Only comments seeking to improve the quality and accuracy of information on the Orphanet website are accepted. For all other comments, please send your remarks via contact us. Only comments written in English can be processed. A rare ophthalmic disorder characterized by an elevated intra-ocular pressure. The clinical presentation frequently associates an increase in the size of the eye, as well as corneal edema. Congenital glaucoma CG is the most common glaucoma of infancy.
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Correlations between different tonometries and ocular biometric parameters in patients with primary congenital glaucoma. Keratometry, pachymetry, biometry and corneal diameter measurements were performed after the IOP measurement. The order of the tonometries was randomized. The other Delta-IOPs showed no correlation with any of the biometric characteristics evaluated.
Intraocular pressure IOP reduction is currently the only effective treatment for glaucoma ; thus, good tonometric accuracy is of great importance in the majority of cases. IOP measurement may be affected by some factors, such as corneal curvature, thickness and biomechanical characteristics hysteresis. Goldmann applanation tonometry GAT is considered the gold standard for IOP measurement; however, the characteristics of this equipment restrict its use in children who are not cooperative and must be examined under general anesthesia in an operating room.
These instruments may help to examine children under these conditions. Tonometry in eyes with congenital glaucoma CG is more susceptible to misinterpretation because of the extreme corneal alterations found in many CG patients Previous studies have demonstrated that central corneal thickness and keratometric changes can lead to false results in adults Subjects with diffuse, moderate or severe local edema or moderate or severe corneal opacification were excluded.
Institutional Review Board approval was obtained for the study protocol. Informed consent was obtained from all of the subjects or their parents. All 46 patients presented reasonable or good corneal transparency conditions, although Haab's striae or slight sectorial edema was observed in some patients. The order of the exams was randomized. Descriptive analyses were performed for all tonometry types and biometric characteristics.
The Pearson product-moment coefficient was used to evaluate correlation. Values were defined as follows: weak 0 to 0. Paired t test was performed to analyze statistical differences between the tonometric mean obtained with each tonometer. Scatter plot of the difference between GAT and other tonometer versus biometric parameters were presented. The ocular biometric characteristics of the sample are described as follow: central corneal thickness The mean IOP measurements were Table 1 shows all of the Pearson product-moment correlation coefficients obtained from the analysis of the Delta-IOPs with biometric parameters.
The majority of correlations between Delta-IOP and central corneal thickness, keratometry, axial length and corneal diameter were weak or not significant. Correct IOP measurement is important for the diagnosis and follow-up of the majority of congenital glaucoma cases.
However, it is often very difficult to determine the IOP in these patients because of the common corneal abnormalities found in this disease, or their inability to cooperate adequately with tonometry. These situations often require the use of others tonometers.
The contact and applanation systems of both are virtually identical. Therefore, the IOP measurements obtained with both tonometers are expected to have similar results with respect to the modification of ocular conditions.
The DCT was designed to fit the corneal contour and produce continuous, real IOP values through its central electromechanical reading system, thus minimizing the error related to corneal changes mean keratometry and thickness in the tonometric measurements. It uses a central tip that requires contact with the corneal surface, activating its reading microprocessor. In this case, for each touch on the corneal surface, repeated electronic measurements 4 to 10 are considered to provide the final IOP result.
Patients with severe corneal abnormalities and dense or generalized edema were excluded from this study because these characteristics could lead to major and non-quantifiable measurement errors. Patients younger than 12 years of age were excluded from this study because underage individuals often do not cooperate with the examination or require examination under anesthesia. These situations could contribute to inaccurate tonometric values.
Differences in central corneal thickness are one of the main sources of error in applanation tonometry There is no agreement about the adjustment factor. Studies have demonstrated errors from 0. Our results indicate no correlation between the central corneal thickness and the Delta-IOP of each tonometer.
This finding can be attributed to the considerable changes in the corneal biomechanical characteristics stromal abnormalities in this sample. A positive correlation between the IOP and axial length or increased myopia has been reported The corneal diameter is the other biometric parameter mentioned as a factor that influences the IOP, but few studies have examined this issue 8,21, In this study, the differences between the IOP results obtained with the three tonometers compared with those obtained with the GAT did not present any correlation with pachymetry, biometry and corneal diameter, meaning that the increase or decrease of any parameter did not influence the difference in tonometry readings.
The corneal curvature appears to influence the IOP reading an increase of 3 diopters overestimates 1 mmHg 14,23 because the greater the corneal curvature, the higher the pressure needed to applanate the corneal surface.
Using mathematic simulation models, other authors have demonstrated that the IOP can be influenced by corneal biomechanical variations of up to 17 mmHg. Those authors have demonstrated that pachymetry was responsible for 2. These results suggest that biomechanical properties have more influence on the IOP than central corneal thickness and corneal curvature isolated in the normal population Our results suggest that the modifications in the corneal stroma biomechanical structure such as Haab's striae formation may lead to changes in the expected correlations between the IOP and biometric parameters.
Therefore, the use of these three other tonometers can be accepted without restrictions attributed to the often biometric characteristics alterations found in these patients. The Ocular Hypertension Treatment Study: a randomized trial determines that topical ocular hypotensive medication delays or prevents the onset of primary open-angle glaucoma. Arch Ophthalmol. Comment in Arch Ophthalmol. The relationship between control of intraocular pressure and visual field deterioration.
Am J Ophthalmol. Classifications of the glaucomas. The glaucomas. Louis: CV Mosby; Diagnosis and treatment of congenital g laucoma. Human corneal thickness and its impact on intraocular pressure measures: a review and meta-analysis approach.
Surv Ophthalmol. Distribution of central corneal thickness and its association with intraocular pressure: the Rotterdam Study. Am J Ophtalmol. The effect of corneal thickness on applanation tonometry. Effect of corneal thickness on dynamic contour, rebound, and goldmann tonometry.
Central corneal thickness and corneal diameter in patients with childhood glaucoma. J Glaucoma. Corneal thickness in congenital glaucoma. Arq Bras Oftalmol. Ocular response analyzer versus Goldmann applanation tonometry for intraocular pressure measurements. Invest Ophthalmol Vis Sci. Mark HH.
Corneal curvature in applanation tonometry. Argus WA. Ocular hypertension and central corneal thickness. Comment in Ophthalmology. Central corneal thickness in normal, glaucomatous, and ocular hypertensive eyes. Applanation tonometry and central corneal thickness. Acta Ophthalmol Copenh.
Epidemiology of intraocular pressure in a population screened for glaucoma. Br J Ophthalmol. Tomlinson A, Philips CI. Applanation tension and axial length of the eyeball. The correlation between intraocular pressure and refractive status.
Effect of corneal thikness on intraocular pressure measurements with the pneumotonometer, Goldmann applanation tonometer and Tono-Pen. Falsely elevated intraocular pressure due to increased corneal thickness. Graefes Arch Clin Exp Ophthalmol. Corneal thickness and curvature in normal-tension glaucoma.
Comment in Am J Ophthalmol. Liu J, Roberts CJ. Influence of corneal biomechanical properties on intraocular pressure measurement: quantitative analysis. J Cataract Refract Surg. Comment in J Cataract Refract Surg.
Correspondence: Marcio H. Submitted for publication: March 4, Accepted for publication: August 21, Funding: No specific financial support was available for this study. Disclosure of potential conflicts of interest: M. Mendes, None; A. Betinjane, None; V. Quiroga, None.
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