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Biomedical subjects

D R Williams

Publications and source records attributed to D R Williams.

At least 145 records · Page 8Linked to original sources
PubMed1993

A multicentre study of the prevalence of diabetic peripheral neuropathy in the United Kingdom hospital clinic population.

A cross-sectional multicentre study of randomly selected diabetic patients was performed using a standardised questionnaire and examination, to establish the prevalence of peripheral neuropathy in patients attending 118 hospital diabetes clinics in the UK. Vibration perception threshold was performed in two centres to compare with the clinical scoring systems. A total of 6487 diabetic patients were studied. 53.9% male, median age 59 years (range 18-90 years). 37.4% Type 1 (insulin-dependent) diabetes mellitus, with a median duration of diabetes 8 years (0-62 years). The overall prevalence of neuropathy was 28.5% (27.4-29.6%) (95% confidence interval) in this population. The prevalence in Type 1 diabetic patients was 22.7% (21.0-24.4%) and in Type 2 (non-insulin-dependent) diabetic patients it was 32.1% (30.6-33.6%). The prevalence of diabetic peripheral neuropathy increased with age, from 5% (3.1-6.9%) in the 20-29 year age group to 44.2% (41.1-47.3%) in the 70-79 year age group. Neuropathy was associated with duration of diabetes, and was present in 20.8% (19.1-22.5%) of patients with diabetes duration less than 5 years and in 36.8% (34.9-38.7%) of those with diabetes duration greater than 10 years. Mean vibration perception threshold measured at the great toe was 21.1 +/- 13.5 SD volts and correlated with the neuropathy disability score, r = 0.8 p < 0.001. In conclusion, diabetic peripheral neuropathy is a common complication associated with diabetes. It increases with both age and duration of diabetes, until it is present in more than 50% of Type 2 diabetic patients aged over 60 years.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1993

Spatial reconstruction of signals from short-wavelength cones.

Because the retinal cone mosaic samples an image only at discrete locations, our continuous visual percept must arise from a spatial reconstruction process. How this process combines information from the three cone types is presently unclear. To investigate, we asked whether L and M cone information can modify the visual system's reconstruction of signals from S cones. In Expt 1, we used a matching paradigm to measure the effect of L and M cone information on filling-in at the foveal tritanopic area. We found that a small luminance disk superimposed at the tritanopic area decreases the amount of filling-in, showing that the reconstruction of S cone signals can be influenced by the spatial pattern seen by the L and M cones. In Expt 2, we asked whether L and M cone information can modify the splotchy low-frequency alias seen when an observer views fine S cone gratings. Here there was no evidence for an interaction. We conclude that though L and M cone information can influence the visual system's reconstruction of S cone signals, this influence may be confined to relatively coarse spatial patterns.

Color Perception↗
PubMed1993

Serial spatial filters in vision.

Observers viewing two superimposed laser interference fringes of nearly equal spatial frequency see an illusory grating of low spatial frequency, even when the spatial frequency of the fringes exceeds the resolution limit. This grating is a product of nonlinear distortion within the visual system [MacLeod, Williams and Makous (1992) Vision Research, 32, 347-363]. By separately manipulating the spatial frequencies of the interference fringes and the distortion gratings, we decomposed the contrast sensitivity function into two serial components separated by the nonlinear process. Losses in the optics of the eye were avoided by use of laser interferometry. Spatial summation preceding the nonlinear stage was restricted to the light-collecting area of individual cones and was directly proportional to the diameters of cone inner segments at three retinal eccentricities; this suggests that light is trapped within cones at the level of their inner segments. Even 30 degrees from the fovea, the nonlinear stage precedes the site where separate signals from individual cones are no longer maintained; this leads us to suggest that the nonlinear process lies within the retina. In addition, spatial antagonism precedes the nonlinear stage; this places the nonlinear process at a site following the outer segments of the cones. Dichoptic presentation of the interference fringes failed to produce illusory gratings; that is, the nonlinearities within the binocular pathway do not produce distortions like those produced by the monocular nonlinearity.

Contrast Sensitivity↗
PubMed1993

Epidemiological and geographic factors in diabetes.

Both non-insulin-dependent (type 2) diabetes mellitus (NIDDM) and insulin-dependent (type 1) diabetes mellitus (IDDM) show a wide variation in incidence and prevalence in different populations. The prevalence of NIDDM varies from close to zero in some populations to 40-50% in the adults of Nauru (Pacific) and in the Pima Indians (North America). The incidence of IDDM in children under 16 years ranges from around 30/100,000 children per year in Finland and other Northern European countries to less than 1/100,000 per year in Japanese children. Many genetic and environmental factors combine to produce this variation. Among the suggestions made for NIDDM are the 'thrifty genotype' and, more recently, the 'thrifty phenotype' hypotheses of Neel and of Hales and Barker respectively. Genetic and environmental factors in IDDM in children have combined to create an apparent gradient of decreasing incidence from northern to southern European countries but with at least one local 'hot spot': the island of Sardinia. The factors responsible for this pattern, and the increasing incidence over time, require further clarification.

Adolescent↗
PubMed1993

Community screening for non-insulin-dependent diabetes mellitus: self-testing for post-prandial glycosuria.

Recent requirements of the GP contract have included the testing of random urine samples from all patients aged 16-75 years for glycosuria. As a means of detecting diabetes this will be both inefficient and wasteful of resources, because of the method employed and age group screened. We examined a simple method for screening large populations for non-insulin-dependent diabetes mellitus using self-testing for post-prandial glycosuria, and compared its performance to that using fasting plasma glucose and haemoglobin A1. The study involved five general practices, screening over 13,000 subjects aged between 45 and 70 years. Of the 13,795 subjects screened, 10,348 responded (75%). Glycosuria was detected in 343 subjects (3.3%), of whom 330 (95.9%) attended for oral glucose tolerance testing. Of these 99 (30%) had newly diagnosed diabetes, 56 (17%) had impaired glucose tolerance. A further 65 subjects were identified as having diabetes but were not on a register of cases. Prevalence of diabetes in this age group rose from 2.26% to 4.1%. In a normal population control group (442 subjects from Ely), this system gave a sensitivity of 43%, specificity of 98% and positive predictive value of 53%, and compared favourably to the use of a fasting plasma glucose of 6 mmol/l, which gave a sensitivity of 65%, but only a specificity of 64% and positive predictive value of 9%. These results support the use of this simple system in large population screening for diabetes.

Aged↗
PubMed1993

Diabetes in the elderly: an under-diagnosed condition.

The proportion of the population aged 65 years and over is increasing. The Coventry Diabetes Study was able to investigate the prevalence of diagnosed and undiagnosed diabetes among elderly (aged > or = 65 years) Europeans (n = 1600) and South Asians (n = 288) in Foleshill, Coventry. The total prevalence of diabetes was higher in South Asians than Europeans among males (30.7 (95% CI 24.0-37.9)% vs 7.7 (95% CI 5.8-10.0)%, respectively) and females (24.0 (95% CI 17.5-32.4)% vs 12.5 (95% CI 10.6-14.8)%, respectively). The proportion with diagnosed diabetes was higher among South Asians than Europeans (55% vs 33%, p < 0.001), possibly because of their earlier onset. These findings suggest that detection of diabetes in elderly Europeans in urban areas may be inadequate.

Age Factors↗
PubMed1993

Modulation transfer of the human eye as a function of retinal eccentricity.

We measured the monochromatic image quality of the eye across a wide visual field (120 degrees), with natural pupil (4 mm) and accommodation (3 diopters). The method is based on the acquisition and the posterior processing of double-pass aerial images of a point source imaged on the retina, which was kept at a fixed distance from the eye at all retinal eccentricities. The two-dimensional modulation transfer functions (MTF's) computed from the aerial images show that astigmatism is the dominant monochromatic aberration in both the fovea and the periphery and is also the major cause of variability among individuals. We found a slower decline in optical quality with eccentricity than had been found by previous measurements. Our foveal results are in close agreement with those of Campbell and Gubisch [J. Physiol. (London) 186, 558-578 (1966)], but off-axis optical quality is much better than found previously by Jennings and Charman [Am. J. Optom. Physiol. Opt. 55, 582-590 (1978); Vision Res. 21, 445-454 (1981)]. The optical system of the eye seems to follow a wide-angle lens design: the optical quality in the center (fovea) is not particularly good (it is far from the diffraction limit at this pupil size), but the modulation transfer function remains roughly constant for a wide visual field.

Accommodation, Ocular↗
PubMed1993

Aberration-free measurements of the visibility of isoluminant gratings.

We developed a new apparatus and psychophysical technique to extend isoluminant contrast-sensitivity measurements to high spatial frequencies. The apparatus consists of two identical laser interferometers that are designed to produce phase-locked two-color interference fringes on the retina without the influence of diffraction and most aberrations in the eye. However, even with interferometry, transverse chromatic aberration of the eye can produce a wavelength-dependent phase shift in the interference fringes, which can be exaggerated by head movements. To reduce the effect of head movements, isoluminant red and green interference fringes of equal spatial frequency and orientation were drifted slowly in opposite directions to guarantee a purely isochromatic (in phase) and a purely isoluminant (out of phase) stimulus during each cycle of stimulus presentation. With this technique we found that observers could resolve red and green stripes at spatial frequencies higher than 20 cycles per degree (c/deg) (20-27 c/deg), substantially higher than has previously been reported. This places a lower bound on the sampling density of neurons that mediate color vision. At all spatial frequencies, even those above the isoluminant resolution limit, a relative phase of the red and the green components could be found that obliterated the appearance of luminance modulation at the fringe frequency. Above the resolution limit, red-green-isoluminant interference fringes are seen as spatial noise, which may be chromatic aliasing caused by spatial sampling at some stage in the chromatic pathway.

Color Perception Tests↗
PubMed1993

Efficiency in detection of isoluminant and isochromatic interference fringes.

We examined the limitations imposed by neural factors on spatial contrast sensitivity for both isochromatic and isoluminant gratings. We used two strategies to isolate these neural factors. First, we eliminated the effect of blurring by the dioptrics of the eye by using interference fringes. Second, we corrected our data for additional sensitivity losses up to and including the site of photon absorption by applying an ideal-observer analysis described by Geisler [J. Opt. Soc. Am. A 1, 775 (1984)]. Our measurements indicate that the neural visual system modifies the shape of the contrast-sensitivity functions for both isochromatic and isoluminant stimuli at high spatial frequencies. If we assume that the high-spatial-frequency performance of the neural visual system is determined by a low-pass spatial filter followed by additive noise, then the visual system has a spatial bandwidth 1.8 times lower for isoluminant red-green than for isochromatic stimuli. On the other hand, we find no difference in bandwidth or sensitivity of the neural visual system for isoluminant red-green and S-cone-isolated stimuli.

Color Perception Tests↗
PubMed1992

Observations on an outbreak of anthrax in pigs in north Wales.

Nineteen pigs in a 500 sow herd died from anthrax over a period of 95 days. This paper describes some of the problems encountered and how they were dealt with, including the risks to public health, the disposal of slurry and the cleansing and disinfection of the premises after the herd was slaughtered.

Animals↗
PubMed1992

Blurring by fixational eye movements.

A complete description of the loss of contrast sensitivity at high spatial frequencies requires an estimate of the role of eye movements, which could blur fine detail. We describe a new technique to isolate their effect. Observers viewed either a 100 c/deg interference fringe, which the cone mosaic aliased to a low frequency zebra stripe, or an artificial zebra stripe. The real and artificial zebra stripes have similar spatial patterns, but differ in the temporal modulation produced by eye movements. Contrast threshold was measured as a function of duration for both stimuli flashed in the dark. The ratio of the contrast thresholds for the real and artificial zebra stripes with long durations, when eye movements could have a differential effect, is always within a factor of two or so of the ratio for 1 msec flashes, when eye movements are eliminated. These results support the view that eye movements are only a minor source of image degradation even at very high spatial frequencies, and provide no support for the view that they improve high resolution tasks.

Contrast Sensitivity↗
PubMed1992

No aliasing at edges in normal viewing.

Although spatial aliasing by the extrafoveal retina can occur under natural viewing conditions, it does not commonly disturb our vision. One possible explanation for this is that real scenes do not have sufficient power in the high frequencies to produce aliasing. We examined whether aliasing distorted the appearance of a high contrast edge, which is a common stimulus in the environment. Observers made a two-interval forced-choice discrimination between low-pass filtered and unfiltered edges at 0, 10, 20, and 40 deg eccentricity. This discrimination could be made only when frequency components were removed below both the cone and ganglion cell Nyquist frequencies at each eccentricity. Since supra-Nyquist frequency components could not be detected in edges, they are incapable of producing aliasing.

Contrast Sensitivity↗
PubMed1992

A visual nonlinearity fed by single cones.

An intensive nonlinearity in the visual system can produce distortion products, or difference frequency gratings, when observers view two high contrast, high spatial frequency interference fringes of slightly different frequency or orientation added together at the retina. These distortion products are visible even when the two fringes imaged on the retina are above the resolution limit. Our experiments take advantage of this nonlinearity to measure the spatial filtering in the visual system following the formation of the retinal image, but preceding the site of the nonlinearity. The point spread function corresponding to this spatial filter is so small that it can be entirely explained by light integration within the apertures of foveal and parafoveal cones. The small size of this point spread function implies that (1) laser interferometry avoids contrast losses inherent in the eye's optics at spatial frequencies as high as 130 c/deg, (2) retinal scatter causes negligible image degradation in the fovea and parafoveal retina, (3) eye movements have little or no effect on contrast sensitivity to the distortion product and (4) that there is no neural spatial summation in the visual system prior to the site of the nonlinearity. Distortion products could also be observed when a bright interference fringe was briefly flashed on the fovea and a test interference fringe was viewed through the resulting afterimage. Measurements of the point spread function at stages in the visual system that precede the generation of this distortion product were similar to those obtained with simultaneous presentation of the two fringes, implying that the aftereffect of light adaptation is extremely local, no larger than the dimensions of single cones.

Adaptation, Ocular↗
PubMed1992

Use of the accident and emergency department by patients with diabetes.

As part of a District-based audit of diabetes care all attendances (of diabetic and non-diabetic patients) at a local Accident and Emergency department were monitored over a 3-month period. A total of 9505 attendances took place of which 62 (0.7%) were by people with diabetes. This was no different from the expected number of attendances (61.5) based on the rates seen in non-diabetic individuals. Of these 62 attendances, 20 (by 15 individuals) were directly related to diabetes (hypoglycaemia 17, hyperglycaemia 1, for supplies of insulin or equipment 2). When these diabetes-specific attendances were removed from the total, the number of attendances (42) was significantly (p = 0.004) less than that expected. Diabetic males aged 0-19 years attended significantly more often than non-diabetic individuals of the same age but this excess was solely attributable to diabetes.

Adolescent↗
PubMed1992

Prevalence of diabetes in different regional and religious south Asian communities in Coventry.

The prevalence of Type 2 (non-insulin-dependent) diabetes in different South Asian (Asian) communities was compared during the Coventry Diabetes Study, a cross-sectional house to house screening programme for diabetes. Screening was by capillary whole blood glucose measurement with oral glucose tolerance tests when concentrations were greater than or equal to 6.0 mmol l-1 within 2 h of a meal or greater than or equal to 5.0 mmol l-1 2 h or more after a meal and a random 10% of others. Of the 4395 resident Asians, 94% were represented by five communities: Punjabi Sikhs, Punjabi Hindus, Gujerati Moslems, Gujerati Hindus, and Pakistani Moslems. Response to screening was 77-89% and to glucose tolerance test was 59-79%. Differences in anthropometry, socioeconomic circumstances, and migratory patterns were found, but all groups had a higher prevalence of Type 2 diabetes than Europeans. Gujerati Moslems had the highest age-adjusted prevalence (per 1000) of Type 2 diabetes (males: 160 (95% CI 107-228), females: 204 (95% CI 144-283)) when compared with the other Asian groups (males: Punjabi Sikhs 89(72-110), Pakistani Moslems 91(67-120), Gujerati Hindus 84(57-120), Punjabi Hindu 113(74-171); females: Punjabi Sikhs 75(60-94), Pakistani Moslems 103(78-133), Gujerati Hindus 88(62-122), Punjabi Hindu 116(77-174)). That all the Asian groups had a high prevalence of diabetes, in spite of their known dietary, cultural, and socioeconomic differences, suggests that the Asian predisposition to Type 2 diabetes is inherited although environmental factors may be necessary for this to be expressed.

Adult↗