Differences in the neural basis of human amblyopia: the distribution of the anomaly across the visual field

Spatio-temporal contrast sensitivities to horizontally-oriented Gaussian-weighted patches of sinusoidal grating stimuli were determined across the nasal and temporal visual fields of strabismic and non-strabismic, anisometropic amblyopes. The visual field distribution of the amblyopic anomaly differs in strabismic and non-strabismic, anisometropic eyes. In strabismus the peripheral region of one or both hemifields is spared; in non-strabismic, anisometropic cases the loss is evenly distributed across the binocular visual field but is not present in the monocular temporal field. These findings suggest that the non-strabismic forms of amblyopia in humans result from binocular competitive imbalance in early life. The strabismic results pose two problems for the present competitive model of amblyopia: in strabismus, amblyopia is mainly limited to central vision and shows an asymmetric distribution.     

Is reduced vernier acuity in amblyopia due to position,contrast or fixation deficits?

Poor vernier acuity, exhibited by amblyopes, may reflect anomalies related to eccentric fixation, deficient position sensitivity, or reduced contour visibility. We have examined these factors by measuring contrast and vernier sensitivities with stimuli consisting of extended sinusoidal gratings of several spatial frequencies. Vernier thresholds were measured using both a classical single step position change and also a grating that was position-modulated sinusoidally along its entire length. For both types of target amblyopes exhibited deficient displacement sensitivity although accurate fixation was not strictly required. The vernier deficits were not critically dependent upon the type of displacement used, and the magnitudes of the vernier and contrast sensitivity deficits were closely related. Both were largest at high spatial frequencies, and those amblyopes with larger contrast sensitivity deficits also had larger vernier acuity deficiencies. Typically, contrast sensitivity and vernier acuity were normal, or nearly so, at very low spatial frequencies. Also, vernier acuities for the amblyopic and non-amblyopic eyes were approximately equal if grating contrast was set at some fixed multiple of detection threshold. We did not find a close relationship between the magnitude of the vernier deficit and reported perceptual distortions.     

Comparison of receptive field properties of neurons in area 17 of normal and bilaterally amblyopic cats

Receptive field properties of extracellularly recorded units in the visual cortex (area 17) of cats made bilaterally amblyopic by a variety of rearing conditions were measured and compared with the properties of units in normal cats. Properties studied included sensitivity to vernier offset, response facilitation to increasing bar length, receptive field size, responsiveness to moving and flashed stimuli, orientation tuning, the relation between mean firing rate and its variance, the amount of overlap of regions of on and off responsiveness in simple and complex cells, and, for flashed stimuli, latency to response onset, time to peak response, and response decay time constant. Behavioural testing of the amblyopic animals showed that spatial resolution was 2–4 times lower and vernier acuity thresholds 10–20 times greater than normal. Despite this, several neuronal response properties did not differ significantly from those in normal animals. These included peak responsiveness to moving stimuli, widths of orientation tuning curves, response variability, and latency to initial response for flashed stimuli. Other properties showed small but significant changes. Sensitivity to vernier offset (impulses per degree of offset) was reduced to nearly half its normal level; receptive field sizes increased by about 24% and an incomplete segregation of regions of on and off responsiveness was found in some cells, which made them hard to classify as simple or complex. Responses to flashed stimuli were smaller and more persistent. Their statistical significance notwithstanding, it seems unlikely that these relatively small response abnormalities in area 17 can fully account for the observed behavioural deficits.     

Contrast and accommodation in amblyopia

Steady-state accommodation as a function of sinusoidal and square-wave grating contrast was measured in amblyopes. The amblyopic eyes exhibited reduced average response levels and required greater contrast to sustain accommodation than the fellow dominant eye or the eyes of visually-normal control subjects. Following therapy, accommodation in the amblyopic eye markedly improved. These results suggest: reduced accommodative controller gain, reduced stimulus effectiveness of target contrast, and considerable residual visual system plasticity with respect to the neurological control pathways of accommodation, in the amblyopic eye.     

Evaluation of photoscreener instruments in a childhood population: 1. Otago photoscreener and Dortmans videophotorefractor

Objective: To evaluate two photoscreeners in a childhood population.
Study design: Double-masked study.
Subjects and method: One hundred and thirteen children aged between 11 and 44 months with either normal vision or known visual disorders were photoscreened without cycloplegia by the Otago and Dortmans (prototype) photoscreeners. Each child had a full ophthalmological examination either on the day of screening or in the preceeding six months. Photoscreen images were reviewed by an independent observer for indicators of amblyopiogenic risk factors, and compared to the full ophthalmological examination to determine sensitivity and specificity for each instrument.
Results: The Otago photoscreener returned a sensitivity of 70% and specificity of 82% for the detection of amblyopiogenic risk factors. The Dortmans photoscreener returned a sensitivity of 70% and specificity of 90%. Both photoscreeners were portable and easily operated. Conclusion: Children can be screened successfully for amblyopiogenic risk factors with these photoscreening systems. Further evaluation is required to determine specificity in a normal population. This would also provide information on the potential usefulness of photoscreeners in a cost effective childhood vision screening program.     

The spatial localization deficit in visually deprived kittens

We measured the spatial localization abilities (alignment accuracy) of visually deprived kittens by use of similar spatially bandpass stimuli (Gaussian blobs) to those employed for the assessment of human amblyopes. The tests of vision were conducted on kittens reared with either strabismus or following different periods of monocular deprivation. As with amblyopic humans, the deficits in alignment accuracy were scaled in proportion to blob size and were not only considerably larger than those of grating acuity but also were not correlated with either the acuity or contrast sensitivity losses. Tests with stimuli of various contrast revealed that the deficits could not be explained in terms of the contrast sensitivity loss in this eye. The positional deficits that arise from anomalous visual development are independent of the contrast sensitivity loss and profound.     

Effect of inferior oblique recession in strabismus sursoadductorius

The aim of this retrospective study was a quantitative analysis of the effect of graded inferior oblique recessions (8mm, 10 mm or maximal) in strabismus sursoadductorius. METHOD 234 patients (2-81 years of age) with unilateral strabismus sursoadductorius and stereopsis were operated between 1990 and 1999. Maximal recession (14.6mm) included an anteroposition to the lateral aspect of the insertion of the inferior rectus. Before and three months after the operation, horizontal and vertical deviations (VD) were assessed by cover testing in five positions of gaze. In 121 patients, additional subjective quantitative assessment, including cyclodeviation measurement, was carried out with Harms' tangent screen. RESULTS The mean effect of the operation on VD in 25° adduction increased from 6° with 8mm recession to 10° with maximal recession. It was related more to the amount of preoperative VD in adduction than to the amount of recession. One patient had a limitation of elevation with a hypotropia of >4°, 7% needed further surgery because of undercorrection. DISCUSSION Maximal recession of the inferior oblique muscle is a suitable procedure even in patients with marked strabismus sursoadductorius. The rate of functional undercorrection is low and there is no risk from general anesthesia or significant postoperative limitation of elevation.     

Effectiveness of the addition of citicoline to patching in the treatment of amblyopia around visual maturity: A randomized controlled trial

Aim:

To study the effectiveness of the addition of citicoline to patching in the treatment of amblyopia in the age group of 4-13 years.

Materials and Methods:

A randomized controlled trial, which included patients who were randomly divided into two groups. Both the groups received patching therapy till plateau was achieved in phase 1 of the study. Then in phase 2, group I received citicoline plus patching and group II continued to receive only patching.

Outcome Measures:

Outcome was measured by the visual acuity in logMAR every month in phase 1 till plateau was achieved and then for 12 months in phase 2.

Results:

No significant difference was found in the mean visual acuities in these two groups in phase 1 till plateau was reached. In phase 2, for the initial four months, there was no significant difference in the visual acuities in these two groups, at the respective intervals. However, five months onward, up to 12 months, there was a significant difference in the visual acuities in these groups. The result was the same in younger patients (< seven years of age) as well as in older patients (> seven years of age). In phase 2, the mean proportional improvement in group I was significantly more than that in group II, at two months and onward, at the respective intervals.

Conclusion:

The improvement in visual acuity with citicoline plus patching was significantly more than that with patching alone, in one year of treatment.     

视知觉学习疗法治疗学龄前儿童屈光不正性弱视的效果

目的探讨视知觉学习疗法治疗学龄前屈光不正性弱视的效果。方法选取3—8岁屈光不正性弱视患儿611名1177眼,给予视知觉学习疗法治疗,比较治疗前后患儿视力情况。结果611例1177眼戴镜视力由治疗前的（0．34±0．23）提高到（0．81±0．15）,差异有统计学意义（t=5．63,P〈0．05）。视力提高2行及以上的眼数为1040只,占总眼数的88．4％。结论视知觉学习治疗是一种安全、有效的治疗学龄前屈光不正性弱视的新方法,在患儿进行视知觉学习训练治疗时给予个性化的护理措施,能提高患儿的治疗依从性和生活质量。     

P-VEP敏感空间频率刺激综合疗法治疗弱视的临床观察

目的：探讨根据图形视觉诱发电位（pattern visual evoked potential,P-VEP）检查出的敏感空间频率进行视觉刺激训练疗法对儿童弱视治疗的有效性。方法：所有患儿随机分两组,对照组110例（126只眼）,采用遮盖法和精细目力训练法治疗,实验组100例（119只眼）,采用P-VEP检查出敏感空间频率,选择该敏感空间频率进行视觉刺激训练加遮盖法的综合治疗。结果：两组总基本治愈率比较,相同年龄段、屈光不正和屈光参差的基本治愈率中比较,实验组高于对照组（P＜0.05）。结论：敏感空间频率刺激加遮盖法综合疗法对弱视治疗有较好的疗效。