Standard Amblyopia Therapy in Adult Amblyopes

Functional amblyopia is an optically uncorrectable decrease in visual acuity, i.e., spatial resolution, with no apparent pathological or morphological cause. There are several etiological origins for amblyopia in the human population; anisometropia and strabismus are the two most prevalent etiological factors. Anisometropic amblyopia is a decrease in visual acuity (spatial resolution) that is the result of a large difference in refractive error between the two eyes. Strabismic amblyopia is characterized by an eye misalignment early in life which is associated with a decrease in visual acuity.

The initial site of the defect in amblyopia is the primary visual cortex. The amblyopic eye is placed at a competitive disadvantage to the nonamblyopic eye which results in a reduction in the synapses or cells responding to the amblyopic eye. The conditions associated with amblyopia must be present prior to the end of the critical period of neural development for amblyopia to develop. The neurophysiological systems of anisometropic and strabismic monkeys have differences in the distributions of binocular cortical cells. Anisometropic amblyopic monkeys only exhibit binocular cells that are tuned to low spatial frequencies. If a neuron has a high spatial resolution, it is unlikely that the neuron will be binocular. However, strabismic amblyopic monkeys exhibit few binocular cells and, moreover, there does not appear to be a spatial frequency dependent distribution of these binocular cells. The strabismic subjects typically displayed few binocular cells, regardless of the spatial frequency tuning of the cell.

If the amblyopic eye is compared to the nonamblyopic eye, two important differences are noted. First, the amblyopic eye has a lower contrast sensitivity at high spatial frequencies (and sometimes at all spatial frequencies) than the nonamblyopic eye. This also results in a decrease in visual acuity for the amblyopic eye. Second, the amblyopic eye demonstrates a longer latency for neural transmission than the nonamblyopic eye.

There is very little published on adult amblyopia therapy. This is because clinicians initially believed that amblyopia could not be treated after the end of the critical period. The prevailing theory was that the synaptic contacts between cells could only be modified during the critical period. In children, the majority of the improvement in acuity (80%) takes place in the first 6 weeks of therapy. There is a direct relationship between the hours of patching and the improvement in acuity. Most of the improvement in acuity occurs after the first 100 hours of patching.

There is a linear dose-response function for amblyopia treatment. The logMAR acuity increases 0.1 log unit or 1 chart line per 120 hours of patching. The response does not differ between the types of amblyopia. It has also been demonstrated that there is only a minimal benefit to patching more than 2 hours/day.