Biomedical subjects

M Yap

Publications and source records attributed to M Yap.

At least 19 recordsLinked to original sources

PubMed2006

Aging and mfERG topography.

AIM: To study the effect of aging retina on the multifocal electroretinogram (mfERG). METHODS: A total of 18 young subjects (age 18-24 years) and 36 elderly subjects (aged 60-85 years) with intraocular lenses (IOLs) were recruited for this study. No subjects had significant eye diseases or media opacities. mfERG was measured in standard conditions using the VERIS system (version 4.1). There were three groups of 18 subjects: (1) 18-25 years, (2) 60-70 years, and (3) 75-85 years. mfERG responses were grouped into central, paracentral, and peripheral regions for analysis. The N1 amplitude, P1 amplitude, N1 latency, and P1 latency of the first-order responses were analysed. RESULTS: Age had no effect on P1 latency, N1 amplitude, and P1 amplitude; however, N1 latencies from central to peripheral regions were significantly longer for group 3 than for group 1. CONCLUSIONS: This study suggests that measured age-related decreases in mfERG responses are due to optical factors (decrease in retinal light levels, scatter) before the age of 70 years, but neural factors significantly affect mfERG topography after the age of 70 years.

Adult↗

PubMed2004

Effects of different degrees of cataract on the multifocal electroretinogram.

AIM: To study the effect of different degrees of nuclear cataract on the multifocal electroretinogram (mfERG). METHODS: mfERGs were recorded from 30 elderly subjects with very mild, mild, or moderate nuclear cataracts using a VERIS System (version 4.1). The subjects were divided into three groups (10 in each group) according to their degree of nuclear cataracts as classified according to the Lens Opacities Classification System III (LOCS III). No subjects had any significant eye disease or degenerative changes except for cataracts. The mfERG responses were grouped into six concentric rings for analysis. Both the N1 and P1 amplitudes and the latencies of N1 and P1 of first-order responses were used for analysis. RESULTS: Amplitudes of N1 and P1 from the central retina (14 degrees) were significantly reduced in patients with mild or moderate cataract when compared with subjects with very mild cataract. However, there was no significant reduction of N1 and P1 amplitudes in the para-central retina (14-40 degrees). There was no difference in the latencies of N1 and P1 in these three groups of subjects. CONCLUSIONS: The mfERG responses from the central retina (central 14 degrees) were affected by the severity of cataract, but responses from the paracentral retina (14-40 degrees) were not affected. This suggests that in interpreting the mfERG in subjects with mild or moderate cataract subjects some care should be taken as reduced amplitudes (N1 and P1) will be expected from the central retina.

Aged↗

PubMed2001

Myopia in Singapore kindergarten children.

PURPOSE: The purpose of this study was to examine whether close-up work was related to myopia in Singapore kindergarten children. METHODS: One hundred twenty-eight children, ages 3 to 7 years, from a kindergarten in Singapore were examined by cycloplegic autorefraction. The parents also completed a questionnaire on the different types of close-up work activities each child was engaged in, socioeconomic status, and parental history of myopia. RESULTS: Myopic children spent 3.0 hours per day (median) on close-up work activity, while nonmyopic children spent 2.0 hours per day (median) on close-up work activity. The prevalence of myopia in the sample was 8.6%. CONCLUSION: Close-up work activity was not related to myopia in pre-school children.

Child↗

PubMed1999

The assessment of obesity: methods for measuring body fat and global prevalence of obesity.

Fat is a normal component of the human body that is stored in adipose tissue. Obesity can be defined as a condition of excessive fat accumulation to the extent that health and well-being are affected. Body fat can be determined in vivo in different ways, using rather accurate laboratory techniques or using simple estimation techniques that can also be applied in field conditions. For population studies, the World Health Organization defines cut-off values for obesity based on the body mass index (BMI): weight/height squared (kg/m2). Generally, for adults, if the BMI exceeds 25 kg/m2, a subject is considered to be overweight, and if the BMI exceeds the value of 30 kg/m2, a subject is considered obese. However, the relationship between body fat percentage and BMI differs between ethnic groups, and, as a consequence, cut-off points for overweight and obesity based on BMI will have to be ethnicity specific. This means that, in some populations, the cut-off points could be lower or higher than the recommended figures. Adapting cut-off point values will have important consequences for prevalence data in some countries as the prevalence of obesity will dramatically increase or decrease. The prevalence of obesity, in children as well as in adults, is high in many countries all over the world and is rising. Given the impact of obesity on health, this is a public health issue that needs to be addressed seriously.

Adipose Tissue↗

PubMed1998

Body mass index and percent body fat: a meta analysis among different ethnic groups.

OBJECTIVE: To study the relationship between percent body fat and body mass index (BMI) in different ethnic groups and to evaluate the validity of the BMI cut-off points for obesity. DESIGN: Meta analysis of literature data. SUBJECTS: Populations of American Blacks, Caucasians, Chinese, Ethiopians, Indonesians, Polynesians and Thais. MEASUREMENTS: Mean values of BMI, percent body fat, gender and age were adapted from original papers. RESULTS: The relationship between percent body fat and BMI differs in the ethnic groups studied. For the same level of body fat, age and gender, American Blacks have a 1.3 kg/m2 and Polynesians a 4.5 kg/m2 lower BMI compared to Caucasians. By contrast, in Chinese, Ethiopians, Indonesians and Thais BMIs are 1.9, 4.6, 3.2 and 2.9 kg/m2 lower compared to Caucasians, respectively. Slight differences in the relationship between percent body fat and BMI of American Caucasians and European Caucasians were also found. The differences found in the body fat/BMI relationship in different ethnic groups could be due to differences in energy balance as well as to differences in body build. CONCLUSIONS: The results show that the relationship between percent body fat and BMI is different among different ethnic groups. This should have public health implications for the definitions of BMI cut-off points for obesity, which would need to be population-specific.

Adipose Tissue↗

PubMed1994

Enhanced IgG production in eRDF media with and without serum. A comparative study.

The performance of three basal media RPMI, DMEM/F12 (DF) and eRDF (enhanced RDF, RPMI:DMEM:F12 in 2:1:1) were evaluated in cultures with and without serum with respect to cell proliferation, metabolism and monoclonal antibody (Mab) productivity. Based on the ease of adaptation, growth rate, maximum cell density and Mab production, the media were ranked as follows: eRDF > DF > RPMI. This was true for serum-free (SF) and serum supplemented (SS) media in static and shaker cultures. Growth performances in static and shaker cultures were consistently 20-50% lower in all three SF media compared to the corresponding SS conditions. Antibody titres in DF/SF and RPMI/SF cultures, irrespective of the culture condition, were generally similar or slightly lower than their SS counterparts. However, eRDF/SF medium yielded a much higher Mab titre (193 mg l-1) compared to eRDF/SS medium (145 mg l-1). This was also six times higher than the lowest titre of 30 mg l-1 in RPMI/SF medium. Hybridomas in eRDF/SF were further adapted to media without bovine serum albumin (eRDF/SF-BSA). Maximum cell densities in these cultures improved with scale up, from 1.1 x 10(6) ml-1 in static, to 1.9 x 10(6) ml-1 in shaker flasks, to 2.5 x 10(6) ml-1 in bioreactors. However, Ig levels remained between 100-130 mg l-1 which were much lower than in eRDF/SF medium. Thus BSA appears to be necessary for Ig production. The manufacturing cost (excluding purification) of Ig using eRDF was calculated to be between 17-50% of the price of the other two media and therefore this is regarded as the best medium for Ig production.

Adaptation, Physiological↗

PubMed1994

PEG-coated lipid vesicles with encapsulated technetium-99m as blood pool agents for nuclear medicine.

Unilamellar lipid vesicles of average diameter 200 nm containing 30 mM glutathione were internally labeled with 99mTc using exametazime (HMPAO) to transport technetium across the lipid bilayer. Vesicles were prepared both in the absence and presence of the lipid-polymer conjugate phosphatidylethanolamine-monomethoxy polyethylene glycol 5000 (PE-MPEG). Labeling efficiency both in the absence and presence of surface polymer was greater than 95% and the vesicles retained greater than 95% of their contents when incubated against 50% human serum at 37 degrees C for 12 h. When introduced intravenously into rabbits at a total lipid concentration of 4 mumol/kg (3.5 mg/kg), radiolabeled vesicles without surface polymer were rapidly cleared from the circulation with a half-life of approx. 30 min and delivered to liver and spleen, however if the lipid vesicles were prepared containing 3 or 4.5 mol percent PE-MPEG the circulation half-life of the label was approx. 5 and 10 h, respectively, and RES uptake was suppressed. These studies confirm a previous report of the utility of exametazime for preparing 99mTc-labeled lipid vesicles and demonstrate that extended circulation half-lives are achievable for 99mTc-labeled vesicles without recourse to high lipid doses and reticuloendothelial blockade. Applications to use exametazime as a blood pool marker in nuclear medicine are discussed.

Animals↗

PubMed1994

Reduction in stereoacuity with age and reduced retinal illuminance.

Stereoacuity has been reported to decrease significantly in the seventh decade of life. Could this decrease be due to a reduction of retinal illuminance in the ageing eye? In this study, we used a more rigorous psychophysical procedure than that used previously, on 35 subjects, aged 21-67 years, to determine if the reported effect of age on stereoacuity could be replicated. We also compared the stereoacuity of 15 healthy young adults under normal and reduced retinal illuminance. A forced-choice algorithm was used to improve the accuracy of measurements. Results suggest that the reduced retinal illuminance resulting from normal ageing is not the cause of the decreased stereoacuity found with ageing.

Adult↗

PubMed1993

Schirmer test. I. A review.

Since its introduction by Schirmer in 1903, the Schirmer test has been widely used for the assessment of the adequacy of tear production. Although the faults of the Schirmer test have been well documented, it is nevertheless still being used to assess tear production, especially in the diagnosis of keratoconjunctivitis sicca (KCS) and for screening potential contact lens wearers. We summarize the findings and conclusions of some of the previous studies.

Contact Lenses↗

PubMed1993

Schirmer test. II. A clinical study of its repeatability.

In most parts of the world, the Schirmer test (ST) is still the main test used by optometrists for the assessment of tear adequacy in the screening of potential contact lens wearers and as an aid in the diagnosis of "dry" eye. We tested the reliability of a 1-min ST in nine normal Hong Kong-Chinese (HK-Chinese) subjects during five to seven visits over a period of 2 weeks. For 4 of the subjects, the test consistently measured 0 mm/1 min, and 68% of the test results were less than 6 mm/1 min, a value suggested as the lower limit for normal asymptomatic eyes. The results obtained were inconsistent and it is concluded that the ST is unreliable and of little clinical value in this population.

Adult↗

PubMed1993

Age, gender, and tear break-up time.

We investigated the effect of age and gender on the tear break-up time (TBUT) of 2 groups of normal, asymptomatic subjects [92 Hong Kong-Chinese (HK-Chinese) and 76 Singapore-Chinese (S-Chinese)] using the conventional fluorescein-instillation technique. The mean TBUT (+/- SD) is 6.5 (+/- 4.0) s for the S-Chinese and 7.8 (+/- 2.4) s for the HK-Chinese. The mode was 7.0 s for HK-Chinese and 5.0 s for S-Chinese. About 80% of the subjects of each group have TBUT less than 10 s. The TBUT of both groups of Chinese decreases with age. It was found that TBUT was significantly different between the two sexes in the S-Chinese but no equivalent effect was observed in the HK-Chinese.

Adolescent↗

PubMed1993

Role of heredity in the genesis of myopia.

Previous studies suggest that myopia in parents may influence the genesis of myopia in their offspring. In this study, we surveyed 2888 children of Chinese descent in China and Hong Kong to determine the relationship between the refractive state of parents and their children. The age range of the children was 7-17 years. Refraction was measured using a Canon R-22 objective infrared autorefractor and a survey on family history of ametropia was conducted using a questionnaire. We found that children with myopic parents are more likely to be myopic.

Adolescent↗

PubMed1993

Glycosylation and high-level secretion of human tumour necrosis factor-beta in recombinant baculovirus-infected insect cells.

Human tumour necrosis factor-beta (TNF-beta) was produced in eukaryotic cells using the insect baculovirus cloning and expression system. A novel insect signal sequence, the honey-bee (Apis mellifera) prepromelittin secretory sequence, was used to aid in the post-translational modifications, glycosylation and secretion of recombinant human TNF-beta. Human TNF-beta cDNA was cloned using the insect baculovirus vector pAcC4s. Expression of the human TNF-beta was regulated by the insect Autographa californica nuclear-polyhedrosis-virus polyhedrin promoter. The 5' end of the TNF-beta cDNA was fused to the honey-bee prepromelittin signal sequence on the baculovirus vector. Insect [Spodoptera frugiperda (Sf9)] cells infected with the recombinant baculovirus secreted high levels of recombinant human TNF-beta into the culture medium. The amount of TNF-beta secreted by the Sf9 cells was estimated to be 28 micrograms of TNF-beta/ml of culture medium at 60-72 h post infection. The secreted human TNF-beta was a 22.5 kDa polypeptide which was glycosylated. Amino acid sequencing of the N-terminus of the recombinant human TNF-beta purified from the infected Sf9-cell culture confirmed that the secreted product was indeed human TNF-beta. This demonstrates that the honey-bee prepromelittin signal sequence was efficiently recognized and accurately cleaved in the Sf9 insect cells. The insect-derived TNF-beta exhibited a high cytotoxic activity similar to that of the native human TNF-beta when assessed by cytotoxic assays using murine L929 cells. Thus the insect baculovirus expression vector can be used for the production of abundant quantities of biologically active, glycosylated human TNF-beta protein.

Amino Acid Sequence↗

PubMed1993

Studies on the chronopharmacology of ethanol.

Male subjects (n = 10) were given ethanol (0.75 g/kg) at four equally spaced times in the 24 hr cycle (9 am, 3 pm, 9 pm 3 am) in random order. Blood ethanol concentrations were monitored by breath analysis and measurements were made of the blood or plasma levels of ethanol, acetaldehyde, acetate, pyruvate, lactate and cortisol. Blood pressure, heart rate and body temperature were measured before and at 60 and 120 min after ethanol administration and the effects of ethanol on a number of behavioural parameters and mood were studied. After ethanol ingestion, there was a significant decrease in body temperature, systolic blood pressure, plasma cortisol and pyruvate levels, whilst acetate levels and the lactate:pyruvate ratio were significantly increased. Standing steadiness, critical flicker fusion threshold and divided attention tracking control were significantly impaired under ethanol and self-report data indicated a significant decrease in alertness, co-ordination, concentration and attentiveness. Although a significantly higher peak blood ethanol concentration was attained at the 9 am session, other time-of-day differences did not reach significance and the pharmacokinetics of ethanol were essentially unchanged. Since the only significant diurnal variations in the response to ethanol identified in this study (apart from the subjective results) were for plasma cortisol concentrations and body temperature (both of which are well known to exhibit diurnal rhythmicity), it appears that major circadian variability in the metabolic and/or behavioural effects of ethanol is unlikely to occur.

Adolescent↗

PubMed1992

Crystalline lens power in myopia.

Growth of the eye shows a coordinated pattern whereby the reduction in refractive power of the cornea and crystalline lens tends to reduce the myopia that would otherwise result from the normal increase in axial length. There is some controversy as to whether the reduction in crystalline lens power is influenced by the refractive state of the eye, i.e., an active role in the emmetropization process, or is simply related to the changing lens dimension occurring with growth. We measured ocular dimensions and determined the crystalline lens powers in 19 myopes and 19 emmetropic subjects matched for age, gender, and ethnic origin. No significant difference was found in corneal radius of curvature for the two groups, but there was a significant difference (p < 0.05) in crystalline lens power of 2.30 D. These results suggest that greater compensation for axial elongation of the eye was afforded by the decrease in crystalline lens power than by corneal flattening.

Adolescent↗

PubMed1992

Reliability of the tear break-up time technique of assessing tear stability and the locations of the tear break-up in Hong Kong Chinese.

The measurement of tear break-up time (TBUT) in assessing stability of the tear film is somewhat controversial, even though the technique is widely used. We examined reliability of the technique within and among examiners and conclude that measurements of TBUT can be made reliably; full-beam observation of the cornea is preferable to scanning the cornea with a narrow slit in making the measurements. There may be differences in TBUT measurements made in the same subjects among examiners, but these differences are smaller with more experienced practitioners. In the Hong Kong Chinese (HK-Chinese), tear break-up appears to be more likely to first occur in the inferior periphery of the cornea. In this population mean TBUT is about 7.20 s; assuming a Gaussian distribution of TBUT values, 2.1 s should be adopted as the lower limit for normal TBUT in HK-Chinese.

Adolescent↗

PubMed1991

Tear break-up time is related to blink frequency.

The relationship between tear break-up time and blink frequency was investigated in a group of 41 subjects. A significant relationship between tear break-up time and blinking was found. This suggests that tear stability plays an important role in stimulating normal involuntary blinking.

Adolescent↗

PubMed1990

Binocular inhibition: psychophysical and electrophysiological evidence.

Binocular summation, defined as an increase in the binocular response compared with the monocular, occurs when the sensitivities of the two eyes are equal. We investigated the psychophysical and electrophysiological binocular response to a difference in monocular retinal illuminance. Different levels of unequal monocular sensitivities were induced by means of neutral density filters placed in front of one eye. Both studies produced similar results. In the absence of filters, maximum binocular summation was produced. With increasing difference in monocular illuminance, the binocular response decreased steadily until it reached a level below the monocular. The clinical implications of binocular inhibition, a perceptual phenomenon similar to Fechner's Paradox, are discussed.

Adult↗