Biomedical subjects

D G Birch

Publications and source records attributed to D G Birch.

At least 73 records · Page 4Linked to original sources

PubMed1993

Heterogeneity in retinal disease and the computational model of the human-rod response.

Abnormal rod-receptor activity can be quantitatively assessed in humans by fitting a computational model of the rod's response to the leading edge of the a wave of the electroretinogram. One purpose of the present study was to compare two procedures for fitting the model to the electroretinogram. A computationally simpler method gives comparable results to the more labor-intensive method used previously. This finding holds for both normal observers and patients with retinodegenerative disease that affects the receptors unevenly. A second purpose of the present study was to consider the effects of a heterogeneous disease process on the parameters of the model. a waves from a heterogeneous retina are computer simulated and are fitted with the receptor model. This analysis suggests that the model will overestimate the change in the healthiest rods and will underestimate the percentage of the rods that are significantly affected.

Adult↗

PubMed1993

S-cone function in patients with retinitis pigmentosa.

PURPOSE: To determine whether short-wavelength-sensitive (S-) cones are more severely damaged in patients with retinitis pigmentosa than long-wavelength-sensitive (L-) and middle-wavelength-sensitive (M-) cones. To determine whether there are differences in the amount of S-cone damage in patients with dominant versus nondominant inheritance patterns. To accomplish these goals with methods that provide information not furnished by previous studies with two-color increment thresholds. METHODS: Acuity mediated by the S-cones was measured in 56 patients with retinitis pigmentosa, and the electroretinogram (ERG) generated by the S-cones was measured in 11 of these patients. Mixed L- and M-cone acuity, mixed L- and M-cone ERGs, and clinical full-field rod and cone ERGs were obtained for all patients. Data for both dominant and nondominant patient groups were compared with data from age-matched normal subjects. RESULTS: Only the nondominant group had reduced S-cone acuity, and 43% of patients in this group had selective reduction of S-cone acuity. In this particular sample the dominant and nondominant groups were comparable in clinical full-field ERG parameters and mixed L- and M-cone acuity, so the difference in S-cone acuities is not due to the dominant group having less advanced retinal degeneration. All 11 patients tested had reduced S-cone ERGs, 6 with significantly greater loss in the S-cone ERG than in the mixed L- and M-cone ERG. CONCLUSIONS: These data provide evidence that retinitis pigmentosa can produce greater loss of S-cones than L- and M-cones, and that this selective loss is primarily seen in patients with nondominant forms of retinitis pigmentosa.

Adolescent↗

PubMed1992

Standardized full-field electroretinography. Normal values and their variation with age.

Full-field electroretinograms were obtained in 269 normal subjects with the International Standardization Protocol endorsed by the International Society for Clinical Electrophysiology of Vision and the National Retinitis Pigmentosa Foundation Inc. Log rod and cone amplitudes decreased exponentially with age in adults; amplitudes declined to one half those in the young adult level (ages 15 to 24 years) by ages 69 and 70 years for rod and cone responses, respectively. B-wave implicit times increased with age for all responses. Lower limits of normal peak-to-peak amplitude and upper limits of normal b-wave implicit time (P < .05) were determined for each decade from birth to age 79 years. Naka-Rushton functions relating rod peak-to-peak amplitude to retinal illuminance were determined in 50 normal subjects. A significant decline in the log maximum asymptotic amplitude with age accounted for most of the amplitude decline in the standard protocol rod response. The average value of log k, the semisaturation constant, was only 0.1 log unit higher at age 70 years than at age 20 years, consistent with previous studies showing little decrease in photopigment optical density with age.

Adolescent↗

PubMed1992

A computational model of the amplitude and implicit time of the b-wave of the human ERG.

To improve the usefulness of the ERG in identifying the sites and mechanisms of adaptation, development, and disease processes, a theoretical framework based upon Granit's analysis of the ERG was evaluated. The framework assumes that the ERG is the sum of two potentials, one, P3, generated by the receptors and the other, P2, generated by the cells of the INL. Hood and Birch (1990a, b) demonstrated that the leading edge of the a-wave can be quantitatively described by a model used to describe the response from single rod receptors. This model provides P3(t), a theoretical receptor response as a function of time, for any given flash intensity. The ERGs from normal observers and patients with retinal diseases were analyzed in this framework, first by deriving P2 by computer subtracting the predicted P3(t) responses. This analysis was successful and a computational model of the ERG was then derived. The model of P2(t) was constructed with linear filters and a static nonlinearity and using P3(t) as the input. The ERG for any given flash intensity is then P3(t) + P2(t). The model describes (1) the change both in implicit times and in trough-to-peak b-wave amplitudes with flash intensity for the normal, dark-adapted observers; and (2) the changes in b-wave implicit times and amplitudes for three patients with retinal diseases. Among the implications drawn from these analyses were as follows: (1) The fits of the Naka-Rushton equation to trough-to-peak b-wave amplitudes must be interpreted with great care. (2) When the INL is affected by retinal disease, the b-wave may be a very poor reflection of INL activity. (3) The implicit time of the b-wave can provide a measure of receptor sensitivity.

Adult↗

PubMed1992

Pattern-reversal electroretinographic acuity in untreated eyes with subfoveal neovascular membranes.

To define further the natural history of visual loss in eyes with age-related macular degeneration (ARMD) complicated by a subfoveal neovascular membrane, pattern-reversal electroretinograms (ERGs) were obtained from patients randomized to no treatment at the Dallas center of the Macular Photocoagulation Study (Texas Retina Associates). Study eyes (n = 20) were tested during the initial visit and at 3-, 6-, and 12-month follow-up visits. Responses were obtained to phase-alternating checkerboards of varying check size. Extrapolation of the best-fit regression line relating the logarithm of the check size to amplitude was used to determine "retinal" acuity (log MAR). The pattern-reversal ERG acuity rating (100 - [50 x log MAR]) was derived for each visit. Pattern-reversal ERG acuity ratings for all patients across visits were correlated significantly with visual acuity ratings derived from the Bailey-Lovie chart (r = 0.61, P less than 0.001) and inversely related to neovascular membrane area (r = -0.55, P less than 0.001). During 1 yr of follow-up, pattern-reversal acuity ratings dropped from 53 to 12, corresponding to an average decrease of approximately 0.2 octaves/month. These results suggest that the pattern-reversal ERG, which samples the resolving power of the central 20 degrees, is a sensitive index of visual loss in age-related macular degeneration.

Aged↗

PubMed1992

Retinal development in very-low-birth-weight infants fed diets differing in omega-3 fatty acids.

Full-field electroretinograms (ERGs) were obtained from very-low-birth-weight (VLBW) neonates to determine whether omega-3 (omega-3) fatty acids are essential for normal human retinal development. Eighty-one infants born at 30.4 (standard deviation, +/- 1.5) wk gestation were, within 10 d of birth, either enrolled to receive mother's milk (naturally containing both omega-6 and omega-3 essential fatty acids) or randomized to receive one of the infant formulas. Corn oil-based Formula A contained mainly linoleic acid (18:2 omega-6) and was low in all omega-3 fatty acids. Soy oil-based Formula B contained ample alpha-linolenic acid (18:3 omega-3) but no long-chain omega-3. Formula C, supplemented with both alpha-linolenic acid and marine oils, was comparable to human milk in long-chain omega-3. Full-field ERGs were obtained in the special care nursery from infants aged 36 and 57 wk postconception. Ten healthy preterm infants born at 35 wk gestation were tested at 36 wk postconception. Significant differences were found among groups in rod ERG function. Post hoc comparisons showed that infants fed Formula A had significantly higher rod thresholds than infants receiving long-chain omega-3 (human milk, Formula C, and intrauterine). Infants receiving Formula B had intermediate thresholds that were significantly higher than those of infants receiving intrauterine nutrition. Analysis of the leading edge of the a-wave showed that b-wave differences originated at the photoreceptor level. Differences were not present in infants at 57 wk postconception. No significant differences among groups were found in cone b-waves at 36 or 57 wk postconception. Oscillatory potentials had significantly longer implicit times at 57 wk postconception in infants fed Formula A than in infants receiving human milk. These findings suggest that retinal function varies with the dietary supply of omega-3 fatty acids in VLBW infants.

Breast Feeding↗

PubMed1992

Dietary essential fatty acid supply and visual acuity development.

The influence of dietary omega-3 fatty acid supply on visual acuity development was evaluated in very low birth weight (VLBW) infants using visual-evoked potential (VEP) and forced-choice preferential-looking (FPL) procedures at 36 and 57 wk postconception. The VLBW infants born at 27-33 wk postconception were randomized to one of three diet groups: corn oil, which provided solely linoleic acid; soy oil, which provided linoleic and alpha-linolenic acids; or soy/marine oil; which was similar to the soy oil formula but also provided preformed long chain omega-3 fatty acids. The VLBW infants in the soy/marine oil group had higher omega-3 levels in erythrocyte membranes and better VEP and FPL acuities at 36 and 57 wk than infants in the corn oil group. The soy oil group had intermediate omega-3 levels in erythrocyte membranes and significantly poorer VEP acuity at 57 wk compared with the soy/marine oil group. Only the soy/marine oil group had acuities comparable to the "gold standards" of VLBW infants fed human milk and preterm infants who were born and tested at 35-36 wk postconception. In addition, VEP and FPL acuity were poorer in a nonrandomized group of formula-fed full-term infants than in breast-fed full-term infants. The results suggest that dietary omega-3 fatty acid supply may play an important role in early human visual development.

Breast Feeding↗

PubMed1992

Pneumatic retinopexy versus scleral buckle. Preferences of Vitreous Society members, 1990.

A survey was conducted among the 1990 members of the Vitreous Society in order to measure their acceptance of pneumatic retinopexy. They were asked which treatment they would prefer should they suffer a hypothetical detachment. The choices were limited to pneumatic retinopexy or scleral buckling (encircling or segmental). The majority of respondents selected a scleral buckling procedure for a phakic retinal detachment with two adjacent superior temporal quadrant tears. Surgeons who had been in practice for 10 years or less (median for entire group = 10 years) were significantly more likely to select a pneumatic retinopexy procedure. As the details of the hypothetical detachment became more complicated with myopia, additional tears, vitreous hemorrhage, or lattice degeneration with a positive family history, the respondents selected a scleral buckling procedure with greater frequency, and the differences between the choices of the surgeons became nonsignificant. This survey shows that many surgeons feel pneumatic retinopexy is an acceptable alternative to buckling surgery in select cases. There were no trends by geographic location.

Humans↗

PubMed1991

Longitudinal measures in children receiving ENCAD for hereditary retinal degeneration.

A hydrolysate of yeast RNA (ENCAD) is used in the Soviet Union for the treatment of hereditary retinal degenerations. We report longitudinal data from three young patients who have made at least two visits to the Soviet Union over a five-year period to receive treatment with ENCAD. Two children were diagnosed with cone-rod degeneration and the third has an isolated (simplex) form of retinitis pigmentosa. Visual function measurements were obtained before and after each visit to Moscow. In the comparison of previsit and postvisit visual acuity, 30 Hz flicker amplitude, and visual fields, ENCAD treatment had no significant short-term effect. Despite treatment with ENCAD, each patient has shown a significant decrease in visual function over the 5-year period. The rate of progression in these patients appears similar to previously published data on the natural history of their retinal degenerative disorders.

Adjuvants, Immunologic↗

PubMed1991

Cataract extraction after brachytherapy for malignant melanoma of the choroid.

Thirteen eyes of 55 consecutive patients treated with brachytherapy for malignant melanoma of the choroid developed postirradiation cataracts. Cataract development was more common in older patients and in patients with larger and more anterior tumors. Eleven eyes had extracapsular cataract extraction and intraocular lens implantation. Initial visual improvement occurred in 91% of eyes, with an average improvement of 5.5 lines. Visual acuity was maintained at 20/60 or better in 55% of the eyes over an average period of follow-up of 24 months (range, 6 to 40 months). These data suggest that, visually, cataract extraction can be helpful in selected patients who develop a cataract after brachytherapy for malignant melanoma of the choroid.

Adult↗

PubMed1990

Vitrectomy in eyes at risk for macular hole formation.

Fifteen eyes believed to be at increased risk for macular hole formation underwent vitrectomy in an attempt to prevent macular hole formation. Full-thickness macular holes have not developed in 10 of 11 eyes with stage 1 macular holes. Four eyes were noted to have small full-thickness foveal defects (stage 2 macular holes) at the time of vitrectomy. Two of the four eyes have not progressed to macular hole formation and have 20/25 visual acuity. All patients have been followed for a minimum of 13 months (median, 18 months). The 12 eyes that have not experienced macular hole formation have had a significant (P less than 0.001) improvement in vision with seven (58%) attaining visual acuity of 20/25 or better. The postoperative foveal electroretinogram (ERG) amplitude was higher than the preoperative amplitude in five of the six eyes tested.

Aged↗

PubMed1990

A quantitative measure of the electrical activity of human rod photoreceptors using electroretinography.

An electrical potential recorded from the cornea, the a-wave of the ERG, is evaluated as a measure of human photoreceptor activity by comparing its behavior to a model derived from in vitro recordings from rod photoreceptors. The leading edge of the ERG exhibits both the linear and nonlinear behavior predicted by this model. The capability for recording the electrical activity of human photoreceptors in vivo opens new avenues for assessing normal and abnormal receptor activity in humans. Furthermore, the quantitative model of the receptor response can be used to isolate the inner retinal contribution, Granit's PII, to the gross ERG. Based on this analysis, the practice of using the trough-to-peak amplitude of the b-wave as a proxy for the amplitude of the inner nuclear layer activity is evaluated.

Adult↗

PubMed1990

Effect of dietary omega-3 fatty acids on retinal function of very-low-birth-weight neonates.

Retinal function was assessed by electroretinogram in 32 neonates randomly assigned to formulas of different omega-3 fatty acid content and in 10 infants fed human milk. All neonates had a birth weight of 1000-1500 g and were fed study diets from d 10 to 45 or discharge. Group A received formula containing predominantly 18:2 omega-6. Group B received a balanced mix of 18:2 omega-6 and 18:3 omega-3. Group C was given a formula containing both essential fatty acids and supplemented with marine oil to provide 22:6 omega-3 content similar to that of human milk. The fatty acid composition of plasma and red blood cell (RBC) lipids were similar for all groups on entry but marked diet-induced differences were found after feeding the study diets. Group C was comparable to the human milk-fed group, but group A had lower 22:6 omega-3 and omega-3 long-chain polyunsaturated fatty acids (LCPUFA) in plasma and RBC membranes. Cone function was not affected by dietary essential fatty acids. Rod electroretinogram thresholds were significantly higher for group A relative to the human milk-fed group and group C and significantly correlated with RBC omega-3 LCPUFA (r = -0.63, p less than 0.0001); 44% of the variance could be explained by RBC and plasma omega-3 LCPUFA content.(ABSTRACT TRUNCATED AT 250 WORDS)

Double-Blind Method↗

PubMed1990

The A-wave of the human electroretinogram and rod receptor function.

The amplitude of the leading edge of the a-wave of the human electroretinogram (ERG) was compared with predictions from a computational model of the light-induced responses of rod mammalian receptors. According to this model, a linear process describes the amplitude and time course of the response to relatively low flash intensities and at brief times after the onset of the flash. At higher flash intensities, a nonlinear process, described by the Naka-Rushton function or a saturating exponential, is involved. The primary focus here is on intensity-response data recorded with a clinical ganzfeld apparatus. The leading edge of the rod a-wave recorded from normal observers and patients with congenital stationary night blindness (CSNB) was described by a linear process for flash intensities up to the maximum available flash intensity, 2.0 log scot td-sec. This finding is consistent with the model of the rod's response. It suggests, however, that when ERGs are recorded with clinical systems limited to 2.0 log scot td-sec, these data cannot be used to distinguish between changes in the parameters (eg, semisaturation intensity versus maximum response) of the human rod receptors. Responses to flash intensities up to 3.4 log scot td-sec were recorded using a custom, high-intensity ganzfeld system. Both the linear and nonlinear components of the model were needed to fit the ERGs recorded with this system. This suggests that changes in different receptor parameters can be distinguished with higher intensity flashes.

Electroretinography↗

PubMed1990

Rod visual fields in cone-rod degeneration. Comparisons to retinitis pigmentosa.

Dark-adapted visual fields to short- and long-wavelength stimuli were obtained from 20 patients with cone-rod degeneration, 20 patients with retinitis pigmentosa, and ten normal subjects. Patients were selected because they retained rod electroretinographic (ERG) function over a sufficient range for the Naka-Rushton analysis of retinal illuminance versus amplitude functions. Patients with cone-rod degeneration retained a relatively normal field topography although overall sensitivity was reduced. The mean sensitivity loss was consistent with a small elevation in ERG semisaturation constant and minimally delayed rod b-wave implicit times. Rod visual fields from patients with retinitis pigmentosa retaining rod ERG function were consistent with log sensitivity profiles reported previously for type 2 patients. Sensitivity loss was greatest in midperipheral regions, with most patients showing least loss in the far periphery. Disproportionate loss in the midperiphery is consistent with a large elevation in the ERG semisaturation constant and prolonged rod b-wave implicit times.

Adolescent↗

PubMed1989

The focal electroretinogram in the clinical assessment of macular disease.

Focal electroretinograms (ERGs) were obtained from the fovea in 142 eyes with a variety of macular diseases and 50 age-matched eyes with acuity loss in which macular disease was ruled out. Across all eyes, the accuracy rate of discriminating macular disease from other causes of acuity loss was 87%. Among all eyes with macular disease, the sensitivity rate of the focal ERG to the presence of macular disease was 85%. Log focal ERG amplitude was significantly correlated with log Snellen acuity, except in those eyes with macular holes. The sensitivity rate increased to 94% when eyes with 20/40 or greater acuity and eyes with macular holes were excluded.

Adult↗

PubMed1988

The focal electroretinogram in fellow eyes of patients with idiopathic macular holes.

In a prospective study of macular hole formation, focal electroretinograms (ERGs) were obtained from both eyes of 35 patients with a unilateral, idiopathic, full-thickness macular hole. Foveal cone ERG amplitude was significantly correlated with hole diameter at the initial visit. Twenty-six patients had normal foveal cone ERGs in the fellow eye at the baseline visit and for the duration of the study (mean follow-up, 35 months; range, 24 to 56 months). None of these eyes developed a macular hole. Seven eyes had significantly reduced foveal cone ERGs in the fellow eye, despite good visual acuity and a normal-appearing macula on the initial visit. Four of these eyes subsequently developed a full-thickness macular hole during follow-up (mean follow-up, 35 months; range, 25 to 46 months). Foveal ERG amplitude was significantly related to subsequent macular hole formation, suggesting that this test can provide an objective measure of macular function to help identify eyes at risk for macular hole formation.

Aged↗

PubMed1988

Focal cone electroretinograms: aging and macular disease.

Focal cone electroretinograms were obtained with a 3-degree flickering stimulus from 100 normal eyes and 134 eyes with known macular disease. Responses were obtained during direct visualization of the fundus with a hand-held stimulator-ophthalmoscope. Mean foveal cone amplitude for 100 normal eyes was 0.31 microV, with 95% of all amplitudes greater than 0.18 microV. There was a significant inverse correlation between amplitude and age for responses obtained from the fovea (r = -0.91; p less than 0.001) but not for responses obtained from the parafovea (midway between fovea and disk, r = -0.53; not significant). In eyes with known maculopathy, mean foveal cone amplitude was correlated with Snellen acuity. Even after correcting for normal decreases in amplitude with age, responses were significantly reduced in 88/94 (94%) of all eyes with 20/40 or poorer acuity, suggesting that the focal electroretinogram is a sensitive test for detecting macular disease.

Adolescent↗