Involuntary Movements following Stereotactic Operations for Parkinsonism with Special Reference to Hemi-chorea (ballismus).

When one examines stained sections of the thalamic area of the brain and sees the mass of fibres and cells in this area interconnecting almost every structure in the vicinity and receiving and sending millions of fibres to remote parts of the brain and spinal cord one marvels at the temerity of the early stereotactic surgeons who dared to place electrodes in this area and make massive destructive lesions. One might imagine that such lesions would have a profound effect on gait and motor control, on sensation and even mental function but, in practice, the clinical effects of such massive lesions are hardly detectable. Whether this reflects the high degree of adaptability in the brain or the relative unimportance of this area is difficult to determine. The main difficulty encountered by the surgeon probing this area is to produce any effect at all. All varieties of test procedures are used to determine the efficacy of lesions -stimulation, micro-electrode recording, 'trial lesions' with heat or cold-and yet, in some instances, no clinical effect can be produced. Most of such lesions are made for the treatment of involuntary movements, particularly the tremor and rigidity of Parkinsonism. The results in general are excellent, yet these abnormal movements are presumed to be the result of cell and fibre destruction and it is a matter for great surprise that such lesions do not in themselves invoke involuntary movements. That we find such concepts difficult to understand may well reflect wrong thinking on our part concerning the organization of the nervous system. When we see a discrete tract of fibres, or a collection of cells of similar size and connexions, we tend to think that they must serve some single function. We may accept the idea that this function differs from time to time and may well depend on the state of other parts of the nervous system at that time. Our minds, however, are not sufficiently alive to the possibility that such a discrete system may well be serving several functions, often directly opposed to each other. Fibres which facilitate or inhibit certain function may well run side by side and the physical destruction of both could well have no apparent clinical effect at all. Involuntary movements are most often the result of infections, of degenerations of unknown cause, or of developmental anomalies. It is easy to conceive that such processes may affect only facilitatory or inhibitory fibres, but not both, and that the gross physical lesions made by the surgeon destroy both and cancel out the imbalance. Tremor and rigidity, ballismus and athetosis, are very rarely caused by discrete physical lesions such as tumour growth, though occasional cases are reported and this suggestion may well account for the fact that the surgeon destroying large areas of the globus pallidus, lateral thalamus, and adjacent structures relatively seldom induces involuntary movements or makes them worse. Such complications do, however, occur, in a small proportion of cases, about 2-5% overall, and the situation of such lesions, together with the preexisting state of the nervous system, seemed a worthwhile study and forms the basis of this paper.