This is a continuation of the concept of how patients diagnosed with the umbrella term of “convergence insufficiency” can (somewhat paradoxically) show esophoria at near under certain conditions. We laid the groundwork for this in an original post in 2015, followed by a redux in 2016. I didn’t anticipate adding anything to that until I came across a monograph on Visual Function while spring cleaning recently, authored by the late, great Daniel Woolf, O.D., Ph.D.
When I first arrived at SUNY in 1982, recruited to serve as Chief of VT Services, Dr. Woolf still had an office on the second floor though with one foot out the door. I regrettably didn’t have the opportunity to chat with him much, but I knew that he was held in high regard. The monthly papers he wrote for OEP in the mid 1960s still hold up well 55 years later. As an aside, many of these papers are archived online, though not the ones of interest here.
The intriguing byline on this particular set of papers reads: “A basic course that will attempt to create a framework within which the concepts of both the Optometric Extension Program and conventional optometry can be placed.” And within that framework, Dr. Woolf posed the question in his seven part series: What’s Become of Convergence Insufficiency?
In Part 5, Dr. Woolf poses this provocative question: Can convergence insufficiency reflect a developmental problem? After reviewing considerable literature, Dr. Woolf concludes: “Thus we find that convergence insufficiency is not a failure to develop convergence, but a failure to maintain facility in convergence for near objects while the child is developing abilities for distance vision. We have illustrated in previous papers the need to develop inhibition of convergence for achievement in reading.”
Apparently Dr. Woolf monitored binocularity while children were reading passages in the Ophthalmograph, a very mid-20th century sci-fi-looking-gizmo that filmed eye movements binocularly.
There is no reference to these papers that I can locate, but in Part 6 Dr. Woolf elaborates about esophoria frequently exhibited during reading by children as measured by the Ophthalmograph, who are exophoric at near on standard clinical tests and identified as convergence insufficiency cases. He writes: “Another example concerns the peculiar and unexpected occurrence while reading of overconvergent disparity at the beginning of each line of print, in cases that clearly test exophoric on near phoria tests … This situation points up the error in assuming that the exophore in the chair is an exophore reading.”
The Ophthalmograph hasn’t survived as a clinical tool, but the idea of approximating binocular projection while reading, rather than in a state of rest (as is phoria) is reflected in the Mallett target still commonly used in the UK, its country of origin. As you can see, the green polarized fixation disparity nonius lines are embedded within an appropriately crowded array of print. If fixation disparity is easily nudged toward zero, particularly if stability and symmetry is readily attained through lenses and/or prisms, then this is the preferred initial treatment before vision therapy.
Dr. Woolf concluded that convergence insufficiency is best thought of as a condition with several subtypes with its origins either accommodative, developmental, attentional, or stress-related in nature. The closest we have come to preserving this is the notion of “pseudo-convergence insufficiency” which acknowledges that in some cases the primary source of convergence difficulty is rooted in the accommodative system. But the use of fixation disparity principles is making a comeback, and can help explain why base-in prism does not help some cases of CI, as noted by Dr. Woolf, or paradoxically why base-out, yoked prism, or plus lenses do help. These are the patients who exhibit over-convergence while reading even though they do not show esophoria on conventional measures such as cover test or dissociated phoria testing at near.