Neurophysiological Assessment of Spinal Excitability in Chronic Low Back Pain

Overview

Chronic low back pain (CLBP) is established by the persistence of low back pain beyond 3 months of symptom initiation . The overwhelming element of treatment is physical exercise . Other methods of treatment like cognitive therapy, behavioural therapy and multidisciplinary rehabilitation can also lead to significant improvements 1. Prevalenc of CLBP increases linearly from the third decade of life on, until the 60 years of age, being more prevalent in women.2 CLBP is a common condition affecting many individuals at some point in their lives.3 The estimation is that between 5.0% and 10.0% of cases will develop CLBP, which is responsible for high treatment costs, sick leave, and individual suffering, in addition to being one of the main reasons for people to seek health care services.4 CLBP and related disorders represent a wide spectrum of syndromes that are associated with changes in the pain processing pathways of the central nervous system .5 Those syndromes affect many systems in the body, and the associated plasticity changes in the CNS can lead to augmentation of pain transmission and processing circuits. 6,7 The pain in patients with CLBP occurs as a result of a process called central sensitization (CS), which refers to increased excitability of the neurons in the dorsal horn of the spinal cord. This increased excitability is associated with increased spontaneous neuronal activity, expanded receptive fields, and enhanced responses to the impulses transmitted by both large and small-fiber sensory afferents.8 The Hoffman reflex , F-wave and Somatosensory evoked potential (SSEP) are often used to measure spinal excitability in various physiological and pathological states.9-10 The H-reflex is elicited by stimulation of type Ia afferent sensory fibers. These fibers synapse directly onto the alpha motor neurons in the anterior horn of the spinal cord, forming a monosynaptic reflex arc. Submaximal stimulation of this reflex arc elicits a compound muscle action potential (CMAP) known as the H wave. This reflex arc appears to depend on the balance between excitatory and inhibitory neurons in the spinal cord . F wave is a late response that follows the motor response (M) and is elicited by supramaximal electrical stimulation of a mixed or a motor nerve.11 The study of the F waves is particularly useful for the diagnosis of proximal nerve lesions that would be otherwise inaccessible to other routine NCSs.12 SSEPs are also important complementary diagnostic methods in the electrophysiologic evaluation of CLBP. SSEPs are a very sensitive measure of the functional integrity of the neuroaxis, including peripheral and central structures. When used in diagnostic mode they can provide additional information regarding the probable areas of dysfunction. Since a change in spinal excitability is one of the main mechanisms underlying the hypothesis of CS in CLBP, performing these tests might be an easy, widely available, cheap, and objective method for assessing spinal excitability in patients with CLBP.