A coordinate-based meta-analysis of acupuncture for chronic pain: Evidence from fMRI studies

Chronic pain is one of the most common and persistent health problems affecting the quality of life of millions of people worldwide. Unlike acute pain, which functions as the body's warning signal, chronic pain persists for prolonged periods and can alter the very functioning of the brain. These brain changes progressively make natural pain control more difficult, creating a cycle that perpetuates suffering. Over the last decades, acupuncture has gained recognition as an effective complementary therapy for various types of chronic pain, and has even been endorsed by the National Institutes of Health in the United States since 1998.

Neuroimaging studies have demonstrated that acupuncture can modify the activity of different brain networks related to pain processing, but the heterogeneity of results between different studies made it difficult to systematically understand how exactly acupuncture acts on the brain.

This study represents the first coordinate-based meta-analysis of brain regions to investigate the neurological mechanisms of acupuncture in the treatment of chronic pain. The researchers used a methodology called ALE (Activation Likelihood Estimation), which is a robust statistical technique for combining results from multiple neuroimaging studies. The search was conducted in several scientific databases through September 2022, including publications in both English and Chinese. The inclusion criteria were rigorous: only studies with adults diagnosed with chronic pain, treated with real acupuncture (insertion of needles into the skin), that used resting-state functional magnetic resonance imaging and performed whole-brain analyses with standardized coordinates.

Two independent reviewers evaluated the methodological quality of the studies using validated tools. The ALE analysis allowed identification of which brain regions are consistently affected by acupuncture in different types of chronic pain, overcoming the limitations of individual studies with small samples.

The analysis included 14 studies of moderate quality, encompassing 524 patients with different types of chronic pain, including migraine, neck pain, low back pain, knee osteoarthritis, and shoulder pain. The results revealed a specific pattern of how acupuncture modulates brain activity. Before treatment, patients with chronic pain showed decreased activity in regions such as the caudate nucleus and thalamus, and increased activity in areas of the frontal cortex and fusiform gyrus compared with healthy individuals. After acupuncture treatment, decreased activity was observed in specific regions, including the precuneus, posterior cingulate cortex, right inferior parietal lobule, right superior temporal gyrus, cingulate gyrus, superior frontal gyrus, and medial frontal gyri.

These regions are part of two important brain networks: the default mode network (which includes areas active when we are not focused on specific tasks) and the frontoparietal network (involved in attention and cognitive control). Reducing the hyperactivity in these areas suggests that acupuncture may restore a more balanced pattern of brain functioning, contributing to pain relief and improvement of associated emotional aspects.

These findings have important implications for patients and health professionals. For patients, the results offer a scientific explanation of why acupuncture may be effective in treating chronic pain, providing an objective neurological basis for its therapeutic effects. This may help reduce skepticism and increase confidence in the treatment. The study demonstrates that acupuncture does not act only locally at the point of needle insertion, but produces systemic changes in complex brain networks involved in the processing of pain, emotions, and cognition.

For professionals, these data suggest that acupuncture may be particularly useful for patients whose chronic pain is associated with emotional and cognitive alterations, such as anxiety, depression, and concentration difficulties. The identification of specific affected brain networks may also guide future research on biomarkers to predict which patients will respond best to treatment and to objectively monitor therapeutic response through neuroimaging exams.

It is important to recognize the limitations of this research. The study included only 14 works, a relatively small number that may limit the generalizability of the results. The methodological quality of the included studies was considered moderate, with many presenting limitations in the randomization process, allocation concealment, and blinding of participants — factors that may introduce bias into the results. In addition, different types of chronic pain were included, which may have distinct pathophysiological mechanisms, although this also represents a strength of the study by demonstrating consistent effects of acupuncture across different conditions.

Heterogeneity in experimental designs and demographic characteristics between studies also requires caution in interpreting the findings. Future research should include studies with greater methodological rigor, larger samples, and sensitivity analyses to confirm these results. Despite these limitations, this work represents an important advance in the understanding of the neurological mechanisms of acupuncture, providing the first systematic synthesis based on neuroimaging evidence on how this ancient therapeutic practice modulates the brain networks involved in the chronic pain experience.