Errors in the three-step test in the diagnosis of vertical strabismus.

The Parks three-step test is the standard for diagnosing which isolated cyclovertical muscle is palsied. It does not, however, tell the examiner if in fact one is dealing with a palsy of one cyclovertical muscle. Numerous other causes of vertical strabismus may have a positive Bielschowsky head tilt test. The use of the three-step test in these clinical situations may lead to incorrect diagnosis and treatment. These clinical conditions include contracture of the vertical recti, paresis of more than one vertical muscle, dissociated vertical divergence, previous vertical muscle surgery, skew deviation, myasthenia gravis, and small nonparalytic vertical deviations associated with horizontal strabismus. Several diagnostic steps in addition to the three-step test are necessary to tell if one is in fact dealing with a palsy of a single cyclovertical muscle.