PURPOSE. To study whether the variation in maximum oblique muscle size accounts for individual variation in the Bielschowsky head tilt phenomenon (BHTP) in clinically diagnosed superior oblique (SO) palsy. METHODS. Seventeen subjects with clinically diagnosed earlyonset or idiopathic SO palsy and 14 normal subjects were enrolled in the study. Magnetic resonance imaging (MRI) in coronal and sagittal planes was used for quantitative morphometry of inferior oblique (IO) and SO muscles. Maximum crosssectional area of the SO and IO cross section at the mid-inferior rectus crossing were determined in central gaze and compared with paretic eye hypertropia on ipsilesional versus contralesional head tilt. RESULTS. Mean (ϮSD) maximum SO cross section was 18.1 Ϯ 3.2 mm 2 in normal subjects, 14.2 Ϯ 6.8 mm 2 ipsilesional to SO palsy, and 19.2 Ϯ 4.5 mm 2 contralesional to SO palsy. The ipsilesional SO cross section was significantly smaller than the contralesional (P ϭ 0.004) and normal (P ϭ 0.01) ones. The mean IO cross section was 18.3 Ϯ 3.5 mm 2 in normal subjects, 21.3 Ϯ 7.9 mm 2 ipsilesional to SO palsy (P ϭ 0.43), and 22.0 Ϯ 6.7 mm 2 contralesional to SO palsy (P ϭ 0.26). Hyperdeviation varied with head tilt by 20.1 Ϯ 5.5°in subjects with SO atrophy, and 10.3 Ϯ 5.6°in subjects without SO atrophy (P ϭ 0.003). Although oblique muscle cross sections did not correlate with BHTP, subjects with clinically diagnosed SO palsy segregated into groups exhibiting normal versus atrophic SO size. CONCLUSIONS. SO size does not account for the variation in BHTP in clinically diagnosed SO palsy, supporting the proposition that the BHTP is nonspecific for SO function. (Invest Ophthalmol Vis Sci. 2009;50:175-179) DOI:10.1167/iovs.08-2393 P atients with early onset or idiopathic superior oblique (SO) palsy are heterogeneous. Only when orbital imaging shows a large asymmetry in cross-sectional areas of the SO muscles is actual muscle weakness 1-4 likely. SO palsy may not necessarily be neuropathic, because abnormalities of the SO tendon, 5-8 or of orbital pulleys may cause incomitant vertical strabismus mimicking SO palsy. 9,10 For this reason, the gold standard for the diagnosis of SO palsy is ultimately radiographic. Nevertheless, much clinical literature on SO palsy is based on clinical, not radiographic, criteria. If clinical criteria are nonspecific for SO palsy, then some beliefs about SO palsy may benefit from reexamination. The Bielschowsky head tilt phenomenon (BHTP) consists of a greater hypertropia during head tilt to the ipsilesional than contralesional shoulder in patients seated upright and is used as a clinical lateralizing test for SO palsy. The biomechanical basis of the BHTP is not fully understood, but probably includes loss of downward and intorsional torque of the palsied SO in compensatory ocular counterrolling (OCR). 11 The BHTP is considered by many clinicians to be the defining clinical criterion for SO palsy. Oblique muscles have both vertical and torsional actions. Contractility of the SO can be radiographically determined by evaluating the change in SO cross-sectional area during gaze shift from supraduction to infraduction. In patients with SO palsy, SO contractility is well correlated with maximum SO cross-sectional area in central gaze. 12 Further, MRI evidence of SO muscle contractile change in vertical gaze shift resembles similar MRI findings during ocular counterrolling. 13 During static ocular counterrolling, 13 the posterior SO cross section was found to be greater during head tilt to the ipsilateral than the contralateral side, reflecting SO contraction to implement ocular torsion. 2 Because changes in SO cross section due to vertical duction resemble changes associated with OCR, we sought to analyze whether variation in SO size accounts for variation in BHTP in SO palsy. Recognizing that maximum SO cross-sectional size in the central gaze is highly correlated with SO contractility, 12 we supposed that the BHTP would also correlate with SO size if this diagnostic test directly reflects SO function. METHODS Subjects Subjects with clinically diagnosed congenital or idiopathic SO palsy, including presumably decompensated cases, were recruited from a prospective study of extraocular muscle function at Okayama University Hospital. The subjects agreed to participate and gave written informed consent according to a protocol conforming to the tenets of the Declaration of Helsinki. Diagnosis of SO palsy was based on clinical criteria including: ipsilesional hypertropia greater in the contralesional than the ipsilesional version, and greater during head tilt to the ipsilateral than the contralateral shoulder when seated upright (Bielschowsky head tilt test); a deficit in infraduction when the ipsilesional eye was adducted; and results of Hess screen testing performed by strabismologists confirming greater hypertropia in deorsumversion and V pattern. All participants underwent complete ophthalmic examinations, including measurement of heterophorias with prism and cover testing. The BHTP was defined quantitatively to be the difference From th
