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Biomedical subjects

T H Williamson

Publications and source records attributed to T H Williamson.

At least 19 recordsLinked to original sources
PubMed2000

Papillofoveal traction in macular hole formation: the role of optical coherence tomography.

OBJECTIVES: To determine the validity of the assumption that optical coherence tomographic scans of macular holes have a discrete linear signal (DLS) that represents a detached posterior vitreous face, and to analyze the DLS in macular hole pathogenesis. METHODS: Optical coherence tomographic scans were taken of 3 situations in which the vitreous conditions were known: (1) dissected intact vitreous, (2) clinically evident Weiss rings, and (3) maculae before and after saccades in eyes without a biomicroscopic posterior vitreous detachment. In addition, 70 eyes of 35 patients with macular holes underwent clinical examination and optical coherence tomographic scanning that passed through the optic disc and the fovea or macular hole. RESULTS: Spatial properties of the DLS matched those of the posterior vitreous face in the situations examined. Of the 70 eyes, 16 (23%) had a biomicroscopic posterior vitreous detachment, whereas a DLS was demonstrated in 40 (57%). Of the 54 eyes without a biomicroscopic posterior vitreous detachment, 18 (33%) had a DLS attached focally to the optic disc margin and the fovea or macular hole. All 7 of the "can opener" holes examined had a nasally "hinged" central flap, 6 with a focally attached DLS. CONCLUSIONS: The DLS corresponds to the posterior vitreous face. Anteronasal papillofoveal traction may generate some macular holes.

Fovea Centralis↗
PubMed2000

Watzke-Allen slit beam test in macular holes confirmed by optical coherence tomography.

OBJECTIVE: To examine the role, validity, and interpretation of Watzke-Allen slit beam testing in patients with idiopathic senile macular holes. METHODS: Thirty-seven consecutive patients with 40 full-thickness macular holes, confirmed on optical coherence tomography, were prospectively recruited. The Watzke-Allen slit beam test was used centrally and on the rim of the macular hole in both vertical and horizontal orientations. RESULTS: In 24 eyes, the beam was reported as thinned in both vertical and horizontal orientations when placed directly over the center of the macular hole. In 9 eyes, the Watzke-Allen slit was reported as broken in both vertical and horizontal orientations. In 6 eyes, the beam was reported as broken in one orientation and thinned in the other. In 1 eye, the beam was reported as kinked but not thinned or broken. When the beam was placed on the edge of the macular hole, all patients reported a displacement or bowing of the beam away from the center of the hole. CONCLUSIONS: These findings confirm tangential traction of photoreceptors from a central foveal dehiscence as the causative mechanism in the development of the majority of macular holes. Careful interpretation of the Watzke-Allen sign may offer a technique for preoperatively determining visual prognosis. Arch Ophthalmol. 2000;118:1059-1063

Aged↗
PubMed1999

Relationship of the distal optic nerve sheath to the circle of Zinn.

BACKGROUND: This study was designed to determine the relationship of the vascular circle of Zinn (CZ) and its branches to the termination of the subarachnoid space surrounding the optic nerve sheath, with particular reference to optic disc size. METHODS: Serial sections of 29 normal human optic nerves were performed at 6-microm intervals. The position of the CZ and subarachnoid space were measured with WILD Heerbrugg objective graticules. The results were analysed with reference to the optic disc size. RESULTS: The position of the CZ was variable but two major types were recognised. In type 1 the circle was located anterior to the distal sheath, and in type 2 posterior to the sheath. There were more small optic discs observed in type 2. CONCLUSION: In this study the CZ was more posteriorly located in small optic discs. This location may induce risk factors contributing to the development of ischaemic optic neuropathy. The combination of small discs, posterior placement of the CZ, and anatomical variations in the vascular pattern may predispose to ischaemic events.

Cadaver↗
PubMed1999

Automated localisation of the optic disc, fovea, and retinal blood vessels from digital colour fundus images.

AIM: To recognise automatically the main components of the fundus on digital colour images. METHODS: The main features of a fundus retinal image were defined as the optic disc, fovea, and blood vessels. Methods are described for their automatic recognition and location. 112 retinal images were preprocessed via adaptive, local, contrast enhancement. The optic discs were located by identifying the area with the highest variation in intensity of adjacent pixels. Blood vessels were identified by means of a multilayer perceptron neural net, for which the inputs were derived from a principal component analysis (PCA) of the image and edge detection of the first component of PCA. The foveas were identified using matching correlation together with characteristics typical of a fovea-for example, darkest area in the neighbourhood of the optic disc. The main components of the image were identified by an experienced ophthalmologist for comparison with computerised methods. RESULTS: The sensitivity and specificity of the recognition of each retinal main component was as follows: 99.1% and 99.1% for the optic disc; 83.3% and 91.0% for blood vessels; 80.4% and 99.1% for the fovea. CONCLUSIONS: In this study the optic disc, blood vessels, and fovea were accurately detected. The identification of the normal components of the retinal image will aid the future detection of diseases in these regions. In diabetic retinopathy, for example, an image could be analysed for retinopathy with reference to sight threatening complications such as disc neovascularisation, vascular changes, or foveal exudation.

Algorithms↗
PubMed1998

Identification and voluntary reduction of vancomycin use for perioperative antibiotic prophylaxis during coronary artery bypass graft surgery.

Vancomycin prophylaxis for coronary artery bypass graft surgery without prosthetic valve implantation voluntarily decreased from 94% to 18% in one medical center. Median hospital stay (10 vs 9 days, P = .30) and number of postoperative infections (17.0% vs 14.3%, P = .60) did not differ among patients who received vancomycin and those who did not.

Adult↗
PubMed1998

An inner city preschool visual screening programme: long-term visual results.

AIMS: The aim of this study was to investigate the long-term outcome of the treatment of amblyopia as a sequel to preschool screening, which has not hither-to been described. METHODS: All patients originally referred from a preschool screening programme were recalled for examination by letter. 255 patients were reviewed at least 4 years after discharge of which 88 were definitely amblyopic at presentation and 107 were not amblyopic at presentation and were used as controls. RESULTS: 79% of the amblyopes improved or maintained visual acuity after discharge but this was reduced to 42% after an age induced increase (estimated from the controls) was compensated for. The mean drop in visual acuity in the amblyopic eyes which deteriorated was 0.23 (SD 0.15) logMAR units. Stepwise multiple linear regression showed that the best single predictor of post-discharge deterioration in visual acuity was the improvement in visual acuity seen during treatment (R2 = 19%). Eccentric fixation at time of follow up (increasing R2 to 47%) and good presenting acuity (further raising R2 to 57%) contributed additional information, and were both associated with greater post-discharge deterioration in visual acuity. CONCLUSIONS: The majority of amblyopes who attended for follow up maintained or improved their visual acuities after discharge. Those patients who demonstrated deterioration of their amblyopia had usually improved well during the programme and were often fixating eccentrically at follow up.

Amblyopia↗
PubMed1997

Electroretinography of central retinal vein occlusion under scotopic and photopic conditions: what to measure?

INTRODUCTION: In the management of cases of central retinal vein occlusion it is important to identify those patients who will develop the complication of iris neovascularisation. Electroretinography is of proven use in this role but there is some confusion about which parameters to measure and at what stimulus intensities. METHODS: A variety of ERG parameters in scotopic and photopic conditions were tested by ROC analysis for the prediction of the development of this complication in 39 patients with CRVO. RESULTS: The results indicated that a single flash and measurement of the b/a ratio under photopic conditions is as effective in this aim as measuring multiple parameters. CONCLUSION: A simplified and shortened protocol could be used in the examination of this condition thereby making the application of ERG more practical in the clinical setting.

Adaptation, Ocular↗
PubMed1996

The value of serial Doppler imaging in central retinal vein occlusion: correlation with visual recovery.

A prospective study of 80 patients with central retinal vein occlusion (CRVO) was performed to determine whether Doppler flow measurements can predict visual outcome. All patients at presentation, had full ophthalmological examination, fluorescein angiography, relative afferent pupillary defect test and electroretinography to distinguish between ischaemic and non-ischaemic occlusions. In addition, all patients were examined at presentation with colour Doppler ultrasound using a 7.5 MHz linear array probe. Both the clinical examinations and colour Doppler ultrasound were repeated at one year following initial presentation in a smaller group of 20 patients. A significant reduction in velocity within the central retinal vein was noted in the ischaemic compared with non-ischaemic CRVO affected eye at the time of the baseline scan. The minimum venous velocity within the central retinal vein was most severely affected. No velocity difference was present within the ophthalmic artery. Follow-up at one year with colour Doppler ultrasound showed an increase in blood velocity values within the central retinal vein in the CRVO affected eye but this was of no prognostic value and did not correlate with clinical outcome. Serial colour Doppler ultrasound examinations, therefore, have no clinical application in patients with CRVO and are of no prognostic value.

Adult↗
PubMed1996

Colour Doppler imaging in giant cell (temporal) arteritis: serial examination and comparison with non-arteritic anterior ischaemic optic neuropathy.

The ophthalmological manifestations of giant cell arteritis (GCA) present a difficult diagnostic and management problem to the ophthalmologist. The orbital circulation is frequently involved in the disease process. The effects of GCA on orbital blood flow assessed by colour Doppler imaging (CDI) were investigated in this study. Serial CDI examinations of the orbital blood vessels were performed (at presentation, 2 days, 1 week and at 1 month) on 7 patients with GCA. CDI examination at presentation was also performed on 4 patients with non-arteritic anterior ischaemic optic neuropathy (AION) for comparison. Blood flow could not be detected in up to three arteries in the affected (ipsilateral) orbit of 6 GCA patients at presentation. Five of these patients were also found to have undetectable blood flow in the posterior ciliary arteries of the contralateral orbit. Serial CDI examination revealed blood flow alterations in arteritic patients despite treatment. Return of blood flow to normal was slow, and related to the clinical features. By contrast, only 1 of the patients with non-arteritic AION showed undetectable blood flow in a posterior ciliary artery. GCA leads to widespread and prolonged alterations in orbital blood flow. CDI allows the detection and monitoring of such alterations in orbital blood flow, which correlate with the clinical features of GCA. Serial CDI examinations in GCA may be used to aid management decisions.

Aged↗
PubMed1996

The role of understaffing in central venous catheter-associated bloodstream infections.

OBJECTIVE: To determine risk factors for central venous catheter-associated bloodstream infections (CVC-BSI) during a protracted outbreak. DESIGN: Case-control and cohort studies of surgical intensive care unit (SICU) patients. SETTING: A university-affiliated Veterans Affairs medical center. PATIENTS: Case-control study: all patients who developed a CVC-BSI during the outbreak period (January 1992 through September 1993) and randomly selected controls. Cohort study: all SICU patients during the study period (January 1991 through September 1993). MEASUREMENTS: CVC-BSI or site infection rates, SICU patient clinical data, and average monthly SICU patient-to-nurse ratio. RESULTS: When analyzed by hospital location and site, only CVC-BSI in the SICU had increased significantly in the outbreak period compared to the previous year (January 1991 through December 1991: pre-outbreak period). In SICU patients, CVC-BSI were associated with receipt of total parenteral nutrition [TPN]; odds ratio, 16; 95% confidence interval, 4 to 73). When we controlled for TPN use, CVC-BSI were associated with increasing severity of illness and days on assisted ventilation. SICU patients in the outbreak period had shorter SICU and hospital stays, were younger, and had similar mortality rates, but received more TPN compared with patients in the pre-outbreak period. Furthermore, the patient-to-nurse ratio significantly increased in the outbreak compared with the pre-outbreak period. When we controlled for TPN use, assisted ventilation, and the period of hospitalization, the patient-to-nurse ratio was an independent risk factor for CVC-BSI in SICU patients. CONCLUSIONS: Nursing staff reductions below a critical level, during a period of increased TPN use, may have contributed to the increase in CVC-BSI in the SICU by making adequate catheter care difficult. During healthcare reforms and hospital downsizing, the effect of staffing reductions on patient outcome (i.e., nosocomial infection) needs to be critically assessed.

Aged↗
PubMed1996

Automatic detection of diabetic retinopathy using an artificial neural network: a screening tool.

AIMS: To determine if neural networks can detect diabetic features in fundus images and compare the network against an ophthalmologist screening a set of fundus images. METHODS: 147 diabetic and 32 normal images were captured from a fundus camera, stored on computer, and analysed using a back propagation neural network. The network was trained to recognise features in the retinal image. The effects of digital filtering techniques and different network variables were assessed. 200 diabetic and 101 normal images were then randomised and used to evaluate the network's performance for the detection of diabetic retinopathy against an ophthalmologist. RESULTS: Detection rates for the recognition of vessels, exudates, and haemorrhages were 91.7%, 93.1%, and 73.8% respectively. When compared with the results of the ophthalmologist, the network achieved a sensitivity of 88.4% and a specificity of 83.5% for the detection of diabetic retinopathy. CONCLUSIONS: Detection of vessels, exudates, and haemorrhages was possible, with success rates dependent upon preprocessing and the number of images used in training. When compared with the ophthalmologist, the network achieved good accuracy for the detection of diabetic retinopathy. The system could be used as an aid to the screening of diabetic patients for retinopathy.

Diabetic Retinopathy↗
PubMed1996

Blood viscosity, coagulation, and activated protein C resistance in central retinal vein occlusion: a population controlled study.

BACKGROUND: The role of blood viscosity and haemostasis has been investigated in mixed groups of patients with branch and central retinal vein occlusion (CRVO) with conflicting results. This may have partly been due to the different aetiologies of these two types of vein occlusion. METHODS: In this study viscosity and coagulation (including activated protein C resistance) were examined in 87 patients with CRVO and compared with the results from an age-matched, population based control group. RESULTS: Viscosity variables were higher in CRVO than in controls which suggested that reduced red cell deformability was associated with the occurrence of CRVO. A higher percentage of the patients with CRVO (12%) had activated protein C resistance than controls (5%). Patients who developed the complication of iris neovascularisation had relatively low antithrombin III, factor VII, and tissue plasminogen activator indicating both a tendency to thrombus formation and a reduction in fibrinolytic activity. CONCLUSION: Increased blood viscosity may contribute to the production of CRVO by inducing stasis of blood flow, with thrombus formation in at risk individuals who go on to develop iris neovascularisation.

Adult↗
PubMed1996

Functional imaging of the retina using the multifocal electroretinograph: a control study.

BACKGROUND: A new technique exists that enables functional mapping of the retina. A control population was examined to obtain normative values and to assess the reproducibility of this new test. METHODS: Twenty healthy volunteers were tested using a 61 hexagonal array stimulus with a 14 minute recording period. Median 5th and 95th percentiles were determined for implicit times and amplitude measures for the 61 test areas. Repeat measurements were performed on 10 individuals. Wilcoxon and Bland and Altman techniques were used to quantify reproducibility of the test. RESULTS: The implicit time of the wave-form components was not found to vary over the retina (peak or b-wave component, 35.52 (1.4) ms; trough or a-wave component, 17.76 (0.8) ms). Reproducibility was found to decrease with eccentricity (coefficient of repeatability 17.4% for the central area increasing to 30.3% for the peripheral ring). CONCLUSIONS: The findings suggest that reproducibility, although variable with eccentricity, is comparable with conventional electrophysiology. These limits of variation were used to assign confidence intervals to individual retinal areas and will be used (future work) in the examination of diseased states.

Adolescent↗