Applanation Pressures Research Articles

New Approach to the Analysis of Raw Data from the Ocular Response Analyzer

Purpose: The aim of this research was to present a new approach for the potential characteristics of the anterior eye, based on numerical analysis of the raw data from multiple measurements on 1 eye captured with use of the Ocular Response Analyzer (ORA). Methods: The measurements were carried out 6 times on every eye, one by one, on 29 patients using the ORA. Dependencies between values of 6 applanation pressures P₁ and P₂ from 1 eye and the new proposed time-dependent parameters sm and smm, obtained from the raw data, were used to determine new characteristic features of the anterior eye. Results: Obtained values for multiple applanation pressures P₁ and P₂ and their average value P_a from all measurements on 1 eye show strong linear dependencies with the new proposed time-dependent parameter smm. Conclusions: The new parameters based on slope coefficients a₁ and a₂, their difference Δa, and respective y-intercept values b₁, b₂, and Δb show some characteristic stability for a multiply measured eye and can be used for its characterization. These parameters can be used for differentiation of the eyes with higher accuracy than those obtained from a single measurement. They are likely related to some biomechanical properties of the anterior eye.

Assessment of the Ocular Response Analyzer as an Instrument for Measurement of Intraocular Pressure and Corneal Biomechanics

Purpose: The purpose of this study is to provide better understanding of the Ocular Response Analyzer (ORA) and how reliable it is to produce intraocular pressure (IOP) measurements that are free of the effects of corneal stiffness parameters, and stiffness estimates that are independent of IOP.Materials and Methods: A numerical parametric study that closely represents the in-vivo conditions of the human eye and the ORA procedure was conducted to determine the correlation coefficient r2 between ORA output and the values of true IOP and a number of stiffness parameters, namely corneal thickness, curvature and age. For the purpose of this exercise, the ORA output was put in the form k1P1+k2P2 where k1 and k2 were variables and P1 and P2 were ORA’s measured applanation pressures. Two separate clinical datasets involving Moorfields Eye Hospital, London and the University of New South Wales, Sydney participants, respectively, were used to validate the numerical results.Results: The numerical study results show a strong association between (k1P1 + k2P2) and the true IOP over a wide range of k1 and k2 values apart from a narrow region approximately extending from (k1 = +2, k2 = −2) to (k1 = −2, k2 = +2). On the other hand, (k1· P1 + k2· P2) was found to have a strong association with CCT, R and age (the stiffness parameters) over the same narrow region, beyond which the association was weak. Similar trends were found with the two clinical datasets.Conclusions: The results of this study show the potential of the ORA to provide reliable IOP measurements with weak dependence on the cornea’s stiffness parameters and the considerably reduced reliability in producing stiffness estimates that are unaffected by IOP values.

Hipermetropia após ceratotomia radial: flutuação da refração e da acuidade visual entre manhã e tarde e correlações com a pressão ocular e o estado biomecânico da córnea

PURPOSE: To verify the morning to evening variations of refraction, visual acuity, intraocular pressure and biomechanical parameters on patients operated by radial keratotomy who presented for refractive re-treatments; and to correlate the biomechanical parameters with refraction, visual acuity and their variations among morning and evening. METHODS: 19 patients were examined, respectively thirty-eight eyes in the morning (9 am) and evening (6 pm), recording sphere-cylindrical dynamic refraction, visual acuity (logMAR) without correction (AVsc) and corrected (AVcc) and ORA (Ocular Response Analyzer) parameters: corneal hysteresis (CH), corneal resistance factor (CRF), intraocular pressure calibrated for Goldmann (IOPg) e corneal compensated (IOPcc). Variables had no normal distribution (Kolmogorov-Smirnov test), so that the Wilcoxon signed rank was used for testing the significance on the differences for each variable between morning and afternoon. The Spearman test was used for assessing the correlations between the ORA parameters and refraction, visual acuity in the morning and afternoon, as well as to verify the correlations between the ORA parameters and the variations on refraction and visual acuity. RESULTS: Sphere (E), spherical equivalent (EE), defocus equivalent (ED), AVsc, IOPcc e IOPg varied significantly (Wilcoxon, p<0.05) between morning and evening. There was more hyperopia, worse visual acuity, higher pressure and lower CH in the morning measurements. In the morning measurements, there was a positive correlation (Spearman, p<0.05) between EE and ED and IOPcc (rs=0.39 and 0.34 respectively), but not with IOPg. In the evening measurement, there were no correlations between the refractive and pressure measurements. Negative correlations were observed (Spearman, p<0.05) between AVsc (logMAR) and CH in the morning and in the evening (rs= -0.48 e rs= -0,51), between E and CH in the morning and in the evening (rs= -0.66 and -0.76), between E and CRF in the morning and in the evening (rs= -0.40 and -0.47), between EE and CH in the morning and in the evening (rs= -0.70 and -0.68), between EE and CRF in the morning and in the evening (rs= -0.41 and -0.46), between ED and CH in the morning and in the evening (rs= 0.64 and -0.54) and between ED and CRF in the morning and in the evening (rs= -0.35 and -0.34). There was a significant negative correlation between the morning measurement of CRF and the variation of defocus equivalent (p = 0.05; rs = -0.30) and the variation of AVsc (p = 0.04; rs = -0.33). CONCLUSION: More hyperopia was recorded in the morning which was associated with higher compensated pressure (IOPcc) but not with IOPg ten years after RK. Lower biomechanical parameters (CRF e CH) were associated with higher hyperopia and worse visual acuity. A trend was observed for having higher fluctuation on weaker corneas. New studies involving the variables derived from the waveform signals, beyond CH and CRF (derived from the applanation pressures) along with data from corneal tomography and wavefront aberrometry are necessary.

Assessment of the Ocular Response Analyzer as a Tool for Intraocular Pressure Measurement

The ocular response analyzer (ORA) is a new indentation tonometer that subjects the cornea to an increasing then decreasing air pulse, and uses the corresponding two applanation pressures P1 and P2 to estimate the intraocular pressure (IOP). The present study aims to improve the accuracy of IOP estimation through representative numerical simulation of the ORA procedure. A parametric study has been carried out to consider the effect of corneal thickness, curvature, age, and true IOP on the P1 and P2 measurements. Based on the obtained database of input and output parameters, an equation has been suggested relating the P1 and P2 measurements to the value of IOP. The equation is intended to make IOP estimates independent of corneal stiffness, which varies with size and age. The equation has been tested against a clinical data set obtained at Moorfields Eye Hospital involving 144 subjects, and found to produce IOP estimates that are less dependent on corneal size and age than published equations.

062 Modifications biomécaniques et topographiques de la cornée après « cross-linking » par Riboflavine-UV dans le kératocône

The aim of this study was to evaluate the biomechanical corneal properties after UV- Riboflavin cross-linking in keratoconic eyes. The Ocular Response Analyzer (ORA) provides a new parameter which is called corneal hysteresis. A rapid air impulse leeds to inward and outward movement of the cornea. During that movement an electro-optical system records two applanation pressures. The difference of those two preassures results in corneal hysteresis (CH). Nine eyes of 9 patients were included in this study. Each patient received a UV- Riboflavin cross-linking treatement to stabilize the cornea. CH, corneal resistance factor (CRF) and corneal topography were measured prior and post treatement. The minimum follow up was set for 3 months. Mean preoperative values as follows: age 33.36 ± 6.93 years, spherical equivalent −1.68 ± 2,24 diopters (D), topographic astigmatism −4.01 ± 1.60 D, CH 7.72 ± 1.56 mmHg, CRF 6.44 ± 1.29 mmHg. Mean follow up was 4.85 ± 1.51 months. Mean postoperative values as follows: spherical equivalent −2.40 ± 2.82 D, topographical astigmatism -4.09 ± 1.87 D, CH 7.63 ± 1.14 mmHg, CRF 6.64 ± 1.26 mmHg. Corneal UV-Riboflavin cross-linking didn’t change the biomechanical corneal properties. In particular cases we observed a reduction in topographic astigmatism.

Corneal Thickness- and Age-Related Biomechanical Properties of the Cornea Measured with the Ocular Response Analyzer

The Ocular Response Analyzer (ORA) is a new instrument that measures the corneal biomechanical response (corneal hysteresis, CH) to rapid indentation by an air jet. CH is the difference in applanation pressures (P1, P2) between the rising and falling phases of the air jet. The investigation had two parts: a characterization study and a validation study. In the characterization study, the purposes were to investigate the intraocular pressure (IOP)-dependence of CH and to characterize the performance of the ORA. In the validation study, the purposes were to investigate the association between CH and both age and central corneal thickness (CCT) and the agreement between ORA and Goldmann applanation tonometer (GAT) IOP measurements. For the characterization study, data were collected from 105 untreated subjects (45 ocular hypertensive patients and 60 normal subjects; mean age, 60 years, range, 26-82). GAT and ORA measurements were performed before and after IOP lowering of one randomly selected eye with apraclonidine drops. The change in P1 and P2 (arbitrary units) in relation to change in GAT IOP was analyzed to calibrate the instrument. The relation between P1, P2, and CCT was explored and ORA IOP was derived from the analyses. For the validation study, ORA and GAT IOP and CCT were measured in 144 eyes of 144 untreated subjects (mean age, 58 years; range, 19-83). The characterization calculations were applied to the dataset and values of CH and ORA IOP were calculated. The relationship between CH and both subject age and CCT was determined. The associations between CH and CCT and between ORA and GAT IOPs, were investigated by linear regression analysis. The agreement between measuring devices was calculated. In the characterization study, P1 changed by 6.41 arbitrary units for every 1-mm Hg change in GAT IOP. CH (P1 - P2) changed by -1.60 arbitrary units for every 1-mm Hg change in GAT IOP. For each unit change in P2, P1 changed by 1.27 units. From this association a new IOP-independent corneal factor was derived [P1 - (P2/1.27)] and is termed the corneal constant factor (CCF; mm Hg). ORA IOP normalized for CCF was defined as P2 - CCF (mm Hg). The CCF (mm Hg) was associated with CCT (micrometers) and with age: CCF = [(0.036 . CCT) - (0.028 . age)] + 1.06 (adjusted r2 = 0.34; P < 0.0001 for CCT, P = 0.007 for age). Normalized ORA IOP measurements were not associated with CCT. GAT IOP was associated with CCT and CCF-more strongly with the latter: GAT IOP = (0.03 . CCT)+1.52 (r2 = 0.06, P = 0.002); GAT IOP = (0.65 . CCF) + 4.5 (r2 = 0.13, P < 0.0001). The mean difference (95% limits of agreement) between GAT and normalized ORA IOP was 0.1 (-6.6 to +6.8) mm Hg. The CCF describes an IOP-independent biomechanical property of the cornea that increases with thicker CCT and decreases with greater age. It is moderately strongly associated with CCT and yet explains more of the interindividual variation in GAT IOP than does CCT. Normalized ORA IOP measurements are not associated with CCT.

Bilateral Ciliochoroidal Detachments After Valsalva Maneuver

We had the opportunity to examine a woman with bilateral ciliochoroidal detachments. Report of a Case. —A 63-year-old white woman presented 3 days after several severe and prolonged episodes of emesis from a presumed viral gastroenteritis. She complained of a droplet-shaped scotoma in the right eye and blurred vision in both eyes. Her ocular history was significant for cataract surgery with intraocular implant in her right eye in 1985 and in her left eye in 1986. Best-corrected visual acuity was 20/25 OD and 20/20 OS. Results of slit-lamp examination revealed well-centered posterior chamber implants. Applanation pressures were 15 mm Hg OU. Results of fundus examination of the right eye were remarkable for a dome-shaped orange-brown choroidal elevation in the inferonasal quadrant (Fig 1) associated with a low-lying inferior and nasal choroidal detachment. The disc, macula, and vessels were otherwise normal. Results of examination of the periphery of the left eye

Ocular Effects of Gravity Inversion

To the Editor.— Friberg and Weinreb 1 recently reported the occurrence of increased intraocular pressure with the head-down position, and suggested that patients with certain conditions, including glaucoma, should refrain from inversion altogether. Report of a Case.— We recently saw a 68-year-old athletic physician in excellent health who was first examined in January 1975, for an increase in his cup-disc ratio of 0.6 on the right and 0.7 on the left. Pressures at that time were 18 in both eyes on six separate occasions, and tonography revealed a C value of 0.24 on the right and 0.13 on the left. Visual fields were normal. From 1975 through 1980, applanation pressures taken at different times during the day were always below 21 mm Hg. At the time of reevaluation in February 1981, a small arcuate scotoma was identified in the left eye and early posterior subcapsular cataracts were identified in both

Ocular rigidity and intraocular pressure in keratoconus.

Eighty-five patients with keratoconus and 20 normal controls were studied. Ocular rigidity, calculated using the Friedenwald nomogram, correlated with corneal thinning (p less than 0.001) and area of cone (p = 0.05). Schiøtz pressures with increasing weights fell more than normal in Grades 0 to 3 keratoconus, but returned to normal following corneal grafting, as did ocular rigidity. Goldmann applanation pressures were low over the apex of the cone, varying most in eyes with lowest ocular rigidity and with greatest corneal thinning, but also in large sagging cones which had only minimal corneal thinning. Pressures by Mackay-Marg tonometry were normal in keratoconus. The axial length of eyes with keratoconus even after corneal grafting was longer than normal and correlated with myopia. It is concluded that abnormalities of ocular rigidity, Schiøtz and Goldmann pressures in keratoconus are determined by abnormal corneal rigidity. If corneal rigidity is low or corneal thinning is present, Goldmann applanation pressures are falsely low and Mackay-Marg tonometry more closely approximates the true intraocular pressure. The cone develops by stretching with limited loss of tissue and the lower rigidity and reduced apical stress decrease the likelihood of rupture.

The Normal Cup-Disk Ratio

We determined the cup-disk ratios of 580 individuals, ranging in age from 4 to 91 years, with a Hruby lens; we also determined the cup-disk ratios of 289 of the 580 with a direct ophthalmoscope. All had applanation pressures of 18 mm Hg or less. There was no linear relationship (covariation) between cup-disk ratio and refractive error or intraocular pressure, but there was a tendency toward increasing cup-disk ratio with increasing age. The mean cup-disk ratio in the group examined with the Hruby lens was 0.38 and that in the group examined with the direct ophthalmoscope was 0.25. With either method almost all eyes had cup-disk ratios of 0.7 or less. In 9% of the eyes the cup was ovoid. In only 10% of these cases was the vertical cup-disk ratio greater than the horizontal cup-disk ratio, and in only one case was this difference 0.2 or more. The Hruby lens method consistently gave a slightly larger cup-disk ratio than the direct ophthalmoscope did. However, there was seldom a disparity of more than 0.2. The Hruby lens or biomicroscopic lens should be used to determine the fullest extent of the contour of the cup. Any cup-disk ratio of 0.7 or more, any vertical cup-disk ratio larger than the horizontal cup-disk ratio, and any disparity between the direct ophthalmoscope estimation and Hruby lens estimation of more than 0.2 should be viewed with suspicion.

The immediate response in applanation pressure to intravenous acetazolamide in primary glaucomas and glaucoma suspects.

The decays in the applanation pressures of both eyes, after a reduction of the aqueous secretion by means of intravenous acetazolamide, are studied with respect to shape and steepness by comparison with reference curves calculated from different values of the facility of aqueous outflow. The pressure decays in three out of four eyes with narrow-angle glaucoma exhibited "breaks" possibly resulting from a stepwise reopening of the chamber angle during the decompression. Twenty patients with intraocular hypertension and open angles could be divided into two groups according to the facility of aqueous outflow (in mul mmHg-1 min-1), estimated from approximately matching reference curves: Nine patients with facilities above 0.15 had normal visual fields and nearly indentical pressure curves of the two eyes. Out of 11 patients with facilities below 0.15, seven patients had glaucomatous visual field defects and excavated optic discs. The pressure curves of eye pairs in this low facility group showed asymmetry, and breaks were seen in approximately half of the curves. These might be caused by changes in the outflow facility due to a decompression of collapsed outflow channels. However, the morphological basis for such an interpretation is not as well founded as in the case of narrow-angle glaucoma.

Systemic steroids and ocular fluid dynamics. I. Analysis of the sample as a whole. Influence of dosage and duration of therapy.

Sixty patients treated for various systemic diseases with long‐term steroids of varying dosages and durations showed a significantly higher mean applanation pressure than both a normal control group and literature populations. There was no significant difference between these groups for tonographic C values before drinking water, but lower values for C and higher values of the log of the Po/C ratio were found in the steroid group after water drinking. Cumulative per cent‐frequency plots for applanation pressures of the steroid patients revealed an obvious deviation to the right from a straight line in the upper end of the curve starting at the point of 60%, representing patients with pressures of 20 mmHg or higher. Presence of a higher mean applanation pressure and absence of an adequately reduced outflow facility in the systemic steroid group are interpreted as evidence of an increased formation of aqueous humor in individuals treated with steroids by this route. Analysis of the influence of dosage and duration of treatment disclosed that differences in the mean applanation pressure between these subgroups were not due either to duration or dosage of treatment but to the presence or absence of individuals within the group responding to steroids with an increase of pressure. The proof of this fact will be presented in the second part.

Some factors affecting the distribution of intraocular pressures in a population.

The rise in ocular tension with age is dependent on a concomitant rise in systemic blood pressure. Sex, season, time of day and systemic blood pressure have independent effects on the distribution of applanation pressures in a population, but are to the best of our knowledge not associated with glaucoma. Their combined effects are large enough to support the opinion that they should be taken into account when evaluating the results of applanation tonometry.

Blood in schlemm's canal in glaucoma suspects. A study of the relationship between blood-filling pattern and outflow facility in ocular hypertension.

The blood filling of Schlemm's canal was studied in 44 patients with open chamber angles who had experienced ocular hypertensive episodes but suffered no visual damage. The findings were compared with tonographic data. A majority of those with normal blood filling in both eyes were relatively young (mean age ±54.7) and showed good outflow facilities (mean C, 0.26 ± 0.078); those with abnormal filling in one eye only were relatively old (mean age, 60.37) and showed poor outflow facilities (mean C, 0.186 ± 0.041); and, those with abnormal filling in both eyes were even older (mean age, 69.94) and also showed poor outflow facilities (mean C, 0.18 ± 0.051). The applanation pressures did not vary significantly between the three groups of individuals.

Combined Medical and Surgical Therapy for Complicated Cases of Peripheral Uveitis

Trapdoor diathermy beneath the exudative mounds of peripheral uveitis reduced the inflammation. Five eyes were operated on because of secondary glaucoma and were controlled by diathermy and medical therapy. One eye with detachment was reattached, and the exudative mass regressed. The exudative masses may disappear for years following the diathermy. No surgical complications were encountered. Unilateral operations of the most severely involved eye, in cases of bilateral disease, resulted in the operated eye being quieter than the unoperated eye. Posterior pole edema and glaucoma frequently occurred in these cases. Applanation pressures were significantly higher than Schiotz. Peripheral uveitis is common and should be suspected in cases of cyclitis without a posterior chorioretinal scar. When steroids produce side effects or fail to control the inflammation of peripheral uveitis, antimetabolites (ie, cyclophosphamide) have proven effective.

CORTICOSTEROIDS AND INTRAOCULAR PRESSURE.

Introduction The finding of γ-globulin and plasma cells in the trabecular meshwork of glaucomatous eyes suggested an immunogenic component. 1,2 The present study was undertaken to determine, under controlled conditions, the effect of topical corticosteroids in the eye with primary open-angle glaucoma. Corticosteroids have been incriminated in the exacerbation or production of the glaucomatous state. 3-7 Such reports were largely concerned with glaucoma occurring during the course of uveitis and were difficult to interpret. Several authors have demonstrated no effect of steroids on intraocular pressure. 8,9,10 In a controlled study of topical steroids in nurses, Linner 11 noted a small but significant increase in intraocular pressure without change in outflow facility. Francois 12 and Goldmann 13 have recorded glaucomatous states after prolonged topical steroid therapy, but these were considered rare occurrences. Recently Bernstein and Schwartz 14 found that patients on long-term systemic corticosteroid therapy showed significantly higher mean applanation pressures