Role of the sclera in the development and pathological complications of myopia

Myopia is one of the most prevalent ocular conditions and is the result of a mismatch between the power of the eye and axial length of the eye. As a result images of distant objects are brought to a focus in front of the retina resulting in blurred vision. In the vast majority of cases the structural cause of myopia is an excessive axial length of the eye, or more specifically the vitreous chamber depth. In about 2% of the general population, the degree of myopia is above 6 dioptres (D) and is termed high myopia. The prevalence of sight-threatening ocular pathology is markedly increased in eyes with high degrees of myopia ( > -6 D). This results from the excessive axial elongation of the eye which, by necessity, must involve the outer coat of the eye, the sclera. Consequently, high myopia is reported as a leading cause of registered blindness and partial sight. Current theories of refractive development acknowledge the pivotal role of the sclera in the control of eye size and the development of myopia. This review considers the major biochemical mechanisms that underlie the normal development of the mammalian sclera and how the scleral structure influences the rate of eye growth during development. The review will characterise the aberrant mechanisms of scleral remodelling which underlie the development of myopia. In describing these mechanisms we highlight how certain critical events in both the early and later stages of myopia development lead to scleral thinning, the loss of scleral tissue, the weakening of the scleral mechanical properties and, ultimately, to the development of posterior staphyloma. This review aims to build on existing models to illustrate that the prevention of aberrant scleral remodelling must be the goal of any long-term therapy for the amelioration of the permanent vision loss associated with high myopia.