Biomedical subjects
Publications and source records attributed to M B Landers.
An inexpensive and reliable fluid-gas exchange system has been developed for vitreous surgery. The system relies on a fish-tank air pump and was compared with the Grieshaber Air System in several laboratory and operating room situations. No significant differences were found between the two systems. Details of the Davis Ocular Air Pump are provided along with a cost analysis.
Subretinal blood within the macula may cause visual loss in a number of macular diseases. The clinical and histopathologic effects of experimental subretinal hemorrhage were evaluated in the cat. Subretinal hemorrhages were produced by creating a focal neurosensory retinal detachment with micropipette techniques, then inserting a needle tip transsclerally to allow choroidal blood to fill the bleb. Experimental lesions were examined clinically and with light and electron microscopy during a 14-day postoperative period. Initial observations included clot organization with retraction of fibrin strands. In six of nine clots more than 1 hour old, fibrin was associated with tearing of sheets of photoreceptor inner and outer segments. Later degeneration progressed to involve all retinal layers overlying the densest areas of fibrin in the clots. Hemorrhages into subretinal blebs containing tissue plasminogen activator did not form fibrin strands or cause photoreceptor tearing. These findings highlight the potential for improved retinal survival if organized subretinal clot can be eliminated soon after formation.
To explore the feasibility of using human recombinant tissue-type plasminogen activator in the management of subretinal hemorrhage, we examined the toxic effects of a commercial preparation of human recombinant tissue-type plasminogen activator in the subretinal space of the holangiotic retina of the cat. Various concentrations of human recombinant tissue-type plasminogen activator and controls were infused into the subretinal space with a glass micropipette (40- to 60-microns tip) to form a retinal bleb. Concentrations from 2.5 mg/L to 200 mg/L were well tolerated without ultrastructural toxic effects, while 1000 mg/L of human recombinant tissue-type plasminogen activator caused severe, irreversible toxic effects to the photoreceptor-retinal pigment epithelium complex. This toxic effect appeared to be due to the carrier vehicle rather than the human recombinant tissue-type plasminogen activator protein itself. Our results demonstrate that commercially prepared human recombinant tissue-type plasminogen activator can be safely used in the subretinal space of the cat with the range necessary for fibrinolysis.
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An objective method for reliably estimating the width of retinal vessels is presented. A computerized microdensitometer scans film negatives of fundus photographs and reconstructs black and white images of the fundus on a visual display terminal. After this initial scan, a desired region of a retinal vessel is selected with moveable cursors, and a cross-sectional density profile through the vessel is plotted. Vessel width is determined by the half-height width of the profile of the minimum (least transmitting) and average background film densities. Measurement sites can be accurately reproduced on different negatives by landmark mapping. A series of ten fundus photographs were taken of three healthy human eyes, and the width of the superior temporal vein was measured in two places on alle negatives. The coefficient of variation of the six sets ranged from 1.2 and 3.4% with an average of 2.2%.
Thrombin is a hemostatic factor that induces platelet aggregation and catalyzes the conversion of fibrinogen to fibrin. The potential for its increasing application to a variety of anterior and posterior segment surgery led us to investigate the in vitro effect of thrombin on the corneal endothelium in a sheep model. We examined freshly excised sheep corneas stained with alizarin red and trypan blue after exposure to two different concentrations of thrombin for four hours. The structure of the corneal endothelium appeared to be intact even after prolonged exposure to thrombin at concentrations of 100 and 1000 U/mL. Thrombin appears to be nontoxic to the corneal endothelium in this experimental model.
When silicone oil fills the vitreous cavity of the phakic eye or the entire aphakic eye, the refractive correction can be expected to change by 5-9 diopters and sometimes as much as 14 diopters. Aphakic eyes become less hyperopic when filled with silicone oil, whereas phakic eyes become more hyperopic when the vitreous cavity is filled with silicone oil. Previously emmetropic aphakic eyes that are filled with silicone oil have only 4-6 diopters of hyperopia, whereas the same eyes have 10-12 diopters of hyperopia before insertion or after removal of silicone oil. Phakic eyes develop 5-7 diopters of hyperopia when the vitreous cavity is filled with silicone oil, and this puts them in the same range as the silicone oil filled aphakic eyes. High myopia, incomplete silicone oil filling, and several other factors influence the final optical outcome. The vitreoretinal surgeon must anticipate significant changes in refractive error when silicone is introduced into an eye or removed from the eye. Careful retinoscopy and refraction is essential for obtaining optimal vision in these patients.
The effect of topical phenylephrine 10% solution on anterior chamber oxygen tension (PO2) was studied in cats using an oxygen-sensitive electrode inserted into the midanterior chamber. A decrease in anterior chamber PO2 was usually observed within eight to 25 minutes and declined steadily thereafter. Within two hours, the drug caused a dramatic 60% reduction in anterior chamber oxygen tension (P less than or equal to .0005). We suggest that the mechanism for this phenylephrine-induced anterior chamber hypoxia is reduced blood flow, mediated by the direct vasoconstrictive effect of the drug and compression of the iris vasculature (induced by dilatation). The resulting decrease in the caliber of the iris arteries produced a parallel decrease in blood volume and flow which resulted in diminished PO2. Phenylephrine 10% may contribute to hypoxia and exacerbate the underlying pathology in those disorders where hypoxia plays an important causal role.
We have analyzed the six-month, one-year, and two-year follow-up examinations in 47 patients who had silicone oil injection in combination with revision of vitrectomy for intractable retinal detachment and advanced proliferative vitreoretinopathy. A visual acuity of 5/200 or better was attained in 22 eyes at six months (47%), 17 eyes at one year (36%), and 14 eyes at two years (30%). Of eyes that were completely attached posterior to the buckle at six months, 77% remained attached at two years. In those eyes that were attached at six months, corneal decompensation was the most frequent cause of a loss of vision between six months and two years. Silicone oil was removed in 22 eyes (47%).
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The effect of timolol maleate, a nonspecific beta-blocking agent, on anterior-segment oxygenation was studied using oxygen-sensitive microelectrodes inserted into the mid-anterior chamber of 13 cat eyes. The anterior-chamber oxygen tension was monitored continuously for two hours following the topical application of three drops of timolol 0.5%. Although a small, but significant, decline in oxygen tension of 19% was observed over the initial 90 minutes (P less than or equal to .05), the overall 10% decrease at the end of two hours was not considered either clinically or statistically significant when compared with a group of controls. Timolol, since it does not contribute significantly to hypoxia, may therefore be a more prudent choice for lowering intraocular pressure in disorders where hypoxia is believed to play an important role, such as in neovascular glaucoma and sickle-cell hyphema.
The extraordinarily high rate of blood flow in the cat choroid allows the choroidal oxygen tension to rise to levels close to the arterial PO2, even with oxygen enriched breathing mixtures. The high PO2 in the choroid can elevate the oxygen tension of the inner retina above the saturation PO2 of hemoglobin when oxygen enriched breathing mixtures at one atmosphere or less are used as shown by polarographic oxygen tension measurements in the preretinal vitreous in cats.
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Phenylephrine preparations are widely used in medicine for their vasoconstrictive and mydriatic effects. The effect of phenylephrine, a powerful alpha-receptor stimulant, on anterior chamber oxygen tension (Po2) was studied using a polarographic electrode inserted into the mid anterior chamber of cats. A commercial 10% solution was applied topically. A decrease in anterior chamber Po2 was usually observed in 8-24 minutes and declined steadily thereafter. Prior to treatment, the mean anterior chamber Po2 was 26 +/- 3 torr; 1/2 hour after treatment it decreased to 20 +/- 7 torr. After 1 hour it dropped to 13 +/- 8 torr and was further reduced to 11 +/- 6 torr by 2 hours. By 90 minutes, the drug had caused a 58% reduction in anterior chamber oxygen tension. This drop is similar to that reported for epinephrine. It is suggested that the mechanism for most of this decrease is reduced blood flow, mediated by the direct vasoconstrictive effect of the drug in addition to compression of the iris vasculature induced by dilation. The resulting change (reduction) in caliber of the iris arteries produces a parallel change (reduction) in blood volume and flow resulting in a diminished Po2. The clinical implications of this conclusion in disorders where hypoxia is felt to play a causal role such as neovascular glaucoma, diabetes, pregnancy and hyphema in sickle cell disease are explored.