Clare O'Donnell

Agreement and repeatability of central thickness measurement in normal corneas using ultrasound pachymetry and the OCULUS Pentacam

Purpose: To compare the accuracy and repeatability of the OCULUS Pentacam (a new Scheimpflug-based imaging system) with ultrasound pachymetry in the measurement of central corneal thickness (CCT). Methods: CCT was measured in 21 subjects (21 normal corneas) on 2 separate occasions by the same examiner, using an Allergan-Humphrey 850 ultrasonic pachymeter and an OCULUS Pentacam instrument. Results: Mean values of CCT for both visits for each instrument were 534 ± 47 μm and 528 ± 45 μm using the ultrasonic pachymeter and the Pentacam, respectively. Plots of differences against means displayed relatively good agreement (limits of agreement were −13.0 to +26.6 μm). The repeatability (limits of agreement) of the ultrasound pachymeter was −18.3 to +17.7 μm, while for the OCULUS Pentacam it was −24.1 to +21.1 μm. Conclusions: Our data showed that the Pentacam instrument provided measurements that were slightly but systematically lower than the measurements provided by ultrasonic pachymetry, which is currently the clinical gold standard method. The results, coupled with a unique ability to image and analyze the anterior chamber in vivo, make the OCULUS Pentacam a promising new instrument for anterior eye evaluation.

Subbasal nerve fiber regeneration after LASIK and LASEK assessed by noncontact esthesiometry and in vivo confocal microscopy: Prospective study

PURPOSE: To evaluate recovery of the corneal subbasal nerve plexus and corneal sensitivity after myopic laser in situ keratomileusis (LASIK) and laser‐assisted subepithelial keratectomy (LASEK). SETTING: Manchester Centre for Vision, Royal Eye Hospital, Manchester, United Kingdom. METHODS: Thirty LASEK patients and 20 LASIK patients had slit‐scanning confocal microscopy and noncontact corneal esthesiometry preoperatively and 1, 3, and 6 months after surgery. Images of the subbasal nerve plexus were analyzed using customized software to evaluate nerve regeneration. RESULTS: Central corneal sensitivity decreased significantly 1 month after LASEK and LASIK and returned to normal levels after 3 months. Corneal subbasal nerve fiber density, nerve branch density, nerve fiber length, and nerve fiber width decreased significantly 1 month after LASIK and had not returned to the preoperative levels by 6 months. Nerve fiber tortuosity decreased significantly 1 month after LASEK and returned to the preoperative levels 3 months after surgery. There were no significant differences in nerve fiber tortuosity before and after LASIK. Neither corneal sensitivity nor nerve fiber morphology was different between the 2 groups at any postoperative visit. CONCLUSIONS: Corneal sensitivity and subbasal nerve morphology were adversely affected by LASEK and LASIK. Corneal sensitivity recovered 3 months after the procedure, but subbasal nerves were still abnormal after 6 months. Despite the different forms of surgical trauma to corneal nerves with LASIK and LASEK, there was no apparent difference in the time course of recovery of corneal structure and function.

Repeatability of corneal thickness measured using an oculus pentacam

Purpose. To evaluate the repeatability of corneal thickness measurements over time, obtained with an Oculus Pentacam. Methods. Measurements of corneal thickness (apical and peripheral) were performed at three different sessions in one eye of 23 healthy subjects, using an Oculus Pentacam instrument. In each session, three consecutive measurements were taken (without realignment) by the same observer. Measurements were performed, immediately after a blink. Results. Mean values obtained were in agreement with published literature values for central corneal thickness (mean ± SEM: 546.23 ± 5.24 μm for first session over the first 1 min, 545.13 ± 5.49 μm for second session 1 h later, and 543.23 ± 4.96 μm for the third session measured 1 week later). The data showed an increase in corneal thickness towards the periphery with a decrease in intraobserver reproducibility. The confidence limits in the Bland and Altman charts for pachymetry at the corneal apex ranged between −10.91 μm and 13.00 μm within the first session measured over the first 1 min, −13.43 μm to 11.23 μm between the first and second sessions after 1 h and −11.99 μm to 10.02 μm between the first and third sessions after 1 week. The confidence limits were found to increase towards the peripheral cornea. There was no significant difference in within-subjects variance between the three time scales (p > 0.05 for all parameters measured). Conclusion. Our data showed that the Oculus Pentacam provides repeatable measures of corneal thickness when measured within a few seconds, after 1 h, and 1 week.

Pellucid corneal marginal degeneration: A review

Pellucid marginal corneal degeneration (PMD) is a rare ectatic disorder which typically affects the inferior peripheral cornea in a crescentic fashion. The condition is most commonly found in males and usually appears between the 2nd and 5th decades of life affecting all ethnicities. The prevalence and aetiology of this disorder remain unknown. Ocular signs and symptoms of patients with PMD differ depending on the severity of the condition. Unless corneal topography is evaluated, early forms of PMD may often be undetected however, in the later stages PMD can often be misdiagnosed as keratoconus. Visual signs and symptoms include longstanding reduced visual acuity or increasing against-the-rule irregular astigmatism leading to a slow reduction in visual acuity. In rare cases, patients may present with a sudden loss of vision and excruciating ocular pain due to corneal hydrops or spontaneous perforation. The vast majority of PMD patients are managed using spectacles and contact lenses. Several surgical procedures have been used in an attempt to improve visual acuity when spectacles and contact lenses do not provide adequate vision correction. Since patients with PMD make poor candidates for laser vision correction, an awareness of the topographical and slit-lamp features of PMD will be useful to clinicians screening for signs of corneal abnormality before corneal refractive surgery. This review describes the clinical features of PMD, its differential diagnosis and various management strategies presently available.

Simultaneous continuous recording of accommodation and pupil size using the modified Shin-Nippon SRW-5000 autorefractor

The internal optics of the recent models of the Shin‐Nippon SRW‐5000 autorefractor (also marketed as the Grand Seiko WV‐500) have been modified by the manufacturer so that the infrared measurement ring has been replaced by pairs of horizontal and vertical infrared bars, on either side of fixation. The binocular, open field‐of‐view, allowing the accommodative state to be objectively monitored while a natural environment is viewed, has made the SRW‐5000 a valuable tool in further understanding the nature of the oculomotor response. It is shown that the root‐mean‐square of model eye measures was least (0.017 ± 0.002D) when the separation of the horizontal measurement bars were averaged twice. The separation of the horizontal bars changes by 3.59 pixels/dioptre (r2 = 0.99), allowing continuous on‐line analysis of the refractive state at up to 60 Hz temporal resolution to an accuracy of <0.001D, with pupils >3 mm. The pupil edge is not obscured in the diagonal axis by the measurement bars, unlike the ring of the original optics, so in the newer model pupil size can be measured simultaneously at the same rate with a resolution of <0.001 mm. The measurements of accommodation and pupil size are relatively unaffected by eccentricity of viewing up to ±10° from the visual axis and instrument focusing inaccuracies over a range of 10 mm towards the eye and 5 mm away from the eye. The resolution and temporal properties of the analysis are therefore ideal for the simultaneous measurement of dynamic accommodation and pupil responses.

Repeatability of corneal and ocular aberration measurements and changes in aberrations over one week

Background:  Both Shack‐Hartmann aberrometry (IRX3, Imagine Eyes, Orsay, France) and Scheimpflug photography (Oculus Pentacam, Oculus Inc Wetzlar, Germany) are known to provide repeatable measurements. The variability in measurements of corneal and ocular aberrations obtained with these instruments over one week had not been assessed. The aim was to study the variability in corneal and ocular aberrations in the human eye over one week and to determine the impact of age on corneal and crystalline lens aberrations and on the variability of these measurements.

Biomechanical Properties of Human Corneas Following Low- and High-Intensity Collagen Cross-Linking Determined With Scanning Acoustic Microscopy

PURPOSE To assess and compare changes in the biomechanical properties of the cornea following different corneal collagen cross-linking protocols using scanning acoustic microscopy (SAM). METHODS Ten donor human corneal pairs were divided into two groups consisting of five corneal pairs in each group. In group A, five corneas were treated with low-fluence (370 nm, 3 mW/cm(2)) cross-linking (CXL) for 30 minutes. In group B, five corneas were treated with high-fluence (370 nm, 9 mW/cm(2)) CXL for 10 minutes. The contralateral control corneas in both groups had similar treatment but without ultraviolet A. The biomechanical properties of all corneas were tested using SAM. RESULTS In group A, the mean speed of sound in the treated corneas was 1677.38 ± 10.70 ms(-1) anteriorly and 1603.90 ± 9.82 ms(-1) posteriorly, while it was 1595.23 ± 9.66 ms(-1) anteriorly and 1577.13 ± 8.16 ms(-1) posteriorly in the control corneas. In group B, the mean speed of sound of the treated corneas was 1665.06 ± 9.54 ms(-1) anteriorly and 1589.89 ± 9.73 ms(-1) posteriorly, while it was 1583.55 ± 8.22 ms(-1) anteriorly and 1565.46 ± 8.13 ms(-1) posteriorly in the untreated control corneas. The increase in stiffness between the cross-linked and control corneas in both groups was by a factor of 1.051×. CONCLUSIONS SAM successfully detected changes in the corneal stiffness after application of collagen cross-linking. A higher speed-of-sound value was found in the treated corneas when compared with the controls. No significant difference was found in corneal stiffness between the corneas cross-linked with low- and high-intensity protocols.

A hyper-Dk piggyback contact lens system for keratoconus.

Purpose To report the fitting of a hyper-Dk piggyback contact lens system in a case of keratoconus. Methods Case report. Results A patient with keratoconus, suboptimal visual acuity, corneal neovascularization, and marked conjunctival hyperemia, who was intolerant of rigid gas-permeable contact lenses, was fitted with a novel piggyback contact lens system that incorporated a hyper-oxygen transmissible hydrogel contact lens, on top of which was fitted a hyper-oxygen transmissible rigid gas-permeable lens. This novel contact lens system provided a significant improvement in visual acuity, comfort, and lens wear time. There was a marked reduction in corneal vascularization and ocular hyperemia. Conclusions This hyper-Dk piggyback contact lens system satisfied the ocular and visual requirements of this patient, for whom other nonsurgical options for vision correction had been unsuccessful. This system has potential application in the visual rehabilitation of other cases of corneal irregularity, including unsuccessful refractive surgery, ocular trauma, and keratoplasty. Piggyback contact lens systems require the same level of expertise to fit as basic forms of soft and rigid contact lenses. The hyper-oxygen transmissible lens materials available make these dual-lens systems a viable option for the compromised cornea.

Visual performance and optical quality with soft lenses in keratoconus patients

Citation information: Jinabhai A, Radhakrishnan H, Tromans C & O’Donnell C. Visual performance and optical quality with soft lenses in keratoconus patients. Ophthalmic Physiol Opt 2012, 32, 100–116. doi: 10.1111/j.1475‐1313.2011.00889.x

Reduction in Corneal Volume with Severity of Keratoconus

Purpose: To compare the corneal volume in keratoconic and normal eyes to improve our understanding of the tissue distribution associated with the disease. Materials and Method: The Oculus Pentacam tomographer (Oculus Inc., Wetzlar, Germany) was used to analyze the corneal volume contained within discs with diameters of 3, 5, 7, and 10 mm in 21 patients with keratoconus and 21 matched healthy control subjects. Results: Corneal volume was significantly decreased in the keratoconus group (keratoconus vs. control group: 3.44 ± 0.39 vs. 4.05 ± 0.29 mm3, 10.34 ± 0.95 vs. 11.79 ± 0.84 mm3, 22.80 ± 1.73 vs. 25.26 ± 1.74 mm3, and 57.17 ± 3.94 vs. 61.90 ± 4.12 mm3 for the 3-, 5-, 7-, and 10-mm diameter discs, respectively; p < 0.001). As the corneal disc diameter analyzed increased, fewer differences were found between the control corneas and keratoconic corneas at different stages of the disease. Within the 3-mm and 5-mm diameter discs, significant differences were detected between the control group, moderate keratoconus, and the severe keratoconus groups (p < 0.05). However, within the 10-mm discs, differences were only detected between the control group and the severe keratoconus group (p = 0.005). Conclusions: Corneal volume was significantly decreased in keratoconus, particularly in the central and paracentral area. The decrease in corneal volume in moderate and severe keratoconus as detected by the Pentacam tomographer, may be explained by loss of corneal tissue. In the early stages of the disease, the altered metabolic activity may cause tissue stretching and, as the disease progresses, this stretching is then accompanied by tissue loss.