Diagnostic Utility of the Three-Step Test According to the Presence of the Trochlear Nerve in Superior Oblique Palsy

BACKGROUND AND PURPOSE To determine the diagnostic utility of the three-step test in unilateral superior oblique palsy (SOP) according to the presence of the trochlear nerve using high-resolution thin-section magnetic resonance imaging. METHODS In total, 166 patients with congenital and acquired unilateral SOP were included, comprising 87 with a normal trochlear nerve (present group) and 79 without a trochlear nerve (absent group). The sensitivity of each component of the three-step test was evaluated as well as factors related to the sensitivity. RESULTS All three steps were positive in 78% of those in the present group and 72% of those in the absent group, demonstrating no intergroup difference (p=0.471). Superior rectus muscle (SR) contracture was more frequent in patients who exhibited at least one negative step (incomplete group) (p=0.014). The incomplete group was significantly related to larger hypertropia in ipsilateral gaze (p<0.001), smaller hypertropia in contralateral gaze (p<0.001), and smaller differences in hypertropia between ipsilateral head tilt and the primary position (p=0.012). CONCLUSIONS The diagnostic utility of the three-step test in unilateral SOP did not differ according to the presence of the trochlear nerve. SR contracture was the main cause of exceptions of the three-step test in unilateral SOP.