I appreciate the interest of Dr Liu and his colleagues and hope that I can reassure them that the diagnosis of INO does not require demonstration of an impaired vertical vestibulo-ocular reflex. The major features of INO1 (limited or slowed medial rectus action and abduction overshoot and nystagmus) compose one of the most recognizable patterns in neurologic diagnosis, now regularly confirmed by demonstration on magnetic resonance imaging of a lesion in the medial longitudinal fasciculus.2 The principal difficulty in diagnosing INO lies in failure to appreciate mild, or even moderate, slowing of adduction saccades in the presence of a full range of motion.3
Keane JR. Internuclear Ophthalmoplegia—Reply. Arch Neurol. 2006;63(4):626–627. doi:10.1001/archneur.63.4.626-b
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