To search for a new definition of muscle hypoplasia in congenital or idiopathic superior oblique muscle palsy.
Retrospective case-control study.
Cross-sectional areas of the superior oblique and 4 rectus muscles near the eye globe–optic nerve junction were measured by an image analysis software on magnetic resonance images of 50 patients with congenital or idiopathic superior oblique muscle palsy and 45 patients with other disease conditions serving as a control. The paretic side/contralateral normal side ratios of the cross-sectional areas and the left side/right side ratios were calculated for the superior oblique muscle palsy patients and the control patients, respectively.
The 95% confidence intervals in paretic side/contralateral side ratios of cross-sectional areas of the superior oblique muscle were 0.55 to 0.80 in the right-side superior oblique muscle palsy, and 0.48 to 0.75 in the left-side palsy, while the 95% confidence interval in the left side/right side ratios was 0.99 to 1.00 in the control. The 95% confidence intervals in the left side/right side ratios of the 4 rectus muscles were 1.00, both in the superior oblique muscle palsy and in the control.
The muscle hypoplasia could be defined as such when the paretic side/contralateral side ratios of cross-sectional areas of the superior oblique muscle on magnetic resonance images fell outside the 95% confidence interval of the ratios in normal controls.
Congenital or idiopathic superior oblique muscle palsy is a common form of incomitant strabismus. Its etiology remains unknown, but genetic background is suggested based on the familial occurrence or the structural abnormalities, such as muscle aplasia and hypoplasia. Recently, magnetic resonance imaging has been used clinically to assess the status of the superior oblique muscle before surgery. The hypoplasia of the superior oblique muscle is the major finding on imaging, and etiologic classification of the palsy is proposed based on the absence or the presence of muscle hypoplasia.
Some authors have suggested that idiopathic superior oblique muscle palsy with muscle hypoplasia be designated as true muscle palsy while palsy without muscle hypoplasia be designated as simulated muscle palsy. A problem in this classification is the definition of the hypoplasia: for instance, the cross-sectional area of the muscle is less than 50% compared with that on the contralateral side. The extent of the muscle hypoplasia may have a spectrum ranging from aplasia to almost normal. In this study, we measured cross-sectional areas of the superior oblique muscle on both sides of patients with idiopathic superior oblique muscle palsy to obtain basic data for the definition of the hypoplasia.
This study involved 50 patients who were diagnosed with idiopathic or congenital superior oblique muscle palsy and underwent orbital magnetic resonance imaging at Okayama University Hospital over 10 years, from January 1999 to December 2008. Of these 50 patients, 42 underwent surgery while the remaining 8 patients were followed without surgery. During the same 10-year period, 98 consecutive patients in total were diagnosed at Okayama University Hospital as having congenital or idiopathic superior oblique muscle palsy. Of these 98 patients, 48 patients were excluded from this study because magnetic resonance imaging was not done (16 patients) or the films for magnetic resonance images were not available (32 patients; imaging was done at other hospitals in 25 patients and at this hospital in 7 patients). Informed consent for magnetic resonance imaging was obtained in written form from each patient. Patients with acquired superior oblique muscle palsy, such as traumatic, ischemic, and vascular accident-related palsy, were excluded from the study.
All patients were questioned about age at onset, a history of previous head trauma, and family history of strabismus. Photographs at earlier ages were obtained to check abnormal head postures in some patients. Clinical examinations included visual acuity, slit-lamp biomicroscopic and fundus examinations, tonometry, inspection of head posture, deviation measurement at 5 m and 0.3 m by alternate prism and cover test in 9 diagnostic positions of the gaze ( Table 1 ), version, Bielschowsky head-tilt test, vertical fusional amplitude, and TNO stereotest.
|Mean (SD) of Deviation (Prism Diopters) in Primary Gaze With Head Straight, Determined by Alternate Prism and Cover Test a|
|Horizontal Deviation at 5 m||Vertical Deviation at 5 m||Horizontal Deviation at 0.3 m||Vertical Deviation at 0.3 m|
|Superior oblique muscle palsy in total (n = 50)||−6.9 (7.0)||19.9 (9.7)||−10.9 (10.3)||18.9 (11.7)|
|Right-side palsy (n = 29)||−8.4 (7.1)||R/L 20.4 (10.1)||−10.9 (10.3)||R/L 17.8 (9.6)|
|Left-side palsy (n = 20)||−5.3 (7.2)||L/R 19.0 (8.7)||−11.9 (10.7)||L/R 21.0 (14.2)|
|Bilateral palsy (n = 1)||−2.0||L/R 8.0||−2.0||L/R 5.0|
Orbital magnetic resonance imaging was performed preoperatively in all patients to evaluate the status of the superior oblique muscle. The patients were instructed to close the eyes during the imaging. Of the 50 patients, 26 were male and 24 female, with age at magnetic resonance imaging ranging from 2 to 80 years (mean and standard deviation, 30.9 and 22.7 years). The palsy was on the right side in 29 patients, on the left side in 20 patients, and on both sides in 1 patient. The superior oblique muscle aplasia was found on the right side in 3 patients (1 male and 2 female) and on the left side in 2 patients (1 male and 1 female).
For the control, we selected 45 patients who underwent magnetic resonance imaging at Okayama University Hospital over 4 years, from 2003 to 2006, for other reasons: 11 patients suspected with orbital tumors, 10 patients suspected with optic neuropathy, 4 with thyroid diseases, 8 with other types of strabismus, 4 with anterior segment diseases, and 8 with screening for other conditions such as retinal diseases, uveitis, and glaucoma. The findings in the orbital structure were within normal parameters in these 45 patients. The 45 patients who served as controls in this study were 20 male and 25 female subjects, with age at magnetic resonance imaging ranging from 2 to 88 years (mean and standard deviation, 51.0 and 19.7 years). Between the 50 patients with the superior oblique muscle palsy and the 45 control patients, the male-female ratio was not statistically different ( P > .05, χ 2 test), while the age of the 50 patients with the muscle palsy was significantly younger than that of the 45 control patients ( P = .0001, Mann-Whitney U test).
A coronal section slice at the nearest location posterior to the eye globe and optic nerve junction was chosen from T1-intensified orbital magnetic resonance images. These magnetic resonance images were taken by different machines with different slice thickness and were printed on films with varying magnifications. Of the 50 patients with the superior oblique muscle palsy, the coronal images were at 3-mm intervals in 41 patients and at 4-mm intervals in 9 patients, while the images were at 3-mm intervals in 2, at 4-mm intervals in 39, and at 5-mm intervals in 4 of the 45 control patients. The selected slices of the magnetic resonance imaging films placed on a show case were captured by a digital camera at the highest magnification possible with good focus, and captured images were transferred to a computer. Using Scion Image for Windows software (Scion Corporation, Frederick, Maryland, USA), cross sections of the superior oblique muscle and 4 rectus muscles were encircled manually with a mouse and the number of square pixels in the encircled area was measured to obtain the area of the cross section. The measurements were repeated 5 times and a mean was calculated to get a representative value for the cross-sectional area of the muscle.
For the superior oblique muscle of patients with either right-side or left-side superior oblique muscle palsy, the value on the paretic side was divided by the value on the contralateral normal side to obtain the paretic side/contralateral normal side ratios. For the superior oblique muscle of control patients and 1 patient with bilateral superior oblique muscle palsy, the value on the left side was divided by the value on the right side to obtain the left-to-right ratio for statistical analysis. The left side/right side ratios of the cross-sectional areas were calculated for the 4 rectus muscles both in the superior oblique muscle palsy patients and in the control patients.
The paretic side/contralateral normal side ratios of cross-sectional areas of the superior oblique muscle varied widely in 50 patients with idiopathic superior oblique muscle palsy. In contrast, the left side/right side ratios for the superior oblique muscle in a control group of 45 patients with other conditions showed no variation at all ( Table 2 , Figure ).
|Paretic Side/Normal Side Ratio or Left Side/Right Side Ratio of Muscle Cross-Sectional Areas a|
|Group||Muscle||Mean||SD||Median||Range||95% Confidence Interval|
|Idiopathic superior oblique muscle palsy group (n = 50)||Superior oblique (SO)|
|In total (n = 50)||0.66||0.33||0.67||0–1.03||0.57–0.75|
|In total excluding SO aplasia (n = 45)||0.73||0.26||0.72||0.19–1.03||0.66–0.80|
|Right-side palsy (n = 29)||0.67||0.35||0.80||0–1.03||0.55–0.80|
|Right-side palsy excluding SO aplasia (n = 26)||0.75||0.27||0.86||0.19–1.03||0.65–0.86|
|Left-side palsy (n = 20)||0.61||0.31||0.64||0–1.02||0.48–0.75|
|Left-side palsy excluding SO aplasia (n = 18)||0.68||0.23||0.66||0.19–1.02||0.57–0.79|
|Bilateral palsy (n = 1)||1|
|Control group (n = 45)||Superior oblique||1.00||0.02||1.00||0.96–1.07||0.99–1.00|