Biomedical subjects

S Shady

Publications and source records attributed to S Shady.

4 recordsLinked to original sources

PubMed1998

Multifocal rod electroretinograms.

PURPOSE: To assess the feasibility of obtaining reliable multifocal rod electroretinograms (ERGS) and to compare them to full-field ERGs. METHODS: Multifocal rod ERGs were recorded using a stimulus array of 61 hexagons. The minimum number of dark, blank frames between flashes was varied from 0 (a minimum of 13.3 msec between flashes) to 21 (a minimum of 293 msec between flashes). Full-field ERGs were obtained using trains of flashes designed to simulate the multifocal sequences. Flashes were blue (W47B), except in a few cases in which red (W26) was used to check for cone intrusion. Flash intensities varied from -1 to 1.7 log scot td-s. RESULTS: Dark-adapted, multifocal ERGs to blue flashes had a small, early component followed by a larger, late component. The early component showed little change in amplitude with increasing intensity. Comparisons with the full-field ERGs indicated that the early component was the focal response. The larger, late component was the response to stray light, and it can be suppressed with the addition of a surround. The focal response was from a relatively circumscribed retinal region. This is shown by comparing the multifocal rod responses from a patient with retinitis pigmentosa to her behaviorally measured rod visual field. CONCLUSIONS: By choosing conditions (namely, flashes of moderate intensity with a surround) to minimize the effects of stray light, multifocal rod ERGs can be recorded with sufficient localization to be clinically useful. However, the signal-to-noise ratio of these multifocal rod ERGs was poorer than for multifocal cone responses for comparable recording periods because of the need for blank frames and the slower recovery of the rods to successive presentations.

Adult↗

PubMed1995

Rod phototransduction in retinitis pigmentosa. Distinguishing alternative mechanisms of degeneration.

PURPOSE: To test alternative hypotheses concerning the mechanisms of rod degeneration in retinitis pigmentosa (RP). METHODS: Full-field rod electroretinograms and rod visual fields were measured for 15 patients with RP and a normal control group. The rod a-wave was fitted with a computational model based on known transduction biochemistry. The values of td (the initial delay), S (a sensitivity parameter), and RmP3 (the maximum amplitude) were estimated. Rod b-wave amplitudes were fitted with the Michaelis-Menten equation, and the parameters Kbw (the semisaturation intensity--a sensitivity parameter) and Vmax (the maximum amplitude) were estimated. RESULTS: The patients all had significantly reduced values of RmP3, indicating rod receptor damage, and a wide range of S values. Three patients had S values in the normal range. Four had abnormal S values but normal thresholds in some locations in their visual field. The remaining patients had abnormal values of S and entirely abnormal visual fields. Three of those had a history of large elevations in Kbw. For all patients, the changes in Kbw and Vmax followed closely the changes in S and RmP3, respectively. CONCLUSION: Retinitis pigmentosa has a varying initial impact on the activation phase of rod transduction. Available evidence suggests that the activation of transduction is initially normal in most patients with RP. In some patients, RP appears to result in a reduced transduction amplification from birth. In all patients, subsequent degeneration of the rods effects progressive reductions in transduction amplification but no other major functional changes. Outer segment shortening and local dropout of rods appear to have little functional impact.

Adolescent↗

PubMed1994

Understanding changes in the b-wave of the ERG caused by heterogeneous receptor damage.

PURPOSE: To understand better the relationship between heterogeneous receptor damage and the changes in the b-wave of the rod ERG. METHODS: A computational model of the b-wave is used to simulate b-waves from retinas with two regions (a healthier and a more affected region) differing in area and sensitivity. The peak-to-peak amplitudes of the simulated b-waves are fitted with a Naka-Rushton equation, and the parameters log K and Vmax are estimated. Insights gained from these simulations are tested against rod ERGs and rod visual fields from patients with autosomal dominant retinitis pigmentosa (n = 11) and cone-rod dystrophy (n = 17). RESULTS: In the simulated retinas, Vmax decreases when the more affected region of receptors is less sensitive than the healthier region by more than 0.5 log unit. However, the relative change in Vmax does not match the relative area of the more affected region unless this region is depressed by 2.0 log units or more. As the more affected region loses sensitivity, log K at first increases but then decreases and approaches the log K of the healthier region for losses greater than 2.0 log units or so relative to the healthier region. For the patients' data, the simulations predict the general relationships observed between the summary statistics of the visual fields (e.g., area of field and mean of log sensitivity changes in the field) and the changes in log K and Vmax. CONCLUSION: A decrease in Vmax indicates that regions of the patient's retina have lost 0.5 log unit or more of sensitivity. An increase in log K indicates that either the healthiest part of the patient's retina is abnormal by at most delta log K, a large part of the retina has lost considerable sensitivity (i.e., 0.5 to 2.0 log units) but is still contributing to the response, or both.

Adolescent↗

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↗