Sensitivity and Specificity of a New "Upright-Supine Test" to Differentiate Skew Deviation From Other Causes of Vertical Strabismus

Abstract

Purpose: : In a previous study, we demonstrated that the vertical strabismus in patients with skew deviation decreased substantially when they changed from an upright to supine position, whereas in patients with trochlear nerve palsy, the vertical strabismus changed minimally between these two positions. The purpose of this study is to determine the sensitivity and specificity of this new "upright-supine test" to differentiate skew deviation from other causes of vertical strabismus in a large number of patients.

Methods: : Seventeen patients with skew deviation, 40 with trochlear nerve palsy, and 25 with other causes of vertical strabismus (e.g., dysthyroidism, orbital trauma, Brown syndrome, myasthenia gravis, childhood strabismus, peripheral oculomotor nerve palsy) were recruited. Vertical strabismus was measured by the prism and alternate cover test using a near target at 1/3 meter in both the upright and supine position. A vertical strabismus that decreased by ≥50% from the upright to supine position constituted a positive test.

Results: : Thirteen patients with skew deviation had a positive "upright-supine test", giving the test a sensitivity of 76%. No patients with trochlear nerve palsy or other causes of vertical strabismus had a positive test, giving the test a specificity of 100%. All four patients with skew deviation who had a negative test exhibited other neurologic signs and had a lesion in the midbrain on MRI, which may have caused a vertical strabismus from a combination of skew deviation, trochlear and/or oculomotor nerve palsy.

Conclusions: : The upright-supine test is highly specific for the detection of skew deviation. Because skew deviation can mimic a trochlear nerve palsy during the "three-step" test, the upright-supine test should be added as a fourth step; if it is positive, neuroimaging should be ordered to rule out a skew deviation even if the three-step test indicates a trochlear nerve palsy.

Keywords: strabismus: diagnosis and detection • neuro-ophthalmology: diagnosis • strabismus