To analyze the volumetric and density characteristics of the extraocular muscles (EOM) in patients with Myasthenia Gravis (MG).

MG is an autoimmune disease that mainly involves EOM. Myopathies affecting extraocular muscles (MEOM) are a common differential diagnosis. Frequently, complementary methods are required to differentiate between them. There are no published data from the volumetric and density characteristics of the EOM in these pathologies.

Case-control study in adult patients, evaluated in FLENI between 2017/2018, with diagnosis of MG and MEOM, compared with healthy controls adjusted for age and sex.

Orbit CT scan was obtained in primary position of the gaze. Manual segmentation of the EOMs of both eyes was done using the ITK.SNAP Software.

Volume (mm3) and average density (Hounsfield Unit) of each EOM were evaluated. References of tendon density of the superior oblique muscle (SO) and intraorbital fat were taken.

We included 18 patients with MG, 7 with MEOM and 18 controls.

No differences were observed in the individual and total muscle volume between patients with MG and controls. Patients with MEOM showed lower volume in all the EOM compared to MG and controls (p<0.0001).

The muscle density of patients with MG was significantly lower (p<0.001) regarding the controls; muscle density in the MEOM was lower than the MG group and controls (p<0.001 respectively).

No differences were observed in SO tendon density and intraorbital fat among the 3 groups.

Volumetric and density analysis of the EOM by CT has been shown to be a feasible procedure and an interesting approach towards neuromuscular diagnosis in cases of MG and MEOM. In our experience, it allows to demonstrate differences in the muscular density of patients with MG with respect to MEOM and controls. In the same way, MEOM present differences in volume and muscle density with respect to MG and healthy controls.