Corneal thickness

Rachymetry measures central corneal thickness thin corneas (less than 540 microns) are considered a glaucoma risk factor. 

The corneal epithelium is a stratified squamous epithelium like the epidermis of the skin but is nonkeratinized like other mucosal epithelia such as the intestinal and airway epithelia. Although the corneal epithelium, five to seven cells thick, represents less than 10% of the entire corneal thickness, it provides as much as 99% resistance to the diffusion of small electrolytes such as Na+ and Cl [55-57],... 

T reatment is indicated for ocular hypertension if the patient has a significant risk factor such as IOP greater than 25 mm Hg, vertical cup-disk ratio greater than 0.5, or central corneal thickness less than 555 micrometers. Additional risk factors to be considered include family history of glaucoma, black race, severe myopia, and presence of only one eye. 

Corneal thickness Increase In thickness was dose-dependent, and cornea returned to normal in 2 d after CR at up to 2X, In 13 d after CR at 10X. 

Reinsten, D.Z., et al. 1994. Epithelial and corneal thickness measurements by high frequency ultrasound digital signal processing. Ophthalmology 101 140. 

The epithelium is built up of several layers of cells and makes up about 10% of the total corneal thickness in man, and a similar proportion in many other mammalian species. There are 5 layers in man with... 

The stroma makes up approximately 90% of the corneal thickness and is mainly composed of hydrated collagen fibrils. It is highly hydrophilic, porous, and can be considered as an open structure, as it allows free passage of hydrophilic substances with a molecular weight below 500,000Da but acts as a diffusion barrier to all lipophilic drugs [4,12],... 

TABLE 39.2. Relative index values for corneal thickness measured by ultrasonic pachymetry over time ... 

Some individuals demonstrate significant increases or decreases ( 10 mcm) in corneal thickness after the instillation of topical benoxinate. This effect must be considered when performing preoperative pachymetry before corneal refractive surgery. 

As with benoxinate, corneal thickness instability can occur for about 5 minutes after proparacaine administration. 

These changes in corneal thickness should be considered when obtaining measurements for refractive surgery or when performing pachymetry in glaucoma patients. 

Asensio 1, Rahhal SM, Alonso L, et al. Corneal thickness values before and after oxybuprocaine 0.4% eye drops. Cornea 2003 22 527-532. 

Nam SM, Lee HK, Kim EK, et al. Comparison of corneal thickness after the instillation of topical anesthetics proparacaine versus oxybuprocaine. Cornea 2006 25 51-54. 

Kaminski S, Hommer A, Koyuncu D, et al. Influence of dorzo-lamide on corneal thickness, endothelial cell count and corneal sensibiUty.Acta Ophthalmol Scand 1998 76 78-79. 

Sodium chloride is a component of all body fluids, including tears. A solution of 0.9% is approximately isotonic with tears. Of the various concentrations tested, 2% to 5% formulations have proven effective, with an irritation level acceptable to most patients. Studies comparing various hyperosmotic agents in human subjects have confirmed the usefulness of hypertonic sodium chloride in the treatment of corneal edema. Use of 5% sodium chloride in ointment form can be effective in reducing corneal thickness and in improving vision.The maximum reduction in corneal thickness occurs 3 to 4 hours after instillation of the ointment (Figure 15-1). 

Figure 15-1 Percent reduction in corneal thickness after application of 5% sodium chloride ointment (triangles = central unfilled circles = nasal filled circles = temporal). (Modified from the American Journal of Ophthalmology 1971 71 847-853. Copyright The Ophthalmic Publishing Company.)...

Glucose solutions ranging from 30% to 50% have been used topically on the eye to treat corneal edema. The dehydrating action of a 30-minute glucose bath eliminates corneal epithelial edema and reduces corneal thickness. The effect lasts 3 to 4 hours. 

Ratio of Anterior Chamber Depth to Corneal Thickness Grade (van Herick) Implication... 

The stroma constitutes approximately 90% of the total corneal thickness and is primarily composed of collagen fibers, keratocytes, and glycosaminoglycans. The imiform arrangement of the collagen fibers is the major determinant of corneal transparency, in contrast to the opaque and less regularly arranged fibers of the sclera. Disruption of the stromal layer regularity results in loss of corneal transparency and potential scar formation. 

Contact lenses can produce changes in corneal shape and/or corneal thickness substantive enough to cause lOP measurement error. This may be particularly true of patients who are prescribed orthokeratology for the management of refractive error. In addition, there is evidence that many soft lens wearers may develop corneal edema during the day. Low levels of contact lens-related edema (<5%) may produce a stiffening of the corneal tissue with a corresponding measured increase in lOP When edema levels increase beyond 6% to 10% (which is less common in contact lens wear), the cornea becomes substantially softer with subsequent lower measured lOP... 

Even in the absence of contact lens wear, the central corneal thickness (CCT) undergoes normal dim-nal variation. CCT is thicker when awakening from sleep and decreases exponentially over approximately 2 homs. 

Summary. Measured lOP varies with corneal thickness, rigidity, curvature, eccentricity on the cornea, time of day, type of tonometer used, position of the patient, use of medications known to influence lOP, adherence to medication regimen, and a host of other variables. For such an important measure, it has several potential sources of error. 

Vertical cupping of the optic nerve head Pattern standard deviation on visual fields Thin central corneal thickness < 555 mcm... 

Aghaian E, Choe JE, Lin S, Stamper RL. Central corneal thickness of Caucasians, Chinese, Hispanics, Filipinos, African American, and Japanese in a glaucoma clinic. Ophthalmology 2004 111 2211-2219. 

Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocular pressure measures a review and meta-analysis. Surv Ophthalmol 2000 44 367-408. 

Outline the importance of central corneal thickness in glaucoma practice. 

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