Abstract
Cerebellar tonsils normally extend up to 5 mm below the foramen magnum (FM). Some individuals with tonsillar ectopia (TE) measuring under 5 mm may present with symptoms of Chiari type 1 malformation (CM1). We analyzed MRI data of the posterior cranial fossa (PCF) in affected patients to assess TE's impact on clinical manifestations. The retrospective study examined 83 adults with 2-4 mm symptomatic cerebellar TE without syringomyelia (SM), 16 with syndromic CM1 without SM, and 19 controls. Participants with TE were divided into three groups: syndromic (TEs, n = 15), incomplete syndromic (TEis, n = 37), and non-syndromic (TEns, n = 31). Cranial MRIs assessed TE, obliterated cisterna magna (CM), and small PCF, along with clinical-radiological correlations. TE and CM1 patients had a smaller PCF on MRI than controls, with obliterated CM found in 53% of TEs and 81% of CM1s. Both syndromic groups (TEs and CM1s) showed more abnormal MRI findings than TEis and TEns. All clinically syndromic cases reported exertion-induced occipital headaches. TEs group exhibited reduced PCF volume and reduced area surrounding the FM compared to TEns and had a smaller inferior outlet area FM than TEis. PCF width and FM inferior outlet area negatively correlated with Chiari-associated symptoms in TE cohort. The study found that patients with TE without SM show clinical symptoms related to small PCF traits and congenital hypoplasia near the FM. In patients with TE under 4 mm, CM1-type symptoms are associated with TE at the FM, a constricted CM, and hypoplasia around the FM.