Objective Tools For Measuring Central Sensitization And Spinal Cord Reflexes

Nociceptive flexion reflex (NFR) and its role in measuring pain experience in central sensitization and chronic pain

The central sensitization is a state of the nervous system, which is involved in the development and maintenance of chronic pain. This situation activates the noxious and non-noxious stimuli to make the central nervous system hyperactive. This activation of the central nervous system induces the feeling of pain in individuals. The Nociceptive Flexion Reflex (NFR) is a nerve reflex, which is released from the spinal cord to protect the body from the damaging stimuli (Ang et al., 2011). This is the withdrawal response of the body against any pain stimulus and the reflex action is measurable in clinical settings. Measuring NFR is an objective and reliable tool to measure the pain experience in central sensitization and chronic pain in individuals. Desmeules et al. (2003) conducted a research to identify the neurophysiological evidences that lead to central sensitization in patients with Fibromyalgia (FM). Researchers conducted an experiment with total 164 outpatients were included using randomized controlled trial method, in which, 85 patients were suffering from Fibromyalgia and 40 were non-FM patients. The control group did not have any history of chronic pain. The physiotherapist selected 18 tender points in the patient’s body and a digital force of 4 kg was forced on that tender point to assess the myalgia score. The evaluating physician’s global impression (PGI) of the patient’s general status was recorded on a 5-point scale in which one is best or less pain felt (Desmeules et al., 2003). The tools used in this research were aimed to identify pain (nonspontaneous), assessment of the pain (using 2-fold subjective scale) and to access the nociceptive flexion reflex or NFR to measure the reflex due to chronic pain. The results indicated that due to increased level of pain, the group of patients with FM had decreased level of tolerance to pain and the NFR level in those patients also decreased that determines the fact that decrease in reflex is associated with increase in pain intensity. To measure the NFR, a cutaneous electrode was applied to measure the muscular reflex. The nerve, which was chosen for stimulation, was the sural nerve. The NFR reflex test took very less time to perform however; several scales were used to statistically analyze the measure of pain. The test needed very basic instruments used in physiotherapy. Further, a trained or experienced physiotherapist can only perform the test as the nerves or points, which were used to stimulate and get reflex, are known to experienced physiotherapists. Another research done by Bezow et al. (2010) determined that continuous nociceptive input from peripheral myofacial structure induces central sensitization and using NFR to determine the pain threshold in the patients of tension-type headache helped the group of researchers to measure the threshold of pain.

Research by Desmeules et al. on identifying the neurophysiological evidences for central sensitization in patients with Fibromyalgia (FM)

The Hoffman Reflex or H-reflex is the reflective reaction of the muscle after some electrical stimulation of sensory fibers. The H-reflex test is done using electrical stimulator that provides results using a square wave current having short duration and long amplitude. Further, an electromyography machine or EMG is required to collect the data and record the response of muscles (Abeles et al., 2007). This method is used to assess the changes in the excitation level of the motor neuron pool. According to the research of Kumru et al. (2015), the researchers were to identify the impact of spinal cord lesion level and severity of the injury on the level of H-reflex excitability and recovery curve. Changes in the spinal cord reflexes can be originated from different issues including changes in the descending pathway, and to determine the reason, they chose 38 SCI or spinal cord injury patients and 18 normal individuals as control. They aimed to identify the H-max responses and the recovery curve at interstimulus intervals and from 50-1000 ms were recorded in the process. Subjects were seated in a room with temperature between 22-25 degree Celsius As clinical assessment showed that nearly all of the SCI persons had equal irregularity in both lower limbs, the recordings and data examination were made on the right side only. For electrophysiological readings, researchers used reusable bar recording electrodes (Medelec) placed on the soleus muscle. They performed the entire research with a Medelec Synergy electromyograph (Oxford Instruments, Surrey, UK). The assessment of the SCI injury was done with the American Spinal Cord Injury Association (ASIA) Impairment Scale (Kumru et al., 2015). The results of this experiment demonstrated that amplitude of the H-max correlates with the level of injury and severity of the lesion and therefore it was determined that level of severity is the reason behind the changes in the spinal cord reflex. The test utilizes very common instruments of a physiotherapists and the electrical stimulator is the only instrument to measure the electrical impulse. However, the nerves or areas, of which the impulse need to be measured, are known to expert physiotherapists and therefore, this measurement tool can be used by experienced physiotherapists for authentic measurement.

The cutaneous silent period is known as the small interruption in the voluntary contraction of nerves following strong electrical stimulation of the cutaneous nerve. This is a protective reflex of the nervous system, which is mediated by the spinal inhibitory circuits and this is reinforced due to the parallel modulation of the motor cortex. In the study done by Kilinc et al. (2017), it was seen that increased responses to pain stimuli without any spontaneous pain determined the role of hyper-excitability of different pain pathways. To determine that, the researchers used cutaneous silent period parameter to demonstrate the level of spinal pain pathway. For the research, the team of researchers chose 29 patients with Myofacial pain syndrome or MPS and 30 another healthy patients were included as controls in the study. The electrophysiological investigation was carried out in laboratory and after recording the stimulus, the recording electrodes were used to perform the CSP. They conducted three studies for the assessment, such as electrophysiological study, nerve conduction study and CSP (Cutaneous silent period) investigation. The electrophysiological study was performed with superficial recording and stimulus electrode and pain threshold was recorded. The nerve conduction study was done in nerves such as right median, ulnar, radial, sural, and superficial peroneal, and the left medial plantar sensory nerves. Amplitudes, latencies, and further nerve conduction velocity parameters were recorded for all these sensory and motor nerves. Same electrode was used for CSP studies and pain threshold was measured for every individual (Kilinc et al., 2017).  Electromyography was used to statistically analyze the obtained data and it was found that injuries in the spinal cord are responsible for the break or gap in stimulus or reflex flow in human body. Another research done with the patients of fibromyalgia, the reflex mediated by the pain was interrupted due to the presence of spinal cord injury and the injury was in the super spinal level (Baek et al., 2016). However, the experiment lacks technical facility, as EMG audio feedback machine was not present in the setup. Further, the experiment was performed without any clinical objective method and therefore the clinical set up for the experiment need time and cost to be successfully implemented. Further, the electrical impulse was collected from different critical nerves and an experienced physiotherapist knows the points from which the impulses were collected. Therefore, this test can only be performed by experienced physiotherapist or any practice assistance, who has worked with any physiotherapist in the system. General physicians, practiced or registered nurse cannot perform the test on their own.

References

Abeles, A. M., Pillinger, M. H., Solitar, B. M., & Abeles, M. (2007). Narrative Review: The Pathophysiology of FibromyalgiaPathophysiology of Fibromyalgia. Annals of internal medicine, 146(10), 726-734.

Ang, D. C., Chakr, R., France, C. R., Mazzuca, S. A., Stump, T. E., Hilligoss, J., & Lengerich, A. (2011). Association of nociceptive responsivity with clinical pain and the moderating effect of depression. The Journal of Pain, 12(3), 384-389.

Baek, S. H., Seok, H. Y., Koo, Y. S., & Kim, B. J. (2016). Lengthened cutaneous silent period in fibromyalgia suggesting central sensitization as a pathogenesis. PloS one, 11(2), e0149248.

Bezov, D., Ashina, S., Jensen, R., & Bendtsen, L. (2011). Pain perception studies in tension?type headache. Headache: The Journal of Head and Face Pain, 51(2), 262-271.

Desmeules, J. A., Cedraschi, C., Rapiti, E., Baumgartner, E., Finckh, A., Cohen, P., … & Vischer, T. L. (2003). Neurophysiologic evidence for a central sensitization in patients with fibromyalgia. Arthritis & Rheumatology, 48(5), 1420-1429.

Kilinc, O., Sencan, S., Ercalik, T., Koytak, P. K., Alibas, H., Gunduz, O. H., … & Uluc, K. (2018). Cutaneous silent period in myofascial pain syndrome. Muscle & nerve, 57(1).

Kumru, H., Albu, S., Valls?Sole, J., Murillo, N., Tormos, J. M., & Vidal, J. (2015). Influence of spinal cord lesion level and severity on H?reflex excitability and recovery curve. Muscle & nerve, 52(4), 616-622.