The 3-step test. The cyclovertical muscles are represented in their fields of action. A, Step 1: Right eye higher than left suggests weakness in 1 of the 2 depressors of the right eye (RIR or RSO) or in 1 of the 2 elevators of the left eye (LIO or LSR). B, Step 2: If the deviation furthermore worsens on left gaze, this implicates either the RSO or the LSR. Note that at the end of step 2, 1 depressor and 1 elevator of opposite eyes will be identified as the possible weak muscle. C, Step 3: If the right eye is furthermore higher in right head tilt than left head tilt, there is weakness of the RSO: right head tilt induces intorsion of the right eye, which depends on activation of both the RSO (a depressor) and the RSR (an elevator), and extorsion of the left eye, from activation of both the LIO (an elevator) and the LIR (a depressor). This rules out the LSR (which was still a candidate at the end of Step 2), and identifies the RSO as the weak muscle. LIO = left inferior oblique; LIR = left inferior rectus; LSR = left superior rectus; RIR = right inferior rectus; RSO = right superior oblique; RSR = right superior rectus.