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David Keating

Publications and source records attributed to David Keating.

7 recordsLinked to original sources
PubMed2006

Vitamin A deficiency modifies the mfERG: a case study of rod influence on the mfERG.

We report an observational case study which showed that the rod pathway may have a significant effect on the multifocal electroretinogram (mfERG). A 68-year-old lady diagnosed with Vitamin A deficiency underwent standard ERG and mfERG. ERG rod and cone function were analysed and the first order mfERG responses were determined. At presentation, there was no demonstrable rod function on ERG testing and normal cone function. mfERG P1 amplitude responses were normal but mfERG P1 latency responses were significantly delayed. Four weeks after initiating treatment with intramuscular Vitamin A, ERG rod function significantly improved. However, cone amplitude was reduced and the 30 Hz flicker latencies decreased. Furthermore, mfERG P1 latency returned to normal and interestingly mfERG P1 amplitude responses showed a significant reduction. These results alter our understanding of the mfERG response by indicating that there is a possible inhibitory influence on mfERG responses from rod pathway systems.

Aged↗
PubMed2006

Wide field multifocal and standard full field electroretinographic features of hemi retinal vein occlusion.

The purpose of this study was to document the standard full field electroretinographic (ERG) and wide field multifocal electroretinographic (WF-mfERG) findings in eyes with recent onset hemi-retinal vein occlusion (HRVO) and to compare the electro-diagnostic findings in the affected and fellow eyes with reference to normative data. Eight patients with HRVO were assessed using ERG and WF-mfERG. WF-mfERG first order responses from the affected hemi-retinae and the unaffected hemi-retinae in each affected eye were compared. WF-mfERG responses from each affected hemi-retina and from the symmetrical hemi-retina of each fellow eye were compared. ERG responses between affected and unaffected eyes were also compared. All electrodiagnostic tests were compared to normative data (5-95% confidence limits derived from age-related controls). WF-mfERG P1 and N1 implicit times were greater for the affected hemi-retinae than for the unaffected hemi-retinae (p <0.05). WF-mfERG N1 and P1 implicit times were prolonged (p < 0.05) and WF-mfERG P1/N1 amplitude ratios were significantly reduced (p < 0.05) for the affected eyes when compared with the fellow eyes. Maximal b-wave, cone b-wave and flicker implicit times were prolonged (p < 0.05) when comparing affected and fellow eyes. These results indicate that retinal injury due to HRVO culminates in significant delay of both ERG and WF-mfERG implicit times. These results suggest that WF-mfERG in combination with ERG may have a role in the management of HRVO.

Adult↗
PubMed2003

Multifocal electroretinographic features of central retinal vein occlusion.

PURPOSE: To determine the features of wide-field multifocal electroretinography (WF-mfERG) recorded in patients with central retinal vein occlusion (CRVO) and to compare WF-mfERG responses of the affected and fellow eyes. In addition, WF-mfERG responses were also compared by using standard electroretinography (ERG). METHODS: WF-mfERG and ERG responses were recorded from both eyes of 56 patients with CRVO. The WF-mfERG responses, obtained using a custom-built system were grouped into central and peripheral rings. The P1 amplitudes, and P1 and N1 implicit times were grouped and averaged within both rings. Nonparametric statistical analysis was used to compare the ERG results from the affected and fellow eyes. The results were also compared with normative data (5% to 95% confidence limits). RESULTS: CRVO markedly affected all the parameters of the WF-mfERG. In the affected eyes, 98% of the central and 91% of the peripheral P1 implicit times fell outside the normal range, as opposed to 35% of the 30-Hz flicker implicit times. The WF-mfERG responses obtained from eyes with CRVO were significantly different (P<0.01) from those derived from the fellow eye. The central and peripheral P1 implicit times were also abnormal in 59.2% and 46.9% of the fellow eyes, respectively. CONCLUSIONS: WF-mfERG is more susceptible than the standard ERG to changes in the nonlinear dynamics of the eye due to the multiple frequencies of stimulation used to record WF-mfERG responses. WF-mfERG could be a sensitive indicator of the underlying disease affecting the retina in eyes with CRVO and may have a role in the clinical setting.

Adult↗
PubMed2002

The multifocal ERG: unmasked by selective cross-correlation.

The purpose of this paper is to provide the reader with a better insight into the mechanisms of multifocal ERG (mfERG) recording. The construction of the first and second order mfERG responses were examined by recovering the response to specific pulse trains embedded in the m-sequence.A custom built pc based multifocal system driving a LED stimulator was used to record a 61 element mfERG and a global ERG. The global ERG recording was used to enable the recovery of different pulse trains embedded in the m-sequence. Summation of these individual pulse trains was performed and the results compared with the standard full cross-correlation. An isolated pulse response is defined as a flash of light that has no other flashes within two m-sequence base periods before or after the flash. This isolated pulse response was recovered from the raw data and this response input into a simple superposition model to predict the waveform shape for specific pulse trains. The superposition model was compared with the actual selective cross-correlation for a particular pulse train. The summations of the selective cross-correlation components give identical responses to the full cross-correlation. The superposition model also predicts the waveform shapes recovered by the selective cross-correlation procedure. The mfERG response is a complex composite response from a number of different pulse trains. Examination of the individual waveform shapes provides some insight into the origin of the mfERG waveform. The main contributions to the P1 component are the same as for an isolated response and as with the standard ERG this component is likely to be dominated by the mid retina. The N1 component is also likely to have similar origins to that of the isolated response but the amplitude is dominated by contributions from pulse trains where there is no change of state and therefore includes a component from the interaction between two consecutive stimuli. The N2 component is a composite response dominated by the interaction between two successive stimuli two base periods apart and the P1 component of a second stimulus delayed one frame from the first stimulus.

Adult↗
PubMed2002

Repeatability and reproducibility of macular thickness measurements with the Humphrey OCT system.

PURPOSE: To assess the accuracy, precision, repeatability, and reproducibility of measurements made by the Humphrey optical coherence tomography (OCT) system (Humphrey-Zeiss Medical Systems, San Leandro, CA). METHODS: The performance of the system was first investigated by scanning a test object containing an air gap of known size. Measurements were repeated with water or glycerin in the gap. In the clinical setting, macular thickness measurements were obtained from a control group of 20 normal subjects. For analysis, these scans were divided into eight sections, each containing 10 A-scans. RESULTS: The average gap thickness was found to be close to the true value in all cases. The overall coefficients of intersession reproducibility were less than 1% for the test object and 1.51% for the control group. There was no significant difference between scans acquired during different sessions. The overall coefficients of repeatability for the test object were between 0.2% and 1.1% and between 1% and 2% for the control group. The range of normal retinal thickness in terms of the 5th and 95th percentiles was 222 to 248 microm in women and 234 to 257 microm in men. CONCLUSIONS: Measurements made from OCT scans were found to be accurate and precise. Introducing water or glycerin into the test object resulted in considerable degradation of the signal, but measurements of gap thickness were still shown to be accurate, precise, reproducible, and repeatable. Retinal thickness measurements in the macular area were repeatable and reproducible. This demonstrates that OCT is a useful tool in the monitoring of patients with conditions that affect macular thickness, even when there is considerable degradation of the OCT signal.

Adult↗
PubMed2002

Repeatability and reproducibility of corneal thickness measurements by optical coherence tomography.

PURPOSE: To assess the repeatability and interoperator and intersession reproducibility of central corneal thickness (CCT) measurements made by a commercially available optical coherence tomography (OCT) system. METHODS: Intersession and interoperator reproducibility and repeatability were defined according to the guidelines of the British Standards Institution and examined in a control group of 14 normal subjects. An in-house computer program was used to evaluate central corneal thickness from these scans. RESULTS: The coefficient of interoperator reproducibility was 0.18%, whereas that for intersession reproducibility was 1.11%. Wilcoxon analysis (5% level of significance) showed that there was no statistically significant difference between scans acquired during different sessions or by different operators. Coefficients of repeatability were all less than 3%. The average CCT was 526 +/- 28 microm (SD) and the range of normal CCT between 5th and 95th percentiles was 498 to 576 microm. CONCLUSIONS: Although the commercially available OCT scanner was designed for retinal imaging, with a few minor modifications, the system may be used to image the anterior segment. Previous studies have shown that OCT measurements correlate well with those from conventional techniques, and it has the added advantage of being a noncontact technique. This study further demonstrates that the OCT measurements show a high degree of repeatability and reproducibility. Thus, OCT is emerging as a promising tool for evaluation of CCT in the clinical setting.

Adult↗