The Symptomatology of Tumours of the Anterior Visual Pathways

This paper was originally presented as the first J. L. Silversides Lecture in Toronto on May 28, 1981. Tumours of the visual pathways, though rare amongst the causes of blindness, are one of the most important disorders the neurologist encounters. Diagnosed early, they can often be cured; diagnosed late, disabling visual impairment is common. Because the neurologist or ophthalmologist can come close to a definitive diagnosis by the exercise of his clinical skills alone and because effective treatment can so often follow, this branch of neurology has particular attractions. Jack Silversides, whom we honour today, is one of those thoughtful, compassionate physicians whose imagination has been captured by the opportunities which neuro-ophthalmology provides for helping patients and by the intellectual stimulation that comes from seeing confirmation by radiology and surgery of clinical predictions based on the logical application of a knowledge of anatomy, pathology and the natural history of disease. For many physicians, neurology in general and neuro-ophthalmology in particular provide an additional stimulus — that of trying to understand the mechanism of the clinical disorders they see in their patients. In the past, mechanism in relation to our topic today has been comprehended either pathologically — that is, as a concern with the factors influencing the origin, growth and spread of tumours — or anatomically — that is, how tumours in particular locations produce characteristic deficits. We owe our diagnostic skills to the classical investigations within this framework by men such as Cushing, Traquair and Jefferson. But they had little to say about physiological mechanisms. Just how do tumours involving the visual pathways produce the symptoms and signs they did do? What happens to nerve fibres when a tumour grows near or amongst them? These problems have interested me increasingly over the past decade and in this lecture I want to discuss some of the questions raised by tumours affecting the optic nerves and chiasm. The first part of what I shall have to say will lie within the traditional framework. I shall have two themes, the first being the broad similarity in the patterns of visual loss with tumours at different sites; the second will be the feasibility of accurate localisation by recognising certain distinctive clinical features despite the fairly stereotyped nature of the visual symptoms. The latter part of the lecture will have a physiological orientation. I shall enquire how far the visual symptoms produced by these tumours can be explained by our present understanding of the pathology and disordered physiology of central nerve fibres.