With many professionals now becoming aware of and learning more about functional visual deficits, I think it’s important to take time to discuss and outline one of the most common eye-teaming deficits that we see here at the office. As this particular condition can have widespread implications and greatly affect visual performance, understanding how it is properly diagnosed is extremely valuable for many to understand.
Convergence is part of our visual system’s near-vision complex and depends on the integrative function of both cortical and subcortical regions on our brain. Convergence insufficiency is a condition where a patient presents with a reduced ability for their eyes to turn inwards (converge) towards each other, and thus sustain binocular (two-eyed) alignment at near. Common presenting symptoms for someone with Convergence Insufficiency are typically related to prolonged, visually demanding, near centred tasks – although distance complaints are also common. Double vision, eye-stain, blurred vision, loss of place, headaches, closure of one eye or anomalous head postures while reading may all be signs that a deficiency in convergence exists. This diagnosis can come about as a developmental finding in children, but also very commonly following acquired brain injury.
For someone outside of the Optometric profession, diagnosis of Convergence Insufficiency may appear to be a rather straight forward task – have an individual track a pen inwards, and if they report doubling or you see an eye drift when the pen is further than 10cm from their eyes, they have Convergence Insufficiency. Easy! … well, sort of. When we look further into how a diagnosis of Convergence Insufficiency is actually made, there are quite a number of additional factors to consider. The first of these factors is what we call dissociation. When we look at the landmark study of Convergence Insufficiency (http://www.convergenceinsufficiency.org/pdf/CITT_children_Scheiman.pdf), the determination of whether someone was effectively able to converge was actually done while wearing red-green (or dissociating) glasses and a penlight. Why you ask? Well, when someone is converging, they may not actually be able to effectively report to you when doubling is occurring without this dissociation – thus giving the examiner a false negative finding. As a note, once dissociated, tests of convergence should also be performed multiple times in order to determine how someone’s eyes respond to repeated visual stress (2-3x is typically common).
In addition to dissociation, an awareness of what we call one’s “visual posture” is extremely important to know when assessing convergence. When looking at objects, individuals will tend to be focused slightly behind the target (exophoric posture) or slightly in front of the target (esophoric posture). This finding is somewhat embedded into how one visually functions and so is not typically an overly variable finding (although Vision Therapy can change this!). Knowing whether someone is esophoric or exophoric (and how esophoric or exophoric one is) plays into the next critical tool for assessment – fusional vergence ranges. As Optometrists, we have the unique ability to apply “pressure” to one’s visual system and evaluate how one responds. By adding what we call “stressing prism” in front of the eyes and relating this to the aforementioned visual posture, one can determine how robust and effective the vergence system truly is. Much like fitness is not evaluated by asking someone to do one or two push-ups or pullups at the gym, the ability to perform convergence tasks is not best assessed by tracking a pen inwards 2-3 times. You have to evaluate the system under stress – and measure how much pressure is needed to break eye teaming down.
In summary of the paragraph above – one cannot effectively assess how well someone converges their eyes simply by watching them converge their eyes. This is actually a pretty powerful statement and speaks to the many additional tools that we as Optometrists possess for in-depth functional visual evaluation. It’s important to also note that Convergence Insufficiency as a diagnosis does not typically present in isolation. Most individuals with Convergence Insufficiency will also present with additional functional visual deficits that need addressing in unison. While these additional findings, such as accommodative or visual perceptual deficits will be covered in future blogs, it’s important to be aware of their existence when considering a diagnosis of Convergence Insufficiency. For professionals uncertain about whether referral to an Optometrist for in depth assessment of convergence is necessary, the Convergence Insufficiency Symptom Survey (CISS) tends to be a great tool to add to your practice. Scores above 20-25 will typically warrant further assessment and thus prompt referral. If a deficit in convergence is uncovered, then an effective, personalized Vision Therapy program can be created to remediate the deficit and have the individual performing far more efficiently and without visual strain.
Until next month,
Paul Rollett, OD