Influence of alcohol on fusion.

T'HE fact that the consumption of a sufficient quantity of alcohol will cause diplopia, even in subjects who normally show no heterophoria, is well known. Until recently, the author, in common with many others, thought that this was due to a toxic exotropia, such as is seen in the early stages of anaesthesia. Powell (1938) and Colson (1940) have shown that the effect of alcohol on lateral heterophorias is to cause a reduction of exophoria, or an increase of esophoria, but that, in either case, the average change only amounted to about two prism dioptres. These findings have been confirmed by the author; he did not, however, find that alcohol had any effect on vertical heterophorias. The only record that he knows of alcohol influencing vertical heterophorias is a paper by Connolly (1946), whose results seem somewhat inconsistent and difficult to interpret; furthermore the author has not succeeded in confirming them. It is well known that alcohol makes inhibition more difficult, and Bairany and Hallden (1947) have shown that this applies to the two retinal images. This explains why the subject is conscious of the diplopia, but does not explain why it occurs. In view of the small change in heterophoria produced by alcohol, it seemed likely that the cause of alcoholic diplopia was a reduction in the subject's power to overcome any heterophoria present.