The Differential Diagnosis of Diplopia

Hermia-"Methinks I see these things with parted eye, When everything seems double." Helena-"So, methinks, "And I have found Demetrius like a jewel, Mine own and not mine own. "-A Midsummer-Night's Dream. Professor of the Ophthalmological Clinic at the Faculty of Paris. In that lecture Professor Marquez described a method devised by him for the elucidation of certain cases of diplopia. In this paper I propose to explain his method in as simple a manner as possible, in what may seem to be a complicated subject. The sudden onset of double vision in an adult is so distressing and alarming that the sufferer is compelled to seek professional advice at an early moment. It is easy to give relief by the simple occlusion of one eye from vision. To determine the reason of the double vision is not always easy, may be difficult, and indeed sometimes may leave an element of doubt in the mind of the physician. Any method which pretends to facilitate the solution of our difficulties and resolve our doubts is of great interest, and, I think, is worthy of our consideration. The method has its limitations. It only applies to affections of the extraocular muscles acting in the vertical plane, so we may dismiss from our minds such gross lesions as complete paralysis of the third and sixth cranial nerves, monocular diplopia, cases due to displac'ement of an eyeball by local effusions or tumour. The muscles concerned are eight in number, and as each may be affected in one of two ways, by paresis or spasm, we may have to determine one out of sixteen possibilities, a formidable enough task. To understand the working of the method, it is necessary to bear in mind certain definite principles. 1. In the normal eye, rays from an external object fall on the fovea. In a deviating eye these rays fall to one or other side of the fovea, according to the deviation, e.g., in convergent deviation on the nasal side of the fovea, in divergent deviation on the temporal side. The picture there formed is projected into space in the opposite direction, and is there seen by the patient.