Physiological Mechanisms Underlying the Primary Respiratory Mechanism (PRM) and Cranial Rhythmic Impulse (CRI) in Osteopathy: A Systematic Review.

BACKGROUND: Cranial Rhythmic Impulse (CRI) or Primary Respiratory Mechanism (PRM), movement felt on the scalp or the rest of the body, respectively, is a fundamental concept used by osteopaths in their practice for their diagnosis and treatment. However, the physiological basis of this phenomenon remains unclear. Sutherland, the founder of cranial osteopathy, proposed in 1939 that PRM was due to the movement of the cranial bones pulled by the meninges, themselves pushed by the fluctuation of cerebrospinal fluid and the motility of the central nervous system. Since then, Sutherland's theory has become dogma, despite scientific progress refuting it, and few osteopaths have attempted to find better explanations. OBJECTIVE: This systematic review of Medline, Science Direct and the Cochrane Library indexed electronic databases explores current knowledge of the physiological mechanisms underlying the Primary Respiratory Movement (PRM) or Cranial Rhythmic Impulse (CRI). METHODS: We entered the following identified keywords: "osteopathy in the cranial field"; "cranial rhythmic impulse"; and "primary respiratory mechanism". We identified 193 studies, evaluated 115, and identified 28 articles that fulfilled our criteria. We classified the studies in terms of methodological rigor, types of studies and tools used. No study had three good-level items, and only five studies had two good-level items corresponding to the type of study and tools used. The protocol of the review has been registered on PROSPERO-CRD42023488497. RESULTS: Out of the 28 articles, 20 referenced at least one of Sutherland's hypotheses, often quoting the model to critique or challenge it, while 25 of them refer to other hypotheses and/or mechanisms underlying PRM/CRI: 11 concern vasomotion in blood vessels (7) and lymphatic vessels (6), 20 THM waves, 14 heart rate variability, 9 ventilation rate, 2 the extra-cellular matrix and 1 oxidative metabolism. CONCLUSIONS: Although Sutherland's theory remains prevalent in general beliefs, in scientific literature, THM waves driven by autonomous system activity have gained prominence, emerging as the leading hypothesis. The results from this systematic review confirm the need for a paradigm shift for the CRI/PRM in osteopathy, and for more rigorous evaluation and communication on a model in step with evolving scientific data.