**2. Corneal anatomy and histology**

The cornea, the primary refractive ocular structure that contributes to focusing the external images on the retina, measures 11 to 12 mm horizontally, 10 to 11 mm vertically, and is about 0.5 mm thick centrally. The corneal thickness increases gradually toward the periph‐ ery to about 0.7 mm. Corneal nutrition depends on both glucose diffusing from the aqueous humor and oxygen supplied from the air through the tear film and in the peripheral cornea from the limbal blood vessels [11]. The cornea accounts for more than two thirds of the total ocular refractive power. Any slight change in the corneal contour can cause a substantial change in the ocular refractive power. The corneal optical properties are determined by its transparency, surface smoothness, contour, and refractive index.

of the endothelial pump, which is linked to the ion-transport system controlled by enzymes

Therefore, the endothelium prevents corneal edema by both the barrier and the pump func‐ tions. The pump function generates the so-called corneal imbibition pressure (IP), a negative pressure that draws fluid into the cornea. The IP is equal to the SP in the excised cornea. In vivo, however, the IP is lower than the SP because of the compressive effect of the IOP on the cornea. The relationship between these three parameters is described by the equation:

Although the regulation of the corneal hydration is maintained largely by the function of the endothelial pump, the epithelial barrier effect, the surface evaporation, the IOP level, and

The Ocular Hypertension Treatment Study (OHTS) [13] was a multicenter, randomized, prospective clinical trial of the efficacy of topical ocular hypotensive medications in delaying or preventing glaucoma onset in patients with ocular hypertension (OHT). Based on the OHTS, the CCT measured by pachymetry (Figure 1) has become important in glaucoma, and the study showed that the CCT is a significant predictor of the patients with OHT who are at higher risk of developing glaucoma, with a hazard ratio of 1.82 for each 40-µm thin‐

Eyes with a CCT of 555 µm or less had a three-fold greater risk of developing glaucoma compared with eyes that had a CCT exceeding 588 µm. In the multivariate model of baseline characteristics predictive of conversion oh OHT to glaucoma, the CCT had the greatest im‐ pact on the risk. These findings were confirmed in the European Glaucoma Prevention


IP = IOP - SP (1)

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http://dx.doi.org/10.5772/53017

such as Na+

, K+

the SP also play a role.

ning of the CCT.

Study [14].

**3. Impact of CCT on tonometry**

**Figure 1.** Ultrasound Pachymeter DGH 500 (Pachette™)

The cornea is comprised of five layers: the epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium. The epithelium, the most anterior layer, is comprised of nonkeratinizing stratified squamous epithelial cells. The epithelium and tear film form an opti‐ cally smooth surface. The Bowman's layer is the most anterior part of the corneal stroma, and is adjacent to the epithelial basement membrane.

The structural and optical features depend mainly on the structure and composition of the corneal stroma, which represents up to 90% of the corneal thickness. Corneal transparency basically depends on the regular spacial distribution of the stromal cells and the stromal la‐ mellae, and also on the water content of the stroma, that must be kept at a constant level of about 78%. The keratocytes are highly scattered and do not affect transparency. The lattice structure of the corneal collagen fibers, within a distance of 0.5 microns of the visible wave‐ length, is responsible for corneal transparency. Any decrease (dehydration) or increase (ede‐ ma) in this distance results in a loss of transparency. Fibrillar collagen types I and V, which are intertwined with type VI collagen filaments (collagen types III, XII, and XIV have also been found in the stroma) and corneal proteoglycans (mainly decorin associated with der‐ matan sulfate and lumican associated with keratan sulfate), are the fundamental compo‐ nents of the extracellular matrix (ECM).

Negatively charged stromal glycosaminoglycans tend to repel each other, producing the corneal swelling pressure (SP) (of about 50 mmHg in the excised cornea), and can absorb and retain large amounts of water. The keratocytes lie between the corneal lamellae and syn‐ thesize both collagen and proteoglycans.

The diameter of each collagen fiber and the distance between the collagen fibers are homo‐ geneous and measure less than half of the wavelength of the visible light (400-700 nm). This anatomic distribution of fibers is responsible for the fact that the incident light rays scattered by each collagen fiber are cancelled by the interference of other scattered rays, which allows the incident light to pass through the cornea without optical disruption.

Descemet's membrane, the basement membrane of the endothelium, is highly elastic and can withstand high pressure. When injured, it can regenerate.

The endothelium, the innermost corneal layer, is a monolayer of hexagonally shaped endo‐ thelial cells arranged in a mosaic pattern. The integrity of this layer and the correct function of the endothelial pump, which is linked to the ion-transport system controlled by enzymes such as Na+ , K+ -ATPase, are necessary to maintain the stability of the corneal water content. Therefore, the endothelium prevents corneal edema by both the barrier and the pump func‐ tions. The pump function generates the so-called corneal imbibition pressure (IP), a negative pressure that draws fluid into the cornea. The IP is equal to the SP in the excised cornea. In vivo, however, the IP is lower than the SP because of the compressive effect of the IOP on the cornea. The relationship between these three parameters is described by the equation:

$$\text{IP} = \text{IOP} - \text{SP} \tag{1}$$

Although the regulation of the corneal hydration is maintained largely by the function of the endothelial pump, the epithelial barrier effect, the surface evaporation, the IOP level, and the SP also play a role.
