Forced duction, active force generation, and saccadic velocity tests

The forced duction test reveals information about mechanical limitations to full ocular rotation. When voluntary ocular rotation is limited, and the forced duction test is completely free, paresis of an extraocular muscle is suggested. The active force generation test and saccadic velocity measurements both provide information about the active forces available to move the globe. Active force can be felt with the forceps in the force generation test. These forces can be measured quantitatively with the calibrated Scott forceps. The saccadic velocity test measures the work an extraocular muscle performs (the eye movement) but does not measure active force directly. The force available is inferred by comparing saccadic velocity measurements to normal control values. This test is especially useful in infants and children, in whom the force duction test cannot be done without general anesthesia, and whose cooperation is insufficient to allow force generation measurements to be performed. It is likely that future improvements in instrumentation and further knowledge of basic oculomotor mechanisms will increase the value of these techniques in clinical strabismus.