Analysis of Extraocular muscle volumes in idiopathic Hypertrophic Pachymeningitis patients

Background: Idiopathic hypertrophic pachymeningitis (HP) is a rare chronic inflammatory condition without an identifiable cause characterized by fibrous thickening of the dura mater, which can involve the extraocular muscles (EOM). Objective: To evaluate volumetric changes of EOM in idiopathic HP patients compared with healthy controls (HC) and study the correlation with ocular motility disturbance. Materials and methods: A retrospective study of 22 diagnosed idiopathic HP patients and 22 age-matched, sex-matched HC underwent a 3T MRI scan from January 1, 2017, to December 31, 2022. EOM was manually segmented from the T1W image using 3D Slicer software, and volume was calculated using FSL software. T-tests and Mann-Whitney U tests were used to compare EOM volumes between the idiopathic HP and control groups. Pearson's correlation coefficient was then used to assess the correlation between ocular motility and EOM enlargement. Results: In idiopathic HP patients, the average EOM volumes, including the medial rectus (MR), inferior rectus (IR), inferior oblique (IO), right lateral rectus (LR), right superior oblique (SO), and left superior rectus (SR) muscles, were significantly larger compared to those in HC, particularly in the left IR and both MR. However, there was no significant correlation between the enlargement of these 9 EOMs and the extraocular movement limitation. Conclusion: In idiopathic HP patients, significantly larger EOM volumes were found compared to control subjects. This enlargement could be due to the diffuse infiltrative histopathology potentially involving microstructures in the EOM. Extraocular movement limitations may be related to cranial nerve involvement.