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Biomedical subjects

R W Snyder

Publications and source records attributed to R W Snyder.

At least 19 recordsLinked to original sources
PubMed1996

Optical modeling of radial keratotomy incision patterns.

PURPOSE: To determine the optical effects of higher-order corneal shape variations resulting from radial keratotomy. METHODS: Videokeratoscopic height data were obtained postoperatively from several patients who had undergone radial keratotomy. For each of clear central zone sizes 3.00 mm, 4.00 mm, and 4.75 mm, two patients were chosen randomly from the larger study group. Data obtained 2 weeks postoperatively from these six patients were decomposed into the Zernike polynomials, and the low-order expansion terms were removed to disclose corneal height variations (the radial keratotomy artifact). The artifact was applied to a schematic eye model, and exact ray-tracing was used to evaluate visual performance, which was defined as a function of pupil diameter, optical zone (central clear zone) size, and radial keratotomy artifact centration. RESULTS: The radial keratotomy artifact degrades visual performance at midspatial frequencies more than it does at high spatial frequencies. This effect is most pronounced for smaller optical zones and for a pupil diameter of 4 mm. Visual performance remains nearly constant for small decentration (0.5 mm or less) of the radial keratotomy optical zone from the corneal apex. CONCLUSIONS: Residual refractive error, corneal asphericity, and the radial keratotomy artifact all affect visual performance after radial keratotomy. Isolated effects of the radial keratotomy artifact degrade visual performance, with the level of degradation dependent on pupil size, optical zone size, and centration of the procedure. More research is necessary to combine the radial keratotomy artifact with changes in corneal asphericity and to further quantify the optical effects of radial keratotomy.

Contrast Sensitivity↗
PubMed1996

Temperature measurements during phacoemulsification and erbium:YAG laser phacoablation in model systems.

PURPOSE: To compare the potential for thermal injury to ocular structure resulting from phacoemulsification ultrasound energy and erbium:YAG (Er:YAG) laser output. SETTING: Morse Laser Laboratory, Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston. METHODS: Ultrasonic phacoemulsification energy and ER:YAG laser output (10 J/cm2, 10 Hz, 0.3 watts) sufficient for lens removal were applied to model systems and human cadaver eyes. Temperatures were measured with ultrafine thermocouples interfaced to a microcomputer data acquisition system. RESULTS: Although greater than 95% of energy from laser output is converted to thermal energy, temperature rise in model systems and cadaver eyes was 10 to 15 times greater after pulsed application of ultrasound energy than after Er:YAG laser application. At 100% power, approximately 4 watts of ultrasound power is converted to heat. Temperature rise following both laser and ultrasound applications decreased with irrigation in cadaver eyes and increasing volume in a model system. With continuous irrigation (20 cc/min), the temperature rise at 2 minutes measured at the corneal endothelial surface, within the corneal stroma, and in the anterior chamber angle of cadaver eyes was approximately 0.5 degrees Celsius (degrees C) after laser application and 7.0 degrees C after ultrasound application. Without irrigation, temperatures rose 2.5 degrees C after laser application and 35.0 degrees C after ultrasound application. CONCLUSION: At operating parameters sufficient for lens removal, the Er:YAG laser imparted less thermal energy to whole eyes and model systems than ultrasonic phacoemulsification.

Anterior Chamber↗
PubMed1996

Comparison of three videokeratoscopes in measurement of toric test surfaces.

PURPOSE: We compared the accuracy of the Computed Anatomy TMS-1 (1.41), the EyeSys Laboratories Corneal Analysis System (2.1), and the Visioptic EH-270 (3.0) videokeratoscopes in measuring toric surfaces. These non-rotationally symmetric aspheric surfaces served as models of corneal astigmatism. METHODS: Precision diamond-turned toric surfaces modeling 0.00 diopter (D) to 7.00 D of astigmatism were fabricated. A three-dimensional contact profiler was developed to calibrate the aspheric surfaces. Videokeratoscopic data taken at "best focus" were compared to the theoretical shape to quantify device measurement errors. RESULTS: The Computed Anatomy system measurement accuracy shows no statistically significant correlation between measurement error and surface toricity (r2 < 0.13). Measurement error increased linearly with surface astigmatism for the EyeSys Laboratories system (0.12 D rms error per D of astigmatism, r2 > 0.96, p < 0.001 and the Visioptic system (0.03 D error per D of astigmatism, r2 = 0.88, p < 0.001). CONCLUSIONS: This study found systematic performance differences among the three machines. Under ideal alignment conditions, the Computed Anatomy TMS-1 is more accurate at detecting astigmatism. The EyeSys Laboratories Corneal Analysis System apparently underestimates the amount of surface astigmatism because of excessive data smoothing. The Visioptic EH-270 errors are primarily in the central zones and may be due to ring localization errors.

Algorithms↗
PubMed1996

Effects of cocaine on anterior pituitary and gonadal hormones.

Acute and chronic cocaine administration has been reported to change endocrine and neurochemical functions in animals and human drug abusers. This study examined the effects of acute cocaine administration on anterior pituitary and gonadal hormones in male human volunteers without a history of drug abuse. Using a double-blind, randomized study design, luteinizing hormone, follicle-stimulating hormone (FSH), prolactin and testosterone levels were measured in 12 healthy men before and after intranasal administration of 2 mg/kg cocaine or placebo. Each subject was studied twice, serving as his own control. Compared to placebo, both luteinizing hormone, and, to a lesser degree, follicle-stimulating hormone levels increased significantly after cocaine, reaching a peak value 60 min after the administration of the study drug. This pattern is consistent with a possible cocaine induced rise in gonadotropin-releasing hormone and subsequent rise in luteinizing hormone and follicle-stimulating hormone due to stimulation of gonadotroph cells in the pituitary gland. Neither cocaine nor placebo induced a change in testosterone levels. Prolactin levels showed a decrease from base line after both placebo and cocaine administration, with a significantly more pronounced decrease after cocaine. This likely reflects the combination of the physiologic diurnal variation in prolactin secretion and an added inhibitory effect on prolactin due to cocaine. These findings show that the acute administration of cocaine significantly alters anterior pituitary hormonal release patterns.

Administration, Intranasal↗
PubMed1996

Cultured human trabecular meshwork cells express functional alpha 2A adrenergic receptors.

PURPOSE: For the treatment of glaucoma, alpha-2 adrenergic receptor (alpha 2-AR) agonists are thought to lower intraocular pressure primarily by decreasing aqueous humor production. Effects on the outflow pathways, however, also may occur. To begin to examine this possibility, the authors characterized the alpha 2-AR subtypes present in cultures of human trabecular meshwork (HTM) cells using both immunofluorescence microscopy and functional measures of alpha 2-AR activation. METHODS: For immunofluorescence microscopy, subtype-specific polyclonal antibodies that recognize each of the human alpha 2-AR subtypes (alpha 2A, alpha 2B, alpha 2C) were used. Functional studies involved the inhibition of forskolin-stimulated cyclic adenosine monophosphate (cAMP) production, the stimulation of mitogen-activated protein (MAP) kinase activity, and the stimulation of mitotic activity as reflected by the expression of proliferating cell nuclear antigen (PCNA). RESULTS: From the immunofluorescence microscopy, there was evidence for the presence of the alpha 2A subtype, but not alpha 2B or alpha 2C subtype, on HTM cells. The administration of the alpha 2-agonist, dexmedetomidine, to HTM cells resulted in a 90% inhibition of forskolin-stimulated cAMP formation, a twofold stimulation of MAP kinase activity, and a threefold increase in the expression of PCNA. Additionally, preincubation of cells with either of the alpha 2-AR-selective antagonists, rauwolscine or atipamezole, reversed the functional effects of dexmedetomidine. CONCLUSIONS: Functional alpha 2A-ARs are present on HTM cells where they may affect the outflow pathway during the treatment of glaucoma with alpha 2-AR agonists.

Adrenergic alpha-Agonists↗
PubMed1995

Effects of cocaine on cortisol secretion in humans.

The effects of acute cocaine administration on the pituitary adrenal axis in humans without a history of drug abuse are unknown. The authors studied 12 male volunteers twice in a double-blinded, placebo-controlled, randomized fashion. After intranasal administration of 2 mg/kg cocaine, cortisol levels were significantly higher than after placebo administration. The authors concluded that acute administration of cocaine to humans increases cortisol secretion.

Administration, Intranasal↗
PubMed1995

Isolation and culture of human trabecular meshwork cells by extracellular matrix digestion.

Like corneal endothelial cells, human trabecular meshwork cells are believed to be of neural crest origin, but demonstrate physiological properties and an antithrombogenic surface similar to vascular endothelial cells. One current method for isolating trabecular meshwork cells utilizes the motile nature of these cells to migrate away from a trabecular meshwork explant in culture to more distal regions of the culture dish. This 'outgrowth' technique is limited in practice by the relatively small number of cells that migrate per explant per unit time, thus hindering the ability to gather sufficient numbers of cells for comprehensive experimentation. For this reason, we have modified an extracellular matrix digestion technique in current use for the isolation of microvascular endothelial cells to isolate human trabecular meshwork cells. This procedure is both efficient and rapid for isolating large numbers of trabecular meshwork cells and results in the availability of trabecular meshwork cells in sufficient quantities for subsequent experimentation.

Adolescent↗
PubMed1995

Breast engorgement, false positive pregnancy tests, and ectopic gonadotrophin production with bronchogenic carcinoma.

Entopic and ectopic production of hCG is recognized to occur in a number of differing neoplasms. We describe two women in the fifth decade of life who presented with breast engorgement and false positive pregnancy tests because of ectopic hCG production in bronchogenic carcinomas. HCG served as a useful tumor marker in both women. In one patient, the recognition of elevated hCG levels led to identification of an occult, early stage bronchogenic carcinoma that was resected for cure. A search for an occult neoplasm should be undertaken in nonpregnant patients with elevated hCG levels. Because of the increasing prevalence of bronchogenic carcinoma in young women, recognition of this syndrome may become more common and facilitate the earlier diagnosis of bronchogenic carcinoma in such patients.

Adult↗
PubMed1994

Predictive value of prominent pulmonary arterial wedge V waves in assessing the presence and severity of mitral regurgitation.

Indwelling pulmonary arterial catheters are often used to monitor and to guide therapy in critically ill patients. The magnitude of the V wave recorded from the pulmonary arterial wedge (PAW) position is sometimes used to assess the presence and severity of mitral regurgitation (MR). The present study was performed to assess the relation (or lack thereof) between a "prominent" PAW V wave and qualitative and quantitative estimates of MR. In 903 subjects (445 men and 458 women, aged 49 +/- 13 [mean +/- SD] years) referred for cardiac catheterization, an oximetrically confirmed PAW pressure was recorded with a large-lumen stiff catheter, and a left ventriculogram was recorded. In 646 of these subjects (328 men and 318 women, aged 50 +/- 13 years), forward cardiac output was measured by the Fick principle, and a regurgitant fraction was calculated. Prominent PAW V waves--as defined in several ways--were insensitive and had poor positive predictive value in identifying moderate or severe MR. At the same time, the absence of prominent PAW V waves was relatively specific for the absence of moderate or severe MR, and the negative predictive value of small V waves was very good. Thus, the prominence of a PAW V wave cannot be used to assess the presence or severity of MR.

Cardiac Catheterization↗
PubMed1994

Frequency, cause and effect on operative outcome of depressed left ventricular ejection fraction in mitral stenosis.

To assess the incidence, pathophysiology and influence on operative outcome of a depressed left ventricular (LV) ejection fraction (EF) in patients with mitral stenosis (MS), demographic, hemodynamic and cineangiographic data on 72 patients (16 men, 56 women, aged 19 to 75 years) with isolated MS were reviewed. Of the 45 who had mitral commissurotomy or replacement, operative course and functional class before and after surgery were assessed. Of the 72 patients, 21 (29%) had an LVEF < or = 0.50. These 21 were similar to the 51 with an LVEF > 0.50 in age, gender, heart rate, intracardiac pressures, transvalvular gradient and valve area, but they had larger LV end-diastolic (79 +/- 19 [mean +/- SD] vs 59 +/- 15 ml/m2, p < 0.001) and end-systolic volumes (46 +/- 13 vs 23 +/- 8 ml/m2, p < 0.0001). Of the 45 subjects undergoing surgery, operative outcome was similar in the 14 with a depressed and the 31 with a normal LVEF. Thus, about 1/3 of patients with isolated MS have a depressed LVEF. Compared with those with MS and a normal LVEF, these subjects have hemodynamic derangements of similar severity, but they have larger LV end-diastolic and end-systolic volumes, suggesting that impaired LV contractile function or excessive afterload (rather than diastolic underfilling), or both, is the cause of a low LVEF. Those with an LVEF < or = 0.50 who undergo valve surgery have a similar operative outcome as those with an LVEF > 0.50.

Adult↗
PubMed1994

Coronary anatomy and prognosis of young, asymptomatic survivors of myocardial infarction.

PURPOSE: To assess the coronary anatomy and prognosis of young, asymptomatic survivors of myocardial infarction. PATIENTS AND METHODS: The records of all 5,316 patients who underwent cardiac catheterization at Parkland Memorial Hospital from July 1978 to December 1992 were reviewed to identify those patients 40 years old and younger who were catheterized within 60 days of a first myocardial infarction. Of 129 such patients, 48 had no indication for catheterization other than age (group I), and 81 were catheterized for spontaneous or provocable ischemia (group II). Extent of coronary artery disease and long-term follow-up were examined to ascertain the utility of cardiac catheterization in the asymptomatic patients. RESULTS: The 2 groups were similar with respect to clinical variables. The asymptomatic survivors of infarction (group I) had fewer diseased coronary arteries than did those with post-infarction ischemia (group II) (1.0 +/- 0.7 versus 1.5 +/- 1.0 [mean +/- SD] diseased coronary arteries, respectively; P = 0.002) and were less likely to have left-main or 3-vessel coronary artery disease (4% versus 20%, respectively; P = 0.027). Eighty-three percent of the group I patients had one diseased coronary artery, or less, and no patient underwent angioplasty or coronary bypass grafting on the basis of catheterization. After 71 +/- 44 months of follow-up, only 5 (10%) had died of a coronary-related event. CONCLUSIONS: Asymptomatic survivors of myocardial infarction who are 40 years of age or less rarely have left-main or 3-vessel coronary artery disease, and their long-term prognosis with conservative therapy is good. Routine catheterization in these patients is not warranted and should be reserved for those who manifest spontaneous or provocable post-infarction ischemia.

Adult↗
PubMed1994

Phrenic nerve injury following stretch trauma: case reports.

Phrenic nerve injuries from stretch trauma are uncommon. The setting in which they occur is often associated with more severe or obscuring injuries. Electrodiagnostic studies are helpful in establishing the presence of a phrenic nerve injury and in monitoring recovery as demonstrated by the two case reports described here. Mechanisms of injury, the anatomic factors associated with phrenic nerve injury, and management are discussed.

Adult↗