Corneal thickness measurements with contact and noncontact specular microscopic and ultrasonic pachymetry

PURPOSE: To evaluate the central corneal thickness values in normal and postkeratoplasty corneas with the new Topcon SP-2000P noncontact specular microscopic, contact specular microscopic, and the "common standard" ultrasonic pachymetry. METHODS: Central corneal thickness was determined in 119 eyes of 81 patients (73 normal eyes of 44 patients and 46 eyes after penetrating keratoplasty) first with a noncontact specular microscopic (Topcon SP-2000P; Topcon Corporation, Tokyo, Japan), then an ultrasonic (AL-1000; Tomey, Erlangen, Germany), and finally with a contact specular microscopic (EM-1000; Tomey, Erlangen, Germany) pachymetry two times each by the same investigator. RESULTS: Reliability of the central corneal measurements was equally high both in normal and in postkeratoplasty corneas with all of the instruments (Cronbach alpha = 0.99). Noncontact specular microscopic corneal thickness determination correlated significantly both with ultrasonic (r =.86, P <.0001) and contact specular microscopic pachymetry (r =.62, P <.0001). The ultrasonic pachymetry correlated well with the Tomey pachymetry (r =.69, P <.0001). The Topcon normal mean central corneal thickness value (542 +/- 46 microm) was 28 +/- 4 microm lower (P <.0001) compared with the ultrasonic data (570 +/- 42 microm), which was 68 +/- 1 microm lower (P <.0001) compared with Tomey thickness (638 +/- 43 microm). CONCLUSIONS: Central corneal thickness measurements with noncontact specular microscopic, contact specular microscopic, and ultrasonic pachymetry demonstrate that each of the instruments is reliable but cannot be simply used interchangeably.     

Corneal thickness and endothelial cell density measured by non-contact specular microscopy and pachymetry in Rhesus macaques (Macaca mulatta) with laser-induced ocular hypertension

PURPOSE: Sustained increase in intraocular pressure (IOP) in humans results in a loss of corneal endothelial cells and an increase of corneal thickness. The effects of chronically elevated IOP on the corneal endothelium of monkeys with laser-induced ocular hypertension, a commonly used animal model of human glaucoma have not been documented. This study examined the central corneal thickness (CCT), the corneal endothelial cell density (ECD), and the corneal endothelial cell size (ACS) in Rhesus monkeys with experimental ocular hypertension.Materials and methods. Ten male monkeys with argon laser-induced ocular hypertension in one eye for an average duration of 2.4+/-0.7 years, were sedated with ketamine hydrochloride, and the CCT, ECD, and ACS measured at the center of the cornea of both eyes with a Topcon SP-2000P non-contact specular microscope (Topcon America Corporation((R)), Paramus, NJ, USA). CCT was also measured using a DHG-500 Pachette ultrasonic pachymeter (DHG Technology Inc., Exton, PA, USA). Mean and standard deviation (S.D.) of CCT, ECD and ACS for each eye was calculated and statistically compared.Results. Mean CCT in the hypertensive and normal eyes measured by specular microscopy was 0.477+/-0.023mm and 0.468+/-0.020 mm, respectively. Mean ECD in the hypertensive and normal eyes was 2601.7+/-631.8 and 3990.2+/-402.9 cells mm(-2), respectively. The mean size of the endothelial cells was 252.4+/-23.9 micro m(2) in the normal eye and 408.7+/-115.0 microm m(2) in the hypertensive eye. No significant difference in the measurement of CCT was observed between the specular microscope and the pachymeter (p=0.46).No significant difference in the mean CCT was observed between the two eyes (p=0.4820), whereas the mean ECD was significantly lower in the hypertensive eye than in the normal eye (p<0.001). The ECD was inversely related to the length of IOP elevation (p<0.001). CONCLUSIONS: No difference in the corneal thickness measurement was observed between the specular microscopy and the pachymetry techniques. Chronic ocular hypertension did not significantly affect the CCT, but caused a significant loss of endothelial cells in the center of the cornea of the laser treated eyes compared to the normotensive eyes. The duration of elevated IOP was the most important factor affecting the ECD.     

Corneal thickness measurements: scanning-slit corneal topography and noncontact specular microscopy versus ultrasonic pachymetry

PURPOSE: To compare central corneal thickness measurements taken with 3 pachymetry systems: Orbscan scanning-slit corneal topography/pachymetry, Topcon SP2000P noncontact specular microscopy, and Tomey ultrasonic pachymetry. SETTING: Multicenter study, Tokyo, Japan. METHODS: In 216 healthy eyes of 114 subjects, scanning-slit topography, noncontact specular microscopy, and ultrasonic pachymetry were used in that sequence to record central corneal thickness. In another 20 healthy eyes of 13 subjects, 2 sets of measurements were repeated for each pachymetry to assess repeatability. RESULTS: The mean central corneal thickness was compatible between scanning-slit topography (546.9 micrometers +/- 35.4 [SD] ) and ultrasonic pachymetry (548.1 +/- 33.0 micrometers); however, noncontact specular microscopy gave a significantly smaller mean (525.3 +/- 31.4 micrometers) than the other 2 tests (P<.001, Tukey multiple comparison). There were significant linear correlations between scanning-slit topography and noncontact specular microscopy (r = 0.846, P<.001), noncontact specular microscopy and ultrasonic pachymetry (r = 0.897, P<.001), and ultrasonic pachymetry and scanning-slit topography (r = 0.852, P<.001). Noncontact specular microscopy tended to show the best repeatability; however, the difference was not statistically significant (P =.663, repeated-measure analysis of variance). CONCLUSIONS: Corneal thickness readings were comparable between scanning-slit topography and pachymetry; noncontact specular microscopy gave significantly smaller values. The measurements of the 3 methods showed significant linear correlations with one another. All methods provided acceptable repeatability of measurements.     

Comparison of endothelial cell density estimated by contact and non-contact specular microscopy

PURPOSE: To compare a contact and a non-contact specular microscope in the determination of endothelial cell density. SUBJECTS AND METHODS: One hundred and twenty-one eyes from 70 patients who had undergone various degrees of photorefractive keratectomy for myopia were included. The endothelium was imaged by contact (Konan Clinical Specular Microscope) and non-contact (Topcon SP-1000) specular microscopy and the endothelial cell density estimated. RESULTS: The average endothelial cell density achieved by the contact specular microscope was 3011+/-298 cells/mm2 (mean+/-SD, n=121) and by the non-contact specular microscope 3015+/-265 cells/mm2 (n= 121). The difference in endothelial cell density between the contact and the non-contact specular microscope (contact minus non-contact) was -4+/-175 cells/mm2 (t=0.26, 2p>0.05 in a paired t-test). The sampling error on the estimated endothelial cell density was 76 cells/mm2 for the contact specular microscope and 74 cells/mm2 for the non-contact specular microscope. CONCLUSION: The average endothelial cell density and the precision of the measuring technique were similar for the contact and the non-contact specular microscope. Furthermore, the endothelial cell densities estimated by the two instruments at various values of anterior central corneal refractive power and central corneal thickness were similar. The two instruments can be used interchangeably.     

Corneal thickness measurements in normal and keratoconic eyes: Pentacam comprehensive eye scanner versus noncontact specular microscopy and ultrasound pachymetry

PURPOSE: To compare central corneal thickness (CCT) measurements taken with the Pentacam comprehensive eye scanner (CES), noncontact specular microscopy (SM), and ultrasound pachymetry (UP) in normal and keratoconic corneas. SETTING: Department of Ophthalmology, Ankara University School of Medicine, Ankara, Turkey. METHODS: In a prospective study, 3 CCT measurements were taken with the Pentacam CES, SM, and UP in that sequence from 1 eye of 45 consecutive patients with myopia (group A) and 62 consecutive patients with keratoconus (group B). Eyes with keratoconus were further divided into 3 subgroups, mild, moderate, and severe, according to the mean keratometry readings. RESULTS: Pentacam CES (r = 0.994) and UP (r = 0.993) demonstrated very high and comparable reproducibility in group A. In group B, Pentacam CES displayed better reproducibility (r = 0.988) than UP (r = 0.969) and SM (r = 0.901). The mean CCT measurements of Pentacam CES and UP were not significantly different in group A (P = .37) and in eyes with mild keratoconus (P = .29), whereas significant differences between all instrument pairs were evident in group B and in moderate and severely keratoconic eyes (P<.05). Noncontact SM measurements were significantly smaller than those of Pentacam CES and UP in all groups (P<.05) when a measurement could be obtained. There were significant linear correlations between CCT measurements of Pentacam CES, UP, and noncontact SM in groups A, B, and mildly keratoconic eyes (P<.001). CONCLUSIONS: Results suggest that whereas Pentacam CES and UP may be used interchangeably in normal eyes in the clinical setting for the measurement of CCT, one should be cautious interpreting corneal thickness data from Pentacam CES, UP, and particularly SM in eyes with keratoconus. Whereas, in normal and mildly keratoconic eyes, Pentacam CES and UP demonstrated very high and comparable reproducibility, in moderately keratoconic eyes, Pentacam CES readings showed better reproducibility than UP.     

Corneal curvature,pachymetry, and endothelial cell density in Marfan syndrome

Purpose: To evaluate corneal curvature, pachymetry, and endothelial cell density (ECD) in Marfan syndrome (MFS). Methods: A case–control study in which K values, pachymetry, and ECD were compared in 39 MFS eyes and 40 control eyes matched for age and refraction was conducted. MFS eyes with lens subluxation also were compared with eyes without subluxation. Results: The mean K_med value in MFS eyes was lower than in the control eyes, 42.2 ± 1.9 versus 43.4 ± 1.4 dioptres (D), respectively (p = 0.02). Fifteen MFS eyes (38%) and three control eyes (8%) had K_med values below 41.5 D (p = 0.0012). MFS eyes had generally more corneal astigmatism than control eyes, 1.1 ± 0.9 versus 0.8 ± 0.4 D (p = 0.035), and MFS eyes with lens subluxation had more corneal astigmatism than those without, 1.6 ± 1.1 versus 0.6 ± 0.3 D (p = 0.0002). Nine MFS eyes with corneal astigmatism exceeding 1.5 D also had a subluxated lens. No eyes had keratoconus. The mean pachymetry value was lower in MFS eyes compared to the controls, 485 ± 54.5 versus 514 ± 37.3 μm (p = 0.007); 24 MFS eyes (62%) and 10 control eyes (25%) had measurements below 500 μm (p = 0.01). The mean ECD values were similar in MFS and control eyes, 2815 ± 430 versus 2858 ± 458 cells/mm² (p = 0.66). The mean K value, pachymetry, and ECD values did not differ between MFS eyes with and without lens subluxation. Conclusion: Decreased K values and pachymetry could indicate MFS regardless of subluxation. High corneal astigmatism is associated with subluxation in MFS. Subluxation should be identified in MFS eyes with high corneal astigmatism.     

Comparison of endothelial cell count using confocal and contact specular microscopy

Precise examination of the corneal endothelium has become increasingly important due to the growing number of intraocular and corneal procedures. The purpose of this study was to compare prospectively the corneal endothelial cell count in normal eyes obtained by confocal and specular microscopy. Central corneal endothelial cell densities of 42 eyes from 42 patients were measured by confocal and contact specular microscopy. Endothelial cells were analyzed with the same software in a manual, an automated and a semi-automated mode. The mean endothelial cell density obtained by confocal microscopy was (in the manual, automated and semi-automated modes) 3,069 +/- 285, 2,791 +/- 344 and 3,077 +/- 286 cells/mm(2), and obtained by specular microscopy 3,076 +/- 298, 2,796 +/- 271 and 3,082 +/- 282 cells/m(2), respectively. No statistically significant difference of endothelial cell density between confocal and specular microscopy was found. Endothelial cell count was significantly lower in the automated than in the semi-automated and manual analysis both with confocal and with specular microscopy. In conclusion, endothelial cell count measurements with confocal and contact specular microscopy are comparable.     

Contact and noncontact specular microscopy.

A contact specular microscope was compared with a noncontact endothelial camera on 20 eyes of 10 consecutive patients presenting for cataract surgery. No difference was found in the percentage of photographs acceptable for cell counting or the time for photography by one or other camera. A close correlation was found in cell density estimations by the 2 methods. Most patients prefer the noncontact mode of examination.     

Comparison of Pentacam Scheimpflug camera with ultrasound pachymetry and noncontact specular microscopy in measuring central corneal thickness

PURPOSE: Although central corneal thickness (CCT) can be measured by several methods, interchangeability of different modalities has not been fully investigated. CCT is known to correlate with intraocular pressure (IOP). The aim of this study was to evaluate the agreement of Pentacam Scheimpflug system with noncontact specular microscopy (NCSM) and ultrasound (US) pachymetry in measuring CCT and the relation between IOP taken with Goldmann applanation tonometer (GAT) and the CCT measured with these three methods. METHODS: The right eyes of 135 enrolled persons without antiglaucoma drug use (100 females and 35 males), who comprised 32 patients with primary open-angle glaucoma, 14 with ocular hypertension, 45 with primary angle-closure glaucoma, and 44 controls, were studied. Intermethod comparison of CCT was made by the 95% limits of agreement analysis according to Bland and Altman. Linear regression analysis was used to assess the relationship between IOP and CCT taken with each modality. RESULTS: The mean CCT (+/-SD) taken with Scheimpflug, US, and NCSM was 559.49 +/- 38.44 microm, 553.01 +/- 39.33 microm, and 552.04 +/- 42.95 microm, respectively. The average values of CCT taken with the three instruments were not significantly different (one-factor ANOVA; p = 0.26), although the marginal mean difference between Scheimpflug and US or NCSM was statistically significant (paired t test; p = 0.0009 and 0.005, respectively). The 95% limits of agreement were 6.47 +/- 43.21 microm between Scheimpflug and US, 7.45 +/- 58.86 microm between Scheimpflug and NCSM, and 0.98 +/- 51.69 microm between US and NCSM. There was a positive association between IOP and CCT measured with US or NCSM, whereas there was no correlation between IOP and CCT measured with Scheimpflug. CONCLUSIONS: Although CCT values measured with Scheimpflug, US, and NCSM are closely similar, clinicians should keep in mind that these methods are not simply interchangeable.     

Endothelial cell density determined by specular microscopy and scanning electron microscopy.

Human eyes were photographed with a specular microscope and later examined wit a scanning electron microscope. Corneas from patients undergoing corneal transplantation in whom we were able to obtain preoperative specular micrographs were similarly analyzed. An attempt was made to correlate the counts obtained with both microscopic techniques by determining the amount of shrinkage the cornea undergoes while being processed for SEM. All specimens were counted with a planimeter. We found that the specular microscope adequately analyzes the endothelial cell density in the central and paracentral cornea of a normal eye, but because of its small sampling area specular microscopic counts are subject to significant error when dealing with nonhomogeneous populations such as postoperative cases. We found the peripheral corneal endothelial density to be less than the central endothelial density. Furthermore, we found that we could maximize the accuracy of counting by using a variable frame in a nonhomogeneous population, counting a minimum of four photographs per specimen, analyzing different areas, and analyzing larger areas.     

Comparison of central corneal thickness as measured by non-contact specular microscopy and ultrasound pachymetry before and post LASIK

Purpose: To compare central corneal thickness (CCT) measurements made by ultrasound pachymetry and non‐contact specular microscopy in eyes before and post laser in situ keratomileusis (LASIK). Methods: Forty eyes of 20 consecutive refractive surgery candidates were included in this study. The CCT in both eyes was measured before and 3 months after LASIK with a non‐contact specular microscope and a ultrasound pachymeter. Result: Both before and after LASIK, the difference of the mean CCT between non‐contact specular microscopy and ultrasound pachymetry was statistically significant (P < 0.001 before surgery, and P = 0.02 after surgery). The difference between them was −14.41 μm and −3.69 μm, respectively. In both situations, non‐contact specular microscopy and ultrasound pachymetry measurements were highly correlated (r = 0.959, before surgery, and r = 0.979 after LASIK). Bland–Altman analysis showed that either before or post LASIK, the 95% limits of agreement have a span of approximately 30 μm. Preoperatively, the mean standard deviation of repeated measurements was 4.26 μm by ultrasound pachymetry, and 4.02 μm by non‐contact specular microscopy, whereas it was 3.83 μm by ultrasound pachymetry, and 3.89 μm by non‐contact specular microscopy, postoperatively. No statistically significant differences were found between the mean standard deviations by these two devices in both situations (P = 0.561 before surgery, and P = 0.849 after LASIK). Conclusions: Both before and after LASIK, non‐contact specular microscopy is not in complete agreement with ultrasound pachymetry in measuring CCT. Both devices provided comparable repeatability of measurements.     

Corneal endothelial cell density in iridocyclitis

The endothelial cells of both eyes of 60 unilateral iridocyclitic patients were photographed with a specular microscope to establish the possible effect of iridocyclitis on corneal endothelial cells. Chronic severe iridocyclitis with mutton-fat keratic precipitates (KP) lowered the central endothelial cell count. Five patients out of the seven with mutton-fat KPs had a distinctly lower central cell density in the affected eye. In the remaining patients, no significant difference in cell densities could be demonstrated between affected and healthy eyes. Neither the inflammatory process itself nor the round white KPs had a deleterious effect on the central corneal endothelial cell densities.     

Corneal endothelial cell density after trabeculoplasty

Central corneal endothelial photographs were taken with a specular microscope. The cell density was calculated with a computerized image analyzer. No difference between the treated and control eyes could be found. The laser energy level delivered in trabeculoplasty seems to be too small to induce any permanent changes in the endothelial cells.     

Central corneal thickness measurements using Orbscan II scanning slit topography, noncontact specular microscopy, and ultrasonic pachymetry in eyes with keratoconus

PURPOSE: To compare corneal thickness measurements using Orbscan II scanning slit topography, Topcon SP-2000P noncontact specular microscopy, and ultrasonic pachymetry in eyes with keratoconus. METHODS: Central corneal thickness was measured in 22 eyes with keratoconus. Eyes with apparent corneal opacity were excluded. Scanning slit topography, noncontact specular microscopy, and ultrasonic pachymetry were used in this sequence. The acoustic equivalent correlation factor (0.92) was used for Orbscan readings. RESULTS: Three devices gave significantly different corneal thickness readings (P < 0.001, repeated-measure analysis of variance). Measurements with Orbscan scanning slit topography (449.5 +/- 43.2 [SD] mum) were significantly smaller than those of ultrasonic pachymetry (485.0 +/- 29.3 microm; P < 0.001, Tukey multiple comparison) and SP-2000P noncontact specular microscopy (476.7 +/- 28.3 microm; P = 0.002). There were significant linear correlations between ultrasonic pachymetry and scanning slit topography (Pearson correlation coefficient r = 0.741, P < 0.001), between scanning slit topography and noncontact specular microscopy (r = 0.880, P < 0.001), and between noncontact specular microscopy and ultrasonic pachymetry (r = 0.811, P < 0.001). CONCLUSION: In eyes with keratoconus, Orbscan II scanning slit topography system gave significantly smaller corneal thickness readings than the other 2 devices. Measurements taken by noncontact specular microscopy and ultrasonic pachymetry were comparable. Three devices showed significant linear correlations with one another.     

Corneal endothelial cell density following strabismus surgery

BACKGROUND: The short-term impact of eye muscle surgery on the blood-aqueous barrier has been described by iris fluoresceinangiography and laser tyndallometry. The aim of this study was to investigate the long-term effects of eye muscle surgery on the oxygen supply of the anterior segment of the eye by determination of the corneal endothelial cell density. PATIENTS AND METHODS: We investigated the corneal endothelial cell density of 32 patients with unilateral rectus muscle surgery pre- and postoperatively (24 cases of primary surgery on 2 horizontal muscles, 4 revision surgeries on 2 horizontal muscles, and 4 primary eye muscle surgeries on the 2 vertical recti including: 3 transpositions of the lateral halves of the vertical recti additional to recession of the medial rectus and one complete transposition of the two vertical recti). Exclusion criteria were previous intraocular surgery and retinal detachment surgery. Mean age at surgery was 37.5 +/- 16.3 (14.5 to 79.9) years. Mean follow-up time was 5.0 +/- 2.2 (1.9 to 10.1) months. The corneal endothelial cell density (CECD) was quantified by means of a specular microscope (EM-1000, Tomey). RESULTS: The intraocular difference in CECD was not statistically significant either pre- or postoperatively. Mean values were 84 +/- 297 (731 to 700) vs. 52 +/- 357 (600 to 800), p = 0.64. In the subgroups of patients with revision surgery and those with primary eye muscle surgery involving the vertical recti, there was also no significant intraocular difference of CECD. CONCLUSIONS: Based on the corneal endothelial cell density, there was no evidence of a chronically decreased oxygen supply of the anterior segment even after complex eye muscle surgery.     

Corneal endothelial cell density in twins.

Corneal endothelial cell density was measured in normal subjects and in a group of monozygotic and dizygotic twins. Cell density was found to decline with age and to be similar in right and left eyes in normal subjects. Both monozygotic and dizygotic twins were found to have very similar mean endothelial cells densities and particularly so in like-sided eyes.