Fusion Influences the Bielschowsky Head Tilt Test in Superior Oblique Paresis

Five patients used the vertical recti for vertical fusional vergence and showed a mean BHTTD - SD of 22.2 - 8.1 PD. After a 30-minute patch test one of those (the only one in whom the test was performed) showed a decrease of 1 PD in BHTTD. Two patients used the "paretic" superior oblique muscle (SOM) and the contralateral superior rectus muscle (SRM) to fuse, and had a mean BHTTD - SD of 5.5 - 7.8 PD. The BHTTD of one of these, in whom a patch test was performed, increased by 11 PD. The remaining patient used the oblique muscles ("paretic" SOM and contralateral inferior oblique muscle (IOM)) to fuse, and showed a BHTTD of only 3 PD, increasing to 21 PD after patching. One explanation for this behavior in the last patient involves lingering vergence adaptation of the "paretic" SOM and contralateral IOM, which makes these muscles more effective when activated on ipsilateral head tilt, lessening the expected increase in hyperdeviation. Similarly, in our two patients with oblique/rectus-mediated fusion, the verge ce-adapted "paretic" SOM and contralateral SRM are activated on ipsilateral and contralateral head tilt respectively, lessening the hyperdeviation in both directions. In the other five patients, however, the vergence-adapted ipsilateral inferior rectus muscle and contralateral SRM are activated on contralateral tilt, possibly accentuating the BHTTD.