Biomedical subjects

A K Lam

Publications and source records attributed to A K Lam.

At least 19 recordsLinked to original sources

PubMed2001

The repeatability and accuracy of axial length and anterior chamber depth measurements from the IOLMaster .

BACKGROUND: Axial length and anterior chamber depth have been measured clinically using conventional ultrasound biometry. Recently, a non-contact device has become available to measure these parameters. This study evaluated the repeatability and accuracy of this device. METHODS: The axial length and anterior chamber depth were measured by two practitioners on a group of young subjects using the IOLMaster followed by a conventional ultrasound biometer operated by a third practitioner. The accommodation was controlled in ultrasound biometry through a full correction on the non-measured eye and a distant fixation target. RESULTS: There was good repeatability and accuracy of axial length assessment. The mean difference between the IOLMaster and ultrasound biometry was -0.099 mm, with 95% limits of agreement between 0.66 and -0.85 mm. The axial length was slightly shorter from the IOLMaster and the difference was not significant. The anterior chamber depth was repeatable but was shown to be deeper than the ultrasound results. The mean difference in anterior chamber depth between the IOLMaster and ultrasound biometry was 0.15 mm, with 95% limits of agreement between 0.34 and -0.03 mm. It is suggested that the former device is not measuring the axial anterior chamber depth. CONCLUSIONS: The IOLMaster is a non-contact 'optical' A-scan which is simple to use and good for axial length assessment. The anterior chamber depth assessment should be further evaluated.

Adult↗

PubMed2000

Molecular biology of esophageal squamous cell carcinoma.

Esophageal squamous cell carcinoma (ESCC) is the predominant histologic subtype of esophageal cancer and characterized by high mortality rate and geographic differences in incidence. With the advances in the field of molecular biology, our understanding of the pathogenesis, epidemiology and behavior of ESCC continues to evolve. The recent development includes research in etiopathogenesis (viruses and cancer susceptibility genes), keratins, tumor related genes (oncogenes, tumor suppressor genes, genes involved in metastasis and apoptosis genes), proliferation-related factors (nuclear proteins, flow cytometry/morphometry, argyrophilic nucleolar organizer region) and factors related to metastases (cell adhesion molecules and enzymes related to degradation of extracellular matrix). There are ranges of molecular techniques potentially available to complement the traditional approaches in the management of ESCC. On the other hand, critics are needed in the interpretation and translation of these research findings from laboratories to clinics. Further investigations, education and collaborations between the various scientific and clinical disciplines are important to successful application of these molecular findings aiming at improving management of patients with ESCC.

Carcinoma, Squamous Cell↗

PubMed2000

Digital measurement of torsional eye movement due to postural change and its effect on reading performance.

PURPOSE: Posture induced torsional eye movements have rarely been investigated. The current study made use of digital imaging technology to measure the cycloduction resulting from postural change. The effect of cycloduction on reading performance was also investigated. METHODS: Thirty subjects were recruited and pictures of the right eye were captured using a digital camera at three postures, i.e. sitting, 90 degrees tilted to the right and 90 degrees tilted to the left. With the identification of a conjunctival landmark, torsional eye movement was measured. The subjects were then required to read a custom designed near chart while in those three postures, the reading card being rotated 90 degrees clockwise or 90 degrees anti-clockwise, to match with the tilting of the head. The reproducibility of the torsional eye movement and reading performance measure was determined in 12 of those 30 subjects. RESULTS: Incycloduction was induced when tilting to the right and excycloduction when tilting to the left. This method was found to be reproducible with the 95% confidence limits of 0.80 degrees between visits. The mean incycloduction induced was 6.50 degrees (SD 1.51 degrees ) and 6.41 degrees (SD 1.46 degrees ) for excycloduction. No significant difference in amount was demonstrated (paired t-test: t = 0.624, P = 0.538). No significant difference was found in the reading scores at various postures (Repeated measures ANOVA: df = 2, F = 1.881, P = 0.162). CONCLUSIONS: The results presented here demonstrate that we have developed an objective and instantaneous method with good precision, which could be applied in other studies that require the measurement of torsional eye movements. The induced cycloduction did not affect the reading performance.

Adolescent↗

PubMed1999

The characteristics of near prism induced fixation disparity curve in Hong Kong Chinese.

The characteristics of prism induced fixation disparity curve at near were studied in 56 asymptomatic young adult Hong Kong Chinese (average age 21 years). The percentage distribution of types of fixation disparity curves was found to be similar to some previous studies, with a higher prevalence of Type I curve (64.3%), followed by Type II (28.6%) and Type III (7.1%) curves. No Type IV curve was revealed from the sample. Two modified measurement approaches were applied to study the effect of measurement techniques on the parameters of fixation disparity curve obtained. A thorough description of the techniques was given. Although a significant difference in the x- and y-intercepts was obtained between different methods, the curve type and slope were not affected. The average y-intercepts (fixation disparity) were 2.72 min of arc and 3.54 min of arc from two methods respectively. Forty-one subjects had x-intercept (associated phoria) found from the curve. The average values were 2.42 delta base-out and 3.54 delta base-out from two methods respectively.

Adult↗

PubMed1999

A preliminary study on the ocular blood flow (OBF) of Hong Kong Chinese.

The Ocular Blood Flow (OBF) Tonometer is an objective instrument to measure the Pulsatile Ocular Blood Flow (POBF), Pulse Amplitude (PA) and Pulse Rate (PR). It also provides the minimum intraocular pressure (IOPmin) simultaneously. This preliminary study applied the OBF Tonometer on 74 Hong Kong Chinese. They had their POBF measured followed by the Goldmann Applanation Tonometer (GAT). The OBF tonometer overestimates the IOP slightly. The average POBF result was smaller (661.2 microliters/min) than the previous studies. The effect of myopia of our sample (mean spherical equivalent -4.27 D) could be one of the contributing factors. Other factors include age and racial difference. Twenty-two subjects had their POBF measured again by the same practitioner and different practitioner for the reproducibility variation assessments. The POBF, PA and IOPmin were found to be repeatable between sessions. The POBF "norm" in Hong Kong Chinese needs to be established from a larger sample size and from a comparison on an age and refraction matched Caucasian group.

Adolescent↗

PubMed1999

The aging effect on corneal curvature and the validity of Javal's rule in Hong Kong Chinese.

PURPOSE: The current study compared the central corneal curvature and the refractive error of Hong Kong Chinese to study the validity of Javal's rule. METHODS: Subjects without corneal pathology were recruited at different age ranges. Their refractive error and the corneal curvature were measured and compared for the right eye only. Two instruments were used for the measurement of corneal curvature including a conventional Bausch & Lomb (B&L) keratometer and a computer-assisted videokeratoscope (TMS-1). RESULTS: Subjects age ranged from 21 years to 77 years were recruited and were categorized into five groups according to age. The Bausch & Lomb keratometer was found to provide corneal information similar to that from the TMS-1. The corneal astigmatism was found to change from with-the-rule to against-the-rule with advancing age. The spectacle astigmatism followed a similar trend. There was a hyperopic shift in the spherical component of the refractive error with aging as well. A regression equation: Spectacle astigmatism = 0.93 (Corneal astigmatism) + (-0.58D x 90) was found, which is similar to the simplified Javal's rule. There was a shift of 1.03D in hyperopia (spherical equivalent) for each decade. CONCLUSIONS: Both the corneal and spectacle astigmatism demonstrated a shift from with-the-rule to against-the-rule with age. The simplified Javal's rule is more suitable for predicting the spectacle astigmatism from corneal astigmatism in Hong Kong Chinese.

Adult↗

PubMed1998

Ret oncogene activation in papillary thyroid carcinoma: prevalence and implication on the histological parameters.

The Ret proto-oncogene is known to be rearranged in papillary carcinoma of the thyroid. The aim of this study was to investigate the in situ expression of Ret mRNA in thyroid tumors. Formalin-fixed, paraffin-embedded tissue specimens from 45 thyroid lesions were examined by in situ hybridization using manual capillary action technology (MicroProbe Staining System) and a 52-base synthetic biotinylated oligonucleotide probe complementary to the tyrosine-kinase domain of Ret proto-oncogene. The clinicopathological features of these patients with thyroid lesions also were noted. Ret was noted in 17 (43%) of 40 papillary carcinomas. In contrast, none of the three follicular carcinomas, follicular adenoma, nodular hyperplasia, and normal thyroids, showed evidence of Ret mRNA. Our results showed that, in papillary thyroid carcinoma, there is an important role of Ret activation. The Ret staining could be a useful marker for papillary carcinoma.

Adenocarcinoma, Follicular↗

PubMed1998

The corneal-thickness profile in Hong Kong Chinese.

PURPOSE: This study investigated the topographic corneal thickness in four groups of Hong Kong Chinese. METHODS: The corneal thickness was determined with an ultrasound pachometer mounted on a X-Y plate. A head rest was used to reduce further any movement from the subject during the measurement. After the central cornea was measured, the peripheral cornea was determined on nasal, temporal, superior, and inferior regions, 2 and 5 mm from the corneal center. RESULTS: The corneal thickness was not significantly different between the right and left eyes. For the right eye, there was no significant thickness difference for the four quadrants at a midperipheral and peripheral region, respectively. There was a general thinning of the corneal thickness at all regions from aging but no difference between the genders. CONCLUSION: The corneal thickness in our subjects was similar to that in other studies. The mean central corneal thickness varied from 541.7 (m to 560.8 microm, dependent on age. This study provides some information for the future studies of Chinese corneal thickness.

Adult↗

PubMed1998

Could a cycloplegic agent be replaced by a fogging or a corrective lens in the biometric measurement of the crystalline lens?

This study investigated whether a fogging or a corrective lens could be used to replace a cycloplegic agent in the ultrasonic measurement of crystalline lens thickness in myopia. A group of 28 Hong Kong Chinese adults with myopia was recruited. The crystalline lens thickness of the examined eye was measured by A-scan ultrasonography while the fixating eye was in one of three conditions: fog (+2.00 D fogging lens), full corrective lens, or cycloplegia (50 minutes after instillation of 1% cyclopentolate HCl). We found that the mean lens thickness was significantly different between the three conditions in our myopic subjects. The mean crystalline lens thickness under fogging and corrective lens conditions was significantly greater than the cycloplegic condition by 0.09 mm and 0.11 mm, respectively. The 95% limits of agreement compared to cycloplegia (fogging: -0.32 to +0.14; corrective: -0.35 to +0.13) showed marked intersubject variability, indicating that there is a risk of overestimating the lens thickness when substituting cycloplegia with either a fogging or a corrective lens.

Accommodation, Ocular↗

PubMed1997

Does the change of anterior chamber depth or/and episcleral venous pressure cause intraocular pressure change in postural variation?

PURPOSE: Previous studies have found that the intraocular pressure (IOP) variation from postural change is due to the obstruction of aqueous outflow by an increase in episcleral venous pressure. This study investigated if any shift of anterior lens position from postural variation would be another contributing factor. METHODS: Thirty-three Chinese subjects were recruited with their IOP and anterior chamber depth (ACD) measured in the sitting, supine, and prone postures. The IOP was measured using a Pulsair 2000 noncontact tonometer and ACD with a Nidek US-2000 EchoScan unit. RESULTS: The highest IOP was obtained in the prone position and this value was significantly different from the IOP obtained in other postures, whereas there was no significant difference in ACD. CONCLUSIONS: Because no significant variation in ACD was demonstrated, the prone and supine IOP variation could be due to something other than the change in lens position. However, a higher IOP in the prone position rather than in the supine position also suggests that it is not merely the episcleral venous pressure causing the IOP change. Investigation of the entire iris profile at different postures would be more informative in future studies.

Adult↗

PubMed1997

The effect of an artificially elevated intraocular pressure on the central corneal curvature.

Twenty-three optometry students with normal corneal condition were recruited. The intraocular pressure and central corneal curvature of the right eye were measured in a sitting and a 30 degrees head-down posture. The mean (standard deviation) IOPs before and during posture change were 15.6 (2.4) mmHg and 22.1 (2.3) mmHg respectively. This 6.5 mmHg mean rise in IOP was found to be statistically significant which is similar to the results from previous studies. The mean changes in radius of corneal curvature and the orientation of the axis of the vertical principal meridian were 0.02 mm (SD 0.025 mm) and 2.4 degrees (SD 10.4 degrees) respectively. No significant variation was demonstrated on these keratometric results due to the 30 degrees head-down posture. The maximum change in radius of curvature was only 0.055 mm for one subject. Perhaps this amount of pressure rise was not sufficient enough to distort the corneal surface centrally. Another possibility could be an even distribution of the elevated pressure around the cornea, or the distribution of pressure is not even but could not be revealed by a conventional keratometer.

Adult↗

PubMed1997

A pilot study on the measurement of central posterior corneal radius in Hong Kong Chinese using Purkinje image technique.

The central posterior corneal radius was measured by assessing the first and second Purkinje images produced from a modified keratometer. This method was initially assessed by measuring five PMMA lens buttons with back surface radii from 6.2 mm to 7.0 mm. It was reasonably accurate when compared with the measurements made using a conventional radiuscope. The technique was found to be repeatable for human subjects measured on three separate occasions. The mean central posterior corneal radii from a group of 30 Hong Kong Chinese were 6.64 mm (SE 0.05 mm) and 6.39 mm (SE 0.05 mm) along the horizontal and vertical meridians respectively. These results are similar to previous studies using a similar technique. The posterior corneal radii were found to be similar for right and left eyes and there were no gender differences.

Adult↗

PubMed1997

The effect of an artificially-elevated intraocular pressure on corneal thickness in Chinese eye.

We measured the central corneal thickness and the applanation intraocular pressure (IOP) on 45 Hong Kong Chinese. There was no obvious relationship between these two parameters, as different from other literatures. It could be due to either a limited number of subjects with a high IOP level (only six subjects with IOP > or = 22 mmHg), or Chinese has a thicker central cornea in general. The mean central cornea of our subjects was thicker (566 +/- 36 microns) than some previous findings. Thirty subjects had their intraocular pressure further increased by adopting a 40 degrees head-down posture. Their IOP and topographic corneal thickness were measured again. There was no significant change in the central corneal thickness even though the IOP was elevated by 11.7 mmHg. However the nasal cornea demonstrated a thinning effect (by some 18 microns) during the IOP elevation but it returned to the pre-inverted level after returning to a sitting posture for 5 min. Further investigation with more corneal regions being measured would be valuable to evaluate the in vivo effect of IOP elevation from glaucoma attack on corneal thickness.

Cornea↗

PubMed1997

Measurement of posterior corneal asphericity on Hong Kong Chinese: a pilot study.

The posterior corneal p-value and apical radius of 60 Hong Kong Chinese were assessed. The values were derived based on the information of the anterior corneal topography and the corneal thickness in different regions. The mean posterior corneal apical radius along the horizontal meridian was 6.51 mm (SD +/- 0.40 mm) and the p-value was 0.34 (SD +/- 0.38). The apical radius is greater while the p-value is smaller than a previous study using a similar principle. This may indicate a flatter posterior cornea and greater peripheral flattening in Hong Kong Chinese. No significant difference between the nasal and temporal corneal thickness, nasal and temporal posterior p-value and apical radius was demonstrated. The right and left eyes were also similar in different ocular parameters apart from a smaller anterior corneal p-value on the right eye (R eye: 0.70 +/- 0.13; L eye: 0.67 +/- 0.12), but the difference may not be significant clinically. The method used here is simple and the generation of posterior corneal topography is informative.

Adult↗

PubMed1996

Application of a modified keratometer in the study of corneal topography on Chinese subjects.

A corneal topography study was carried out on Hong Kong Chinese subjects using a modified keratometer. The Chinese corneal topography measured by this modified keratometer was similar to other studies using the Wesley-Jessen Photo Electronic Keratoscopy (PEK) System 2000. The mean p-values along the horizontal meridian and the vertical meridian were 0.82 and 0.87, respectively. A negative correlation was found between the p-value and the central corneal radius along the horizontal meridian which may indicate that a steeper central cornea will have a greater p-value. However, the correlation coefficient of 0.56 was not too certain for this conclusion to be drawn. A difference of 0.04 was found between the horizontal p-value and the vertical p-value, which may be a result of the tight lid tension along the vertical meridian. This modified keratometer can be considered as an inexpensive instrument for the study of corneal topography.

Adult↗

PubMed1996

Effect of naturally occurring visual acuity differences between two eyes in stereoacuity.

Stereoacuity was measured in 30 subjects with a naturally occurring visual acuity (VA) difference between the eyes. The stereoacuity was measured by a modified Howard's apparatus using the staircase method and VA was measured with log MAR charts. Stereoacuity was worse in subjects with a large VA difference between the two eyes; the correlation between stereoacuity and VA difference was significant (r = 0.76, P < 0.001). Neither the VA of the worse eye nor of the better eye contributed to the reduction in stereoacuity. The deterioration was more obvious if VA difference between the two eyes was one line or more (correlation coefficient, r= 0.88, P < 0.001). This study also reinforces the use of a > or = 70% stereothreshold when attempt stereoacuity results to compare with other studies.

Adolescent↗

PubMed1995

A hand-held keratometer.

Using model eyes the sensitivity to orientation of an Alcon hand-held autokeratometer was assessed after calibration with steel balls. It was concluded that the keratometer should be used in a vertical position (6:00/12:00) in order for axis references to be accurate. Corneal curvature of optometry students was measured using the autokeratometer and a Bausch and Lomb keratometer. The results showed that the autokeratometer was similar to the Bausch and Lomb keratometer in measuring the vertical and horizontal principal meridians, and the axis of the vertical principal meridian. This hand-held keratometer is useful for screening purposes and measuring corneas of infants.

Adult↗

PubMed1994

Three month study of changes in the cornea after computer-determined and conventionally-determined contact lens fitting.

The conventional rigid-lens fitting method uses fluorescein to assess the tear layer beneath a trial lens on the corneal surface. A more advanced technique, which uses a computer program to determine the back surface specifications of a contact lens from a pre-set tear layer thickness, has been studied. No significant difference was found in terms of corneal physiological changes between the conventionally determined contact lenses and computer-determined contact lenses after a three-month wearing period. The results suggest that there is little difference in the two methods of fitting. However, the computer determined method was found to be more efficient in terms of chair time, and also can aid inexperienced rigid lens practitioners to select the appropriate lens back surface curves.

Computer-Aided Design↗