The Parks–Bielschowsky three-step test,[1] also known as Park's three-step test or Bielschowsky head tilt test,[2] is a method used to isolate the paretic extraocular muscle, particularly superior oblique muscle and trochlear nerve (fourth cranial nerve),[3] in acquired vertical double vision.
[6] When there is a discrepancy in ocular deviation based on which way the head is tilted, the patient is diagnosed with unilateral palsy of the superior oblique muscle due to damage in the Trochlear Nerve.
[citation needed] People with superior oblique palsy on one side experience double vision, which is improved or even abolished by tilting the head towards the shoulder on the unaffected side.
Tilting the head towards the shoulder on the affected side will make the double vision worse by causing increased separation of the two images seen by the patient.
[citation needed] Lateralization of side of defect based on Parks-Bielschowsky three-step test: The physiologic basis of the head tilt test was explained by Alfred Bielschowsky and Hofmann[8] in 1935.