Does Spinal Manipulation Affect Central Nervous System Pain Mechanisms

Part of the Evidence in Action series by Palmer College of Chiropractic

By Zacariah Shannon, DC, MS, Robert Vining, DC, and Stephen Onifer, PhD

Clinical guidelines for adults with acute, subacute, and chronic low back pain support conservative management with spinal manipulative therapy (SMT).¹ Research shows SMT is comparable to other noninvasive treatments in reducing low back pain and disability with relatively low risk for adverse events.² However, the mechanisms leading to pain relief from SMT are poorly understood. Identifying therapeutic mechanisms of SMT can inform treatment strategies and lead to more effective care.

Central sensitization is a physiological process underlying many chronic pain conditions, including low back pain.^3-5 Central sensitization results in amplified neural signaling within the central nervous system causing pain hypersensitivity.³ The progression of acute pain to chronic pain, and the maintenance of chronic pain, are thought to be due, in part, to central sensitization.⁶

Prolonged pain signaling initiates neuroplastic changes in the spinal cord, brainstem, thalamus, limbic system, and cerebral cortex.^3,7 These neuroplastic changes can eventually result in pain signal self-propagation, which might be observed clinically as pain perceived more strongly than it otherwise would be and/or in an area without injury. Because of its clinical significance, a variety of sensory testing methods have been developed to indirectly assess observable signs of central sensitization (Table 1).^4,8

Table 1. Common signs of central sensitization

Signs	Definition	Clinical Indication
Allodynia	Reduced pain threshold resulting in pain from a non-painful, innocuous stimulus such as light touch	Brush with cotton swab feels painful
Hyperalgesia	Increased pain response and prolonged pain after noxious stimulus	Pin prick is more painful in affected area
Secondary hyperalgesia	Expansion of perceived pain area	Pain from pin prick spreads from original location to a larger area

Temporal summation of pain is an indirect clinical and experimental measure of central sensitization.^8,9 Temporal summation is a process by which pain is perceived as more intense over time, despite no change in a repeated noxious stimulus. Thermal, electrical, or mechanical stimuli are used to induce and measure the temporal summation phenomenon in humans. Because there is evidence for enhanced temporal summation in chronic low back pain patients,¹⁰ an important question for doctors of chiropractic is: Does SMT affect temporal summation?

Boal & Gillette hypothesized that SMT relieves chronic low back pain by its afferent, peripheral nervous system input inhibiting or altering neuroplastic changes occurring in spinal cord dorsal horn neurons involved in transmitting or amplifying pain signals.¹¹ Since temporal summation occurs in the spinal cord dorsal horn,¹² investigators at the University of Florida and the Université du Québec à Trois-Rivières addressed this hypothesis by performing five novel studies that evaluated the immediate effect of high-velocity, low-amplitude (HVLA) SMT on temporal summation (Table 2).

Table 2. HVLA SMT and temporal summation studies

Reference	Subjects	Intervention(s)	Stimulus	Test Location	Measure	Results
George et al., 2006⁹	Asymptomatic	1) Lumbar SMT^a 2) Stationary bicycle ride 3) Lumbar extension exercise	Heat pulses	Forearm Hand Leg Foot	NRS^b	SMT and exercise immediately reduced temporal summation >^c stationary bicycle at lumbar innervated sites. No effect at cervical innervated sites.
Bishop et al., 2011¹³	Asymptomatic	1) Upper thoracic SMT 2) Cervical exercise 3) Rest	Heat pulses	Hand Foot	NRS	Only SMT immediately reduced temporal summation at both cervical and lumbar innervated sites.
Bialosky et al., 2009¹⁴	Chronic LBP^d	1) Lumbar SMT 2) Stationary bicycle ride 3) Lumbar extension exercise	Heatpulses	Hand Foot	NRS	Only SMT immediately reduced temporal summation at lumbar innervated site. All interventions immediately reduced temporal summation at the cervical innervated site.
Bialosky et al., 2014¹⁵	Chronic LBP	1) Lumbar SMT 2) Placebo SMT 3) Enhanced placebo SMT 4) Rest	Heat pulses	Foot	NRS	Only SMT immediately reduced temporal summation at lumbar innervated site.
Randoll et al., 2017¹⁶	Asymptomatic	1) Upper thoracic SMT 2) Upper thoracic light mechanical stimulation 3) Control	Electrical pulses	Upper thoracic paraspinals	NRS	Only SMT immediately reduced temporal summation but hypoalgesia did not persist 5 minutes later. Tactile sensation evoked by non-painful electrical stimulation was not affected by any intervention.

SMT^a = spinal manipulative therapy thrusts; NRS^b = Numeric Rating Scale; >^c = greater than; LBP^d = low back pain.

Collectively, these studies show that SMT conveys a short-term hypoalgesic effect on temporal summation in patients with chronic low back pain. Because temporal summation is considered an indirect measure of centralization, SMT may also affect central sensitization. Whether SMT can help reverse or reduce neuroplastic changes associated with temporal summation and central sensitization over the long term is unknown. However, even a temporary effect has an important implication.

Evidence-based practice incorporates research evidence, clinical experience and patient preference into management decisions. SMT’s effect on temporal summation, and potentially central sensitization, provides a mechanism by which SMT helps reduce pain. Understanding that SMT influences the temporal summation phenomenon offers a physiological explanation and a limited, though potentially important, rationale for treatment. This knowledge can also be used to help patients better understand treatment response and be leveraged by both the provider and patient to inform treatment decisions.

Dr. Shannon is a clinical research fellow at the Palmer Center for Chiropractic Research, Palmer College of Chiropractic, Davenport, Iowa.
Dr. Vining is associate professor and senior research clinician at the Palmer Center for Chiropractic Research, Palmer College of Chiropractic, Davenport, Iowa.
Dr. Onifer is an associate professor at the Palmer Center for Chiropractic Research, Palmer College of Chiropractic, Davenport, Iowa.

For an ACA member-exclusive video of Dr. Shannon discussing this article, click here.

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