Superior oblique extraocular muscle shape in superior oblique palsy

Abstract

Purpose: To investigate the superior oblique (SO) extraocular muscle cross section in normal controls and in SO palsy using high-resolution magnetic resonance imaging (MRI).

Design: Prospective observational study.

Methods: At a single academic medical center, high-resolution MRI was obtained at 312 μm in-plane resolution using surface coils in multiple, contiguous, quasi-coronal planes perpendicular to the orbital axis in 12 controls and 62 subjects with SO palsy. Previous strabismus surgery was excluded. Imaging was repeated in central gaze and infraduction. In each image plane along the SO, its cross section was outlined to compute cross-sectional area and the major and minor axes of the best-fitting ellipse. Main outcome measures were SO morphology and ocular motility.

Results: The major and minor axes, cross-sectional area distributions, and volume of the SO belly were subnormal in orbits with SO palsy at most anteroposterior locations (P = .001), but discriminant analysis showed that palsied SO cross sections segregated distinctly into round and elongate shapes representing isotropic vs anisotropic atrophy, respectively. The major axis was relatively preserved in anisotropic atrophy (P = .0146). Cases with isotropic atrophy exhibited greater hypertropia in infraversion than central gaze, as well as greater excyclotorsion, than cases with anisotropic atrophy (P < .05 for all).

Conclusions: Characteristic differences in shape of the palsied SO belly correlate with different clinical features, and may reflect both the degree of differential pathology in the medial vs lateral neuromuscular SO compartments and the basis for diversity in patterns of resulting hypertropia.

Fig. 1

Quasi-coronal MRI of right (left column) and left (right column) orbits in superior oblique (SO) palsy. Each SO cross section is outlined in white. Top row - Subject with right unilateral anisotropic superior oblique (SO) atrophy manifesting elongate, oval shape compared to the unaffected left SO. Middle row – Subject with right unilateral isotropic SO atrophy group manifesting uniform, circular reduction in right cross section compared to the unaffected fellow eye. Bottom row – Normal control subject has similar SO cross sections in both orbits.

Fig. 2

Quasi-coronal MRI in unilateral superior oblique (SO) palsy cropped to show SO muscles exhibiting isotropic atrophy, and corresponding normal fellow muscles.

Fig. 3

Quasi-coronal MRI cropped to show superior oblique muscles exhibiting anisotropic atrophy in superior oblique palsy, and corresponding normal fellow muscles.

Fig. 4

Major and minor axes of the superior oblique (SO) cross section in SO palsy exhibiting isotropic and anisotropic SO atrophy, and in normal control subjects. Image planes are 2 mm thick image and numbered positively anterior and negatively posterior to zero at the globe-optic nerve junction. Significant differences from normal are indicated by single and double asterisks at the 0.05 and 0.01 levels, respectively, by unpaired t testing.

Fig. 5

Discriminant analysis of major and minor axes of the palsied superior oblique (SO) belly at in quasi-coronal image planes at the globe-optic nerve junction (image plane 0) in 62 cases of unilateral SO palsy. Each symbol indicates one case. Two morphometric groups were discriminable. Cases clustered in the left half of the graph exhibited isotropic atrophy (squares), while cases clustered in the right half exhibited anisotropic atrophy (circles).

Fig. 6

Superior oblique (SO) volume in unilateral SO palsy manifesting anisotropic and anisotropic atrophy, and in normal control cases. Error bars indicate standard error of the mean.

Fig. 7

Underdepression in adduction (left) and over-elevation in adduction (right) in superior oblique (SO) palsy. Values are rated on a scale of −4 underaction to +4 overaction, with normal defined as zero. Chi-square testing demonstrated significantly more underdepression in adduction in isotropic than anisotropic SO atrophy.