Video-Oculographic Saccadic Fatigability Distinguishes Myasthenia Gravis From Unilateral Ocular Motor Cranial Nerve Palsy in Diplopia: A Prospective Diagnostic Study.

OBJECTIVE

To determine whether repetition-based saccadic fatigability measured with three-dimensional video-oculography (VOG) distinguishes myasthenia gravis (MG) from unilateral ocular motor cranial nerve palsy (CNP) in patients with diplopia.

METHODS

In this prospective cross-sectional diagnostic study conducted from August 2022 to July 2024, consecutive patients with MG or unilateral third, fourth, or sixth CNP, and healthy controls underwent repetitive horizontal and vertical saccade testing (±°15 targets; 75 cycles at 0.25 Hz). Prespecified index metrics were percentage change in saccadic range and time to target acquisition from the second saccade to the mean of the last five saccades. Diagnostic discrimination was assessed using receiver operating characteristic (ROC) curves.

RESULTS

The cohort comprised 231 participants: 93 with MG, 54 with CNP and 84 healthy controls. MG showed greater repetition-related range decrement and longer time to target acquisition than healthy controls and axis-matched CNP. In the horizontal comparison (MG vs. sixth CNP), AUCs were 0.800 for percentage range change and 0.787 for percentage time-to-target change. In the vertical comparison (MG vs. third/fourth CNP), percentage range change showed the best discrimination (AUC 0.868, 95% CI 0.783 to 0.934); a threshold of -5.95% yielded 79.6% sensitivity and 87.5% specificity. CNP showed early baseline slowing, whereas MG showed progressive hypometria with relatively preserved peak velocity.

CONCLUSIONS

Repetition-based VOG captures a dynamic fatigability signature in MG that differs from the more static deficit of unilateral ocular motor CNP. Quantitative saccadic fatigability may provide a useful objective adjunct to the neuro-ophthalmic evaluation of diplopia.