Introduction: Chiari malformation type I (CMI) is typically defined by a cerebellar tonsil position 5 mm or more below foramen magnum. Low cerebellar tonsil position is a frequent incidental finding, even in asymptomatic individuals. Non-specific symptoms such as headache and neck pain are common in those with low tonsil position and in those with normal tonsil position, leading to uncertainty regarding appropriate management for many patients with low tonsil position and non-specific symptoms. Since cerebellar tonsil position is not strictly correlated with typical CMI symptoms, we sought to determine if other 2D morphometric or 3D volumetric measurements on MRI could distinguish asymptomatic from symptomatic CMI patients.

Methods: We retrospectively analyzed 102 pediatric patients from the University of Michigan clinical CMI database. All patients in this database have cerebellar tonsil position at least 5 mm below foramen magnum. Fifty-one symptomatic and 51 asymptomatic patients were matched for age at diagnosis, sex, tonsil position, and tonsil morphology. Five 2D anatomic MRI measurements and four 3D volumetric measurements of posterior fossa and CSF sub-volumes on MRI were measured.

Results: There were no significant differences observed between symptomatic and asymptomatic CMI patients related to basilar impression (-5.9 mm vs. -7.0 mm; p=0.026), tentorium length (50.3 mm vs. 51.0 mm; p=0.537), supraoccipital length (39.4 mm vs. 42.6 mm; p=0.055), clivus-tentorium distance (52.0 mm vs. 52.1 mm; p=0.964), clivus-torcula distance (81.5 mm vs. 83.3 mm; p=0.257), total posterior fossa volume (PFV; 183.4 mL vs. 190.6 mL; p=0.250), caudal PFV (152.5 mL vs. 159.8 mL; p=0.256), fourth ventricle volume to caudal PFV ratio (0.0140 vs. 0.0136; p=0.649), or CSF volume to caudal PFV ratio (0.071 vs. 0.061; p=0.138).

Conclusions: We did not identify any 2D or 3D measurements that could reliably distinguish pediatric patients with symptoms attributable to CMI from those with asymptomatic CMI.

Patient Care: It is important to correctly diagnose symptomatic CMI in order to avoid unnecessary and ineffective surgery as well as to avoid delayed diagnosis and treatment in truly symptomatic cases. This investigation is a step toward developing a method of identifying true symptomatic CMI among the many patients with low tonsil position and non-specific symptoms.

Learning Objectives: By the conclusion of this session, participants should: 1) Appreciate the uncertainty regarding appropriate management for many patients with low tonsil position and non-specific symptoms. 2) Realize the limitations of MRI measurements in distinguishing pediatric patients with symptoms attributable to Chiari I (CMI) from those with asymptomatic CMI. 3) Consider investigating new methods to reliably distinguish symptomatic from asymptomatic pediatric CMI.

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