Amblyopia: what else beyond patching and critical period?

Amblyopia has been conventionally defined as “a unilateral or bilateral decrease of visual acuity (VA) caused by pattern deprivation or abnormal binocular interaction, for which no cause could be detected by the physical examination of the eye and which, in some cases, could be reversed by therapeutic measures”. Recent understanding emphasizes that, amblyopia could be redefined as a syndrome, “a visual cacophony of deficits in contrast sensitivity, spatial localization, fixation, ocular motility, accommodation, crowding, attention, motion perception and temporal processing in addition to VA loss”. This article aims to review the understanding of amblyopia from the developments in literature in the view of amblyopia mechanisms, treatment and future directions. Amblyopia is the major cause of defective vision in the young with large population studies showing an amblyopia prevalence of 1.6–3.6% with higher rated in medically underserved population. Amblyopia has traditionally been classified as strabismic (SA), anisometropic (AA), refractive or deprivational according to the accompanying conditions thought to be responsible for the acuity loss. There is converging evidence in literature that suggests that amblyopia might be more correctly classified in terms of the visual and oculomotor disturbances noted. Amblyopia can be defined as a developmental abnormality of the visual cortex. There may be a loss of connections (under-sampling) or a distortion or rearrangement of connections (neural disarray) within the visual cortex, and this depends on the etiology that caused the amblyopia and the central nervous system compensates to the loss by means of the above-mentioned mechanisms. The binocular interactions and the associated deficits are more apparent in the central visual field than the periphery and the inhibitory mechanisms are active under binocular viewing. The art of amblyopia treatment lies in balancing the conflicting demands of monocular and binocular vision. During the past 20 years, different critical periods have been demonstrated for different visual functions during the development of the visual system. Critical period is that sensitive period in the life span of an organism where the skills acquired are indispensable. It is believed that, visual functions processed at higher anatomical levels within the system have a later critical period than functions processed at lower levels. Binocular function and stereopsis, if disrupted within this sensitive period, are difficult to retrieve at later age. This general principle suggests that the treatment for amblyopia should follow a logical sequence, with treatment for each visual function starting before its critical period is over. Three periods in the development of the visual system in human infants have been postulated by Daw. These include a pre-stereoptic period (0–4 months), onset of stereopsis (4–6 months) and post-stereoptic period (6 months–2years). These functions are all plastic for a period after they first develop. Based on the understanding of critical period, it could be predicted that anomalous binocular visual experience during early infancy severely disrupts stereopsis. Fawcett et al. used a random dot stereo test to determine the critical period for stereopsis in infantile and accommodative strabismus in children <5 years of age and found that the critical period for susceptibility of stereopsis extends through late infancy and early childhood and continues to at least 4.6 years of age. In more than 200 years, there is better understanding of the neurophysiology and neuropathology of amblyopia than the past, though the treatment has not changed significantly. The mainstay of treatment still consists of forcing the use of the amblyopic eye, most often by occlusion of the sound eye and is probably the oldest treatment modality. Either Saint-Yves in 1722 or de Buffon (1743) is credited with the first documentation of occlusion treatment for amblyopia. There has been adequate research on standardizing the occlusion regimens that ranges from a few minutes of patching a day to all waking hours, often continuing for many months. More successful treatment strategies are being opted currently through well controlled experiments and neurophysiologic research, and recent randomized controlled trials have tried to answer the basic questions, “which of the two eyes should be occluded, and with what, and for how long?” There is a variety of evidence from the literature that the eye contralateral to the amblyopic eye, referred to as the good eye has subtle but measurable deficits in a wide variety of visual functions. In this context, we (Varadharajan & Hussaindeen, 2012) tried to understand the development of VA in the better eye of unilateral amblyopes. A total of 112 children with amblyopia were included (SA, 14; AA, 51; combined Elite School of Optometry, Srimathi Sundari Subramanian Department of Visual Psychophysics Sankara Nethralaya, Pediatric Ophthalmology and Binocular Vision Department, Units of Medical Research Foundation Chennai Birla Institute of Technology & Science, Pilani, Rajasthan