Tilting of the head as an aid in diagnosis of paretic vertical muscles is based on the theory that torsional action of the vertical muscles maintains the vertical meridians of the eyes erect when the head is tilted to the side. Since rotation of the optic disc about the macula occurs in the same direction, approximately to the same degree as the head is tilted, torsion as a normal function of the vertical ocular muscles is challenged. In differentiating between paresis of a superior oblique muscle or the superior rectus of the opposite eye, since paresis of either muscle in diagnostic problem cases will usually demonstrate a positive test for both muscles, tilting of the head as a diagnostic procedure is unreliable and often misleading.
Levine MH. Evaluation of the Bielschowsky Head-Tilt Test. Arch Ophthalmol. 1969;82(4):433-439. doi:10.1001/archopht.1969.00990020435002