Biomedical subjects
Publications and source records attributed to G Lafond.
We report a case of bilateral keratoconus after laser in situ keratomileusis (LASIK). Before surgery, the patient had a forme fruste keratoconus, which evolved rapidly to a severe form of keratoconus in the months following LASIK. From this case, we conclude that forme fruste keratoconus is a contraindication to LASIK.
PURPOSE: To evaluate excimer laser retreatment of central islands and the refractive consequences. SETTING: Department d'Ophtalmologie, Centre Hospitalier de l'Université Laval, Quebec City, and Sir Mortimer B. Davis Jewish General Hospital, McGill University, Montreal, Canada. METHODS: In this study, 22 eyes retreated for central islands were retrospectively analyzed. All initial corrections and retreatments were performed with the Chiron Technolas Keracor 116 laser. RESULTS: A marked decrease in symptoms was observed in almost all cases. Topographic analysis confirmed these results but revealed persistence of minimal islands in several cases. Retreatment of central islands also induced a small myopic correction. Using a 2.5 mm zone, the mean induced refractive effect was 0.21 diopter of myopic correction per diopter of retreatment. CONCLUSION: Retreatment of central islands appeared safe, was relatively easy, and provided good results with a rapid recovery.
Previous electrophysiologic results from this laboratory indicate that apomorphine exerts a differential dose-related effect on rabbit electroretinograms, with low doses increasing the b-wave and higher doses decreasing this parameter. Results were interpreted as reflecting apomorphine's agonistic properties at two different receptors: 1.0 mg/kg acting at the postsynaptic site, and the lower dose, 0.01 mg/kg, preferentially stimulating inhibitory autoreceptors. The purpose of this experiment was to investigate further this hypothesis by determining retinal levels of dopamine, dihydroxyphenylacetic acid, and homovanillic acid in retinas of light- or dark-adapted rabbits treated with saline, 1.0, 0.1, or 0.01 mg/kg apomorphine intravenously. Results indicate that in dark-adapted rabbits only the highest dose tested, 1.0 mg/kg, decreased dopamine concentrations. In animals exposed to light, the lowest dose tested, 0.01 mg/kg, significantly reduced dopamine and metabolite levels, whereas the highest dose unexpectedly increased retinal dopamine turnover. Results are discussed in terms of receptor sites and the influence of lighting conditions.
The interaction of retinal dopamine depletion and partial ischemia on the a- and b-wave amplitudes and implicit times of the electroretinogram was examined in adult pigmented rabbits. Seven days after 6-hydroxydopamine treatment, which resulted in a depletion of the amine, partial retinal ischemia was induced by raising the intraocular pressure. As expected, moderate elevation of intraocular pressure produced increases in both a- and b-wave amplitudes. Amplitude hyperresponses were significantly reduced in dopamine-depleted retinas. These reductions were more prominent with relatively lower intensities. However, response delays were not shortened but lengthened by 6-hydroxydopamine pretreatment. Together, these results point to a selective role of dopamine in partial retinal ischemia induced by moderate elevation of intraocular pressure in rabbits.
The present study was undertaken to determine whether dextromethorphan (DM), a potent N-methyl-D-aspartate antagonist, could attenuate the effects of ischemia on rabbit ERG. Retinal ischemia was induced by increasing intraocular pressure to 120 mm Hg for 30, 60, or 90 min. DM was intravenously administered before ischemia and maintained throughout the entire experimental period. ERGs were recorded prior to, during, and after ischemia. The results indicate that the b-wave hyperresponses and the delays in implicit times induced by 30 min. ischemia were suppressed by the administration of DM. Similar findings were obtained when ischemia lasted for 60 min, except that DM did not improve delayed implicit times, suggesting that cellular injury is still present. ERG changes resulting from 90 min ischemia were not reversed by DM treatment. Effects of DM treatment on a-wave were less prominent. Together, our results further support that DM can to some extent alleviate ischemic injury in the rabbit retina.
Dextromethorphan has been shown to protect against ischemic tissue damage. We investigated the effects of dextromethorphan on electroretinographic oscillatory potentials in retinal ischemia. Retinal ischemia was induced in rabbits by increasing intraocular pressure to 120 mm Hg for 30, 60 or 90 minutes. Dextromethorphan was intravenously administered before ischemia and maintained throughout the whole period of experiments. Oscillatory potentials were recorded before and during ischemia as well as 4 hours of recirculation after ischemia. As expected, all oscillatory potentials were decreased after 60 and 90 minutes of ischemia. However, after 30 minutes of ischemia followed by 4 hours of recirculation, amplitudes of P2 were elevated whereas those of P3 and P4 were decreased with normal P1 amplitudes. Dextromethorphan administration diminished the effects of 30 minutes of ischemia on oscillatory potentials and partially attenuated the effects of 60 minutes of ischemia, whereas the effects of 90 minutes of ischemia could not be reversed by dextromethorphan treatment. These results indicate that electroretinographic oscillatory potentials could be useful indicators to evaluate retinal function in the ischemic condition and that dextromethorphan can attenuate the effects of relatively short periods of ischemia on rabbit electroretinographic oscillatory potentials.
In order to better understand the role of Calcitonin gene-related peptide (CGRP) in mammalian retina, the dose related effects of human CGRP (hCGRP) on rabbit electroretinogram (ERG) were examined in the present study. CGRP was administered intraocularly in doses of 0.1, 1.0 and 10.0 micrograms. ERG A- and B-wave as well as oscillatory potentials (P1, P2, P3 and P4) were recorded. The highest dose of CGRP (10.0 micrograms) significantly increased the amplitudes of the A-wave and OP components (P1, P2, P3 and P4) produced by relatively high stimulus intensity. The same dose of the peptide also enhanced B-wave amplitude at all intensities studied. The effects of the intermediate dose of CGRP (1.0 microgram) on the B-wave amplitudes were dependent on stimulus intensities. B-wave amplitudes at high stimulus intensities were not affected by 1.0 microgram of CGRP but were significantly increased with relatively lower stimulus intensities. The amplitudes of P3, one of OP components, were significantly increased. However, amplitudes of A-wave and other OP components (P1, P2 and P4) were not affected by 1.0 microgram CGRP. The lowest dose of the peptide (0.1 microgram) did not affect any amplitudes of ERG components. Implicit times of A-wave, B-wave and OP components were not significantly affected by the different doses of CGRP. Taken together, these results indicate that CGRP may play a functional role in modulating retinal responses to photic stimulation.
A retrospective study of 126 patients who underwent total laryngectomy during a 10-year period was carried out. The incidence of post-laryngectomy pharyngo-cutaneous fistula was 19%. Pre-operative radiotherapy appears to be a risk factor when surgery is performed within three months of completion of radiotherapy. Intra-operative blood transfusion is strongly associated with fistula development. Fistula formation has an important effect on patient morbidity and mortality post-operatively. It remains a relatively frequent complication following total laryngectomy and may be able to be reduced in incidence if surgery is delayed post radiotherapy and blood transfusion is avoided whenever possible.
PURPOSE: To determine the role of dopamine in experimentally induced retinal ischemia. Experiment 1 was designed to study the effects of ischemic levels on dopamine (DA) metabolism. Experiment 2 evaluated the effects of ischemic duration on DA metabolism. The effects of recirculation time after ischemia on DA metabolism were investigated in experiment 3. METHODS: Ischemia was produced by raising intraocular pressure (IOP) in rabbits. Three levels of ischemia were used--level A, level B, and level C--representing 50%, 75%, and 100%, respectively, of the IOP necessary to produce total ischemia. Retinal levels of DA and its main metabolites, homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC), were measured by high-performance liquid chromatography with electrochemical detection. RESULTS: Experiment 1 showed that at ischemic level B, DA contents were significantly reduced, but neither DOPAC nor HVA concentrations were altered. The reductions of retinal DA, DOPAC, and HVA concentrations were seen at level C without altering the ratio index. Level A ischemia did not alter DA metabolism. In experiment 2, significant reductions in DA, DOPAC, and HVA concentrations were found at both 30 and 60 minutes in the ischemic group, whereas the ratio DOPAC/DA and (HVA + DOPAC)/DA were significantly decreased only in the 60-minute ischemic group. Five-minute total ischemia did not alter DA metabolism. In experiment 3, concentrations of DA were still significantly decreased at 30 minutes of recirculation after ischemia, but DOPAC and HVA levels were back to normal. However, the ratios of DOPAC/DA, HVA/DA, and (HVA + DOPAC)/DA were significantly elevated. After 90 and 240 minutes of recirculation, retinal DA contents had returned to normal values, but DOPAC and HVA concentrations as well as all ratio indices of DA metabolism were still enhanced. CONCLUSION: Dopamine metabolism is altered during and after retinal ischemia. Dopamine may play a contributing role in ischemia.
Extracranial head and neck schwannomas are usually benign and rarely malignant tumors of peripheral nerve sheath origin. The clinical presentation of these tumors is reviewed in our series of 21 patients, emphasizing the diagnosis with dynamic infused CT scans. Management is suggested and outcome analysis is provided.
We have determined the structures and thermodynamic stabilities of the wild type Asn-52 and unusually thermostable mutant Ile-52 yeast iso-1-cytochromes c (Das, G., Hickey, D. R. McLendon, D., McLendon, G., and Sherman, F. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 496-499). Although both structures were similar, Water-166, buried within the wild type protein, is excluded from the Ile-52 mutant, which substantially reorganizes the local hydrogen bonding. Wild type Cys-102 was replaced with alanine or serine to eliminate dimerization in vitro. The Cys-102 (wild type), Ala-102, and Ser-102 proteins were equally stable, whereas the chemically modified Cys-102-SCH3 was less stable. The order of stability observed with replacements at positions 52 and 102 was as follows: Ile-52 Ala-102 greater than Ala-52 Ala-102 greater than Asn-52 Ala-102 ("normal") greater than Gly-52 Ala-102. No significant stabilization was attributed to potential energy interactions expressed as helix-forming propensities of replacements at position 52. A high correlation between differences in free energy changes and transfer free energies suggests hydrophobic interactions are the main factor for enhancing stability in the Ile-52 mutant. Additional possible contributions to the thermostability of the Ile-52 variant are energetic effects due to packing and hydrogen bonding changes surrounding position 52.
This study evaluated the effects of intravitreal injections of 300 micrograms of 6-hydroxydopamine (6-OHDA) on electroretinogram (ERG) amplitudes and implicit response times of adult pigmented rabbits. One eye was injected intravitreally with 300 micrograms 6-OHDA and 600 micrograms ascorbic acid in a 0.3 ml 0.9% NaCl solution; the fellow eye received a similar solution containing only 600 micrograms ascorbic acid. Following this treatment ERG recordings were performed at 1, 4, and 7 days. After the last recordings, animals were sacrificed and retinas were isolated for biochemical analyses. Significant and progressive reductions in retinal concentrations of dopamine (DA) and its main metabolites homovanillic acid (HVA), and dihydroxyphenylacetic acid (DOPAC) were found in treated retinas. Concentrations of norepinephrine (NE), serotonin (5HT), and 5 hydroxyindoleacetic acid (5-HIAA) were not affected, thus demonstrating the specific neurotoxic action of 6-OHDA on retinal dopaminergic neurons. Concurrently, significant increases in ERG a- and b-wave amplitudes as well as in implicit response times were observed. These electrophysiological changes were progressive reaching a maximum 7 days after intravitreal injections. Changes in b-wave amplitudes and response times were more pronounced at low intensities of stimulation. These results clearly show that, in rabbits, selective decreases in retinal DA concentrations result in pronounced ERG changes, which offer additional evidence supporting a role for this transmitter in lateral inhibition in the retina.
We studied the effect of optic nerve section on the pattern electroretinogram (PERG) of 10 pigeons. Base line PERG and flash ERG were measured prior to surgery and then one optic nerve was cut. We measured PERG at various time intervals up to 24 weeks after the section. There was a sharp drop in PERG amplitude immediately following the section. It gradually returned to normal levels in the following 24 weeks. Significant decreases in time to peak were noted at 1 and 16 weeks for 0.5 cycle per degree checks and at 1, 2 and 9 weeks for 0.25 cycle per degree checks. The ERG generated by a TV stimulus did not change in amplitude but had a shorter time to peak at 24 weeks for a high intensity stimulus and at 16, 18, 24 weeks for a low intensity stimulus. Light and electron microscopic sections (at 24 weeks) revealed an almost complete loss of nerve fibers in the optic nerve. Retinal sections revealed only a few surviving ganglion cells and a few nerve fibers. This suggests that in pigeons, PERG is not generated by ganglion cells.
Between January 1982 and December 1983 stereo colour photography covering 10 standard fields of 30 degrees and fluorescein angiography were performed in 64 consecutive patients with diabetes mellitus who had no clinical evidence of diabetic retinopathy. Fifty-two patients were excluded on the basis of the slightest photographic or angiographic evidence of retinopathy or any other ocular disease. Electroretinogram oscillatory potentials were recorded in the remaining 12 patients. Four of the 12 had normal oscillatory potentials, colour photographs and angiograms, while 8 had angiographic evidence of lobular delayed choroidal perfusion and abnormal oscillatory potentials. Although only a small number of patients in the preclinical stages of diabetic retinopathy were studied, the results warrant that further attention be given to lobular delayed choroidal perfusion, which may represent early evidence of choroidal involvement in diabetic retinopathy.
Two series of experiments are reported on incremental increases of ischemia and monitoring of recovery from total ischemia of varying duration. In the first, rods show hyper-responses and cones do not; changes in amplitude and delays appear at higher levels of ischemia in cones than they do in rods. In the second series, cones do not recover up to control values with total ischemias of 30 min or more, while rods do recover and even show hyper-responses. Cones appear more resistant than rods to ischemia, but once affected do not recover as well as rods. Discussion proposes that these differences are specific to receptor systems of each rather than secondary to vascularization patterns.
The purpose of this study was to examine the dose related effects of 6-hydroxydopamine (6-OHDA) on the electroretinographic (ERG) B-wave amplitude and on retinal concentrations of dopamine (DA) and its main metabolites homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC). Doses of 6-OHDA (60, 300 or 1200 micrograms) were dissolved in a 0.9 percent NaCl and ascorbic acid solution and administered intravitreally in 300 microliter volume in one eye of adult pigmented rabbits. The fellow eye received a similar volume of the drug vehicle. With the smallest dose of 6-OHDA (60 micrograms) no changes in ERG B-wave amplitude or retinal concentrations of DA, DOPAC or HVA were found. With the largest dose of 6-OHDA (1200 micrograms) complete extinction of the ERG and almost total disappearance of DA, DOPAC and HVA were observed. With the intermediate dose of 300 micrograms 6-OHDA significant increases in B-wave amplitudes and decreases in retinal DA, DOPAC and HVA concentrations were obtained. This latter dose did not affect retinal concentrations of serotonin (5-HT), 5-hydroxyindole acetic acid (5-HIAA) and norepinephrine (NE). These results demonstrate that selective reductions of retinal dopamine and its metabolites can be obtained with a single intravitreal injection of 300 micrograms 6-OHDA in rabbits. The observed concomitant increase in B-wave amplitudes lends support to the hypothesis that DA acts as a mediator of lateral inhibition in the retina.
We have observed an increase in time to peak and a decrease in amplitude for both the pattern electroretinogram (PERG) and the electroretinogram (ERG) in pigeons following an elevation in intraocular pressure (IOP). The changes in PERG were highly correlated to the changes in ERG. However, they differed in that significant changes from baseline values occurred at lower levels of IOP for PERG than ERG for the amplitude, and in some cases for the time to peak.
Three groups of animals were studied during the four-hour period following their recovery from total occlusion of retinal circulation for periods of 30, 60 and 90 minutes. During incremental increases of ischemia, recovery followed a pattern that is the reverse of that previously reported, inasmuch as wave amplitude is concerned. From total extinction, ERGs recovered and showed hyperresponses. Delays, however, behaved differently: they increased with recovery time--the opposite of what was expected. All signs were inversely proportional to the duration of ischemia. Sixty minutes of occlusion appears to be a threshold beyond which ERGs do not recover as well.