Pharmacological enhancement of treatment for amblyopia

Levodopa/Carbidopa to Augment the Treatment of Amblyopia: A Report by the American Academy of Ophthalmology

PURPOSE

To review the published literature on the use of levodopa/carbidopa to augment the treatment of amblyopia.

METHODS

Literature searches for English language studies were last conducted in October 2022 in the PubMed database with no date restrictions. The combined searches yielded 55 articles, of which 23 were reviewed in full text. Twelve of these were considered appropriate for inclusion in this assessment and were assigned a level of evidence rating by the panel methodologist. Nine studies were rated level I, and 3 studies were rated level II; there were no level III studies.

RESULTS

The duration of treatment was limited to 3 to 16 weeks because of concern about long-term adverse effects such as tardive dyskinesia. This complication was not reported in any of the study participants. The dose of levodopa ranged from 1.5 to 8.3 mg/kg/day, generally divided into 3 daily doses. The carbidopa dose was approximately 25% of the levodopa dose in all treatments. Evidence from these studies indicates that augmenting traditional patch occlusion therapy with the oral administration of levodopa/carbidopa can improve the vision of amblyopic children, but the effect was small (0.17-0.3 logarithm of the minimum angle of resolution [logMAR] units) and only statistically significant when compared with patching alone in 2 of the 12 studies cited. Regression of vision was reported in the majority of studies (9 of 12 reported; range, 0-0.17 logMAR unit regression) after discontinuation of therapy. Short-term side effects of the medications were not consistently reported but were most frequently mild and included headache and nausea.

CONCLUSIONS

The best available evidence is currently insufficient to show that augmenting amblyopia therapy using up to 16 weeks of levodopa/carbidopa will result in meaningful improvement in visual acuity. Given the potential for significant side effects such as tardive dyskinesia with long-term therapy, levodopa/carbidopa does not appear to be a viable option for amblyopia therapy FINANCIAL DISCLOSURE(S): The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Characterization, passive and active treatment in strabismic amblyopia: a narrative review

Strabismic amblyopia is characterized by a distorted spatial perception. In this condition, the neurofunctional disorder occurring during first years of life provoke several monocular and binocular anomalies such as crowding, deficits in the accommodative response, contrast sensitivity, and ocular motility abilities. The inhibition of the binocular function of the brain by the misaligned amblyopic eye induces a binocular imbalance leading to interocular suppression and the reduction or lack of stereoacuity. Passive treatments such as occlusion, optical and/or pharmacological penalization, and Bangerter foils has been demonstrated to be potentially useful treatments for strabismic amblyopia. Recent researches have proved new pharmacological options to improve and maintain visual acuity after occlusion treatment in strabismic amblyopia. Likewise, the active vision therapy, in the last years, is becoming a very relevant therapeutic option in combination with passive treatments, especially during and after monocular therapy, in the attempt of recovering the imbalanced binocular vision.

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.

Cognitive processing of orientation discrimination in anisometropic amblyopia

Cognition is very important in our daily life. However, amblyopia has abnormal visual cognition. Physiological changes of the brain during processes of cognition could be reflected with ERPs. So the purpose of this study was to investigate the speed and the capacity of resource allocation in visual cognitive processing in orientation discrimination task during monocular and binocular viewing conditions of amblyopia and normal control as well as the corresponding eyes of the two groups with ERPs. We also sought to investigate whether the speed and the capacity of resource allocation in visual cognitive processing vary with target stimuli at different spatial frequencies (3, 6 and 9 cpd) in amblyopia and normal control as well as between the corresponding eyes of the two groups. Fifteen mild to moderate anisometropic amblyopes and ten normal controls were recruited. Three-stimulus oddball paradigms of three different spatial frequency orientation discrimination tasks were used in monocular and binocular conditions in amblyopes and normal controls to elicit event-related potentials (ERPs). Accuracy (ACC), reaction time (RT), the latency of novelty P300 and P3b, and the amplitude of novelty P300 and P3b were measured. Results showed that RT was longer in the amblyopic eye than in both eyes of amblyopia and non-dominant eye in control. Novelty P300 amplitude was largest in the amblyopic eye, followed by the fellow eye, and smallest in both eyes of amblyopia. Novelty P300 amplitude was larger in the amblyopic eye than non-dominant eye and was larger in fellow eye than dominant eye. P3b latency was longer in the amblyopic eye than in the fellow eye, both eyes of amblyopia and non-dominant eye of control. P3b latency was not associated with RT in amblyopia. Neural responses of the amblyopic eye are abnormal at the middle and late stages of cognitive processing, indicating that the amblyopic eye needs to spend more time or integrate more resources to process the same visual task. Fellow eye and both eyes in amblyopia are slightly different from the dominant eye and both eyes in normal control at the middle and late stages of cognitive processing. Meanwhile, abnormal extents of amblyopic eye do not vary with three different spatial frequencies used in our study.

Pharmacological therapy for amblyopia

Amblyopia is the most common cause of preventable blindness in children and young adults. Most of the amblyopic visual loss is reversible if detected and treated at appropriate time. It affects 1.0 to 5.0% of the general population. Various treatment modalities have been tried like refractive correction, patching (both full time and part time), penalization and pharmacological therapy. Refractive correction alone improves visual acuity in one third of patients with anisometropic amblyopia. Various drugs have also been tried of which carbidopa & levodopa have been popular. Most of these agents are still in experimental stage, though levodopa-carbidopa combination therapy has been widely studied in human amblyopes with good outcomes. Levodopa therapy may be considered in cases with residual amblyopia, although occlusion therapy remains the initial treatment choice. Regression of effect after stoppage of therapy remains a concern. Further studies are therefore needed to evaluate the full efficacy and side effect profile of these agents.

Amblyopia update: new treatments

PURPOSE OF REVIEW

This review article is an update on the current treatments for amblyopia. In particular, the authors focus on the concepts of brain plasticity and their implications for novel treatment strategies for both children and adults affected by amblyopia.

RECENT FINDINGS

A variety of strategies has been developed to treat amblyopia in children and adults. New evidence on the pathogenesis of amblyopia has been obtained both in animal models and in clinical trials. Mainly, these studies have challenged the classical concept that amblyopia becomes untreatable after the 'end' of the sensitive or critical period of visual development, because of a lack of sufficient plasticity in the adult brain.

SUMMARY

New treatments for amblyopia in children and adults are desirable and should be encouraged. However, further studies should be completed before such therapies are widely accepted into clinical practice.

Levodopa and Other Pharmacologic Interventions in Ischemic and Traumatic Optic Neuropathies and Amblyopia

The visual impairment in traumatic and ischemic optic neuropathy and amblyopia may be permanent. Hence, lots of efforts have been focused on neuroprotection. Dopamine is one of the suggested neuroprotective agents. Besides its important role in the brain, dopamine is found in various cell types of the retina, and is claimed to play a neuromodulator and neurotransmitter role there. The dopamine D1 receptor is the most highly expressed subtype of dopamine receptors, and its activation has been shown to be potentially neuroprotective against oxidative-stress damage in retinal neurons. Levodopa, a precursor of dopamine, can easily breach the blood-brain and blood-retinal barriers, and exerts effective dopaminergic responses in the brain and retina. This article summarizes and discusses the use of levodopa and other pharmacologic agents in the treatment of 3 groups of visual pathway disorders that primarily involve neuronal systems: ischemic optic neuropathy, traumatic optic neuropathy, and amblyopia.

Efficiency of the occlusion therapy with and without levodopa-carbidopa in amblyopic children-A tertiary care centre experience

OBJECTIVES

To assess the role, efficacy and tolerability of levodopa-carbidopa in the management of small and older children with different types of amblyopia.

METHODOLOGY

Prospective randomised placebo controlled clinical study, in which 50 amblyopic patients between 5 and 20 years of age with visual acuity (V/A) < 20/40 were included, was carried on. After having attained the best possible refractive correction, patients were randomly divided into 2 groups. They were prescribed levodopa-carbidopa (10:1) (4-6mg/kg/day in 2-3 divided doses) or placebo, plus full-time occlusion of the sound eye, for a period of three months. Assessment of improvement in V/A, compliance and tolerance was done at follow up visits. Data was analyzed using computer software Ms-Excel and Epi-Info Version 6.0. The statistical significance was assessed by Chi-Square/Fisher's Exact Test.

RESULTS

Visual acuity for the amblyopic eye improved significantly in both groups but there was significant improvement in group 1 than group 2 (P = 0.0001). In a subgroup of patients older than 12 years, levodopa group showed statistically significant improvement in baseline V/A (P = 0.0001). In patients with severe amblyopia, each group showed significant improvement in baseline V/A (p < 0.05), but was significantly more in group 1 (P = 0.0001). Compliance rates were similar among the groups and levodopa-carbidopa at a dose range of 4-6 mg/kg/day was well tolerated.

CONCLUSION

Levodopa-carbidopa can be used as an adjunct to conventional occlusion therapy in amblyopia particularly in older children and severe cases of amblyopia, and it is well tolerated.

Visual Acuity Improvement of Amblyopia in an Adult With Levodopa/Carbidopa Treatment

Amblyopia is the leading cause of visual loss in children, affecting 2% to 3% of the population. Occlusion of the dominant eye is the primary and best treatment, although efficacy decreases after 6 years of age. As a result, levodopa/carbidopa has been explored as an adjunct to conventional therapy and has been shown to have an immediate impact on visual acuity. Several studies to date have shown mixed results on the benefit of supplementing occlusion therapy with levodopa/carbidopa, although they have primarily studied children. The authors describe the oldest patient (46 years old) documented in the literature to have shown improvement in visual acuity using levodopa/carbidopa. He was given a 16-week course at a dose in line with previous studies while being effectively occluded full time due to a glaucomatous right eye with no light perception. On 3-month follow-up, his left eye improved two lines and stabilized at 6 months.

Relationship between monocularly deprivation and amblyopia rats and visual system development

OBJECTIVE

To explore the changes of lateral geniculate body and visual cortex in monocular strabismus and form deprived amblyopic rat, and visual development plastic stage and visual plasticity in adult rats.

METHODS

A total of 60 SD rats ages 13 d were randomly divided into A, B, C three groups with 20 in each group, group A was set as the normal control group without any processing, group B was strabismus amblyopic group, using the unilateral extraocular rectus resection to establish the strabismus amblyopia model, group C was monocular form deprivation amblyopia group using unilateral eyelid edge resection + lid suture. At visual developmental early phase (P25), meta phase (P35), late phase (P45) and adult phase (P120), the lateral geniculate body and visual cortex area 17 of five rats in each group were exacted for C-fos Immunocytochemistry. Neuron morphological changes in lateral geniculate body and visual cortex was observed, the positive neurons differences of C-fos expression induced by light stimulation was measured in each group, and the condition of radiation development of P120 amblyopic adult rats was observed.

RESULTS

In groups B and C, C-fos positive cells were significantly lower than the control group at P25 (P<0.05), there was no statistical difference of C-fos protein positive cells between group B and group A (P>0.05), C-fos protein positive cells level of group B was significantly lower than that of group A (P<0.05). The binoculus C-fos protein positive cells level of groups B and C were significantly higher than that of control group at P35, P45 and P120 with statistically significant differences (P<0.05).

CONCLUSIONS

The increasing of C-fos expression in geniculate body and visual cortex neurons of adult amblyopia suggests the visual cortex neurons exist a certain degree of visual plasticity.

Amblyopia treatment strategies and new drug therapies

Amblyopia is a unilateral or bilateral reduction of visual acuity secondary to abnormal visual experience during early childhood. It is one of the most common causes of vision loss and monocular blindness and is commonly associated with strabismus, anisometropia, and visual deprivation (in particular congenital cataract and ptosis). It is clinically defined as a two-line difference of best-corrected visual acuity between the eyes. The purpose of this study was to understand the neural mechanisms of amblyopia and summarize the current therapeutic strategies. In particular, the authors focused on the concept of brain plasticity and its implication for new treatment strategies for children and adults with amblyopia.

Environment and Brain Plasticity: Towards an Endogenous Pharmacotherapy

Brain plasticity refers to the remarkable property of cerebral neurons to change their structure and function in response to experience, a fundamental theoretical theme in the field of basic research and a major focus for neural rehabilitation following brain disease. While much of the early work on this topic was based on deprivation approaches relying on sensory experience reduction procedures, major advances have been recently obtained using the conceptually opposite paradigm of environmental enrichment, whereby an enhanced stimulation is provided at multiple cognitive, sensory, social, and motor levels. In this survey, we aim to review past and recent work concerning the influence exerted by the environment on brain plasticity processes, with special emphasis on the underlying cellular and molecular mechanisms and starting from experimental work on animal models to move to highly relevant work performed in humans. We will initiate introducing the concept of brain plasticity and describing classic paradigmatic examples to illustrate how changes at the level of neuronal properties can ultimately affect and direct key perceptual and behavioral outputs. Then, we describe the remarkable effects elicited by early stressful conditions, maternal care, and preweaning enrichment on central nervous system development, with a separate section focusing on neurodevelopmental disorders. A specific section is dedicated to the striking ability of environmental enrichment and physical exercise to empower adult brain plasticity. Finally, we analyze in the last section the ever-increasing available knowledge on the effects elicited by enriched living conditions on physiological and pathological aging brain processes.