Search PubMed⌕ Search

Biomedical subjects

J Marshall

Publications and source records attributed to J Marshall.

771 records · Page 43Linked to original sources
PubMedDate not supplied

Excimer laser photorefractive keratectomy for myopia: comparison of 4.00- and 5.00-millimeter ablation zones.

BACKGROUND: To date, there has been no systematic study of the effects of ablation zone diameter on the outcome of photorefractive keratectomy. To address these issues, we examined a series of eyes with bilateral corrections using different-sized ablation zones. METHODS: Thirty-three patients underwent bilateral photorefractive keratectomy (Summit Excimed UV200, Waltham, Mass) with identical dioptric corrections in both eyes, except first eyes had 4.00-millimeter and second eyes had 5.00-millimeter ablation zones. Identical postoperative eyedrop regimens were used in both eyes of each subject and the interval between treatments was 12 months. The mean depth of the programmed central ablation was 24 microns in eyes treated with 4.00-millimeter and 39 microns with 5.00-millimeter zones. RESULTS: There was no statistically significant difference in the preoperative refraction between first and second eyes. Mean changes in refraction at 1, 3, 6, 9, and 12 months were significantly greater in eyes treated with 5.00-millimeter ablation diameters (p < .001). No eyes treated with 4.00-millimeter zones were overcorrected, but five eyes (15%) treated with 5.00-millimeter beams had a refraction greater than +1.00 diopter (D) at 12 months postoperatively. There was no significant difference in the amount of anterior stromal haze between the two eyes at any stage. In 14 patients, less night halo was noticed in the eye treated with a 5.00-millimeter zone. Using a computer program, halo measurements were made in both eyes of 12 patients whose pre- and postoperative refractions were within 0.50 D. The magnitude of halo was significantly less in eyes treated with 5.00-millimeter zones (p < .01). CONCLUSIONS: Despite greater depths of stromal ablation with 5.00-millimeter diameters, there was no increased anterior stromal haze or postoperative regression of refraction. The biological and physical constraints governing the optimum size of the photorefractive keratectomy ablation zone are discussed.

Adult↗
PubMedDate not supplied

Discrimination between the origins and functional implications of haze and halo at night after photorefractive keratectomy.

A series of 84 eyes with up to -6.00 diopters (D) of myopia were treated by photorefractive keratectomy (PRK) using a 5.00 mm ablation zone. Three months postoperatively, 43 eyes (51%) complained of disturbed night vision, compared to 12 (14%) preoperatively. Ten (12%) had significant problems, ie, interference with driving at night. At 12 months, there were 32 patients (38%) with minor disturbances of night vision, 4 (5%) with significant problems.

Automobile Driving↗
PubMedDate not supplied

Model for predicting the optical performance of the eye in refractive surgery.

BACKGROUND: Predicting the outcome of refractive surgical procedures is one of the chief goals of the refractive surgeon. Besides perfecting the change required to nullify a refractive error, the optical quality of the postsurgical eye should be maximized. METHODS: A model eye useful for predicting the likely effects on the optical performance of the eye as a result of refractive surgical intervention is presented. The model features aspheric ocular interfaces, a gradient refractive index within the lens, and a uni-index cornea. The dimensions of the model parameters are taken from the literature. Baker's method of ray tracing through aspheric surfaces is used to predict the lateral spherical aberration of the eye for specific pupil sizes. The results for various operational conditions such as number of lens layers and corneal shape were calculated. RESULTS: The model predicts that optimal optical imagery is produced when the corneal profile is represented by a flattening ellipse (shape factor = .65 to .85). Ideally, in refractive surgery involving the cornea, the postoperative corneal contour should conform to this flattening ellipse.

Cornea↗
PubMedDate not supplied

Shape and radius of posterior corneal surface.

BACKGROUND: The posterior corneal surface is often ignored in predictive models concerned with refractive surgery. In previous studies, the radius of this surface has been measured in a variety of ways, primarily over a large chord diameter of the surface, with the common assumption that the surface is spherical. The asphericity of this surface has not been adequately addressed in the past. METHODS: An algorithm is derived for the calculation of posterior corneal surface apical radius and using characteristics of the anterior corneal surface and topographic corneal thickness variation. Anterior corneal asphericity was measured using a commercially available photoelectric keratoscope. Using a marked soft contact lens, a simple method of locating noncentral corneal sites is described as an aid to ultrasonic pachometry. RESULTS: In a group of 20 normal subjects ranging in age from 19 to 23 years, the average posterior corneal surface apical radius and asphericity (p) was 5.80 mm (SD = +/- 0.42) and 0.64 (SD = 0.37) for the vertical meridian, and 5.82 mm (SD = +/- 0.40) and 0.52 (SD = 0.30) for the horizontal meridian. Average central corneal thickness was 533 mu (SD = +/- 19). CONCLUSIONS: The average asphericity values are below 1, hence the posterior corneal surface is described as a flattening ellipse. The rate of flattening of this surface is greater than the rate of flattening along the averaged anterior corneal surface.

Adult↗
PubMedDate not supplied

Halos--a problem for all myopes? A comparison between spectacles, contact lenses, and photorefractive keratectomy.

After photorefractive keratectomy (PRK) using excimer lasers (193 nm) many patients report the presence of halos around light sources at night. However, halos are not unique to PRK patients, as they are a common observation in myopic contact lens wearers. We present an objective measurement of the halos using a computerized technique. The patient fixated on a red cross within a white circle in the center of a video monitor which served as the halo source. The screen surrounding the circle was not illuminated. The operator controlled the movement of the white spot and moved the spot toward the halo source until the subject indicated when the cursor was at the outer parameter of the halo. Measurements were made at 30 degree intervals around the halo source and expressed as square degrees. The study found that spectacles, soft contact lenses, and excimer laser surgery were superior to hard contact lenses in terms of the size of the halo. A mean value of 2.51 square degrees was obtained for spectacles wearers compared with 3.18 square degrees for soft contact lenses, 3.14 square degrees for excimer laser patients with 4-millimeter ablation zone, 2.76 square degrees for excimer laser patients with a 5-millimeter ablation zone, and 89.5 square degrees for hard contact lenses. It appears that this device is very useful for measuring the halo size after excimer laser PRK. We concluded that halos were not a problem for our patients after excimer laser photorefractive keratectomy.

Adult↗
PubMedDate not supplied

Acute stroke treatment in UK hospitals: the Stroke Association survey of consultant opinion.

The aim of the Stroke Association survey was to document United Kingdom consultant opinion of the immediate treatment for patients with acute stroke. A national postal survey of all UK hospital consultant general physicians, geriatricians and neurologist was carried out in 1992/3. We identified 1,953 consultants who routinely cared for patients with acute stroke; 39% of them regularly used aspirin for patients with acute stroke and 10% used low-dose subcutaneous heparin. Other treatments were rarely used. There was much uncertainty about the effectiveness of all currently available acute stroke treatments; 73% of physicians were prepared to start aspirin before a CT scan, but a much smaller proportion would start heparin therapy without one. Twenty-seven percent of consultants would actively treat hypertension in the initial 24 hours after stroke. Routine aspirin for secondary prevention after ischaemic stroke was widely accepted, but blood cholesterol lowering by drugs was not. In conclusion, aspirin and heparin alone are the only routinely used treatments for the immediate treatment of acute stroke; other treatments are used sparingly or not at all. The great uncertainty about the value of all available acute stroke treatments should encourage participation in randomised controlled trials.

Aspirin↗
PubMedDate not supplied

Torsades de pointes with administration of high-dose intravenous d-sotalol to a patient with congestive heart failure.

A patient with heart failure was administered d-sotalol 3.0 mg/kg infused over 2 minutes. The patient had normal electrolytes and baseline QT. Six minutes after drug administration the QT prolonged to 600 msec, and the patient developed torsades de pointes and required electrical cardioversion. This emphasizes the need to consider both rate of administration and the dosage when evaluating the safety and efficacy of a new class III antiarrhythmic drug.

Aged↗