Underlying neurological mechanisms associated with symptomatic convergence insufficiency

Convergence insufficiency (CI) is the most common binocular vision problem, associated with blurred/double vision, headaches, and sore eyes that are exacerbated when doing prolonged near work, such as reading. The Convergence Insufficiency Neuro-mechanism Adult Population Study (NCT03593031) investigates the mechanistic neural differences between 50 binocularly normal controls (BNC) and 50 symptomatic CI participants by examining the fast and slow fusional disparity vergence systems. The fast fusional system is preprogrammed and is assessed with convergence peak velocity. The slow fusional system optimizes vergence effort and is assessed by measuring the phoria adaptation magnitude and rate. For the fast fusional system, significant differences are observed between the BNC and CI groups for convergence peak velocity, final position amplitude, and functional imaging activity within the secondary visual cortex, right cuneus, and oculomotor vermis. For the slow fusional system, the phoria adaptation magnitude and rate, and the medial cuneus functional activity, are significantly different between the groups. Significant correlations are observed between vergence peak velocity and right cuneus functional activity (p = 0.002) and the rate of phoria adaptation and medial cuneus functional activity (p = 0.02). These results map the brain-behavior of vergence. Future therapeutic interventions may consider implementing procedures that increase cuneus activity for this debilitating disorder.

Vergence Eye Movements

This chapter reviews the stimuli for vergence, the properties of fusional and accommodative vergence, as well as vergence made in combination with saccades or vestibular eye movements, or blinks. Different properties of horizontal, vertical, and torsional vergence are discussed. Current models are presented to account for interactions between vergence and saccades. The neural substrate for vergence movements is reviewed from ocular motoneurons to the midbrain supraoculomotor area, to visual cortical areas such as MST and frontal eye field, including pontine nuclei, cerebellar vermis, and fastigial nucleus. Adaptive properties of vergence are reviewed, especially phoria adaptation, discussing the role of the cerebellum. The bedside and laboratory evaluation of vergence is summarized and the pathophysiology of disorders of vergence discussed, including developmental disorders associated with childhood strabismus and acquired disorders such as convergence spasm, convergence insufficiency, vergence forms of nystagmus such as convergence-retraction nystagmus, and effects of focal pontine lesions.