Potential mechanism of over / under actions of inferior oblique muscles
Reviewed by Fiona Rowe

In this paper, the authors hypothesise that fusional vergence adaptation, in patients who can fuse in at least some gaze positions, can cause curvature of the non-fixing eye movement paths of patients with apparent overaction or under action of the oblique muscles, yielding possibly inaccurate evidence of hypertonicity as the cause of over / under actions. This was a retrospective study which presents four cases - two with and two without fusion. None had surgical correction. The authors postulate that shortened inferior oblique (IO) muscles hold the globes in extorted positions and the apparent IO overaction is primarily from the eyes following their extorted movement paths into side gaze, not requiring abnormally increased tonicity in adduction. They acknowledge the limitations of sample size, heterogenous patient characteristics, this retrospective design and selection bias. They recommend a prospective case control study with objective assessment.

Inferior oblique muscle overaction caused by inferior oblique muscle shortening, not by hypertonicity.
Lagstein O, Guyton DL.
JOURNAL OF PEDIATRIC OPHTHALMOLOGY AND STRABISMUS
2022;59(1):28-34.