A possible aetiology for glaucoma in Negroes.

CHRONIC simple glaucoma is a syndrome well established in ophthalmic practice, but its aetiology is still unknown. Fundamentally this must be related to the question of how the intra-ocular pressure is maintained at a constant level. The many theories which have been advanced to explain the cause of this disease fall into two groups postulating either a hypersecretion ofaqueous humour or an obstruction to its drainage. The latter concept has assumed pride of place ever since Leber (1873) discovered the preferential channel of drainage of the aqueous humour at the angle of the anterior chamber. This was confirmed by Priestley Smith (1891), who stressed that obstruction at the angle of the anterior chamber was primarily responsible for raised tension. This simple mechanical concept was challenged by Duke-Elder (1926), who felt that the obstruction was only an adjuvant factor and that the essential disease process was a vascular disturbance resulting in a state of congestion and stasis of the uveal circulation. Recently a large volume of work has accumulated on the anatomy and physiology of the drainage of aqueous humour from the angle of the anterior chamber, and various sites of obstruction have been suggested, so that the mechanical hypothesis is slowly gaining ground. Sugar (1957) summarized the immediate cause of ocular hypertension as being either vasoconstriction or sclerosis at the outlets of Schlemm's canal, or increased resistance within the trabecular meshwork itself. In considering the importance of the episcleral and aqueous veins in the drainage mechanism, Thomassen (1948) showed that changes in the pressure of episcleral blood veins anticipated changes in the intra-ocular pressure in glaucomatous eyes, and that this was a constant finding. He further concluded that, because the pressure in the aqueous veins is proportional to the intra-ocular pressure, the same relationship must exist between these veins and the episcleral blood veins; therefore, when the intra-ocular pressure is in an increasing phase, the pressure in the blood veins will be high in proportion to the aqueous veins, and the outflow through the latter will accordingly be hampered. This idea was confirmed