About Parks 3 step test
Right vs. left head tilt, the paretic muscle can be detected. Potential paretic muscle: LSO, left superior oblique; LIR, left inferior rectus; RSR, right superior rectus; RIO, right inferior oblique. Reprinted with permission from (22).It has been proposed to add a fourth step, namely, measuring the vertical deviation in upgaze and downgaze (14). For example, with a right fourth nerve palsy, given that the superior oblique depresses the eye, the right hypertropia will increase in downgaze and lessen in upgaze. There are 3 important caveats regarding the Parks–Bielschowsky 3-step test: spread on commitance, the test's sensitivity, and the test's specificity. With a longstanding fourth nerve palsy, the amount of vertical deviation may become similar in all fields of gaze (12). This usually occurs when the fourth nerve palsy affects the dominant eye. For example, in a patient with a right fourth nerve palsy, the right inferior oblique (antagonist of the right superior oblique) requires less innervation to move the eye into its field of action. Following the Hering law, the left superior rectus (yoke muscle of the antagonist of the paretic muscle) also receives less innervation and will seem paretic. This will, in turn, diminish the amount of vertical misalignment. This has been referred to as inhibitional palsy of the contralateral antagonist. An alternate explanation for the spread of comitance may be found in the study by Suh et al (15). Using high-resolution orbital MRI, these investigators demonstrated that displacements in the pulley systems of the rectus muscles can alter motility patterns in superior oblique palsy. The Parks–Bielschowsky 3-step test lacks both sensitivity and specificity. In a series of 7 patients, Kushner (16) suggested that a number of other entities might simulate a fourth nerve palsy with the Parks–Bielschowsky 3-step test including paresis of more than one vertical muscle, dissociated vertical deviation, myasthenia gravis, and previous vertical muscle surgery. Using superior oblique atrophy on MRI as evidence of a fourth nerve palsy, Manchandia and Demer (17) performed the Parks–Bielschowsky 3-step test on 50 patients. They found that the test was diagnostic in only 70% of cases. By reducing the test to 2 steps, the sensitivity increased to 76%–84% but diminished the specificity. Lee et al (18) measured the sensitivity of the 3-step test, depending on the presence or absence of the fourth nerve using high-resolution thin-section MRI. Testing sensitivity was 78% in patients with a fourth nerve and 72% in those without a fourth nerve. There was no statistically significant intergroup difference. Taken together, these reports demonstrate that the Parks–Bielschowsky 3-step test is insensitive in 22%–30% of patients with fourth nerve palsy: CYCLOTORSION Because the primary action of the superior oblique muscle is incyclotorsion, detection of a torsional component of diplopia is helpful in establishing the diagnosis of fourth nerve palsy. In obtaining the history, the clinician should not only elicit the vertical orientation of the diplopic images, but also inquire if one of the images seems “tilted” or “slanted.” For confirmation and quantitation of cyclotorsion, a valuable technique is
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Toward the side of the palsied muscle, there was an increase in the vertical deviation, while tilting to the contralateral side caused either a decrease or resolution of the deviation. This became known as the Bielschowsky Head Tilt Test. Parks expanded on these observations and proposed a stepwise diagnostic schema in patients with vertical strabismus (13). As he pointed out, head tilt because of cyclovertical strabismus does not necessarily indicate binocular vision nor that the palsied muscle is an oblique. For example, head tilting may accompany a vertical rectus muscle palsy. In Parks' schema, the first step reduces the possibility from 8 to 4 cyclovertical muscles, the second step from 4 to 2, and the third step will determine which of the remaining 2 muscles is weak. Figures 4 and 5 illustrate the results of the Parks–Bielschowsky 3-step test for a right and left hypertropia, respectively. The first step determines that the paretic muscle is either one of 2 depressors in 1 eye or 1 of 2 elevators in the other. With the second step, one determines if the vertical deviation increases in right or left gaze. At this point, the 2 suspected muscles are in different eyes, both are always superior or inferior muscles and both are either incyclotorters or excyclotorters. In the third step, the paretic muscle is unable to perform its torsional and vertical actions. Yet, the muscle of the same eye that is able to perform the appropriate torsional movement will also move the eye vertically, thus increasing the vertical misalignment. For example, in a patient with a right fourth nerve palsy and the head tilted to the right, excyclotorsion will occur in the left eye because of contraction of both the left inferior oblique and left inferior rectus muscles. However, the paretic right superior oblique cannot balance the torsional and elevating activity of the right superior rectus, and the right eye will move upward leading to an increase in the vertical deviation (right hypertropia). With head tilt to the left, the paretic right superior oblique is not involved, and there will be either no increase in the vertical misalignment or it will diminish or no longer be detectable.FIG. 4.: Results of the Parks–Bielschowsky 3-step test are shown for a patient with a right hypertropia. In primary gaze, the vertical deviation may be due to paresis of 1 of 4 muscles. Increase in the deviation in right vs. left gaze reduces the possibility to 2 muscles and with measurement in right vs. left head tilt, the paretic muscle can be detected. Potential paretic muscle: LIO, left inferior oblique; LSR, left superior rectus; RIR, right inferior rectus; RSO, right superior oblique. Reprinted with permission from (22).FIG. 5.: Results of the Parks–Bielschowsky 3-step test are shown for a patient with a left hypertropia. In primary gaze, the vertical deviation may be due to paresis of 1 of 4 muscles. Increase in the deviation in right vs. left gaze reduces the possibility to 2 muscles and with measurement in
