Absence of Relationship between Oblique Muscle Size and Bielschowsky Head Tilt Phenomenon in Clinically Diagnosed Superior Oblique Palsy

Abstract

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 early-onset 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 cross-sectional 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 (±SD) maximum SO cross section was 18.1 ± 3.2 mm² in normal subjects, 14.2 ± 6.8 mm² ipsilesional to SO palsy, and 19.2 ± 4.5 mm² contralesional to SO palsy. The ipsilesional SO cross section was significantly smaller than the contralesional (P = 0.004) and normal (P = 0.01) ones. The mean IO cross section was 18.3 ± 3.5 mm² in normal subjects, 21.3 ± 7.9 mm² ipsilesional to SO palsy (P = 0.43), and 22.0 ± 6.7 mm² contralesional to SO palsy (P = 0.26). Hyperdeviation varied with head tilt by 20.1 ± 5.5° in subjects with SO atrophy, and 10.3 ± 5.6° in subjects without SO atrophy (P = 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.