Purpose

Oblique muscle overaction is typically diagnosed based on physical examination, and interferes with binocular vision and quality of life. However, the clinical examination poorly distinguishes SO overaction from other scenarios, such as heterotopic rectus extraocular muscles (EOM) pulleys, or other orbital or structural EOM abnormalities (1). The current progress in magnetic resonance imaging (MRI) permits addressing the functional anatomy of the eye, which was exploited in the studies of SO palsy (2-3). We aim to quantitatively determine the size and contractility of the superior oblique muscle and inferior oblique muscle in primary superior oblique overaction (PSOOA) and primary inferior oblique muscle overaction (PIOOA), to facilitate the differential diagnosis, thus open the way for more targeted treatment.

Methods

A prospective, observational study was conducted on twelve patients with PSOOA, 7 cases with PIOOA, and 10 healthy, orthotropic subjects. Sets of contiguous, 2 mm slice thickness, quasi-coronal and sagittal magnetic resonance imaging (MRI) were obtained during different gazes, giving pixel resolution of 0.391 mm. The globe–optic nerve junction was localized as plane zero for SO measurement (Fig. 1). For IO, the scanning plane was parallel to the long axis of the orbit, and plane zero was defined from the nasal side to the temporal side through the midpoint of the inferior rectus. Cross sectional areas of the superior oblique (SO) and inferior oblique (IO) muscles were determined in primary position, supraduction and infraduction to evaluate size and contractility. The cross sectional areas of SO and IO muscle were compared with those of controls in the primary position to detect hypertrophy or atrophy and changes in contractility could be detected during the vertical gaze. All statistical calculations were performed using PROC MIXED (SAS 9.4).

Results and Discussion

In the primary position, the greatest SO cross-sections occurred about midorbit, 6 mm posterior to the globe-optic nerve junction. In down gaze, the SO cross-sectional area increased, and the plane in which the maximum cross-sectional area observed was more posterior. In up gaze, the SO cross-sectional area decreased, and the plane in which the maximum cross-sectional area observed was more anterior (Fig. 3). There was no difference between the ipsilesional (affected eye), contralesional (unaffected eye) and normal SO muscle cross-sections: 0.176 ± 0.018 cm2, 0.175 ± 0.005 cm2, and 0.173 ± 0.015 cm2, respectively (P=0.82). The maximum contractility of SO muscle on the ipsilesional (affected) side was 0.097 ± 0.024 cm2, and was different than on the contralesional (unaffected) side: 0.067 ± 0.015 cm2 and in control subjects: 0.063 ± 0.018 cm2 (P=0.0002) (Figure 4). For PIOOA, compared IO of the patient group and control group of subjects, ipsilesional IO maximum cross-sectional area (0.150 ± 0.021 cm2) was greater than that of normal IO maximum cross-sectional area (0.131 ± 0.016 cm2) (P=0.023) (Fig. 4). However, ipsilesional IO maximum contractility (0.052 ± 0.013 cm2) did not differ from that of normal subjects (0.0430 ± 0.011 cm2) (P=0.080). It is reported that the cause of EOM overaction includes excessive innervation, an increase in muscle’s cross-sectional area (hypertrophy), and changes in muscle fiber types within the muscle (1, 4). It seems that muscle hypertrophy is the likely reason for PIOOA, not for the PSOOA. On the contrary, a difference was found in contractility between eyes with SO overaction and control eyes, which may indicate abnormal innervation resulting in increased muscle stimulation. The excess innervation may primarily act on the posterior half of SO, where the contractile changes mainly occurred, which is in accordance with the anatomy of SO (5). Later studies would be recommended to further illustrate oblique muscle and other EOMs cross-section, contractility, and pulley locations to get a better understanding of PSOOA and PIOOA, thus helping choose more appropriate operation method.

Conclusions

In PSOOA, the ipsilesional superior oblique is more contractile than the contralesional SO muscle and different than in controls, with no difference in SO muscle size in primary position, which suggests that excessive innervation rather than muscle hypertrophy underlies PSOOA. In PIOOA, IO cross-section area of the case group was greater than that of the normal group, while there was no significantly difference in contractility between the two groups, indicating that primary inferior oblique muscle overaction associated with IO muscle hypertrophy.

Acknowledgements

The study was supported by Department of Science and Technology of Sichuan Province, grant number 2012SZ0138.