Central disruption of fusional amplitude.

The stability of binocular vision depends on good fusional amplitudes. These amplitudes are the result of a reflex arc, the sensory stimulus being caused by the images of the object of regard falling off one fovea. If this image falls outside Panum's area, diplopia will result. The motor response involves a movement of one or both eyes to maintain fusion and avoid diplopia. The afferent sensory stimulus from each eye must reach the cortex for conscious fusion to take place (Moses, I970). The efferent motor limb of the reflex must produce a co-ordinated response from the eye muscles to keep each eye correctly aligned with its fellow. An occipito-mesencephalic nervous pathway conveys these fibres to the mid-brain area for supranuclear control of the eye movements. The horizontal amplitudes of fusion are dependent on convergence and divergence. Convergence or divergence paralysis may be seen as a clinical entity (Walsh and Hoyt, I 969). A combination ofconvergence and divergence paralysis would result in a complete disruption of horizontal fusional motor response in the presence of normal versions. Such a lesion would leave the patient with the ability to fuse the images from the two eyes if they were superimposed but the inability to correct diplopia with any fusional vergence movements. From the practical point of view, therefore, a patient such as this would have constant and incurable diplopia. Fusional amplitudes are also present on a vertical plane and supranuclear disruption may occur. It would seem likely that some cases of skew deviation, where a concomitant vertical separation of the two eyes takes place, may be due to manifestations of a disruption of the vertical fusional amplitude. This seems particularly likely in those cases where the vertical separation is under IO prism diopters. It is postulated that a motor association area, controlling fusional amplitudes, exists possibly in the mid-brain, and that if it is damaged, a loss of fusional amplitude occurs as the main sign, with consequent constant diplopia as the main symptom. The following case reports lend credence to this hypothesis.