This study shows that the upright-supine test with the head centered is not a sensitive method to differentiate an acute or subacute skew deviation from a fourth nerve palsy. Accurate diagnosis is key because a skew deviation can stem from a potentially dangerous etiology, such as stroke, whereas a fourth nerve palsy is often caused by a benign etiology, such as microvascular ischemia.
In this multicenter study, the authors enrolled 37 consecutive patients who presented with an acute or subacute skew deviation (19 patients) or a fourth nerve palsy (18 patients), and evaluated whether the upright-supine test could differentiate the two entities.
Previous work by Wong and colleagues suggested that the upright-supine test is a sensitive and specific tool for detecting a chronic skew deviation, which shows a diminished vertical misalignment in the supine position.
However, the current analysis found that only 1 patient with an acute/subacute skew deviation (5%) had more than a 50% decrease in hypertropia when moving to a supine position with the head centered, while no patients with a fourth nerve palsy showed a 50% change. Therefore, the upright-supine test was 100% specific for a skew deviation, but only 5% sensitive. When the head was tilted to the side during the upright-supine test, there was a larger change in patients with a fourth nerve palsy, which made the vertical strabismus more concomitant in the supine position.
The authors also administered the Parks-Bielschowsky 3-step test, which is the standard for diagnosing fourth nerve palsy. Only 1 patient with skew deviation (5%) had a positive result on the Parks-Bielschowsky 3-step test, while 16 patients with a fourth nerve palsy (89%) had positive results (sensitivity 88%; specificity 95%).
Finally, 95% of patients with skew deviation showed additional ocular motor signs and/or neurological abnormalities; these were not observed in patients with fourth nerve palsy.
This study was limited by the lack of a control group, the relatively small sample and the inclusion of only acute- and subacute-onset vertical strabismus. The previous study that reported an improvement in vertical strabismus in the supine position among patients with skew deviation, was done in patients with a chronic skew deviation.
As speculated by the authors, the discrepancy of the current study may be explained by the chronicity of symptoms: It is possible that the upright-supine test is not sensitive in an acute setting but may be sensitive in a chronic setting because potential adaptive vestibular mechanisms take time to develop. However, this theory could not be tested in this study because only acute and subacute patients were included.
This study shows that the upright-supine test with the head centered is not a sensitive method to differentiate an acute or subacute skew deviation from a fourth nerve palsy. This is an important finding because differentiating these 2 entities in the acute setting is more important than in the chronic setting; therefore, it is helpful to know that this test cannot be relied upon.
Instead, a fourth nerve palsy may be indicated by an incomitant vertical misalignment with head tilts in the upright position that converts to a concomitant vertical strabismus in the supine position. In addition, fourth nerve palsies usually fulfill the Parks-Bielschowsky 3-step test and will not typically have accompanying ocular motor and/or neurologic signs.