Convergence insufficiency is a frequent anomaly. Probably 8 per cent. of our eye patients show some degree of it, and this proportion would certainly be increased if every patient that presented himself were carefully tested. This statement by no means implies that in all or even a majority of these cases the convergence insufficiency is responsible for the symptoms. On the contrary, in a great many instances convergence insufficiency — especially the accommodative form — seems to cause no trouble whatever. Still it does cause trouble often enough to make it important for us to consider carefully its causation, varieties and treatment.
In considering a condition like this it is obviously essential that first of all we should have clear ideas of its nature. We may start, therefore, with a definition and say that convergence insufficiency is a condition the distinctive feature of which is that the eyes converge imperfectly
DUANE A. CONVERGENCE INSUFFICIENCY. JAMA. 1914;LXIII(9):721-729. doi:10.1001/jama.1914.02570090007003