MRI evidence of extraocular muscle atrophy and fatty replacement in myasthenia gravis

Purpose: The aim of the study was to evaluate extraocular muscle (EOM) atrophy and fatty replacement in ocular myasthenia gravis (OMG) and generalized myasthenia gravis (GMG) patients with chronic and untreated ocular symptoms or with inadequate response to immunotherapy and unprovoked ocular exacerbations despite chronic immunotherapy.

Methods: Nineteen patients with either OMG or GMG and 19 healthy age-matched controls underwent an orbital MRI. Visually obvious muscle atrophy and muscle fatty replacement were evaluated by two raters independently. Maximum thickness of EOM was measured. Measurements of the muscles of each participant were added up, in order to calculate the total thickness.

Results: Eleven patients suffered from AChR-positive GMG, and 8 patients from OMG. All patients had chronic ocular symptoms or inadequate response to corticosteroids and unprovoked ocular exacerbations in spite of immunotherapy. Fatty replacement was reported in 6/19 (31.6%) patients and 0/19 (0%) controls (p = 0.02). Obvious atrophy in at least one muscle was reported in 8/19 (42.1%) patients and 1/19 (5.3%) controls (p = 0.019). Statistically significant differences between the two groups were also found in the mean total thickness, as well as in the thickness of superior recti, levator palpebrae, inferior recti, and superior oblique muscles.

Conclusion: EOM atrophy and fatty replacement were seen frequently in our series of MG patients with treatment difficulties and frequent relapses of ocular involvement.