Strabismus can be comitant or incomitant. With comitant cases, the deviation of the eyes remains constant with changes in the angle of gaze. With incomitant cases, there is a change in the angle of deviation in different positions of gaze, or according to which eye is used for fixation.

Common examples of incomitance are the “alphabet” or “letter” patterns seen during ocular motility. By definition, a V-pattern esotropia leads to divergence on gazing up and convergence on gazing down, but is the result of anomalous muscle insertions or structures rather than any defect of the muscles or their nerve supply (McCullough, 2018a).

Congenital incomitances are often diagnosed in early childhood and include Duane retraction syndrome, a rare neurogenic condition that prevents the sixth cranial nerve from developing properly, and Brown’s syndrome, caused by abnormal development and function of the superior oblique muscle tendon (McCullough, 2018b). Note that acquired Brown’s syndrome can sometimes be caused by pathological inflammation or scarring of the trochlea or tendon.