THE UVEOSCLERAL OUTFLOW ROUTES

As there is no epithelial barrier between the anterior chamber and the ciliary muscle, aqueous humour may freely pass between the ciliary mu scle bundles into the supraciliary and suprachoroidal spaces, from whi ch it is drained through the sclera. This uveoscleral outflow of aqueo us humour accounts for 40-60% of the total outflow in monkeys, whereas it is considerably less in (3-8%) in cats and rabbits. Direct measure ments in human eyes have suggested that less than 15% is drained by th e uveoscleral routes. However, indirect calculations have given a valu e of about 35% in young adults and 3% in elderly persons (>60 years). Under normal conditions, in monkeys, the uveoscleral outflow is insens itive to changes in the intraocular pressure, but cyclodialysis and ex perimental uveitis increase the uveoscleral outflow and make it more p ressure sensitive. The uveoscleral outflow is decreased by contraction (pilocarpine) and increased by relaxation (atropine) of the ciliary m uscle. Thus, changing the tone of the ciliary muscle may redistribute aqueous humour between the conventional and uveoscleral outflow routes . Prostaglandins decrease the intraocular pressure by increasing the u veoscleral outflow. Two mechanisms seem to contribute to this effect: relaxation of the ciliary muscle and changes in extracellular matrix, causing decreased resistance in the uveoscleral outflow routes.