Functional MRI of amblyopia before and after levodopa

Durable recovery from amblyopia with donepezil

An elevated threshold for neuroplasticity limits visual gains with treatment of residual amblyopia in older children and adults. Acetylcholinesterase inhibitors (AChEI) can enable visual neuroplasticity and promote recovery from amblyopia in adult mice. Motivated by these promising findings, we sought to determine whether donepezil, a commercially available AChEI, can enable recovery in older children and adults with residual amblyopia. In this open-label pilot efficacy study, 16 participants (mean age 16 years; range 9-37 years) with residual anisometropic and/or strabismic amblyopia were treated with daily oral donepezil for 12 weeks. Donepezil dosage was started at 2.5 or 5.0 mg based on age and increased by 2.5 mg if the amblyopic eye visual acuity did not improve by 1 line from the visit 4 weeks prior for a maximum dosage of 7.5 or 10 mg. Participants < 18 years of age further patched the dominant eye. The primary outcome was visual acuity in the amblyopic eye at 22 weeks, 10 weeks after treatment was discontinued. Mean amblyopic eye visual acuity improved 1.2 lines (range 0.0-3.0), and 4/16 (25%) improved by ≥ 2 lines after 12 weeks of treatment. Gains were maintained 10 weeks after cessation of donepezil and were similar for children and adults. Adverse events were mild and self-limited. Residual amblyopia improves in older children and adults treated with donepezil, supporting the concept that the critical window of visual cortical plasticity can be pharmacologically manipulated to treat amblyopia. Placebo-controlled studies are needed.

Amblyopia: progress and promise of functional magnetic resonance imaging

Amblyopia is a neurodevelopmental disorder characterized by functional deficits in the visual cortex. Functional magnetic resonance imaging (fMRI) is the most commonly used neuroimaging technique for investigating amblyopia. Herein, we systematically searched a PubMed database from inception to December 2021 to highlight the current progress and promises about fMRI technology in amblyopia; amblyopia's neural mechanism, the comparison of different types of amblyopia, and the evaluation of the therapeutic effect were explored. Relevant articles published in English and appropriate cross-references were considered for inclusion, including basic studies, imaging techniques, clinical diagnostic and therapeutic studies, case series, and reviews.

Impact of Amblyopia on the Central Nervous System

Amblyopia is a common perceptual disorder resulting from abnormal visual input during development. The clinical presentation and visual deficits associated with amblyopia are well characterized. Less is known however, about amblyopia's impact on the central nervous system (CNS). While early insights into the neuropathophysiology of amblyopia have been based on findings from animal models and postmortem human studies, recent advances in noninvasive magnetic resonance imaging (MRI) techniques have enabled the study of amblyopia's effects in vivo. We review recent retinal and neuroimaging research documenting amblyopia's structural and functional impact on the CNS. Clinical imaging provides some evidence for retinal and optic nerve abnormalities in amblyopic eyes, although the overall picture remains inconclusive. Neuroimaging studies report clearer changes in both structure and function of the visual pathways. In the optic nerves, optic tracts, and optic radiations of individuals with amblyopia, white-matter integrity is decreased. In the lateral geniculate nuclei, gray matter volume is decreased and neural activity is reduced. Reduced responses are also seen in the amblyopic primary visual cortex and extrastriate areas. Overall, amblyopia impacts structure and function at multiple sites along the visual processing hierarchy. Moreover, there is some evidence that amblyopia's impact on the CNS depends on its etiology, with different patterns of results for strabismic and anisometropic amblyopia. To clarify the impact of amblyopia on the CNS, simultaneous collection of retinal, neural, and perceptual measures should be employed. Such an approach will help (1) distinguish cause and effect of amblyopic impairments, (2) separate the impact of amblyopia from other superimposed conditions, and (3) identify the importance of amblyopic etiology to specific neural and perceptual deficits.

An update on pharmacological treatment options for amblyopia

Amblyopia is a common cause of visual impairment in children and young adults. The cornerstone in the management of this disorder is based on increasing visual stimulation of the amblyopic eye by occlusion, by administering atropine or by causing optical penalization of the dominant eye. All these treatment options have shown some limits in terms of efficacy, due to the suboptimal treatment adherence for the patients and the lack of long-term clinical outcomes. Moreover, although it is well known that clinical efficacy decreases with age, new evidence is suggesting that cortical plasticity can be induced also in older children. For these reasons, new treatment options are being studied, in order to extend the "treatment window" beyond the critical period also in older patients. In this review, we will discuss all the most promising novel pharmacological agents in the management of amblyopia.

Evaluating the Acute Effect of Stereoscopic Recovery by Dichoptic Stimulation Using Electroencephalogram

Amblyopia is a common developmental disorder in adolescents and children. Stereoscopic loss is a symptom of amblyopia that can seriously affect the quality of patient's life. Recent studies have shown that the push-pull perceptual learning protocol had a positive effect on stereoscopic recovery. In this study, we developed a stereoscopic training method using a polarized visualization system according to the push-pull protocol. Dichoptic stimulation for 36 anisometropic and amblyopic subjects and 33 children with normal visual acuity (VA) has been conducted. Electroencephalogram (EEG) was used to evaluate the neurophysiological changes before, during, and after stimulation. For the anisometropic and amblyopic subjects, the statistical analysis demonstrated significant differences (p < 0.01) in the beta rhythm at the middle temporal and occipital lobes, while the EEG from the normal VA subjects indicated no significant changes when comparing the results before and after training. We concluded that the dichoptic training in our study can activate the middle temporal visual area and visual cortex. The EEG changes can be used to evaluate the training effects. This study also found that the beta band EEG acquired during visual stimulation at the dorsal visual stream can be potentially used for predicting acute training effect. The results facilitated the optimization of the individual training plan.

Levodopa Plus Occlusion Therapy versus Occlusion Therapy Alone for Children with Anisometropic Amblyopia

Purpose

This study aimed to compare the effects of short-term administration of levodopa plus occlusion therapy versus occlusion therapy alone in preschool children with hyperopic anisometropic amblyopia.

Methods

This comparative interventional study included 40 eligible preschool children aged 6 to 7 years with hyperopic anisometropic amblyopia. The primary outcome measure was the logarithm of the minimum angle of resolution (logMAR) best-corrected visual acuity recorded at baseline, 3 weeks after the treatment initiation and 12 weeks after the treatment termination. The results were compared between the two groups.

Results

No statistically significant intergroup difference was observed in baseline logMAR visual acuities (P = 0.92). The mean logMAR visual acuities of the amblyopic eyes were significantly better in both groups three weeks after the treatment initiation than the baseline (P< 0.01 in both groups). At 12 weeks after treatment termination, the logMAR visual acuities of the amblyopic eyes were significantly better than the baseline values (P< 0.001 in the placebo group and P = 0.09 in the levodopa group). Intergroup comparisons revealed no statistically significant difference in visual acuities 3 weeks after the treatment initiation (P = 0.11) and 12 weeks after the treatment termination (P=0.10). Twelve weeks after the treatment termination, visual acuities regressed 0.037 logMAR in the placebo group and 0.042 logMAR in the levodopa group. These regression rates were not significantly different (P = 0.89).

Conclusion

The results of this study provide evidence that adding short-term administration of levodopa to occlusion therapy in hyperopic anisometropic amblyopia offers no additional benefit in visual outcomes and provides no advantage in terms of the regression rate.

Selective serotonin reuptake inhibitors may lead to improved cataract surgery outcomes in patients with amblyopia

Purpose

To evaluate the effects of selective serotonin reuptake inhibitor (SSRI)/serotonin norepinephrine reuptake inhibitor (SNRI) medications in combination with cataract surgery in treating amblyopia in adult patients.

Patients and methods

A retrospective chart review study was conducted on patients who had undergone cataract surgery at the Johns Hopkins Hospital Wilmer Eye Institute. Six inclusion criteria were used to assess patient eligibility: 1) >18 years of age, 2) diagnosis of amblyopia, 3) diagnosis of cataract and treatment with surgery, 4) electronic medical record contains pre-surgery and post-surgery visual acuity (VA) measurements, 5) electronic medical record contains information on whether the patient was ever prescribed a SSRI/SNRI and the treatment duration, and 6) interocular VA difference of two lines or more on Snellen chart prior to cataract surgery. From each record, preoperative VA, postoperative VA, date of surgery, date at which postoperative VA was measured, and age at surgery were collected.

Results

A total of 237 patients were included, with 38 of them being on SSRI/SNRI. The mean improvement in VA after surgery was not significantly greater in patients on SSRI/SNRI (SSRI/SNRI: −0.276 logMAR, control: −0.192 logMAR, p=0.15). Multivariable regression was subsequently performed and while holding all other variables constant, demonstrated a statistically significant improvement in VA in patients on SSRI/SNRI (95% CI: −0.194, −0.0116, p=0.03). The regression analysis further demonstrated that advanced age has an adverse effect on the change in post-op VA (CI: 3.34×10⁻³ logMAR, 9.77×10⁻³ logMAR, p<0.005). Worse baseline VA is associated with a greater improvement in post-op VA (95% CI: −0.659 logMAR, −0.463 logMAR, p<0.005) but adverse effect on the absolute post-op VA (95% CI: 0.341 logMAR, 0.544 logMAR, p<0.005).

Conclusion

This study suggests that patients with amblyopia undergoing cataract surgery may potentially have a greater visual improvement when treated with SSRI/SNRIs.

Using magnetic resonance imaging to assess visual deficits: a review

Purpose

Over the last two decades, magnetic resonance imaging (MRI) has been widely used in neuroscience research to assess both structure and function in the brain in health and disease. With regard to vision research, prior to the advent of MRI, researchers relied on animal physiology and human post‐mortem work to assess the impact of eye disease on visual cortex and connecting structures. Using MRI, researchers can non‐invasively examine the effects of eye disease on the whole visual pathway, including the lateral geniculate nucleus, striate and extrastriate cortex. This review aims to summarise research using MRI to investigate structural, chemical and functional effects of eye diseases, including: macular degeneration, retinitis pigmentosa, glaucoma, albinism, and amblyopia.

Recent Findings

Structural MRI has demonstrated significant abnormalities within both grey and white matter densities across both visual and non‐visual areas. Functional MRI studies have also provided extensive evidence of functional changes throughout the whole of the visual pathway following visual loss, particularly in amblyopia. MR spectroscopy techniques have also revealed several abnormalities in metabolite concentrations in both glaucoma and age‐related macular degeneration. GABA‐edited MR spectroscopy on the other hand has identified possible evidence of plasticity within visual cortex.

Summary

Collectively, using MRI to investigate the effects on the visual pathway following disease and dysfunction has revealed a rich pattern of results allowing for better characterisation of disease. In the future MRI will likely play an important role in assessing the impact of eye disease on the visual pathway and how it progresses over time.

What's new for us in strabismus?

Strabismus is one of the most challenging subspecialties encountered in the field of ophthalmology. The concept of etiology of strabismus is being advanced with the development of newer imaging modalities and increased understanding of the genetics of strabismus. Imaging is also being used to aid in the planning of strabismus surgery. Newer horizons are being explored in the amblyopia management. The good old eye-pad is being replaced with the iPad. Early detection of loss of stereopsis is being used to decide the timing for strabismus surgery. Improvement of binocular summation has been discovered as a benefit of corrective strabismus surgery. Newer surgical techniques such as new transposition procedures are being developed to correct complex strabismus. Strabismus surgeries aided by adjustable sutures have increased the precision of a strabismologist. A new light has been thrown on the psychosocial impact of strabismus. A present-day strabismologist has advanced from the goal of ocular alignment to a bigger perspective "to regain the paradise lost: stereopsis."

Medical Therapies of Amblyopia: Translational Research to Expand Our Treatment Armamentarium

Amblyopia is a developmental brain disorder in which vision is lost due to asymmetric or inadequate visual stimulation early in life. Although amblyopia is responsive to treatment if therapy is initiated early, treatment of older children and adults is usually unsuccessful due to closure of a window of cortical brain plasticity. Extensive basic research has been devoted to understanding modulators in shaping the visual cortex during the critical period of plasticity, and to providing potential clinical applications of neurotransmitters in the treatment of amblyopia. Current pharmacological treatments are reviewed from basic science research extending into clinical use, focusing on the acetylcholinesterase inhibitor donezepil, serotonin receptor inhibitor fluoxetine, dopamine precursors carbidopa-levodopa, and catecholamine modulator citicoline.

Does levodopa improve vision in albinism? Results of a randomized, controlled clinical trial

BACKGROUND

Dopamine is an intermediate product in the biosynthesis of melanin pigment, which is absent or reduced in albinism. Animal research has shown that supplying a precursor to dopamine, levodopa, may improve visual acuity in albinism by enhancing neural networks. This study examines the safety and effectiveness of levodopa on best-corrected visual acuity in human subjects with albinism.

DESIGN

Prospective, randomized, placebo-controlled, double-masked clinical trial conducted at the University of Minnesota.

PARTICIPANTS

Forty-five subjects with albinism.

METHODS

Subjects with albinism were randomly assigned to one of three treatment arms: levodopa 0.76 mg/kg with 25% carbidopa, levodopa 0.51 mg/kg with 25% carbidopa, or placebo and followed for 20 weeks, with best-corrected visual acuity measured at enrollment, and at weeks 5, 10, 15, and 20 after enrollment. Side-effects were recorded with a symptom survey. Blood was drawn for genotyping.

MAIN OUTCOME MEASURES

Side-effects and best-corrected visual acuity 20 weeks after enrolment.

RESULTS

All subjects had at least one mutation found in a gene known to cause albinism. Mean age was 14.5 years (range: 3.5 to 57.8 years). Follow up was 100% and compliance was good. Minor side-effects were reported; there were no serious adverse events. There was no statistically significant improvement in best-corrected visual acuity after 20 weeks with either dose of levodopa.

CONCLUSIONS

Levodopa, in the doses used in this trial and for the time course of administration, did not improve visual acuity in subjects with albinism.

BOLD responses to different temporospatial frequency stimuli in V1 and V2 visual cortex of anisometropic amblyopia

PURPOSE

Functional magnetic resonance imaging (fMRI) is the most advanced neuroimaging technique. The aim of this study was to investigate the blood oxygenation level-dependent (BOLD) of V1 and V2 visual cortex in anisometropic amblyopia with fMRI and explore the neural mechanism of amblyopia.

METHODS

fMRI was performed with a 3.0-T MRI scanner during reversal checkerboard visual stimulation with different spatial frequencies (SF) of 0.4, 2, and 8 cpd in 2 states of temporal frequencies (TF) of 6 Hz and 8 Hz in a group of patients with anisometropic amblyopia (n=5) and a group of normal observers (n=4). Data were processed by SPM software offline. Responses of different eyes were compared in different conditions.

RESULTS

The BOLD signal magnitude in V1 and V2 visual cortex of amblyopic eyes was significantly lower than the fellow eyes with anisometropic amblyopia at low SF (0.4-2 cpd) (p<0.05), but it was significantly higher than the fellow eyes at high SF (8 cpd) (p<0.05). The BOLD signal magnitude in V1 and V2 visual cortex of amblyopic eyes was significantly lower than the nondominant eyes in normal subjects in all conditions (p<0.001).

CONCLUSIONS

There are cortical deficits in V1 and V2 visual cortex of anisometropic amblyopia, which may be useful for selecting an optimum stimulus at proper temporospatial frequency.

Levodopa versus non-levodopa brain language fMRI in Parkinson's disease

OBJECTIVE

To identify the effect of levodopa in language areas in Parkinson's disease patients.

METHODS

We evaluated 50 patients with mild to moderate Parkinson's disease, age and gender paired to 47 healthy volunteers. We selected two homogeneous groups of 18 patients taking levodopa and 7 no levodopa patients. The functional magnetic resonance imaging verbal fluency task, with low and high cognitive demands, was performed at a 3T magnetic resonance imaging equipment. Data was analyzed with XBAM software for group maps and ANOVA comparison.

RESULTS

Patients without levodopa had more activation than the ones with levodopa in the medial frontal and in the left frontal and parieto-occipital areas. The striatal activation in patients taking levodopa had similar result of the activation detected in the healthy volunteer group. Parieto-occipital areas were less activated in the levodopa group than in the no levodopa one.

CONCLUSION

Parkinson's disease patients without levodopa replacement, during a verbal fluency effort, had more diffuse and intense cerebral activation in left hemisphere, mainly in the frontal and parieto-occipital areas. The striatal activation in verbal fluency of patients with levodopa intake was more similar to the activation found in healthy volunteers. These initial evidences suggested a role of levodopa inhibiting activation in parieto-occipital compensating areas.

The reorganized brain: how treatment strategies for stroke and amblyopia can inform our knowledge of plasticity throughout the lifespan

Neural plasticity plays a crucial role in human development. During development, neural networks are shaped by experience-dependent processes that selectively strengthen and prune connections so that those that remain match the environment and process it optimally. Over time, neural connections become more stable, forming widely distributed, interconnected networks involving balanced excitation and inhibition and structural stabilizers like myelin. It was long believed that the potential for organization or reorganization existed only during early development. However, the successful treatments for adults with stroke or amblyopia discussed in this issue suggest that the potential for significant reorganization persists well into adulthood. Thus, development can be thought of as the stabilization of connections to match the current environment but with considerable residual plasticity that can be revealed if there is a shift in the excitatory: inhibitory balance or the removal of the structural stabilizers.

Neuroimaging in Human Amblyopia

Functional magnetic resonance imaging (fMRI), positron emission tomography (PET) and magnetoencephalography (MEG) have been the principal neuroimaging tools used to assess the site and nature of cortical deficits in human amblyopia. A review of this growing body of work is presented here with particular reference to various controversial issues, including whether or not the primary visual cortex is dysfunctional, the involvement of higher-order visual areas, neural differences between strabismic and anisometropic amblyopes, and the effects of modern-day drug treatments. We also present our own recent MEG work in which we used the analysis technique of synthetic aperture magnetometry (SAM) to examine the effects of strabismic amblyopia on cortical function. Our results provide evidence that the neuronal assembly associated with form perception in the extrastriate cortex may be dysfunctional in amblyopia, and that the nature of this dysfunction may relate to a change in the normal temporal pattern of neuronal discharges. Based on these results and existing literature, we conclude that a number of cortical areas show reduced levels of activation in amblyopia, including primary and secondary visual areas and regions within the parieto-occipital cortex and ventral temporal cortex.

Amblyopie fonctionnelle : évaluation en IRM fonctionnelle de la réponse corticale visuelle après traitement

PURPOSE

Functional MRI evaluation of the cortical response in treated amblyopic patients.

MATERIAL AND METHODS

Clinical and functional MRI exploration of ten patients, seven men and three women aged from 21 to 59 years, with strabismus management during childhood. Functional evaluations were performed on a 1.5 Tesla MR device, with four monocular functional sessions, two stimulations per eye. Alternating rest and active phases displayed still and flickering black and white checkerboards with spatial and temporal frequencies of 1 degree/8Hz and 15'/4Hz. Anatomical realignment and statistical analysis were performed using SPM99 (Statistical Parametric Mapping) to compare the four sessions in individuals.

RESULTS AND DISCUSSION

In patients presenting a visual acuity of the amblyopic eye less than 0.7, stimulation of this eye induced lower response in V1, V3, and V5 in comparison with the contralateral eye stimulation. Unexpectedly, in patients recovering normal or subnormal acuity, the amblyopic eye gave comparable or enhanced response in these areas. Additional response was found in the secondary visual cortex, the cuneus, the lingual gyrus, and in parietal, frontal, and orbitofrontal areas. These results suggest a variation in cortical response depending on the efficacy of the treatment. Recovered amblyopic eye, even with acuity less than the contralateral eye, may induce a reinforced cortical sensitivity to visual stimulus. Secondary visual areas may contribute to an attentional process in image perception and analysis. Cortical plasticity may be observed several years after amblyopia treatment.

CONCLUSION

Our study substantiates the importance of an effective and early treatment of functional amblyopia, inducing cortical plasticity with reinforced attention and sensitivity to visual perception.

Medical treatment of amblyopia: present state and perspectives

A brief review is provided on the role of experimental modulation of the visual system during its plastic period. The importance of several substances has been demonstrated. Potential clinical applications of available studies on the treatment of amblyopia are summarized. The role of the dopaminergic system and particularly of dopamine in amblyopia are also discussed. The results of the use of cytidine-5'-diphosphocholine (citicoline) in amblyopia are evaluated in detail. The advantages of and indications for an association of citicoline with part-time occlusion are also discussed. Finally, new administration modalities of citicoline, more detailed evaluation of its efficacy, and the use of new substances are briefly analyzed.

The effects of L-dopa on the functional magnetic resonance imaging response of patients with amblyopia: a pilot study

PURPOSE

We investigated the effects of a single dose of levodopa (L-dopa) on the level and extent of visual cortex activation of subjects with amblyopia and normal subjects using blood oxygenation level-dependent functional magnetic resonance imaging (fMRI).

SUBJECTS AND METHODS

Six patients with amblyopia and 9 control patients were recruited. A baseline fMRI session was followed by a second session 90 minutes after the dose of L-dopa. Visual stimuli included vertical sinusoidal gratings with spatial frequencies of 1 and 2 cpd that were counterphased at 4 Hz. Stimuli were presented monocularly and binocularly. The fMRI response was characterized by the total volume and the average level of activation within the occipital cortex. An interocular absolute difference (IDIF) was defined in terms of said measures for between-population analysis of monocular data.

RESULTS

After the administration of L-dopa, visual acuity improved significantly ( P = 0.026) from 0.72 +/- 0.21 (mean +/- SD) to 0.64 +/- 0.24 LogMAR in the amblyopic eye, although remaining the same in the dominant eye and in the eyes of control subjects. The response to L-dopa was found to be population-specific, as indicated by a significant treatment-by-population interaction for the volume of activation IDIF ( P = 0.018) measure: subjects with amblyopia exhibited a post-treatment increase in the volume of activation IDIF whereas control subjects showed a less prominent decrease. This post-treatment increase of IDIF in subjects with amblyopia may be explained by the decrease in the volume of activation found for the amblyopic eye after L-dopa ( P = 0.038). No L-dopa-related activation changes were detected for dominant eye or binocular stimulation in the amblyopic group, and no change was detected in control subjects.

CONCLUSION

L-dopa elicits a population-specific modulation of the fMRI response, namely, a reduced total volume of activation from the amblyopic eye despite improvement in visual acuity.

Amblyopia treatment studies

PURPOSE OF REVIEW

Amblyopia continues to be a major cause of vision loss in children. Several different treatment modalities exist. Research in the field of amblyopia has been very active over the past few years, and clinical practice patterns are changing.

RECENT FINDINGS

Atropine has been found to be effective in the treatment of amblyopia. Less intense patching regimens have been found to be effective as well. Many other aspects of amblyopia therapy are being investigated.

SUMMARY

Herein are current reviews of the major clinical amblyopia studies. Various treatment options are reviewed, and controversies in the field are discussed.