Patients with long-standing unilateral strabismus, such as "sensory" exotropia in the absence of fusion, or esotropia with unilateral amblyopia, typically show bilateral deviations under anesthesia, often symmetric. Forced ductions usually show symmetric muscle tightness. Changes in extraocular muscle lengths thus appear to occur primarily bilaterally, whether fusion is present or not. With skeletal muscles responding to changes in stimulation by the gain or loss of sarcomeres, it is likely that abnormal or unguided vergence tonus, which changes the lengths of the extraocular muscles bilaterally, is largely responsible for changes in the angle of strabismus over time. This mechanism helps explain the development of [1] increasing "basic" deviations in accommodative esotropia; [2] torsional deviations with apparent oblique muscle "overaction/underaction" and A and V patterns; [3] recurrent esotropia with early presbyopia; [4] occasional divergence insufficiency in presbyopes; and [5] basic cyclovertical deviations that mimic superior oblique muscle paresis.