The 3-Step Test
There are 8 cyclovertical extraocular muscles (4 in each eye): the 2 depressors of each eye are the inferior rectus (IR) and superior oblique (SO) muscles; the 2 elevators of each eye are the superior rectus (SR) and inferior oblique (IO) muscles. Cyclovertical (especially superior oblique) muscle weakness often causes vertical deviations. The 3-step test (also called the Parks-Bielschowsky 3-step test) is an algorithm that helps identify a weak cyclovertical muscle. However, it is not always diagnostic, and results can be misleading, especially in patients with 1 of the following: more than 1 paretic muscle, previous strabismus surgery, skew deviation, or restrictions or dissociated vertical deviation (see Chapter 11). The 3-step test is performed as follows (Fig 7-8; see also Chapter 11, Fig 11-4):
Step 1: Determine which eye is higher using the cover-uncover test (see Fig 7-1). Step 1 narrows the number of possible underacting muscles from 8 to 4. In the example shown in Figure 7-8, the right eye is higher than the left eye. This indicates weakness in 1 of the 2 depressors of the right eye (RIR, RSO) or 1 of the 2 elevators of the left eye (LIO, LSR). Draw an oval around these 2 muscle groups (see Fig 7-8A).
Step 2: Determine whether the vertical deviation is greater in right gaze or in left gaze. In the example, the deviation is larger in left gaze. This implicates 1 of the 4 vertically acting muscles used in left gaze. Draw an oval around these (see Fig 7-8B). At the end of step 2, the 2 remaining possible muscles (1 in each eye) are either both intortors or both extortors and are either both superior or both inferior muscles (1 rectus and 1 oblique). In the example shown in Figure 7-8B, the increased left-gaze deviation eliminates 2 inferior muscles and implicates 2 superior muscles.
Step 3: Known as the Bielschowsky head tilt test, the final step involves tilting the head toward the right shoulder and the left one during distance fixation. Head tilt to the right stimulates intorsion of the right eye (RSR, RSO) and extorsion of the left eye (LIR, LIO). Head tilt to the left stimulates extorsion of the right eye (RIR, RIO) and intorsion of the left eye (LSR, LSO). Normally, the 2 intortors and the 2 extortors of each eye have opposite vertical actions that cancel one another. If 1 intortor or 1 extortor is weak, the vertical action of the other ipsilateral torting muscle becomes manifest during the torsional response to head tilt.
In the example shown in Figure 7-8C, the right hypertropia increases when the head is tilted to the right. This suggests that the vertical action of the right superior rectus muscle is unopposed, causing the right eye to move upward as it attempts to intort to maintain fixation; this indicates that the right superior oblique muscle is the weak muscle. (See also Chapter 11, Fig 11-3.)
Figure 7-8 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.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.