tonic convergence
Tonic convergence is the convergence that brings the eyes from their anatomical position of rest, generally believed to be one of divergence, into the physiologic position of rest.1 The extraocular muscles are never without electrical activity in the awake person, and their tonus presumably produces tonic convergence as an inherent innervational tone to the medial recti.1,2
Tonic convergence is one of four components of reflex convergence, along with accommodative, fusional and proximal convergence.1,2 In the clinic, the distance phoria stands for tonic vergence, and one classification of binocular vision disorders pairs the distance phoria with the AC/A ratio to give nine possible diagnoses.3
More detail
Without quantitative data on the absolute position of rest, tonic convergence cannot be stated in numbers. In the visually adult person, tonic convergence appears to remain rather stable throughout life, and its apparent increase after sustained fixation reflects slow decay of fusional vergence rather than a change in tonic innervation.1 Ethanol increases convergence at distance, an effect read as an increase in tonic convergence.1
The existence of tonic convergence has been questioned, on the argument that no vergence mechanism operates without retinal stimuli, but the evidence appears to favor it.1 Excessive tonic convergence has been proposed as the only explanation for a nonaccommodative convergence excess esotropia with a normal or low AC/A ratio, and it has been blamed for the recurrence of some comitant esotropias after repeated maximal surgery.1 Nothing is known about the source and trigger mechanism of this innervation, yet the concept remains useful in the clinic.1
Sources (5)
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 5
- Kanski, Clinical Ophthalmology (2024), ch. 18
- Scheiman and Wick, Clinical Management of Binocular Vision, ch. 2
- Cacho-Martínez, P., García-Muñoz, Á., & Ruiz-Cantero, M. T. (2016). Extraocular Muscles Tension, Tonus, and Proprioception in Infantile Strabismus: Role of the Oculomotor System. BioMed Research International, 2016, 1-11. https://doi.org/10.1155/2016/6965658
- Rouse, M. W., Borsting, E. J., Mitchell, G. L., Scheiman, M., Barnhardt, C., Berman, E., ... & Kulp, M. T. (2004). Accommodative and vergence dysfunction. Optometric Clinical Practice Guidelines, 1-57. https://www.aoa.org/AOA/Documents/Practice%20Management/Clinical%20Guidelines/Consensus-based%20guidelines/Care%20of%20Patient%20with%20Accommodative%20and%20Vergence%20Dysfunction.pdf
Reviewed October 2026