Highlights
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We classified AOS into three classes differing from previous classification methods.
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Discrete anomalous bands most commonly involve the IR, while abnormal EOM connections primarily occur between the SR and MR.
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AOS involving vertical rectus muscles can contribute to eyelid malposition.
PURPOSE
To evaluate and classify the magnetic resonance imaging (MRI) features of anomalous orbital structures (AOS) and correlate these findings with clinical manifestations.
DESIGN
Retrospective, observational case series.
METHODS
The detailed clinical data from 35 patients diagnosed with congenital restrictive strabismus and AOS were retrospectively reviewed between July 2010 and October 2024. All patients underwent brain stem and intraorbital MRI.
MAIN OUTCOME MEASURES
The morphological features of AOS, extraocular muscles (EOMs), and ocular motor nerves were examined.
RESULTS
This retrospective study included 35 cases (21 males and 14 females). The mean ± SEM age was 6.9 ± 0.8 years (range: 2-22 years). Sixteen patients had right-eye involvement, and 19 had left-eye involvement. AOS were categorized into three types: Type 1, discrete anomalous bands extending from the rectus muscles to the posterior sclera (25.7%), with the inferior rectus (IR) being the most affected; Type 2 refers to anomalous connections observed between the extraocular muscles (EOMs) (65.7%), often occurring in the area between the superior rectus (SR) and medial rectus (MR); Type 3, abnormal connection between the posterior sclera and the surrounding optic nerve sheath and extending to the equatorial region of the globe (8.6%). Varying degrees of restricted eye movements and strabismus were observed in all patients. Most patients also exhibited amblyopia and eyelid malposition in the affected eye.
CONCLUSIONS
Discrete anomalous bands most commonly involve the IR, while abnormal EOM connections primarily occur between the SR and MR. Eyelid malposition could suggests the possible presence of AOS.
INTRODUCTION
Congenital restrictive strabismus is a form of ocular misalignment characterized by limited eye movement primarily due to neuropathic or myopathic mechanical forces affecting the extraocular muscles (EOMs). , This condition arises from various underlying factors, including congenital cranial dysinnervation disorders (CCDDs) and structural anomalies of the EOMs or adjacent tissues, leading to significant motility restrictions and may manifest as globe retraction, pronounced vertical and horizontal strabismus, abnormal head positions, or difficulties with visual fixation. ,
Structural anomalies of the EOMs or adjacent tissues are commonly termed anomalous orbital structures (AOS). Some of these structures attach to the globe and cause mechanical restrictions. , , In a review conducted by Lueder, these anomalous structures were classified into three distinct types—the first arise from EOMs and insert in abnormal locations, the second are fibrous bands located beneath the rectus muscles, and the third are discrete anomalous muscles originating in the posterior orbit and inserting in abnormal locations on the globe. , , The exact causes of AOS remain unclear; however, several theories suggest that they may arise from early disturbances in the development of the EOM anlage or from the presence of atavistic retractor bulb muscles.
In clinical practice, we have encountered patients with restrictive strabismus associated with AOS. These cases often pose diagnostic challenges, as they are initially misdiagnosed as conditions such as CCDDs in outpatient settings. This observation prompted us to explore whether AOS follows certain patterns, such as which EOMs are most commonly affected or whether their involvement correlates with the embryological sequence of EOM development. The embryology of the EOMs, which follows a specific temporal and spatial order, provides a potential framework for understanding the pathogenesis of AOS.
Imaging studies are often recommended for patients exhibiting atypical strabismus to effectively identify underlying anomalies. Magnetic resonance imaging (MRI) enables noninvasive and precise visualization of these structures, including their course and location, significantly improving the diagnosis of AOS. This capability has not only improved diagnostic accuracy but also provided new opportunities for investigating the clinical and embryological significance of AOS.
Despite these advances, AOS have mostly been described in isolated case reports, and current knowledge remains limited. In this study, we reviewed the clinical characteristics and imaging findings of 35 patients with congenital restrictive strabismus associated with AOS. Additionally, we discuss the potential clinical implications of AOS on ocular motility and their possible pathogenic mechanisms.
METHODS
PATIENTS
Comprehensive ophthalmic examination records were reviewed to analyze clinical manifestations. MRI scans of the orbits were evaluated to assess the EOMs and adjacent tissues; MRI of the brainstem was used to measure the ocular motor nerves. This retrospective study was conducted in accordance with the tenets of the Declaration of Helsinki. The research protocol was approved and appropriate consent was obtained from the Beijing Tongren Hospital Research and Ethics Committee. Patient information was anonymized prior to analysis.
The inclusion criteria were: (1) a history of congenital ocular movement restrictions; and (2) MRI scans revealing anomalous orbital structures. The exclusion criteria were: (1) abnormalities of the ocular motor nerves on MRI; or (2) ocular movement restrictions caused by acquired factors such as trauma or metabolic diseases. Twenty age- and sex-matched individuals with normal imaging findings of the ocular motor nerves and intraorbital structures were selected as the control group. These individuals had previously undergone MRI scans at our hospital for other clinical indications and were included for comparison of ocular motor nerve diameter and developmental characteristics.
CLINICAL ASSESSMENT
Clinical information was obtained from medical records, including birth history, family history, and results of detailed ophthalmic examinations. Examinations included best-corrected visual acuity (BCVA) measured at 6 meters using the Snellen chart and converted to LogMAR values, as well as ocular motility, eyelid position and ocular alignment in the primary position.
All thirty-five patients had a high-resolution MRI examination with a General Discovery MR750 3.0-T Twinspeed scanner. Different sequences were adopted in brain, cranial nerves and orbit. Children under 10 years old were given oral chloral hydrate (0.8 ml/kg) 30 min before examination (less than a total of 20 ml). (1) Scanning of the brain structure was conducted with a phased array coil in head-cross section and the parameters were as follows: Fast spin-echo/T2-weighted images (FSE/T2WI); section thickness, 6.0 mm; layer spacing, 1.0 mm. (2) Imaging of cranial nerves at the brainstem was conducted with an 8HR brain coil and adopted the three-dimensional fast imaging employing steady—state acquisition (3D-FIESTA) sequence. Parameters were as follows: axial scanning; repetition time (TR), 4.28 ms; echo time (TE), 2.04 ms; flip angle (FA), 60°; the field of view (FOV), 16 × 16 cm; Matrix, 256 × 320; section thickness, 0.8 mm; number of excitation (NEX), 4. (3) Orbital imaging was performed on the quasi-coronal plane (perpendicular to the orbital axis) using the following parameters: FSE/T2WI; TR, 3000 ms; TE, 90 ms; FOV, 18 × 18 cm; matrix size, 256 × 256; section thickness, 3.0 mm; slice spacing, 0.3 mm; NEX, 2.
Based on the imaging, AOS were divided into three categories according to their adjacency to the EOMs and surrounding tissues. Type 1: discrete anomalous structures inserting into the posterior sclera; Type 2: connections between two or more EOMs. Type 3: abnormal connection between the posterior sclera and the surrounding optic nerve (ON) sheath.
QUANTITATIVE MRI ANALYSIS
Digital Imaging and Communication in Medicine (DICOM) images were analyzed using Radiant DICOM Viewer Software (Version 2020.2, 64-bit, Medixant, Poznan, Poland. URL: https://www.radiantviewer.com ) to evaluate the morphological features of the AOS and EOMs. Multiplane reconstruction (MPR) technology was used to reconstruct images along any plane parallel to the ocular motor nerves. Imaging of the oculomotor nerve (CN3) and abducens nerve (CN6) was evaluated, and the diameters of the thickest cistern segments of CN3 and CN6 were measured.
STATISTICAL ANALYSIS
All statistical analyses and drawings were performed using SPSS software (Version 26.0; IBM Corp., Armonk, NY, USA). Group differences were analyzed using the Kruskal–Wallis test. P-values < 0.05 were considered statistically significant.
RESULTS
PATIENTS
A total of 35 cases that represent a spectrum of clinical forms of congenital restrictive strabismus caused by AOS were identified, comprising 21 males and 14 females with an average age of 6.9 ± 0.8 years (mean ± SEM, range, 2-22). The right eye was affected in 16 cases, and the left eye in 19 cases. All patients exhibited unilateral AOS.
Clinical manifestations varied among the 35 cases, and a detailed summary of ocular alignment is presented in Supplementary Tables S1-S3. The clinical characteristics of these 35 cases were as follows: all cases had restricted ocular motility; 25 cases (71%) had horizontal strabismus; 30 cases (86%) had vertical deviation; 22 cases (63%) had a BCVA with at least two lines less than the contralateral eye (some patients were too young to be tested for visual acuity); 17 cases (49%) had eyelid abnormalities.
Classification of AOS: AOS cases were classified into three types based on imaging results: type 1, comprising nine cases, showed discrete anomalous bands originating from one or more EOMs and inserting into the posterior sclera (9/35, 26%) ( Figure 1 ); type 2, with 23 cases, showed abnormal interconnections between multiple EOMs (66%) ( Figure 2 ); and type 3, consisting of three cases, demonstrated abnormal connection between the posterior sclera and the surrounding optic nerve sheath and extending to the equatorial region of the globe (9%) ( Figure 3 ).
