Elsevier

Physiotherapy

Volume 96, Issue 4, December 2010, Pages 317-323
Physiotherapy

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

https://doi.org/10.1016/j.physio.2010.02.005Get rights and content

Abstract

Objectives

To investigate impairments in sensory function in chronic non-specific low back pain patients, and the relationship between any impairment and the clinical features of the condition.

Design

A cross-sectional case–control study.

Setting

Laboratory-based study.

Participants

Nineteen chronic non-specific low back pain patients and 19 healthy controls.

Main outcome measures

Tactile threshold, two-point discrimination distance and accuracy at a task involving recognising letters drawn over the skin of the lower back (graphaesthesia) were assessed over the lumbar spine in both groups. Pain duration, pain intensity, physical function, anxiety and depression were assessed by questionnaire in the back pain group.

Results

No difference was found in tactile threshold between the two groups [median difference 0.0 mg, 95% confidence interval (CI) −0.04 to 0.04]. There was a significant difference between controls and back pain patients for two-point discrimination (mean difference 17.9 mm, 95% CI 5.9 to 29.8) and graphaesthesia accuracy (mean difference 6.1, 95% CI 1.3 to 11.0). Low back pain patients had a larger lumbar two-point discrimination distance threshold and a greater letter recognition error rate. In the back pain group, no relationship was found between clinical profile and sensory function, and no relationship was found between the sensory tests.

Conclusions

These data support existing findings of perceptual abnormalities in chronic non-specific low back pain patients, and are suggestive of cortical rather than peripheral sensory dysfunction. Amelioration of these abnormalities may present a target for therapeutic intervention.

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:

  • Do CNSLBP patients demonstrate a deficit in graphaesthesia ability over the lower back?

  • Does graphaesthesia performance relate to other sensory measures, specifically lumbar tactile acuity and simple tactile threshold?

  • 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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