Understanding Eye Structure

Introduction

The cornea is the outer surface and protective covering of the eye. Of relevance to glaucoma risk factors and the cornea, are 'central cornea thickness' or CCT. Generally, a lower CCT is considered to correlate with a larger risk factor for glaucoma, but there are new recognized factors related to the cornea which may be more important diagnostically. There has been new research and understanding related to the bio-mechanical factors of the cornea such as corneal hysteresis or CH. Bio-mechanical factors can be measured by newer instruments such as the Reichert Ocular Response Analyzer (ORA). 

The anterior part of the eye is of interest in understanding glaucoma and ocular hypertension because it constitutes the eye anatomy and physiology where aqueous humour is generated and then drained to the outside of the eye. It is failure in the balance of aqueous humour (AH) production (inflow) vs drainage (outflow) which accounts for the largest known risk factor in the progression of glaucoma,  high intraocular pressure. The AH is produced in the ciliary body where it travels in front of the lens, through the pupil, and over the top of the iris to the Trabecular meshwork or 'drain.' There can be numerous causes for increased intraocular pressure in this process. In ocular hypertension (OH) or primary open angle glaucoma (POAG), there can be too much AH produced, but the main problem is usually the failure of the TM to properly drain the fluid. Another source of failure in Closed Angle Glaucoma (CAG) when either the AH, because of a constricted angle, can not bypass the pupil (pupillary block), and/or cannot reach the drain because the closed angle obscures it.  In the latter case, the drain itself may be functioning normally, but there is an overlap of conditions where POAG and CAG occur simultaneously. This is discussed in more detail in the Narrow Angle Glaucoma article.

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