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Biomedical subjects

D M Silver

Publications and source records attributed to D M Silver.

At least 19 recordsLinked to original sources
PubMed2001

Acute effect of latanoprost on pulsatile ocular blood flow in normal eyes.

PURPOSE: To determine the effect of an acute dose of 0.005% latanoprost on intraocular pressure and pulsatile ocular blood flow in normal eyes. METHODS: Nineteen volunteers received a single dose (two drops) of latanoprost 0.005% in one eye and placebo in the fellow control eye, randomized and masked to the observer. Intraocular pressure, perfusion pressure, pulsatile ocular blood flow, and systemic circulatory parameters were measured before and 8 hours after dosing. RESULTS: The mean (+/- SE) intraocular pressure, perfusion pressure, and pulsatile ocular blood flow before treatment were 17 +/- 1 mm Hg, 46 +/- 3 mm Hg, and 13 +/- 1 microl per second, respectively, in both the treated and control eyes. The mean intraocular pressure reduction was 4.9 and 2.1 mm Hg (28% and 12%) in the treated and fellow eye, respectively. The mean perfusion pressure increase was 5.6 and 2.8 mm Hg (12% and 6%) in the treated and fellow eye, respectively. The mean pulsatile ocular blood flow increase was 2.7 and 0.2 microl per second (20% and 1%) in the treated and fellow eye, respectively. The treated eye change in pulsatile ocular blood flow was not correlated to the change in perfusion pressure. Simultaneous reduction of intraocular pressure and increase or no significant change in pulsatile ocular blood flow occurred in 15 of 19 (79%) of the treated eyes. The systemic blood pressure and pulse rates remained in normal ranges over the 8-hour period. CONCLUSION: In this study, topical latanoprost significantly reduces intraocular pressure and increases ocular blood flow in normal eyes 8 hours after dosing. These effects of latanoprost may be beneficial in the management of glaucoma patients.

Aged↗
PubMed2000

Pressure-volume relation for the living human eye.

PURPOSE: The pressure-volume relation for an eye is the mathematical equation that relates changes in intraocular pressure to changes in intraocular volume. This relation is useful for calculating outflow facility from tonography and pulsatile ocular blood flow from intraocular pressure pulsations. The present work develops a new relation by culling together all the published direct manometric rigidity measurements on living human eyes. METHODS: A total of 182 data items taken from 21 eyes are available in the 1958-62 literature of Ytteborg, Prijot, Eisenlohr, Langham and Maumenee. The approach was (i) to perform an error analysis based on the various experimental conditions, (ii) to assume general mathematical forms for the relation, (iii) to use least-squares analysis and statistical measures to find the optimal data representation, and (iv) to introduce the total volume of the eye into the formulation. RESULTS: A new formula for the pressure-volume relation for the living human eye is derived relating DeltaV, the change in volume, to P, the corresponding intraocular pressure: DeltaV = V (C + C(0) x ln P + C( 1) x P), where V is the volume of the eye, C, C(0) and C(1) are numerical parameters. This equation gives the most statistically significant fit to the experimental data. CONCLUSION: The new equation for the pressure-volume relation derived from all the currently available ocular rigidity data on the living human eye gives a larger volume increment for a given increment of pressure than Friedenwald's equation based on measurements performed on cadaver eyes.

Data Interpretation, Statistical↗
PubMed2000

Plasminogen activator activity in tears after excimer laser photorefractive keratectomy.

PURPOSE: To quantify changes of plasminogen activator activity in tear fluid during corneal re-epithelialization after excimer laser photorefractive keratectomy (PRK). METHODS: Tear samples were collected with glass capillaries from 77 eyes of 42 patients immediately before and immediately after PRK treatment and on postoperative days 3 and 5. In 20 patients, the contralateral eye was similarly sampled to serve as control. Plasminogen activator activity in the tear samples was measured by a spectrophotometric method using human plasminogen and chromogenic peptide substrate, D-valyl-L-leucyl-L-lysine-p-nitroanilide (S-2251). RESULTS: In tears of all eyes that underwent PRK, the plasminogen activator activities were lower immediately after PRK than were the preoperative values. For patient eyes with normal wound healing, tear plasminogen activator activities were significantly elevated above the preoperative level on the third postoperative day and then returned to the preoperative level by the fifth postoperative day. In contrast, tear plasminogen activator activities remained low through the third postoperative day in all (six) eyes in which haze developed after 3 to 6 months. The contralateral control eyes showed no appreciable change in plasminogen activator activity over the 5-day period. CONCLUSIONS: Plasminogen activator activity levels measured in tears of excimer laser PRK-treated eyes may serve as a predictor of wound healing. Extended low levels of plasminogen activator activity through the third postoperative day correlate with the development of corneal healing abnormalities (haze). The low plasminogen activator activity could be not only an accompanying sign but also a cause of defective corneal wound healing.

Adolescent↗
PubMed1999

Pulsatile ocular blood flow in diabetic retinopathy.

PURPOSE: To assess circulatory properties of eyes with progressive stages of diabetic retinopathy. METHODS: The intraocular pressure, pulse amplitude (PA) and pulsatile ocular blood flow (POBF) were measured with a pneumatonometer (OBF Labs UK Ltd). The eyes were grouped: (a) normal control, n = 26, (b) diabetes with no observable diabetic retinopathy (NDR), n = 18, (c) mild to moderate non-proliferative diabetic retinopathy (NPDR), n = 20, and (d) very severe pre-proliferative and proliferative diabetic retinopathy (PPDR/PDR), n = 12. RESULTS: The PA and POBF values were lower than normal values in the earliest stage (NDR). The POBF increased but was still below normal levels at the NPDR stage, and then increased to an above normal level in the PPDR/PDR stage of diabetic retinopathy. The PA was at normal levels in these later two stages. CONCLUSION: An initial decrease in pulsatile ocular blood flow is present with the onset of diabetes where no diabetic retinopathy has yet occurred. Subsequently, the pulsatile ocular blood flow increases with the severity of the retinopathy.

Aged↗
PubMed1998

Reproducibility of topographic measures of the glaucomatous optic nerve head.

AIMS/BACKGROUND: Laser scanning tomography provides an assessment of three dimensional optic disc topography. For the clinical purpose of follow up of glaucoma patients, the repeatability of the various measured variables is essential. In the present study, the reproducibility of morphometric variables calculated by the topographic scanning system, TopSS (Laser Diagnostic Technology, San Diego, CA) was investigated. METHODS: Two independent measurements (30 minutes apart) each consisting of three complete images of the optic disc were performed on 16 eyes of 16 glaucoma patients using a TopSS. The instrument calculates 14 morphometric variables for the characterisation of the optic disc. RESULTS: From the two tailed paired tests, all variables were seen to have good reproducibility. However, correlation and regression analyses showed that only the three variables, volume below, half depth area, and average cup depth, are acceptably reproducible. CONCLUSION: The TopSS provides three variables which describe the physiological shape of the optic disc that have high reproducibility. These three variables might be useful for following the progression of optic disc changes in glaucoma patients.

Aged↗
PubMed1997

The mechanism of intraocular pressure rise during cyclocryotherapy.

PURPOSE: Intraocular pressure (IOP) spikes that occur during cyclocryotherapy for advanced glaucoma may further injure the already damaged glaucomatous optic nerve and be responsible for visual impairment that may occur after this treatment. The authors investigated the mechanism of pressure rise to see whether it can be avoided and thus prevent further optic nerve injury. The authors postulated that intraocular ice forms during the cryo procedure and causes the pressure changes. METHODS: Intraocular pressure was monitored using a pneumatonometer during 15 cryocycles of four patients with advanced glaucoma and 21 cryocycles of five normal rabbits. A simple thermal model was developed to analyze the relation between volume expansion and pressure rise in the eye. The physical effect of freezing rabbit eye structures was investigated in vitro. RESULTS: The largest pressure spikes observed during the cryocycles in this work were increases of 32 mm Hg for humans and 25 mm Hg for rabbits. The mean value of the IOP immediately before and after the cryo freezing stage was 53 +/- 1 and 68 +/- 2 mm Hg, respectively, for humans and 22 +/- 1 and 32 +/- 1 mm Hg for rabbits. The parameters of the thermal model were determined from the observed IOP spikes. Calculated thaw times were consistent with measured times for return to precryo IOPs. In vitro cryoapplication (rabbit eye) showed the formation of an ice ball internal to the eye. CONCLUSIONS: Volumetric increase of the intraocular content related to the formation of an ice ball in the eye, is the mechanism of pressure spikes during cyclocryotherapy. Because this complication is unavoidable, other cyclodestuctive methods may be more prudent, particularly in patients with advanced glaucoma.

Animals↗
PubMed1994

Validity of pulsatile ocular blood flow measurements.

The determination of average net pulsatile ocular blood flow form measurements of the intraocular pressure (IOP) pulse is based upon 1) an accurate measurement of IOP and its time variation; 2) a knowledge of the relation between the volume of the living eye and its IOP; 3) a physical model for the flow of blood through the eye: and 4) a concept of steady venous outflow from the eye. Each of these premises needs to be examined. The present analysis assesses the validity of the pneumatic tonometer for measuring the pressure in a flowing column of gas that is directed toward a thin membrane that is in contact with the surface of the cornea. The pressure of this stream of gas exerts a force against the cornea that depresses the corneal surface against the opposing force of the IOP. The balance of these forces and the resultant effect on the tonometer pressure sensor is described by the theories of elasticity, thermodynamics and fluid mechanics. In this treatment, differential equations are solved for the elastic deflections of the cornea subject to the opposing intraocular and tonometer pressures. This permits the pressure in the chamber of the pneumatic tonometer to be related through first principles to the IOP. The response of the tonometer pressure sensor to a range of IOPs (5-60 mmHg) is obtained, as well as the dynamical response of the tonometer to IOP oscillations. The conclusion is that the pneumatic tonometer provides a high fidelity, noninvasive measure of the IOP and its time variation.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood Flow Velocity↗
PubMed1993

Heparin-induced thrombocytopenia in children.

Two children aged 3 months and 14 years developed heparin-induced thrombocytopenia whilst receiving heparin. The clinical course was complicated by renal vein thrombosis in the 3-month-old infant and subcutaneous haemorrhage requiring resuscitation in the 14-year-old child. Anticoagulant therapy was discontinued immediately in the 3-month-old child. In the 14-year-old child, because of the need for continued anticoagulation therapy, low molecular weight heparin was used until warfarin could be substituted.

Adolescent↗
PubMed1991

Arthroscopically-assisted semitendinosus and gracilis tendon graft for anterior and posterior cruciate ligament deficiency.

A simple, reliable, arthroscopically-assisted procedure is presented in which a doubled and suture woven semitendinosus and gracilis free tendon graft is used in the anterior or posterior cruciate ligament deficient knee. A ligament guide system is used for isocentric placement of tibial and femoral drill holes. The procedure is performed using a button and staple technique that allows accurate placement of the ligament substitution and facilitates early ambulation. At an average follow-up of 2.5 years, good and excellent results have been demonstrated in 97% of 74 knees in which this technique was used, with only a 3% risk of graft failure.

Adolescent↗
PubMed1989

Estimation of pulsatile ocular blood flow from intraocular pressure.

A relationship has been derived between intraocular pressure and pulsatile blood flow in the eye. Measurements of intraocular pressure show a time variation that is associated with the pulsatile component of arterial pressure. Experimental results provide a means of transforming intraocular pressure changes into ocular volume changes. The eye is represented by a chamber with elastic walls, a pulsatile incoming flow of incompressible fluid (blood), and a steady outgoing flow of blood. Under these conditions, the rate of pulsatile blood flow through the eye can be approximated from the instantaneous intraocular pressure measurements. Data from a healthy human eye are used to illustrate the analysis.

Blood Flow Velocity↗
PubMed1988

Tears of the meniscus as revealed by magnetic resonance imaging.

Forty-four patients who were examined by magnetic resonance imaging and arthroscopy to detect tears of the meniscus of the knee were studied. Twenty-eight patients (Group I) had had no previous operations on the knee. Sixteen patients (Group II) had had a previous operation on the knee that had been performed after a meniscal injury. In Group I, the percentage of correct diagnoses by magnetic resonance imaging was 45 per cent. If the false-negative results are excluded, the rate of accuracy was 65 per cent. Similarly poor results were found in Group II: a rate of accuracy of 49 per cent and, if we eliminate the false-negative results, the rate of accuracy was only 55 per cent.

Adolescent↗
PubMed1986

Minor articular fractures of the knee: an update.

Early management of minor articular fractures of the knee results in the best possible prognosis with the least period of disability. The authors stress the need for early arthroscopic examination when history, physical examination and x-ray suggest possibility of minor intra-articular fracture.

Arthroscopy↗
PubMed1981

Differential behavior of cytotoxic effector cells against HLA antigens in strong genetic linkage disequilibrium.

Five sets of cytotoxic effector cells were generated, using haplo-identical, first degree relatives in five different families, against the HLA-A3; B7 serological determinants combined with different DR antigens. When tested against a panel of cells bearing combinations of the HLA-A, -B and -DR antigens it was shown that the HLA-B7 antigen was as strong a CML target determinant alone as it was in the presence of HLA-A3. The strength of the HLA-A3 antigen as target determinant varied. With effector cells primed to the HLA-A3; B7; DR2 haplotype, the A3 antigen alone behaved as a weak target determinant. When the same target cells were tested with the effector cells generated against HLA-A3; B7 without DR2, the A3 antigen behaved as a strong target determinant. A number of target cells lacking the serologically detectable HLA determinants present on the sensitizing HLA haplotype were identified as being killed by specific effector cells. These data suggest either a number of new CML target determinants controlled by different loci or the presence of a single, new locus with multiple alleles controlling CML targets.

Cytotoxicity, Immunologic↗
PubMed1981

Polygenic control of the immune response to F antigen.

The ability to produce an autoimmune response to F antigen in mice is under H-2-linked and non-H-2-linked Ir-gene control. There is an absolute requirement for a k allele at H-2K or I-A in order to produce antiF antibodies. Low and high responsiveness is controlled by a non-H-2-linked Ir gene which behaves in a similar fashion to Ir-3, in that as the dose of F-antigen is lowered, low responders behave as high responders and vice versa. This conversion from low to high responders and vice versa. This conversion from low to high responsiveness also occurs within a month after ATX.-Most F1 hybrids derived from (responder X nonresponder) parents bearing identical F-types behave as dominant nonresponders. As a result of ATX, such F1 mice convert to high responders . This conversion occurs if the animals are not immunized before day 90. If they receive F antigen prior to that time, they remain nonresponders for 7-9 months. One F1 combination showed--AKD2--behaves as an dominant higher responder. Genetic analysis showed that the presence of a K allele at H-2K or I-A, a non-H-2-linked Ir gene inherited from the AKR mice determined dominant responsiveness. No manipulation of the immune response or combination of genes converted nonresponders lacking a k allele into responders. Such complex genetic control suggests regulation by a number of independently segregating loci whose function it is to limit the autoimmune response to F antigen.

Animals↗
PubMed1981

Cytotoxic effector cells against HLA antigens in strong linkage disequilibrium: identification of a strong, new CML determinant.

Three sets of cytotoxic effector cells were generated against the A1, B8, DR3 haplotype using haploidentical individuals in three different families. The three sets of effector cells generated against this haplotype showed excellent reproducibility testing, strong cytotoxicity against their specific targets, low autologous kill, and segregation with the sensitizing haplotype within the family. When tested against a panel of cells bearing all combinations of the A1, B8, DR3 antigens, a hierarchy of contribution of the individual HLA antigens as CML target determinants was seen. A new strong target cell determinant was identified by cytotoxicity with one of the effector cells not explicable in terms of the A1, B8, DR3 antigens or known HLA cross-reactivity. A family study demonstrated that this determinant clearly segregates with HLA. The success of this approach in defining new CML determinants may result from the generation of effector cells across a single haplotype in strong linkage disequilibrium or from the presentation of CML determinants in the context of self.

Analysis of Variance↗