Ocular Weakness in Myasthenia Gravis: Changes in Affected Muscles are a Distinct Clinical Feature

INTRODUCTION: In this study we quantitatively describe ocular weakness patterns in myasthenia gravis (MG) to help neurologists in making the clinical diagnosis and to investigate how the current outcome measures reflect ocular weakness in MG. METHODS: We investigated ptosis and diplopia patterns in a retro- and prospective cohort of 306 MG patients. Diplopia was systematically examined by testing extra-ocular muscle (EOM) fatigability in two horizontal and four oblique directions for 60 seconds. RESULTS: Of patients with initial symmetric ptosis, 40% developed asymmetric ptosis at the second visit. Changes in form of ptosis occurred less often in seronegative MG patients (50%) than in patients with acetylcholine receptor (AChR) antibodies (70%) or muscle-specific kinase (MuSK) antibodies (69%) (p = 0.038). Of patients with diplopia on the first visit, double vision contained both a vertical and horizontal component in 95%. At the second visit, 83% manifested diplopia in other gaze directions. The mean time (in seconds) to diplopia was 11.6±14.0 and the mean time to ptosis was 27.6±19.8. Diplopia or ptosis manifested within 30 seconds in 87% and 58%, respectively. Patients who manifested diplopia after 30 seconds, reported no limitations due to diplopia. DISCUSSION: Changes in the gaze directions in which diplopia occurs or ptosis side occur frequently in MG. In diagnostically challenging cases, we recommend testing ptosis and diplopia in multiple gaze directions for 30-60 seconds during at least two follow-up visits to maximize the chance of observing changes in ocular weakness patterns.