tactile acuity but greater cortical manipulation of the sensory stimulus to construct an image of which letter has been drawn [13]. There are currently no data on graphaesthesia performance in CNSLBP patients, or evidence of whether deficits in cortical sensory function extend beyond problems with tactile acuity. The authors were interested in establishing whether patients demonstrate a deficit in graphaesthesia and the relationships between graphaesthesia performance, tactile acuity and simple tactile thresholds.
The specific research questions investigated in this study were as follows:
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Do CNSLBP patients demonstrate a deficit in graphaesthesia ability over the lower back?
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Does graphaesthesia performance relate to other sensory measures, specifically lumbar tactile acuity and simple tactile threshold?
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Is graphaesthesia performance related to the severity of the clinical condition?
It was hypothesised that CNSLBP patients would have a normal tactile threshold, but would demonstrate deficits in graphaesthesia and two-point discrimination. Furthermore, it was predicted that graphaesthesia performance would be related to tactile acuity and the severity of the clinical condition.
Section snippets
Participants
A convenience sample of 19 volunteers with CNSLBP was recruited from the neurosurgical waiting list of a district general hospital in Perth, Western Australia and from a private physiotherapy clinic. Subjects were screened by a physiotherapist and included in the study if they were aged between 20 and 55 years, had experienced non-specific low back pain for more than 6 months, were proficient in written and spoken English, and were able to provide written informed consent. Participants were
Sample characteristics
Table 1a, Table 1b, Table 1c provide a summary of the characteristics of the study sample.
Methodological checks
The two groups did not differ with respect to age or gender. There was no significant correlation between age or gender and any of the sensory tests. There were no significant within-participant side-to-side differences for any sensory test in either the control subjects or in the bilaterally distributed low back pain patients (data not shown). As a result, in these participants, the mean of left and right
Discussion
The two-point discrimination detection threshold was larger and the graphaesthesia error rate was greater over the lumbar spine in patients with CNSLBP compared with a control group of similar age and gender. However, simple tactile thresholds were not significantly different between groups. These results confirm previous findings that patients with CNSLBP demonstrate specific deficits in sensory function over the lumbar spine. Moseley [11] and Luomajoki and Moseley [12] demonstrated impairment
Acknowledgements
The authors would like to thank the staff of the pain and neurosurgical clinics at The Sir Charles Gairdner Hospital, the staff of Physcare Fremantle WA, and David Maskill and Dr. Lorimer Moseley for their review of the manuscript.
Ethical approval: Human Research Ethics Committee of the University of Notre Dame Australia and the Ethics Review Board of The Sir Charles Gairdner Hospital.
Conflict of interest: None declared.
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