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Ronald E Carr

Publications and source records attributed to Ronald E Carr.

7 recordsLinked to original sources
PubMed2004

Test-retest reliability of the multifocal electroretinogram and humphrey visual fields in patients with retinitis pigmentosa.

We examined the reliability of Humphrey visual field thresholds and multifocal electroretinogram (mfERG) amplitudes and timing in a group of patients with Retinitis Pigmentosa (RP). Eight patients with RP and seven control subjects were tested five times: at baseline (visit #0), at three weekly follow-up visits (visits #1 - #3), and at three months (visit #4). For the Humphrey thresholds, differences between dB values on repeat visits were obtained. Differences between log values on repeat visits were calculated for mfERG amplitude and implicit time. We used the standard deviations of these difference scores as a measure of reliability and the means of the difference scores as a measure of progression. We found that the majority of the patients' repeat data were more variable than that of the control subjects for both the Humphrey and mfERG. We found no single factor that predicted the magnitude, or the variance, of the SD of differences scores for the patients. We recommend that each patient's reliability be assessed individually. Ultimately, the choice of an outcome measure must be guided by its reliability, as well as its ability to assess the visual function of interest.

Adult↗
PubMed2004

Rod and cone photoreceptor function in patients with cone dystrophy.

PURPOSE: To determine the extent of rod and cone photoreceptor dysfunction in patients with cone dystrophy using psychophysical and electrophysiological tests. METHODS: Ten patients with cone dystrophy participated. Rod and cone system psychophysical thresholds were measured as a function of retinal eccentricity. Bright-flash full-field electroretinograms were obtained under dark-adapted (rod-mediated) and light-adapted (cone-mediated) conditions. The a-wave data were fitted with a model based on photopigment transduction to obtain values for log Rmax (maximum response) and log S (sensitivity). b-Wave parameters were also examined by fitting a nonlinear, saturating function (the Naka-Rushton equation) to the rod-mediated responses. Oscillatory potentials were measured to the cone-mediated high-intensity flashes. RESULTS: On average, the rod-mediated psychophysical thresholds were elevated by 0.5 log unit. These threshold elevations did not differ significantly with retinal eccentricity. In contrast, cone-mediated psychophysical thresholds were elevated up to 3.0 log units. Threshold elevation was greatest in the central retinal locations. For rod-mediated conditions, the a-wave Rmax parameter was significantly reduced in three patients; the a-wave log S parameter was within normal limits. The rod-mediated b-wave Rmax parameter was reduced in six patients; log k was abnormal in one patient. For cone-mediated conditions, the a-wave Rmax parameter was reduced in six patients and the a-wave log S parameter was reduced in two patients. The cone system oscillatory potentials were abnormal in nine patients. CONCLUSIONS: Patients with cone dystrophy show different patterns of psychophysical rod versus cone system sensitivity losses with retinal eccentricity. The full-field electrophysiological data indicate that most of the patients had abnormal cone photoreceptor function. Some patients also showed rod photoreceptor abnormalities. The rod system changes were smaller than the cone system changes.

Adult↗
PubMed2003

Multifocal electroretinography as a function of age: the importance of normative values for older adults.

PURPOSE: To determine the influence of age on local electroretinographic responses in humans. METHODS: Multifocal electroretinograms (mfERGs) were obtained from 62 normally sighted subjects ranging in age from 21 to 81 years. A stimulus array of 103 scaled hexagons was used to measure electrical signals within a retinal area approximately 46 degrees in diameter. Commonly reported mfERG methods were used to quantify the responses: peak-to-peak amplitudes and implicit times, scalar product amplitude, and amplitude and time scales derived from the algorithm of Hood and Li, published in 1997. RESULTS: Regression analysis showed significant linear relationships of amplitude and timing measures with age. The rates of losses were 10.5% per decade for peak-to-peak amplitude, 11.7% per decade for scalar product amplitude, and 9.5% per decade for a-scale. The rate of amplitude reduction was highest in the central 3 degrees. Age had less influence on implicit time measures. The rates of timing losses were 1.4% per decade for the N1 component and 1.0% per decade for both the P1 component and the t-scale measure. Using predicted interval ranges, the age was calculated at which 50% of the expected values would fall below the lower 95% prediction interval band of younger subjects. CONCLUSIONS: The age-associated mfERG alterations are presented to emphasize the importance of appropriate normative data in interpretation of mfERGs.

Adult↗
PubMed2002

A method for comparing psychophysical and multifocal electroretinographic increment thresholds.

The multifocal electroretinogram (mfERG) has been commonly used as a method for obtaining objective visual fields. Although qualitative comparisons have been good, quantitative comparisons between the results from mfERG and the results from Humphrey Visual Field Analyser (HVFA) have found variable degrees of agreement depending upon the mfERG response parameter examined and/or the disease studied. Lack of agreement may be due to differences in methodology, differences in the sites of response generation, and/or differences derived from comparing suprathreshold versus threshold responses. In addition, the two procedures are performed at different levels of adaptation. We developed an approach for matching stimulus parameters and compared mfERG and psychophysical thresholds to assess the effects of technique and level of adaptation on the two responses. Psychophysical and mfERG thresholds were obtained as a function of the adaptation level (1.5-4.0 log td) and retinal location. The derived increment threshold-versus-intensity functions for both measures were fitted using the equation logT=logT(0)+log((A+A(0))/A(0))(n). We found that the values of A(0) for the mfERG data were one log unit higher than those for the psychophysical data. In addition, the value of the slope (n) for the mfERG data was shallower (0.8) than that of the psychophysical data (1.0). Predictions were made about comparisons of HVFA threshold and mfERG amplitude data in patients with retinal disease based upon a two-site model of adaptation. The data for some groups of patients could be best-fitted with a model of a disease acting at a site distal to all gain changes, whereas data from other patients were best fitted with a model of a disease acting at a site proximal to all retinal gain. The relationship between the Humphrey visual field threshold losses and mfERG amplitude reductions depends upon the site and mechanism of a particular disease process and the model of retinal gain assumed. In no case is a one-to-one relationship between the losses in the two measures predicted.

Adaptation, Ocular↗
PubMed2002

Local cone and rod system function in progressive cone dystrophy.

PURPOSE: To compare the patterns of local cone and rod system impairment in patients with progressive cone dystrophy (CD) using psychophysical and electrophysiological techniques. METHODS: Local cone system function was assessed by measuring cone system thresholds (visual fields) and cone-mediated multifocal electroretinograms (mfERGs). Rod system function was assessed by measuring rod system thresholds (visual fields) and rod-mediated mfERGs. The results in a group of eight patients with CD were compared with those in an age-similar control group. RESULTS: All the patients had abnormal cone system visual field thresholds and cone-mediated mfERGs. Cone system psychophysical thresholds were elevated for targets presented within the central 10 degrees, but were within normal limits for targets at peripheral locations. Cone-mediated mfERG measures of amplitude scale and time scale were abnormal for most of the hexagons tested. Most of the rod-mediated psychophysical thresholds and mfERGs were within normal limits. Rod system losses tended to be patchy and scattered throughout the area tested. CONCLUSIONS: There was poor correspondence among local measures of cone and rod system losses in these patients with CD. The results suggest that the spatial pattern of cone system losses in this disease differs from the spatial pattern of rod system losses.

Adolescent↗