Park's 3-step test is used to evaluate palsy of which cranial nerve?

• A. Cranial nerve III palsy
• B. Cranial nerve IV palsy
• C. Cranial nerve VI palsy
• D. Cranial nerve II palsy

Answer: (B)

Park's three-step test is used to pinpoint which cyclovertical muscle is underactive, with a special focus on diagnosing trochlear nerve (cranial nerve IV) palsy. The superior oblique, the muscle innervated by CN IV, mainly depresses the eye when it is adducted and also intorts it. When this muscle is weak, the eye tends to sit higher in primary gaze because the depressor is lost. In the test, you first note which eye is hypertropic in straight-ahead gaze. Then you see how the vertical misalignment changes when the patient looks to either side—in trochlear palsy, the hypertropia worsens when the affected eye is adducted. Finally, you assess the pattern with head tilt; the misalignment typically worsens when tilting the head toward the side of the affected muscle. The combination of these findings consistently points to a palsy of the superior oblique, i.e., dysfunction of the trochlear nerve. Other cranial nerves produce different patterns of misalignment that Park’s test does not localize as specifically.