One-and-a-half syndrome is a syndrome characterized by horizontal movement 
disorders of the eyeballs, which was first reported and named by Fisher in 1967. 
It presents a combination of ipsilateral conjugate horizontal gaze palsy (one) 
and ipsilateral internuclear ophthalmoplegia (INO) (a half). On the basis of the 
one-and-a-half syndrome, there are a series of related rare syndromes called the 
one-and-a-half syndrome spectrum disorders. This article reviews rare cases of 
one-and-a-half syndrome spectrum disorder, describes the clinical and 
pathological features of different syndromes, and summarizes their nomenclature.