Refined sensory measures of neural repair in human spinal cord injury: bridging preclinical findings to clinical value

Afferent input and sensory function after human spinal cord injury

Spinal cord injury (SCI) often disrupts the integrity of afferent (sensory) axons projecting through the spinal cord dorsal columns to the brain. Examinations of ascending sensory tracts, therefore, are critical for monitoring the extent of SCI and recovery processes. In this review, we discuss the most common electrophysiological techniques used to assess transmission of afferent inputs to the primary motor cortex (i.e., afferent input-induced facilitation and inhibition) and the somatosensory cortex [i.e., somatosensory evoked potentials (SSEPs), dermatomal SSEPs, and electrical perceptual thresholds] following human SCI. We discuss how afferent input modulates corticospinal excitability by involving cortical and spinal mechanisms depending on the timing of the effects, which need to be considered separately for upper and lower limb muscles. We argue that the time of arrival of afferent input onto the sensory and motor cortex is critical to consider in plasticity-induced protocols in humans with SCI. We also discuss how current sensory exams have been used to detect differences between control and SCI participants but might be less optimal to characterize the level and severity of injury. There is a need to conduct some of these electrophysiological examinations during functionally relevant behaviors to understand the contribution of impaired afferent inputs to the control, or lack of control, of movement. Thus the effects of transmission of afferent inputs to the brain need to be considered on multiple functions following human SCI.

Usefulness of laser-evoked potentials and quantitative sensory testing in the diagnosis of neuropathic spinal cord injury pain: a multiple case study

STUDY DESIGN

A retrospective study.

OBJECTIVES

The aim of this study was to investigate the contribution of laser-evoked potentials (LEPs) and quantitative sensory testing (QST) to the diagnosis of neuropathic pain in patients with spinal cord injury (SCI) and inconclusive magnetic resonance imaging (MRI) findings.

SETTING

A multidisciplinary pain center.

METHODS

QST (DFNS protocol) and Tm-YAG-laser stimulation of the skin were applied within the pain site corresponding with dermatomes of altered sensation. Available MRI scans were reviewed.

RESULTS

Thirteen individuals (50±16 years) with SCI were examined. In four cases with no detectable neural lesion on MRI, all QST but three LEP were abnormal. In four patients with poorly defined spinal lesion on MRI, all QST but three LEP only were abnormal. In four cases where pain was not matching adequately with MRI lesions, all patients had abnormal LEP and QST. In one patient showing a spinal cord atrophy, LEP was normal but QST was abnormal. Findings supported the diagnoses at-level (n=5) and below-level (n=8) SCI pain. Spinothalamic tract function assessed by LEP was normal in three cases, but QST was abnormal in all cases.

CONCLUSIONS

As QST is a psychophysical examination depending on patient cooperation, we suggest that the combination of QST and LEP might be a valuable diagnostic tool to detect lesions of the somatosensory system in a subgroup of patients with neuropathic spinal cord injury pain and inconclusive MRI findings.

Assessment of Spinothalamic Tract Function Beyond Pinprick in Spinal Cord Lesions

Background. Although a mainstay of clinical sensory examination after damage in the spinal cord, pinprick sensation represents only one afferent modality conveyed in the spinothalamic tract. As an objective outcome, complementary information regarding spinothalamic tract conduction may be elucidated by measuring contact heat evoked potentials (CHEPs). Objective. To assess the value of CHEPs to measure spinothalamic tract function in spinal cord disorders compared with pinprick scoring. Methods. CHEPs were examined using a standard (35°C) and increased baseline (42°C) contact heat temperature. Pinprick sensation was rated as absent, impaired, or normal according to the International Standards for the Neurological Classification of Spinal Cord Injury. Results. Fifty-nine dermatomes above, at, and below the sensory level of impairment were analyzed in 37 patients with defined spinal cord disorder. In dermatomes with absent or impaired pinprick sensation, CHEPs using a standard baseline temperature were mainly abolished (3/16 and 8/35, respectively). However, when applying an increased baseline temperature, CHEPs became recordable (absent: 11/16; impaired: 31/35). Furthermore, CHEPs with increased baseline temperature allowed discerning between dermatomes with absent, impaired, and normal pinprick sensation when using an objective measure (ie, N2P2 amplitude). In contrast, the pain perception to contact heat stimulation was independent of pinprick scores. Conclusion. Applying pinprick testing is of limited sensitivity to assess spinothalamic tract function in spinal cord disorders. The application of CHEPs (using standard and increased baseline temperatures) as an objective readout provides complementary information of spinothalamic tract functional integrity beyond pinprick testing.