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Biomedical subjects

N X Nguyen

Publications and source records attributed to N X Nguyen.

At least 73 records · Page 4Linked to original sources
PubMed1994

Scotopic threshold response and scotopic PII in glaucoma.

PURPOSE: The value of the ERG scotopic threshold response (STR), which is known to originate in the proximal retina and of the near-threshold scotopic PII(B-wave), was studied in glaucoma. METHODS: Full-field single flashes were presented under scotopic conditions in two subject groups, a glaucoma group (n = 30) and a normal control group (n = 35). The intensity level for evoking STR was where the maximum response occurs, whereas the intensity level for evoking scotopic PII was where the response begins to appear. RESULTS: The STR is only slightly reduced in glaucoma (P = 0.046), but scotopic PII is significantly diminished (P < 0.0001). CONCLUSIONS: The relatively intact STR in glaucoma suggests that structures responsible for its generation are less damaged in glaucoma than those responsible for scotopic PII.

Dark Adaptation↗
PubMed1994

Protein quantification and electrophoresis in aqueous humor of pseudoexfoliation eyes.

PURPOSE: Pseudoexfoliation (PSX) eyes frequently show clinical signs of blood-aqueous barrier impairment. To analyze these alterations, the authors examined aqueous humor of human eyes with and without PSX. METHODS: After aqueous humor samples had been obtained during cataract or filtering glaucoma surgery, a modified Pierce-bicin choninic acid assay was used to quantify total aqueous protein concentration in 27 PSX eyes and 37 eyes without clinical signs of PSX (12 cataract eyes and 25 eyes with primary open-angle glaucoma). In addition, aqueous protein composition was analyzed by sodium dodecylsulfate polyacrylamide gel electrophoresis, silver staining, and laser densitometry in 27 PSX eyes and 59 eyes without PSX. RESULTS: Aqueous protein concentration was significantly higher in PSX (mean 0.42 +/- 0.16 mg/ml) than in normal cataract eyes (0.22 +/- 0.08 mg/ml, P < 0.0001) and in eyes with open-angle glaucoma (0.26 +/- 0.09 mg/ml, P < 0.0001, Wilcoxon-Mann-Whitney test). Electrophoresis revealed a characteristic increase of a 12.5-kDa band in 15 of 27 PSX eyes but in only 1 of 59 eyes without PSX (P < 0.00001, chi-square test). CONCLUSIONS: These results substantiate increased aqueous protein concentration and aqueous barrier impairment in PSX. The additional finding of an increased 12.5-kDa band in 56% of PSX eyes may be related to the pathogenesis of PSX in the anterior ocular segment.

Aged↗
PubMed1994

VEP test of the blue-sensitive pathway in glaucoma.

PURPOSE: The blue-sensitive pathway in normal subjects and in patients with primary open-angle glaucoma (POAG) was tested with the pattern visual evoked potential (VEP) method under selective adaptation. METHODS: Recording of pattern-onset VEP in response to blue (460-nm) stripes (0.88 c/deg) presented either without or with a bright yellow (570-nm) adaptation light (Maxwellian view, 33 degree diameter). Amplitude and peak times were evaluated, and the mean tritan score of the Farnsworth 100-hue test was determined. Age-matched normal subjects (n = 34) and (n = 32) patients with POAG were examined. RESULTS: The amplitude and peak time of the VEP without selective adaptation did not discriminate normal subjects from the POAG group. With selective adaptation, the amplitude was reduced (P = 0.002) and its peak time delayed (P < 0.0001) in POAG, yielding a sensitivity of 75% and a specificity of 94%. The VEP measures only under selective adaptation correlated significantly in patients with POAG with the mean perimetric defect, with the optic disc damage, and with the 100-hue test. CONCLUSIONS: Recording the blue-on-yellow VEP is a useful test in glaucoma research.

Adult↗
PubMed1994

Hospital markets and competition: implications for antitrust policy.

Antitrust and competitive issues are intertwined with efforts to control health care costs. Whether hospital competition holds down costs is an issue of much controversy and is the topic of this article. The findings are consistent with the hypothesis that hospitals compete primarily on a nonprice basis. This suggests that a more flexible application of antitrust policy to hospital mergers will reduce cost increases and will improve efficiency. A step in this direction may be necessary if the managed care plans discussed by the Clinton administration create highly concentrated provider networks.

Antitrust Laws↗
PubMed1993

Spatial and chromatic interactions in the human pattern electroretinogram.

The spectral sensitivity and the spatial selectivity was studied both psychophysically and electroretinographically using the pattern onset-offset paradigm. All measurements were made under intensive yellow adaptation. The spectral sensitivity functions of both measures were in close agreement. They showed a peak at 460 nm (blue-sensitive mechanism) and a shoulder around 550 nm (red-green-sensitive mechanism). The luminance curves of the pattern onset ERG obtained with long wavelengths had a steeper slope and reached larger amplitudes than those obtained with short wavelengths. In addition the response-peak times were longer with short wavelengths. When the spatial frequency of the pattern was varied the 460 nm-onset responses showed very little or no spatial tuning and long peak times (around 60 msec). This was ascribed to the contribution from only one type of ganglion cell, namely the blue-yellow opponent receptive fields lacking a center-surround organization. The 550 nm-onset responses showed a clear spatial tuning (4 c/deg) and an increase in peak time (40-50 msec) with increasing spatial frequency (0.26-9.2 c/deg). This was ascribed to different types of receptive fields having a center-surround structure.

Adaptation, Ocular↗
PubMed1993

Interactions of spectral, spatial, and temporal mechanisms in the human pattern visual evoked potential.

The human pattern onset-offset visual evoked potential (VEP) was studied with different colours and spatial frequencies presented on a steady homogeneous intensive yellow background. Under this condition a broad, late negative wave (N2) dominated by the blue-sensitive mechanism and a sharper earlier positive wave (P1) dominated by the red-green-sensitive mechanism can be observed. With a 460 nm pattern N2 shows a strong amplitude tuning at low-medium spatial frequencies. With 550 nm an early negative component (N1) is added showing an amplitude tuning at high spatial frequencies. Different spatial resolutions of the two colour mechanisms are thus indicated. With pattern-reversal stimuli the 550 nm stimulus shows a higher temporal resolution than the 460 nm stimulus.

Color Perception↗
PubMed1993

[Correlation between tyndallometry with the "laser flare cell meter" in vivo and biochemical protein determination in human aqueous humor].

INTRODUCTION: The laser flare-cell meter allows noninvasive measurement of aqueous flare in humans. In vitro studies have shown good correlations between flare values and albumin concentrations. In this study, we compared aqueous flare values and aqueous protein concentrations in living human eyes. PATIENTS AND METHODS: We measured aqueous flare and the total aqueous protein concentration in 52 eyes of 52 patients (age 58.4 +/- 18.8 years) that underwent cataract extraction (22 eyes) or trabeculectomy (30 eyes). Before surgery, aqueous flare was determined with the laser flare-cell meter (Kowa FC-1000). During surgery, 50 microliters of primary aqueous humor was aspired, and total aqueous protein concentration was performed using a modified Pierce-BCA assay. Linear regression analysis was calculated. RESULTS: There was a high correlation between aqueous flare values and total aqueous protein concentration (y = 0.52x + 0.15, r = 0.98, p < 0.00001). This correlation was observed both in eyes with and without cataract. CONCLUSION: Our results indicate that aqueous flare values as determined with the laser flare-cell meter reflect the true aqueous protein concentration quite accurately. Lens opacities appear not to significantly alter the results of aqueous flare measurements. As expected by theoretical assumptions, flare measurements overestimate aqueous protein concentrations in eyes with severely impaired blood-aqueous barriers because, according to Rayleigh's law, larger protein molecules cause more light scattering. However, transformation and expression of flare values as "albumin concentration equivalents" according to in vitro-calibration may still be useful, especially if the results of different instruments are to be compared.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1993

[Blue-sensitive VEP test in diagnosis of glaucoma].

BACKGROUND: Glaucomatous eye diseases can be associated in many cases with a disturbance of blue-color vision. Also visual evoked potentials (VEPs) can be altered in glaucoma. In the present study both tests were combined in order to increase the sensitivity of the method, i.e. the pattern VEP was examined under conditions isolating the blue-sensitive part of the visual pathway. METHOD: 34 normals, 14 patients with ocular hypertension (OHT), 15 patients with a so called pre-perimetric primary open-angle glaucoma (PPOAG), and 32 patients with primary open-angle glaucoma (POAG) were examined by presenting in the onset-offset mode blue stripe patterns 1. without an adaptive background light (blue- "black" pattern = BB) and 2. with an intensive yellow adaptation light (blue-on-yellow pattern = BY) suppressing the red- and green-sensitive cones. Amplitudes and peak times of the onset response were evaluated. RESULTS: BB responses are of positive, BY responses of negative polarity (inion-ear). In normals the latter responses are significantly larger and have a significantly longer peak time. While in the patients groups the BB responses were not significantly altered, the BY responses were significantly reduced only in POAG. The peak time was significantly delayed in both PPOAGs and POAGs. The BY peak time proved to be most significant: Sensitivity 87.1%, specificity 82.4%. CONCLUSION: The recording of the VEP with blue stimuli presented under selective yellow adaptation is a sensitive test for early glaucoma diagnosis.

Adult↗
PubMed1993

Aqueous flare and cells in eyes with retinal vein occlusion--correlation with retinal fluorescein angiographic findings.

The laser flare cell meter (LFCM) was used to evaluate alterations of the blood-aqueous barrier in eyes with retinal vein occlusion (RVO). In 42 eyes of 42 patients with RVO (30 branch vein occlusions, 12 central vein occlusions) aqueous flare and aqueous cells were determined and retinal fluorescein angiography was performed. Flare and cell values were compared with 59 normal age and sex-matched control eyes and correlated with fluorescein angiographic findings. Both aqueous flare (12.321 (SD 6.717) photon counts/ms) and aqueous cells (mean 3.37 cells/0.075 x 10(6)/l) were significantly increased in eyes with RVO in comparison with the normal control group (flare 4.288 (SD 1.144) photon counts/ms, cells 0.54 cells/0.075 x 10(6)/l, p < 0.0001 and p < 0.005). Flare values in eyes with central RVO (19.517 (SD 7.762) photon counts/ms) were significantly higher than in eyes with branch RVO (9.443 (SD 3.307) photon counts/ms, p < 0.0001). Significant correlations were found between flare values and area of RVO (r = 0.70, p < 0.0001) and between flare values and area of retinal non-perfusion (r = 0.76, p < 0.0001). There were also significant correlations between flare values and number of cotton wool spots (r = 0.54, p < 0.0002) and between cell counts and degree of cystoid macular oedema (r = 0.46, p < 0.006). Our findings show that aqueous protein concentration, as indicated by flare, and aqueous cells are increased in RVO and that changes of aqueous flare correlate with fluorescein angiographic findings that reflect the severity of RVO. Measurements with LFCM may be useful non-invasively to quantify changes of the ocular barrier functions in RVO.

Adult↗
PubMed1992

Quantitative assessment of aqueous flare and aqueous 'cells' in pseudoexfoliation syndrome.

We used the laser flare-cell meter to measure aqueous flare and aqueous 'cells' in 38 eyes of 38 patients with pseudoexfoliation, in 36 normal control eyes of 36 subjects, and in 19 eyes of 19 patients with chronic open-angle glaucoma unrelated to pseudoexfoliation. In pseudoexfoliation eyes, both aqueous flare (0.61 +/- 0.55 mg/ml human albumin equivalent) and aqueous 'cells' (mean 10.70, range 0-50.6 cells/0.075 mm3) were significantly higher than in the normal control group (flare 0.15 +/- 0.06 mg/ml, 'cells' 0.43, range 0-2, P less than 0.0001) and in the glaucoma group without pseudoexfoliation (flare 0.19 +/- 0.08 mg/ml, 'cells' 0.80, range 0-2.4, P less than 0.0001 and P less than 0.005). No significant difference could be found between the flare and cell counts of normal eyes and glaucoma eyes without pseudoexfoliation (P greater than 0.09, P greater than 0.05) and between PSX eyes with (18 eyes) and without (20 eyes) open-angle glaucoma (P greater than 0.99, P greater than 0.4). Our findings indicate that the blood-aqueous barrier is impaired in eyes with pseudoexfoliation, and that the laser flare-cell meter may be a useful tool to quantify these changes. These alterations of the blood-aqueous barrier need to be considered in medical therapy and intraocular surgery.

Aged↗
PubMed1992

Quantitative measurement of aqueous flare and aqueous "cells" in eyes with diabetic retinopathy.

In order to evaluate the applicability of the laser flare-cell meter to diabetic oculopathy, we measured aqueous flare and aqueous "cells" in 84 diabetic eyes of 84 patients and 50 normal control eyes of 50 age-matched subjects. Using fluorescein angiography, diabetic retinopathy was divided into background retinopathy (11 eyes), preproliferative retinopathy (38 eyes), and proliferative retinopathy (35 eyes). In diabetic eyes, the occurrence of both aqueous flare (0.73 +/- 0.39 mg/ml human albumin equivalent) and aqueous cells (mean 2.96, range 0-35.7 cells/0.075 mm3) was significantly greater than in the normal control group (flare 0.14 +/- 0.06 mg/ml, cells 0.39, range 0-2 cells/0.075 mm3, P < 0.0001 and P < 0.0002, respectively). The flare values also showed differences between the diabetic subgroups, with flare values being higher in more advanced stages of diabetic retinopathy. No significant correlation could be found between the flare values and the kind of diabetic therapy, the duration of the diabetes, and the number of or the time interval since previous retinal laser coagulations. A flare value of more than 0.5 mg/ml was found to represent probably a "critical value" indicating a tendency towards preproliferative and or proliferative changes. The laser flare-cell meter is a valuable instrument for noninvasive, quantitative assessment of alterations of the blood-aqueous barrier in diabetes. The increase of flare values seems to parallel the progression of diabetic retinopathy.

Adult↗
PubMed1991

[Neuroretinal rim in eyes with simple optic atrophy].

In comparison with normal eyes, eyes affected by glaucomatous optic nerve damage are characterized by a decreased neuroretinal rim area and an increased optic cup size. This study was performed to find whether eyes with nonglaucomatous optic nerve atrophy and normal eyes differ with respect to the neuroretinal rim. In all, 143 eyes with nonglaucomatous descending optic nerve atrophy and 563 normal eyes were biomorphometrically examined. The area and form of the neuroretinal rim and optic cup and the horizontal and vertical cup-to-disc ratios did not differ significantly between the two groups. These results are in part not supported by data in the literature. If they are confirmed by further studies, they will indicate that measurement of the neuroretinal rim and estimation of the cup-to-disc ratios will not be useful for the diagnosis of damage to the descending nonglaucomatous optic nerve. It will be helpful for the differentiation of eyes with glaucomatous and nonglaucomatous optic nerve atrophy.

Adult↗
PubMed1989

Role of intravenous isoproterenol in the electrophysiologic induction of atrioventricular node reentrant tachycardia in patients with dual atrioventricular node pathways.

To assess the role of intravenous isoproterenol for the facilitation of electrophysiologic induction of atrioventricular (AV) node reentrant tachycardia, 20 patients with dual AV node pathways who lacked inducible AV node reentrant tachycardia at control study had a constant isoproterenol infusion administered and underwent repeat study. Six (30%) of 20 patients (group I) had inducible AV node reentrant tachycardia during isoproterenol infusion whereas the other 14 (70%) patients (group II) did not. Paroxysmal supraventricular tachycardia was clinically documented in all 6 group I patients compared to 3 (21%) of 14 group II patients (p = 0.002). The sensitivity and specificity of isoproterenol-facilitated induction of AV node reentrant tachycardia were 67 and 100%, respectively. The isoproterenol-facilitated induction of sustained AV node reentry was mediated by resolution of the weak link in anterograde slow pathway in 2 (33%) patients, in retrograde fast pathway in 3 (50%) and in both anterograde slow and retrograde fast pathways in 1 (17%) patient. Four group I patients were given intravenous propranolol, 0.2 mg/kg body weight, and had complete suppression of isoproterenol-facilitated induction of AV node reentry. Thus, intravenous isoproterenol is a rather sensitive and highly specific adjunct to electrophysiologic induction of AV node reentrant tachycardia in patients with dual AV node pathways but without inducible sustained AV node reentry.

Adrenergic beta-Antagonists↗
PubMed1989

Electrophysiologic manifestations of the excitable gap of orthodromic atrioventricular reciprocating tachycardia demonstrated by single extrastimulation.

To assess the electrophysiologic characteristics of the excitable gap, 12 patients with orthodromic atrioventricular (AV) reciprocating tachycardia were studied. During tachycardia, 8 patients used a left-sided and 4 patients a right-sided anomalous bypass tract for retrograde conduction. QRS complex-synchronized single extrastimuli were delivered from high right atrium, right ventricular apex and coronary sinus, respectively, scanning the whole cycle length of tachycardia. An excitable gap was determined to be present if tachycardia resetting or tachycardia termination occurred. The duration of the excitable gap varied among different pacing sites and occupied 0 to 48% (mean 17 +/- 16) of basic tachycardia cycle length (240 to 480 ms, mean 327 +/- 70). Three patterns of tachycardia resetting were observed: the sum of coupling interval and return cycle being (1) less than a fully compensatory pause in 12 of 12 patients, (2) more than a fully compensatory pause in 5 of 12 patients and (3) equal to a fully compensatory pause in 2 of 12 patients, depending on extent of AV nodal conduction delay exhibited in return cycle. Tachycardia termination was possible when extrastimuli were delivered from right ventricular apex and coronary sinus but not from high right atrium, and only when basic tachycardia cycle length was greater than or equal to 290 ms in 7 of 12 patients. Tachycardia termination was accounted for by development of orthodromic conduction block in AV node in 7 of 7 patients and in bypass tract in 2 of 7 patients. Therefore, site of extra-stimulation and basic tachycardia cycle length affect electrophysiologic manifestations of excitable gap. Further, functional properties of the AV node influence patterns of tachycardia resetting and are primarily responsible for tachycardia termination during programmed single extrastimulation.

Adult↗
PubMed1989

The retinal nerve fiber layer in normal eyes.

The retinal nerve fiber layer is different in normal and glaucomatous eyes. The authors used red-free photographs to examine the retinal nerve fiber layer in 234 normal eyes. The retinal nerve fiber layer was most visible in the inferior temporal arcade, followed by the superior temporal arcade, then by the temporal macular area, and finally the nasal area. This distribution was significantly (P less than 0.0001) correlated to (1) the configuration of the neuroretinal rim, which was significantly broadest at the inferior disc pole followed by the superior one, (2) the juxtapapillary caliber of the retinal vessels, which were significantly wider in the inferior temporal arcade than in the superior temporal arcade, and (3) the location of the foveola 0.53 +/- 0.34 mm inferior to the middle point of the vertical optic disc axis. The retinal nerve fiber layer decreased with age. No correlation occurred with sex or right or left eye. No localized retinal nerve fiber layer defects were seen. These features of the normal retinal nerve fiber layer are important for diagnosis of retinal nerve fiber layer changes secondary to optic nerve damage in the diseased eye.

Adolescent↗
PubMed1989

[The retinal nerve fiber layer in normal eyes and in glaucoma. I. Semiquantitative data of 398 eyes with glaucoma].

Atrophy of the optic nerve is associated with changes of the retinal fiber layer (RNFL). Using red-free photographs the authors examined the RNFL of 398 eyes with chronic primary open-angle glaucoma and compared it with the RNFL of 234 normal eyes. The glaucoma group was divided into five stages and the fundus into four sectors. Differences between the normal and glaucoma eyes were: (1) The sequence of the sectors, with regard to the best visibility of the retinal nerve fiber bundles, was changed. In the normal eyes the nerve fiber bundles were most often best visible in the inferior temporal sector, followed by the superior temporal sector, the temporal horizontal area and finally the nasal region. In the glaucoma group the nerve fiber bundles were significantly more often best detectable in the superior temporal sector and the temporal horizontal area. (2) The degree of visibility of the retinal nerve fibers decreased significantly with increasing glaucoma stage. (3) Localized defects were seen in 15% of the eyes with glaucoma and none of the normal eyes. The specificity of this qualitative parameter was, therefore, 100%. The defects were found most often in the superior and inferior temporal regions. These differences between normal and glaucomatous eyes were also significant for the first glaucoma stage of this study. The localization of the foveola below the optic disk center (0.53 +/- 0.34 mm in the glaucoma group and 0.55 +/- 0.29 mm in the normal eyes) was not significantly different.

Adolescent↗
PubMed1989

[The retinal nerve fiber layer in normal and glaucomatous eyes. II. Correlations].

The retinal nerve fiber layer is different in normal and glaucomatous eyes. We correlated semi-quantitative data of the retinal nerve fiber layer of 398 eyes with chronic primary open-angle glaucoma and of 234 normal eyes with the intra- and parapapillary morphometric signs and with the perimetric indices. The three parameters "sequence of the fundus sectors concerning the best visibility of the retinal nerve fiber bundles", "visibility of the nerve fiber bundles", and "localized defects" were significantly (p less than 0.001) correlated to 1) area of the neuroretinal rim as a whole and in four different optic disc sectors, 2) neuroretinal rim width determined every 30 degrees, 3) optic cup area, diameters and form, 4) horizontal and vertical cup/disc ratios and the quotient of the horizontal to vertical cup/disc ratio, 5) area and width of zone "Alpha", zone "Beta", and the total parapapillary chorio-retinal atrophy, 6) diameter of the retinal vessels, 7) grade of a "tesselated fundus", and 8) the visual field loss. If only the inferior temporal and the superior temporal sectors were considered, the retinal nerve fiber bundles were less visible in that sector with the largest notch in the neuroretinal rim, the smaller neuroretinal rim area and width, the thinner retinal vessels, and the larger zone "Alpha", zone "Beta", and total parapapillary chorio-retinal atrophy. The glaucomatous changes in the retinal nerve fiber layer are correlated in time and location with the intra- and parapapillary and the perimetric alterations. Evaluation of the retinal nerve fiber layer is a useful method to detect a glaucomatous optic nerve damage.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗