Denervation of rectus capitis posterior minor as neglected factor in Chiari malformation type I revealed by double blinded prospective study.

Chiari malformation type I (CMI) typically manifests with Valsalva-induced occipital headaches and commonly co-occurs with syringomyelia. The disruption of cerebrospinal fluid (CSF) dynamics at the craniocervical junction (CCJ) is a key pathophysiological feature. The rectus capitis posterior minor (RCPmi), innervated by the C1 nerve root's posterior branch, significantly facilitates CSF flow at the CCJ, correlating closely with occipital headaches. This study aims to explore RCPmi functionality in CMI patients compared to healthy controls using needle electromyography (nEMG). Data from adult CMI patients and a health group collected from January 2023 to May 2024 were analyzed. Both groups underwent bilateral RCPmi nEMG testing, assessing mean duration, amplitude, multiphasic wave ratio, recruitment phase amplitude, and spontaneous potentials during Valsalva maneuvers. We conducted a double-blinded evaluation, with additional subgroup analyses based on headache presence, tonsillar herniation relative to the C1 vertebra, and syringomyelia involvement. The study included 40 CMI patients and 30 healthy controls with no demographic differences. Healthy controls displayed stable RCPmi-nEMG parameters, with intense electrical activity during Valsalva maneuvers. In contrast, CMI patients exhibited substantial denervation damage in bilateral RCPmi, particularly during Valsalva maneuvers, characterized by insufficient electrical signal response and sparse motor units. Subgroup analysis revealed increased denervation in patients with headaches, extensive tonsillar herniation, and syringomyelia involving C1. RCPmi plays a critical role in maintaining cranio-cervical stability and modulating intracranial pressure, especially during Valsalva maneuvers. Compared to controls, CMI patients show widespread denervation damage in RCPmi, strongly linked to enhanced obstruction of CCJ-CSF flow and typical headache symptoms. This denervation damage, likely related to pathological factors like C1 nerve root compression by herniated cerebellar tonsils and inflammatory mediator release at the CCJ, highlights the functional failure of RCPmi as a novel target for understanding the headache mechanism in CMI and for developing pain interventions.