The influence of lingering fusional adaptation on the Bielschowsky head tilt test in superior oblique paresis

Purpose

To investigate how fusion influences the Bielschowsky head tilt test (BHTT) in unilateral superior oblique paresis (SOP).

Methods

In eight fusing patients, we correlated haploscopic-determined fusion mechanisms with BHTT differences (BHTTD).

Results

Five patients used the vertical recti for vertical fusional vergence and had a mean±SD BHTTD of 22 ± 8PD. After a 30-minute patch test one of those, in whom the test was performed, showed a decrease of 10PD. Two patients used the “paretic” superior oblique muscle (SOM) and the contralateral superior rectus muscle (SRM) to fuse, and had a mean ± SD BHTTD of 6 ± 8PD. The BHTTD of one, in whom a patch test was performed, increased by 11PD. The remaining patient used the “paretic” SOM and contralateral inferior oblique muscle (IOM) to fuse, and had a BHTTD of only 3PD, increasing to 21PD 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 patients with oblique/rectus-mediated fusion, the vergence-adapted “paretic” SOM and contralateral SRM are activated on ipsilateral and contralateral tilt respectively, lessening the hyperdeviation in both directions. In the other five patients, however, the vergence-adapted ipsilateral IRM and contralateral SRM are activated on contralateral tilt, accentuating the BHTTD.

Conclusions

Fusion influences the BHTTD, either decreasing or increasing it depending on the particular muscles used for fusion. The absence of a positive BHTT should not be relied upon to rule out the diagnosis of SOP. In suspected SOP patients with fusion, performing the BHTT after a patch test may be necessary to bring out the BHTTD supporting the diagnosis.