The validity and reliability of the handheld SW-100 autokeratometer

Background: The agreement of new instruments or clinical tests with other instruments or tests defines the possibility of these being used interchangeably.Aim: To investigate the validity and reliability of the SW-100 autokeratometer using a Bausch & Lomb (B&L) keratometer as the ‘gold standard’. Methods: Eighty subjects (80 right eyes) aged between 21 and 38 years were recruited. For intra-test repeatability, two measurements of the corneal radius of curvature were taken with the SW-100 and B&L keratometers. Forty of the 80 subjects participated in the inter-test repeatability measurement.Results: Corneal radius of curvature was found to be statistically different between the two instruments (p < 0.001), with the SW-100 providing slightly flatter values of 0.11 mm and 0.05 mm for the horizontal and vertical meridians, respectively, than the B&L keratometer. The average corneal curvature was 0.07 mm flatter with the SW-100 autokeratometer than with the B&L device. Agreement between the SW-100 and B&L keratometers’ axes was 45% within ± 5°, 60.3% within ± 10°, 78.8% within ± 15°, 80.3% within ± 20°, and 88.7% within ± 40°. Intertest repeatability was better for the B&L device than the SW-100 and showed no significant difference between the two sessions. Both instruments demonstrated comparable intrasession repeatability. As such, both instruments were comparatively reliable (per coefficients of repeatability). The range of limits of agreement of ± 0.14 mm (horizontal meridian) and ± 0.17 mm (vertical meridian) between the SW-100 and B&L devices showed good agreement.Conclusion: The results suggest that the SW-100 autokeratometer is a reliable and objective instrument that, however, provides flatter radii of curvature measurements than the B&L keratometer. A compensating factor incorporated into the instrument could reduce the difference between the two instruments and make them more interchangeable. 

Central corneal thickness and axial length in an adult Nigerian population

Purpose

The aim of this study was to determine the relationship between central corneal thickness (CCT) and axial length (AL) in adult Nigerians.

Methods

A total of ninety-five (n  = 95) subjects aged between 20 and 69 years consisting of 56 males and 39 females were recruited for this study. The CCT was measured by ultrasound pachymetry using SW-1000P ultrasound pachymeter (Tianjin Suowei Electronics Technologies LTD, China) and the axial length by A-Scan ultrasonography using I-2100 A Scan Biometer (Cima Technology Inc., USA).

Results

The average CCT and AL were 547.0 ± 29.5 μm and 23.5 ± 0.70 mm, respectively. Age inversely correlated with CCT (r  = −0.32, p  = 0.02), but had no effect on AL (r  = 0.082, p  = 0.43). The association between CCT and AL showed an inverse trend but was not statistically significant (r  = −0.10, p  = 0.32). Neither CCT nor AL was affected by gender (p  = 0.11; p  = 0.63, respectively).

Conclusion

CCT decreases with age, but AL was not affected by age. The association between CCT and AL was not significant. Gender did not show any statistically significant effect on CCT and AL.

Age, gender, corneal diameter, corneal curvature and central corneal thickness in Nigerians with normal intra ocular pressure

Purpose

To investigate the relationship between age, gender, corneal diameter, central corneal curvature, central corneal thickness (CCT) and intraocular pressure in Nigerians with normal intraocular pressure.

Methods

One hundred and thirty eyes from 130 subjects (mean age = 47.8 ± 16.8 years) including 77 males and 53 females were recruited. CCT was measured by ultrasound pachymetry, intraocular pressure was determined by non-contact tonometry, horizontal and vertical corneal diameters were measured with a ruler and central corneal curvature was measured by keratometry.

Results

The mean values obtained were as follows: CCT = 548.97 ± 34.28 μm, IOP = 15.61 ± 2.69 mmHg, average corneal curvature (AVK) = 42.98 ± 1.19 D, horizontal corneal diameter (HVID) = 11.39 ± 0.69 mm and vertical corneal diameter (VVID) = 10.51 ± 0.50 mm. There was a significant effect of age on CCT (r = −0.35, p < 0.001). A 10-year-increase in age was associated with a 7.0 μm decrease in CCT. Males had significantly wider HVID than females (p = 0.03). Subjects in older age groups have narrower HVID and VVID. Corneal curvature, corneal diameter and gender did not significantly affect CCT.

Conclusion

CCT of normotensive Nigerian adults decreases with increasing age. There was no correlation between CCT and IOP in normotensive subjects. CCT was not significantly influenced by gender, corneal curvature and corneal diameter.

Anthropometry, amplitude of accommodation, and spherical equivalent refractive error in a nigerian population

Aim. The aim of this study was to investigate the association between anthropometry, amplitude of accommodation assessed by minus-lens to blur (AAPUB) and push-up to blur (AAMLB), and spherical equivalent refraction (SEQ). Method. A total of two hundred and one subjects aged between 17 and 70 years with mean age of 34.2 ± 13.3 years, consisting of 93 males and 108 females were recruited for this study. Anthropometric variables were measured with standard instruments like the free-standing rod for height, weighing balance for body weight, and body mass index (BMI) calculated. The refractive error was measured by static retinoscopy and subjective refraction. Result. An inverse correlation was found between age, AAMLB and AAPUB (r = -0.84, -0.81, both P < 0.0001). BMI increased with age (r = 0.32, P < 0.0001). There was an inverse correlation between BMI, AAPUB and AAMLB (r = -0.27, -0.25, both P < 0.0001), respectively. However, the association between SEQ and anthropometry was not significant (P > 0.05). The AAPUB and AAMLB decreased with age while BMI increased. AAPUB and AAMLB decreased with BMI, but were not affected by the SEQ. Conclusion. BMI increased with age while AA measured by the two methods decreased with age, and BMI increased with decreasing AA.

The relationship between central corneal thickness and corneal curvature in adult Nigerians

Purpose: The aim of this study was to provide average values for central corneal thickness (CCT) and corneal curvature (CC) and also to determine a regression model for the relationship between CCT and CC in adult Nigerians without glaucoma.Methods: A total of 95 subjects consisting of 56 males and 39 females aged between 20 and 69 years with mean age of 47.1 ± 14.1 years were recruited for the study. Central corneal thickness was measured by ultrasound pachymetry (SW-1000P ultrasound pachymeter, Tianjin Suowei Electronic Technology, China) and corneal curvature was measured by keratometry (Bausch & Lomb keratometer H-135A, USA).Results: The average values of 550.1 ± 33.1µm and 43.0 ± 1.1 D were obtained for CCT and CC respectively. CCT significantly correlated with age (p=0.01), and the regression model predicts a decrease of 6.0 µm in CCT per decade. No significant association was found between CC and age (p=0.56). Also, no significant association was found between CCT and CC (p=0.07). Female subjects had significantly steeper corneas than their male counterparts.Conclusion: Central corneal thickness decreases with increasing age. Neither CCT nor age appear to be significantly correlated with corneal curvature. (S Afr Optom 2011 70(1) 44-50)

The role of axial length-corneal radius of curvature ratio in refractive state categorization in a nigerian population

The aim of this study was to investigate the association of axial length (AL)/corneal radius of curvature (CRC) ratio (AL/CRC) with spherical equivalent refractive state (SER) in young adults. A total of seventy (n = 70) subjects consisting of 31 males and 39 females participated in this study. Subjects were categorized into emmetropia, hyperopia and myopia using the spherical equivalent refraction. The axial length was measured with I-2100 A-Scan ultrasonography/Biometer (CIMA Technology, USA), the corneal radius of curvature with Bausch & Lomb H-135A (Bausch & Lomb Corp., USA), and the refractive state by static retinoscopy and subjective refraction. The mean AL, CRC and AL/CRC ratio of all subjects were 23.74 ± 0.70 mm, 7.84 ± 0.19 mm, and 3.03 ± 0.14, respectively. Myopes had significantly longer AL, steeper CRC and higher AL/CRC ratio than the emmetropes and hyperopes. There was statistically significant inverse correlation between AL and CRC (r = −0.53, P < 0.0001), SER (r = −0.64, P < 0.0001), and between SER and AL/CRC (r = −0.78, P < 0.0001). A significant positive correlation was found between CRC and SER (r = −0.69, P < 0.0001). The categorization of the refractive state of an individual is better done by using the AL/CRC ratio index.

The relationship between central corneal thickness and intraocular pressure: a comparative study of normals and glaucoma subjects

The purpose of the study is to determine whether central corneal thickness (CCT) is a better predictor than intraocular pressure (IOP) in early identification of those at higher risk of developing glaucoma. Sixty-five subjects were categorized into normals, ocular hypertensives and glaucoma subjects based on clinical characteristics of ocular risk factors. The lOP was assessed with slit-lamp mounted Goldmann applanation tonometer. Prior to applanation tonometry, the central corneal thickness (CCT) of both eyes was assessed with Sonomed PacScan 300AP Biometric/pachymeter. The difference in mean IOP between normals and glaucoma subjects was statistically significant (unpaired t-test; p<0.05). Similarly, there was a significant difference in mean CCT between normals and glaucoma subjects (p<0.05). The association between CCT and Age was not significant in normals but slightly significant in glaucoma subjects and the linear regression predicts a decrease of 7.0 ìm in CCT for every 10 years. A strong association was found between CCT and IOP for ocular hypertensives with a prediction of increase of 0.70 mmHg for every 10 ìm corneal thickening. The association between CCT and IOP for glaucoma subjects was weak, with an indication of an increase of 0.35 mmHg in intraocular pressure for every 10 ìm corneal thinning. The central corneal thickness is a better predictor than intraocular pressure in identifying those at higher risk of developing primary open-angle glaucoma when combined with some ocular risk factors.