PAPER

Motor neurone excitability in back muscles assessed using mechanically evoked reflexes in spinal cord injured patients

A Kuppuswamy^1,3, S Theodorou^1,3, M Catley^1,3, P H Strutton², P H Ellaway^1,3, A H McGregor² and N J Davey^1,3

¹ Division of Neuroscience and Psychological Medicine, Imperial College Faculty of Medicine, Charing Cross Campus, London W6 8RP, UK
² Department of Orthopaedic and Trauma Surgery, Imperial College Faculty of Medicine, Charing Cross Campus, London W6 8RP, UK
³ The National Spinal Injuries Centre, Stoke Mandeville Hospital, Aylesbury, Bucks HP21 8AL, UK

Correspondence to:
Correspondence to:
Dr N J Davey
Department of Movement and Balance, Division of Neuroscience and Psychological Medicine, Imperial College Faculty of Medicine, Charing Cross Campus, St Dunstan’s Road, London W6 8RP, UK; n.davey{at}imperial.ac.uk

Objective: The clinical and functional assessment of back muscles in human spinal cord injury (SCI) has received little attention. The aim of this study was to develop a method to assess the level of a thoracic spinal cord lesion based on the reflex activation of back muscles.

Methods: In 11 control subjects and in 12 subjects with clinically complete thoracic SCI (T2–T12), either a spinous process or an erector spinae muscle was prodded to elicit short latency reflexes recorded electromyographically at the spinal level of stimulation. An electromagnetic servo, attached to a blunt probe, applied stimuli at a frequency of 1 Hz and amplitude of 3 mm. Two trials of 50 mechanical prods were conducted at each site.

Results: Reflexes were evoked in control subjects in 82% of trials when the spinous process was prodded, and in 80% of trials when the muscle was prodded. In contrast, reflexes in SCI subjects could be elicited in 90–100% of trials two segments either above or below the lesion. Reflex responses in control subjects had a mean (SEM) latency of 5.72 (0.53) ms when the spinous process was prodded, and 5.42 (0.42) ms when the muscle was prodded. In the SCI subjects, responses had slightly (but insignificantly) longer latencies both above and below the lesion to either stimulus. The amplitude of reflex responses, expressed as a percentage of the background EMG, was on average 2–3 times larger at the three vertebral levels spanning the lesion in SCI subjects than at sites above or below the lesion or at any level in control subjects.

Conclusion: We propose that the size of these mechanically evoked reflexes may be useful in determining the level of thoracic SCI. Furthermore, the reflexes might provide a valuable tool with which to monitor recovery after an intervention to repair or improve function of a damaged spinal cord.

Abbreviations: ASIA, American Spinal Injury Association; ES, erector spinae; SCI, spinal cord injury

Keywords: stretch reflex; electromyography; interspinous ligaments; mechanoreceptors

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