The Binocular Syndrome of Amblyopia – Part 1

In May of this year, Dr. Dan Fortenbacher updated us on some of the latest research into amblyopia.  Extending that, I highly commend to you the September issue of Vision Research, an international journal for the functional aspects of vision.  The special issue is titled Amblyopia: A window into visual cortex development and recovery of vision.

Virtually every article is a gem, beginning with the Preface to this special issue by its Guest Editors,
Benjamin Thompson, Susana T.L. Chung, Lynne Kiorpes, Timothy Ledgeway, and Paul V. McGraw.  They state in the opening paragraph that amblyopia is characterized by a loss of visual acuity in the affected eye and impaired or absent binocular visual function.  This theme that amblyopia is a binocular problem recurs throughout the issue, and sets the stage for updated thinking regarding treatment that moves well beyond patching.  A few other statements and concepts caught my eye:

1. Amblyopia represents the most common cause of visual impairment in childhood and has a significant impact on qualify of life.
2. It is suggested that amblyopia may alter the white matter property of early visual pathways.
3. The impact of amblyopia is not limited to the primary visual cortex, but extends to extra striate visual cortex.
4. Suboptimal pooling of neural information from the amblyopia eye within V1 and V2 is a potential mechanism for a reduction in the relative proportion of signals to extrastriate areas.
5. Characteristics of fixational eye movement abnormalities limits visual stability and acuity.
6. Amblyopia disrupts visuo-motor coordination, and difficulties in acquiring reliable visual information regarding the shape and location of target objects appear to play a role in the visuo-motor deficits associated with amblyopia.
7. Impairments in motion-defined form perception and multiple object tracking occurs not only in the amblyopic eye, but in the fellow eye.
8. Crucially, these deficits were not improved by occlusion therapy, even when the acuity of the amblyopic eye improved significantly.
9. A binocular approach to amblyopia therapy, particularly when embedded in a video game format, resulted in significant improvements in visual acuity, stereopsis, contrast sensitivity and reading speed.
10. Taken together, these items underscore that amblyopia is more than an eye problem.  Amblyopia should be viewed as a syndrome manifest as a diffuse brain problem in the visual pathways leading to and beyond the striate cortex with disruption in overall performance.

Perhaps the strongest clinical article in this special issue is the first one, Amblyopia and the binocular approach to its therapy, by Robert Hess and Benjamin Thompson.  It is important to note that in their binocular approach to therapy, no patients have reported diplopia because they are always working under conditions where fusion is operating.  However as they note, their approach to date has been limited to patients with anisometropic amblyopia or small angle strabismic amblyopia characterized as less than 10 prism diopters.  It is Hess and Thompson who refer to amblyopia as a syndrome, and make the following statement:  “Suppression is an important part of the amblyopia syndrome and the positive correlation between suppression and the depth of amblyopia indicates that binocular dysfunction is the primary problem.”

I very much like the terminology that Hess and Thompson use in referring to their therapeutic approach as binocular re-balancing therapy.  They refer readers to earlier behavioral optometric approaches, referenced to an article I authored in the COVD journal in 1981.  I explored these connections a bit further here a few months ago.