Biomedical subjects

Chi D Luu

Publications and source records attributed to Chi D Luu.

8 recordsLinked to original sources

PubMed2006

Electrophysiological findings in patients with dengue-related maculopathy.

OBJECTIVE: To assess visual function in patients with dengue maculopathy using electrophysiological tests. METHODS: Fifteen consecutive patients with dengue fever who experienced vision loss between July 2004 and July 2005 were included in this review. Full-field electroretinograms (ERG), pattern ERGs, and multifocal ERGs were performed. RESULTS: The most common electrophysiological finding (60%, 9/15) was a normal or mildly abnormal full-field ERG with reduced pattern ERG P50 amplitude and abnormal mfERG. Typically, multifocal ERG demonstrated a focal area of decreased macular response (especially between the fovea and optic nerve). Pattern ERG suggested normal optic nerve function in all but 1 case. Four patients had more severely reduced full-field ERG responses with reduced a-wave amplitude (suggestive of photoreceptor dysfunction), 3 of whom had an electronegative maximal response (suggestive of additional postreceptor dysfunction). Repeat multifocal ERG showed little change in 7 patients and incomplete resolution in 2 patients over 3 to 10 months. CONCLUSION: Retinal dysfunction associated with dengue maculopathy was localized mainly around the foveal region. It appeared to affect the outer and middle retina more severely with relative sparing of the inner retina. Retinal dysfunction may persist for several months. Longer follow-up is required to determine whether these changes are permanent.

Adolescent↗

PubMed2006

Multifocal electroretinogram in adults and children with myopia.

OBJECTIVE: To determine the correlation between multifocal electroretinogram (mfERG) parameters and the severity of myopia in adults and children. DESIGN: Observational study. METHODS: Multifocal electroretinograms were recorded using the VERIS system from randomly selected eyes of 104 children and 31 adults with various degrees of myopia. Dawson, Trick, and Litzkow fiber electrodes were used and the pupil was dilated with 1% tropicamide. Subjective refraction was performed under cycloplegia and axial length measurement was determined by A-scan ultrasonography. The N1 (first negative trough), P1 (first positive peak), and N2 (second negative trough) components of the first-order kernel response of the mfERG were measured and correlated with the refractive data. MAIN OUTCOME MEASURES: First-order kernel mfERG responses. RESULTS: The N1, P1, and N2 amplitudes were significantly correlated with the severity of myopia in adult subjects (N1, r = 0.591, P = .001; P1, r = 0.682, P<.001; N2, r = 0.732, P<.001). The response amplitudes of N1, P1, and N2 decreased as the dioptric power of myopia increased. However, there were no significant correlations found between N1 (r = 0.073, P = .30), P1 (r = 0.071, P = .31), and N2 (r = 0.052, P = .46) amplitudes and the severity of myopia in children. The severity of myopia was also significantly correlated with N1 (r = -0.750, P<.001), P1 (r = -0.769, P<.001), and N2 (r = -0.664, P<.001) implicit times in adults with myopia, however, only the P1 (r = -0.166, P = .02) implicit time was significantly correlated with children with myopia. CONCLUSIONS: There is a significant correlation between the refractive error and mfERG amplitude in adults with myopia; however, such a relationship is absent in children with myopia. These findings suggest that the severity of myopia has little influence on the ERG amplitude, at least in children.

Adolescent↗

PubMed2006

Electrophysiological findings in persons with nyctalopia.

INTRODUCTION: Ophthalmologists are occasionally confronted with an individual presenting with nyctalopia (i.e., a relatively greater difficulty seeing at night). When there is no accompanying abnormality seen in the fundus, visual electrophysiology becomes useful as an objective means of assessing rod (scotopic) photoreceptor function or pathway defects. MATERIALS AND METHODS: A retrospective study was performed on 50 consecutive patients, aged less than 40 years, with seemingly normal fundi and good vision [visual acuity (VA) >6/12] presenting to the Visual Electrodiagnostic Unit, Singapore National Eye Centre, for the investigation of nyctalopia over a 2-year period. Subjective scotopic threshold sensitivity (STS) and objective full-field electroretinogram (ERG) were performed. Persons with abnormal test results were identified. RESULTS: Normal ERG scotopic responses were obtained in 74% of subjects. There was no significant difference in age, refraction and STS levels between subjects with abnormal and normal ERG. In the group with abnormal scotopic ERG responses, 9 were identified to have nonspecific rod dysfunction, 2 had rod-cone dystrophies and 2 had ERG changes suggestive of congenital stationary night blindness (CSNB). CONCLUSION: A large number of subjects presenting with nyctalopia had normal ERG findings. We can only assume that in these patients, no significant rod pathway dysfunction exists and that optical (e.g., night or instrument myopia) and psychological aetiologies should be considered. The fact that an abnormal result occurs in 26%, however, suggests that ncytalopia should be evaluated with electrophysiolgoical testing even when the fundi appear normal.

Adult↗

PubMed2005

Visual function in Vogt-Koyanagi-Harada patients.

BACKGROUND: Vogt-Koyanagi-Harada (VKH) disease presents with anterior segment inflammation, choroiditis and exudative retinal detachment. Following resolution of the inflammation, VKH patients have been noted to complain of visual disturbances despite good visual acuity. We therefore investigated the visual function deficits of convalescent VKH patients. METHODS: A cross-sectional observational nonrandomized controlled study of convalescent VKH patients from the Uveitis Service of the Singapore National Eye Centre, and normal subjects was performed. The best-corrected visual acuities (BCVA) and multifocal electroretinograms (mfERGs) of VKH patients with and without peripapillary atrophy (PPA) were compared with those of the normal eyes. The mfERG results were subdivided into those obtained from the peripapillary area and those from the rest of the macular. RESULTS: Eleven VKH eyes with large PPA to disc ratios (PPA/D ratio >2), 15 VKH eyes with PPA/D ratios<1 and 6 normal eyes were included in the study. Five eyes (54.5%) of VKH patients with PPA/D>2 had a BCVA of less than 20/40. All the other eyes had 20/20 vision. Nine of the 11 VKH eyes with PPA/D>2 also had large areas of chorioretinal atrophy. The mfERG responses of VKH eyes with PPA/D ratio >2 were markedly reduced in amplitude (p<0.001) and delayed in implicit time (p<0.001) throughout the entire macular area. VKH patients with PPA/D ratio<1 had significantly reduced mfERG amplitudes throughout the entire macular area, as well as delayed implicit times at the peripapillary region (p=0.026). Sub-division of VKH eyes with PPA/D<1 into eyes with no PPA and eyes with a small PPA, showed that both groups had a similar reduction in response amplitude over the entire macular region. However, the implicit time was significantly delayed in eyes with small PPA when compared to those without PPA (p<0.03). CONCLUSIONS: VKH patients with large PPA have clinically significant visual dysfunction. VKH patients without PPA also have subclinical retinal dysfunction. The mfERG may be a useful adjunct in the management of VKH by detecting early retinal damage.

Cross-Sectional Studies↗

PubMed2004

Clinical application of the multifocal visual evoked potential.

BACKGROUND: Measures of visual function thresholds such as visual acuity and visual fields are generally dependent on subjective responses and assume maintenance of fixation, attention and motivation. In the young, elderly, cognitively impaired or malingering populations, such measures may be inaccurate or difficult to obtain. The Visual Evoked Response Imaging System (VERIS) has been claimed to give more objective topographic recordings of retinal and cortical function. This paper aims to explore the adequacy of this technique in four unusual, unrelated, clinically difficult cases. METHODS: Multifocal visual evoked potentials (mfVEPs) recorded on the VERIS System 3.01 are used to assess visual function in four cases with contradictory clinical findings or unreliable subjective responses. RESULTS: Patient 1 had sustained a head injury and had normal ocular and pupil examination but light perception in the right eye and 6/5 acuity in the left. Multifocal VEPs showed a marked depression of the right visual field with little macular response. Patient 2 had sustained a head injury, had a left field hemianopia, possible macular sparing and loss of much of the right field, reduced but variable visual acuities, good near vision and normal ocular fundi. Multifocal VEPs showed a severe depression in both visual fields (L more than R) with little macular response. Patient 3 had a left optic nerve meningioma and experienced great difficulty with visual field assessment. mfVEPs showed a bilateral depression in the superior field particularly the left field, with a larger deficit in the left eye. Patient 4 had unexplained visual acuity and peripheral field deficits. mfVEP results were inconclusive in this case. DISCUSSION: Where there is difficulty performing traditional techniques or conflicting clinical findings, mfVEPs may provide additional objective information to aid in the assessment of patients.

Adult↗

PubMed2003

The plasticity of vertical motor and sensory fusion in normal subjects.

PURPOSE: It is generally agreed that vertical prism bar training can increase vertical fusion amplitude. However, little is known as to whether the increased vertical fusion amplitude following vergence exercise is due to improvement of the sensory or the motor system or both. The main aim of the present study was to determine the effect of vertical prism bar training on the motor and sensory components of vertical fusion. METHODS: Fifty normal subjects between 17 and 25 years of age were given vertical prism bar training to improve their vertical fusion amplitude. Vertical vergence eye movements were recorded using an infrared reflectance eye tracker system. The sensory component was assessed using a Sheedy disparometer. RESULTS: The findings of the present study showed that the average increase in vertical motor fusion was approximately 30% following vertical prism bar training. The maximum magnitude of VFD remained unchanged in the majority (73%) of subjects. CONCLUSIONS: The increase in the vertical fusion amplitude following vertical prism bar training is largely due to improvements in the vertical motor fusion. The prism training has little effect on the sensory component.

Adolescent↗

PubMed2002

Separation of contour and area dependent components in the first and second order kernels of the multifocal pattern appearance evoked potential.

The non-linearities of the diffuse flash visual evoked potential (VEP) have been shown to generate separable waves corresponding to Magnocellular and Parvocellular contributions. The authors wished to investigate whether, under conditions of isoluminance, the chromatic pattern appearance VEP shows waveforms that are related to both the processing of contours (the pattern) and the processing of the diffuse stimulation (from the area lying between the contours). The VERIS multifocal system was used to record monocular VEP. Standard 2-ring, 3-ring, 5-ring and 7-ring VERIS pattern stimuli were employed with a cardboard circular aperture used to blank the stimulus monitor to a 9-cm diameter field. The results for four subjects were recorded. The first order kernel responses showed a significant increase in amplitude with total contour length. In comparison, the amplitudes of the first and second slices of the second order kernel did not increase with contour length. It appears that the contour-dependent component has negligible non-linear components. It is proposed that the constant, non-zero responses of the higher order kernel components reflects contributions from the diffuse chromatic VEP.

Adult↗

PubMedDate not supplied

The ON/OFF-response in retinopathy of prematurity subjects with myopia.

Retinopathy of prematurity (ROP) is often associated with myopia. Electrophysiological findings have shown that an imbalance of the ON- and OFF-response in subjects with ROP is associated with their refractive status. Nevertheless, the extent of these functional changes is still unclear. The aim of this study was to determine the extent of the ON- and OFF-response attenuation in ROP subjects with myopia. Fast and slow m-sequence multifocal electroretinogram were recorded on 14 eyes in 8 subjects with various degrees of myopia using the VERIS system. The spherical equivalent of refractive error ranged from -0.25 to -13.50 D. All ERG recordings were made using DTL electrodes with dilated pupils. A 19 retinally scaled hexagon stimulus was used. The findings showed that the ON- and OFF-response are reduced differently in ROP subjects with myopia. The ON-response attenuation is dependant on the severity of myopia, whereas, the OFF-response attenuation is influenced by retinal eccentricity as well as the degree of myopia. The contributions of these findings toward the understanding of retinal mechanisms controlling ocular growth need to be further explored.

Adolescent↗