The relationship between measurement method and corneal structure on apparent intraocular pressure in glaucoma and ocular hypertension.

PURPOSE: To analyse the relationship between measured intraocular pressure (IOP) and central corneal thickness (CCT), corneal hysteresis (CH) and corneal resistance factor (CRF) in ocular hypertension (OHT), primary open-angle (POAG) and normal tension glaucoma (NTG) eyes using multiple tonometry devices. METHODS: Right eyes of patients diagnosed with OHT (n=47), normal tension glaucoma (n=17) and POAG (n=50) were assessed. IOP was measured in random order with four devices: Goldmann applanation tonometry (GAT); Pascal dynamic contour tonometer (DCT); Reichert ocular response analyser (ORA); and Tono-Pen XL. CCT was then measured using a hand-held ultrasonic pachymeter. CH and CRF were derived from the air pressure to corneal reflectance relationship of the ORA data. RESULTS: Compared to the GAT, the Tonopen and ORA Goldmann equivalent (IOPg) and corneal compensated (IOPcc) measured higher IOP readings (F=19.351, p<0.001), particularly in NTG (F=12.604, p<0.001). DCT was closest to Goldmann IOP and had the lowest variance. CCT was significantly different (F=8.305, p<0.001) between the 3 conditions as was CH (F=6.854, p=0.002) and CRF (F=19.653, p<0.001). IOPcc measures were not affected by CCT. The DCT was generally not affected by corneal biomechanical factors. CONCLUSION: This study suggests that as the true pressure of the eye cannot be determined non-invasively, measurements from any tonometer should be interpreted with care, particularly when alterations in the corneal tissue are suspected.

Changes of corneal biomechanics with keratoconus.

PURPOSE: To perform advanced analysis of the corneal deformation response to air pressure in keratoconics compared with age- and sex-matched controls. METHODS: The ocular response analyzer was used to measure the air pressure-corneal deformation relationship of 37 patients with keratoconus and 37 age (mean 36 ± 10 years)- and sex-matched controls with healthy corneas. Four repeat air pressure-corneal deformation profiles were averaged, and 42 separate parameters relating to each element of the profiles were extracted. Corneal topography and pachymetry were performed with the Orbscan II. The severity of the keratoconus was graded based on a single metric derived from anterior corneal curvatures, difference in astigmatism in each meridian, anterior best-fit sphere, and posterior best-fit sphere. RESULTS: Most of the biomechanical characteristics of keratoconic eyes were significantly different from normal eyes (P < 0.001), especially during the initial corneal applanation. With increasing keratoconus severity, the cornea was thinner (r = -0.407, P < 0.001), the speed of corneal concave deformation past applanation was quicker (dive; r2 = -0.314, P = 0.01), and the tear film index was lower (r = -0.319, P = 0.01). The variance in keratoconus severity could be accounted for by the corneal curvature and central corneal thickness (r = 0.80) with biomechanical characteristics contributing an additional 4% (total r = 0.84). The area under the receiver operating characteristic curve was 0.919 ± 0.025 for keratometry alone, 0.965 ± 0.014 with the addition of pachymetry, and 0.972 ± 0.012 combined with ocular response analyzer biomechanical parameters. CONCLUSIONS: Characteristics of the air pressure-corneal deformation profile are more affected by keratoconus than the traditionally extracted corneal hysteresis and corneal resistance factors. These biomechanical metrics slightly improved the detection and severity prediction of keratoconus above traditional keratometric and pachymetric assessment of corneal shape.

The use of the Ocular Response Analyser to determine corneal hysteresis in eyes before and after excimer laser refractive surgery.

PURPOSE: To compare corneal biomechanical parameters and two measures of intraocular pressure (IOP) in eyes before and after excimer laser refractive surgery, with the Ocular Response Analyser (ORA). MATERIALS AND METHODS: Eighty normal eyes of 41 patients undergoing excimer laser refractive surgery in Birmingham, U.K. were recruited into three groups: Laser Assisted-Epithelial Keratomileusis (LASEK) (Myopes), Laser Assisted in Situ Keratomileusis (LASIK) (myopes) and LASIK (hyperopes). The preop and 3 months postop Goldmann correlated IOP (IOPg), corneal compensated IOP (IOPcc), corneal hysteresis (CH), and corneal resistance factor (CRF) were measured by the ORA. Central corneal thickness (CCT) was measured using ultrasonic pachymeter. The differences of the changes in IOPg, IOPcc, CH, CRF and CCT between the three groups were estimated. A General Linear Model was selected to investigate the influence of gender, age, initial conditions (CH, CRF, CCT, IOPcc and IOPg) and changes in CCT on the measured IOP. RESULTS: The differences between the mean IOPg, CH and CRF after refractive surgery were statistically significant for all three groups. The hyperopic LASIK group had a significantly smaller change compared to the other groups (which had no statistical significance). The preop IOPg, preop CH and gender were significant predictors of the changes in measured pressure and biomechanical parameters after surgery in the myopic groups only. CONCLUSION: CH and CRF were found to decrease after both myopic and hyperopic refractive surgery. CH and CRF measurement may prove important tools to clarify the role of corneal biomechanics for refractive surgery.

Ocular response analyser to assess hysteresis and corneal resistance factor in low tension, open angle glaucoma and ocular hypertension.

PURPOSE: The aim of this study is to compare the hysteresis and corneal resistance factor (CRF) in normal tension glaucoma (NTG), primary open angle glaucoma (POAG) and ocular hypertension (OHT) eyes measured by the ocular response analyser (ORA). METHODS: This is a prospective, cross-sectional and comparative clinical trial. The setting was a teaching hospital in Birmingham, England. PATIENTS: 216 eyes with POAG, 68 eyes with NTG and 199 eyes with OHT. Observational procedures: Goldmann applanation tonometry and intraocular pressure (IOP), hysteresis and CRF measured by ORA and central corneal thickness (CCT) by ultrasonic pachymetery. The main outcome measures were IOP, CCT, hysteresis and CRF. RESULTS: The hysteresis in NTG, POAG and OHT eyes was 9.0 +/- 1.9, 9.9 +/- 2.1 and 10.2 +/- 2.0 mmHg; CRF was 9.1 +/- 2.2, 10.6 +/- 2.0 and 12.0 +/- 2.0 mmHg; IOP by Goldmann applanation tonometry and ORA was 14.7 +/- 2.8 and 15.3 +/- 4.2 mmHg, 16.7 +/- 4.0 and 16.9 +/- 4.6 mmHg and 20.5 +/- 4.1 and 20.0 +/- 4.5 mmHg; CCT was 526.5 +/- 42.2, 537.0 +/- 36.0 and 563.4 +/- 35.9 microm, respectively. The difference for CRF, IOP and CCT for NTG, POAG and OHT eyes was statistically significant. CONCLUSION: Hysteresis and CRF were highest in OHT eyes. These factors may prove to be useful measurements of ocular rigidity and may help to understand role of the corneal rigidity in monitoring the progress of conditions such as NTG, POAG and OHT.

A 10-year follow up of ocular hypertensive patients within the Bolton Corneal Thickness Study. Can measured factors predict prognostic outcomes?

OBJECTIVE: This is a case note review of a cohort of patients examining the effects of central corneal thickness (CCT), presenting intraocular pressure (IOP), age and gender on the risk of progression of ocular hypertension (OHT) to primary glaucoma (POAG) over a 10-year period. DESIGN: Cohort study with retrospective case note review. PARTICIPANTS AND METHODS: 58 case notes from a cohort at the Bolton Royal Infirmary initially assessed 10 years ago were reviewed again. Presenting CCT, IOP, age and gender were recorded. Development of POAG was assessed by visual field and/or optic disc changes being present. MAIN OUTCOME MEASURES: The effects of CCT, IOP, age and gender on the risk of progression of OHT to POAG were analysed in a multivariate logistic regression model following a preliminary univariate analysis. RESULTS: 50 out of 116 eyes developed primary open angle glaucoma over the 10-year period. Thinner CCT (odds ratio 0.985 associated with each 1mum increase in CCT), higher presenting IOP (odds ratio 1.131 associated with each 1mmHg increase in IOP) and increasing age (odds ratio 1.062 associated with each 1 year increase in age) were found to be associated with progression to POAG. CONCLUSION: Patients with a CCT of 579microm or more, a presenting intraocular pressure of 26mmHg or less and age 75 years or less had a lower risk of developing POAG within this cohort of patients.

Diurnal variation of ocular hysteresis in normal subjects: relevance in clinical context.

BACKGROUND: This study was conducted to assess the diurnal variation in ocular hysteresis, as measured by the Ocular Response Analyser to establish a relationship between diurnal hysteresis variation and diurnal intraocular pressure (IOP) variation. METHODS: Forty-two normal eyes of 21 colleagues and staff in a teaching hospital in Birmingham, UK, were recruited. The IOP and hysteresis were measured by the Ocular Response Analyser. The central corneal thickness (CCT) was measured using a hand-held ultrasonic pachymeter in the mid-pupillary axis. RESULTS: The mean ocular hysteresis at 8 am was 12.7 +/- 2.3 mmHg, at 11 am was 12.2 +/- 2.0 mmHg, at 2 PM was 12.7 +/- 2.1 mmHg and at 5 PM was 12.7 +/- 1.7 mmHg; the difference between the values at any time of measurement was not statistically significant (P > 0.9, repeated measures). IOP as measured by non-contact tonometry was 18.4 +/- 2.8 mmHg, 17.9 +/- 3.3 mmHg, 16.9 +/- 3.1 mmHg and 16.8 +/- 3.2 mmHg, respectively, for the same time period; the difference between the values in the morning and afternoon was statistically significant (P < 0.0001, repeated measures). The CCT was 548.8 +/- 29.5 microm, 547.0 +/- 31.4 microm, 548.2 +/- 29.6 microm and 548.6 +/- 29.4 microm, respectively; the difference between the values was not statistically significant at any time points. Multiple regression analysis showed the relationship between IOP and hysteresis was not statistically significant (P = 0.9). CONCLUSION: The ocular hysteresis reading was almost constant throughout the day, whereas the IOP readings showed highest values in the morning with a reducing trend being lowest in the afternoon. The CCT values were almost stable throughout the day. IOP appears to vary independently of a variation in hysteresis or CCT.

The tear (lacrimal) meniscus height in human eyes: a useful clinical measure or an unusable variable sign?

PURPOSE: To review notes and reports in the literature on assessments of human tear meniscus height (TMH) with the view to defining what the current state of knowledge is, and allow comparisons with data obtained on elderly individuals. METHODS: TMH data was obtained from 97 elderly individuals without significant eye disease. Photo-slit-lamp views, perpendicular to the lower marginal tear strip, were used to obtain close-up images by videography over approximately 30s. Repeated measures at the same location separated by a few seconds (time-dependent TMH measures, tTMH), or repeated measures along the length of the middle portion of the lower eyelid (position-dependent TMH measures, pTMH) were made. The results were compared to those from published reports of TMH. RESULTS: For elderly individuals, tTMH averaged 0.185+/-0.088 mm, while pTMH averaged 0.189+/-0.093 mm. The distribution of TMH measures however included a number of data sets with a much higher average, and which could be separated from the main group of data using cluster analyses approach (> 0.23 mm, P < 0.001). Literature-reported values for TMH, assessed by a perpendicular view, average 0.215 mm (range of average values from 0.163 to 0.350 mm), whereas those made from a tangential view average 0.343 mm (range 0.215-0.460 mm). CONCLUSIONS: The average TMH value of close to 0.2mm in the elderly, as compared to the small number of individuals with rather greater TMH values of up to 0.7 mm, might therefore be considered as indicative of their having normal tear volume, with a few individuals having reflex tearing. Based on the data and literature values, a lower cut-off limit for normal would be /=0.25 mm would indicate reflex tearing and/or sub-optimal tear drainage. An alternative interpretation is that elderly individuals have a subnormal tear film volume, with just a few individuals having adequate tear volume. Further work is clearly needed to resolve this issue.