Strabismus fixus is a rare condition in which one or both eyes are anchored in an extreme position and cannot be moved. The condition can be congenital or acquired. Acquired strabismus fixus has been reported to be associated with amyloidosis, high myopia, and myositis.1–3 The congenital form is generally thought to be caused by congenital fibrosis of the extraocular muscle,4 anomalous orbital structures,5 or congenital orbital fibrosis.6 Treatment of strabismus fixus is challenging, but surgery is helpful and some cosmetic and functional improvement can be accomplished.7
We describe three specific cases of congenital monocular strabismus fixus with limited ocular movements in all directions. Magnetic resonance imaging (MRI) was used to see if it could show unusual or unexpected causes for restriction of ocular rotation. Interestingly, retrobulbar areas of low signal noise in the muscle cone were found on MRI in the three cases, which involved the extraocular muscles, posterior Tenon's capsule and globe, and would be indicative of fibrosis and adhesion. The clinical characteristic was that surgery was ineffective for these cases. This study further suggests that orbital imaging plays an important role in the diagnosis and treatment of atypical restrictive strabismus.
Patients and Methods
The medical records and MRIs of all cases of congenital monocular strabismus fixus were reviewed between January 1, 2006, and December 31, 2016. Ophthalmologic examinations and the MRI results were abstracted. The research protocol was approved and appropriate consent was obtained from the Beijing Tongren Hospital Research and Ethics Committee.
MRI of the ocular motor nerves was performed with a General Electric 1.5-T Twinspeed scanner, as reported previously.8 The nerves in the cistern were displayed by the three-dimensional fast imaging employing steady-state acquisition (FIESTA) sequence (TR = 4.8 ms; TE = 1.4 ms; FOV = 18 × 18 cm; matrix = 256 × 256; NEX = 4; slice = 0.8 mm). Nerves in the orbits, including innervating extraocular muscles, and their associated connective tissues, were acquired separately by dual-phased coils with T1-weighted fast spin-echo (TR = 440 ms; TE = 12 ms; slice thickness = 2 mm; interslice gap = 0.3 mm; FOV = 10 × 10 cm; matrix = 224 × 256; ETL = 2; NEX = 2).
A total of three cases were identified in Beijing Tongren Hospital over the 10-year period.
A 2-year-old boy presented with restrictive esotropia and a slight enophthalmos in the right eye since birth. The appearance and movement were normal in the left eye. Anterior segment and dilated fundus examinations of both eyes also were normal. The right eye had 30 prism diopters (PD) of restrictive esotropia and enophthalmos with severe limited ocular movements in all directions. A forced duction test revealed marked restriction in all horizontal and vertical movements. There was no family history of strabismus or systemic abnormalities.
On follow-up at the age of 12 years, the esotropia showed no progression when compared to previous examinations. The best corrected visual acuity (BCVA) was 20/40 in the right eye after occlusion therapy and 20/20 in the left eye. Cycloplegic refraction was −2.75 diopters sphere (DS) −0.50 diopters cylinder (DC) × 140 in the right eye and −4.25 DS −1.50 DC × 180 in the left eye. An MRI of the head and orbit was unremarkable at the time of that examination. A 6-mm recession of the medial rectus muscle was performed in an attempt to improve the primary position of the right eye. However, the eye remained in the same primary position as preoperatively by 1 day and 1 year after surgery. Further MRI of the oculomotor nerve and orbit was performed to investigate the possible cause of this condition. The results showed abnormally low signals around the optic nerve in the right eye. An axial MRI revealed the retrobulbar triangular area of low signal in the right eye (Figure 1). An MRI of the brainstem showed that the cranial nerves were normal.
Magnetic resonance imaging (MRI) of case 1. (A) An axial T1-weighted MRI showing a retrobulbar triangular area of low signal in the right eye (large arrow), with adipose tissue inserted in the medial rectus (small arrow). (B) An oblique-coronal T1-weighted MRI of the deep orbit demonstrates a crescent fiber mass around the optic nerve (ON) in the right eye (arrow).
A 2-year-old boy presented with mild ptosis, enophthalmos, and limited movement in the right eye since birth. The patient was born uneventfully at full term with no significant medical or family history. There was 100 PD of right exotropia and 50 PD of right hypertropia in the primary position (Figure 2). A 6-mm ipsilateral ptosis, lower eyelid scar, and enophthalmos were noted. An extraocular movement examination revealed marked restriction of the right eye for all horizontal and vertical movements. The axial length was 18.5 mm in the right eye and 22.1 mm in the left eye. Under anesthesia, an MRI scan showed an abnormal signal of the muscle cone within the orbit, which involved the posterior Tenon's capsules, lateral, inferior, and medial rectus muscles, and globe. An MRI of the brainstem showed that the cranial nerves were normal (Figure 3).
A composite 9-gaze photograph of case 2.
Magnetic resonance imaging (MRI) of case 2. (A) An axial T2-weighted MRI of case 2 showing that the space between the globe and the intraconal fat was wider than that of the contralateral healthy eye (arrow). (B) A coronal 3D FIESTA MRI of the deep orbit of case 2 demonstrating abnormally low signals around the optic nerve (arrow). (C–E) MRI images of 0.8 mm thickness showing the normal courses of the ocular motor nerves, trochlear nerves, and abducens nerves in the brainstem, respectively (arrow).
Because the MRI findings were essentially the same as those in case 1, we did not recommend further surgery in this case. However, the parents still strongly requested surgery after full communication. Consequently, a 12-mm recession of the lateral rectus muscle and 10-mm recession of the superior rectus muscle combined with medial rectus muscle fixing to the periosteum was performed, which revealed severe limitations of ductions in all directions during the surgery. Postoperatively, the eye remained hypertropic and exotropic and there was no improvement in ocular motility. At 6 years' follow-up, BCVA was 20/120 in the right eye due to the lack of persistent occlusion therapy and 20/20 in the left eye. Cycloplegic refraction was +8.50 DS + 2.00 DC × 105 in the right eye and +0.50 DC × 95 in the left eye. The patient had the same ocular alignment as preoperatively.
A 10-year-old boy presented with severe limitation of movements in the right eye since birth. The BCVA was 20/30 in the right eye and 20/20 in the left eye. His refraction was +2.5 DS in the right eye and +2.25 DS in the left eye. Anterior segment and dilated fundus examination of both eyes were normal.
He presented with an almost normal primary position of both eyes without binocular vision, whereas the right eye demonstrated marked limitations in all directions (Figure 4). The forced duction test confirmed the immobility of the eye. There was no family history of strabismus or systemic abnormalities. The 4-year follow-up examination showed no progression.
A composite 9-gaze photograph of case 3.
A sagittal T1-weighted MRI on the right orbit showed a retrobulbar triangular area of low signal in the muscle cone, which involved the posterior Tenon's capsules and lateral, inferior, and medial rectus muscles. A T2 axial MRI showed that the right eyeball was mechanically stretched into a square-like shape. The cranial nerve and branches in the orbits were normal (Figure 5). This patient did not have surgery.
Magnetic resonance imaging (MRI) of case 3. (A) A T1-weighted sagittal MRI showing a retrobulbar triangular area of low signal in the right eye (arrow). (B) A coronal T1-weighted MRI showing a thickened superior rectus (SR) and irregular lateral rectus (LR), inferior rectus (IR), and medial rectus (MR), with anomalous extraocular muscle bands between the LR and IR (arrow). (C) An oblique-coronal T2-weighted MRI of case 3 showing that the right eyeball was mechanically stretched into a square. (D–F) MRI images of 0.8 mm thickness showing the normal courses of the ocular motor nerves, abducens nerves, and trochlear nerves in the brainstem, respectively (arrows).
Monocular strabismus fixus is a rare condition. The involved eye is “fixed” in this position and cannot be moved, and the forced duction test will confirm the immobility of the eye. Little has been revealed about its clinical characteristics and MRI findings of extraocular muscles and ocular motor nerves.
Strabismus fixus usually occurs in a convergent form, and high myopia is the most common cause.9 Myopic strabismus fixus is classically characterized by a bilateral progressive esotropia and hypotropia associated with a limited elevation and abduction. There are few other reports of unilateral esotropia associated with low myopia or without myopia, which are considered to be parts of congenital fibrosis of the extraocular muscle.10,11
Congenital fibrosis of the extraocular muscle is a congenital and non-progressive restrictive disorder of ocular motility with one or both eyes involved. Orbital and cranial imaging studies based on MRI have demonstrated hypoplasia and aplasia of cranial nerves III and IV, as well as atrophy of the muscles supplied by them, supporting the neurogenic theory of this disease. Additionally, Souza-Dias et al.12 reported 3 cases of large-angle restrictive unilateral strabismus acquired rapidly during the first months of life in otherwise normal children documented to have normal eye alignment and motility as newborns. Surgery was performed at the age of 5 to 8 months and the primary position improved postoperatively. Myositis causing extraocular muscle fibrosis was a possible cause of the strabismus in these cases. Hertle et al.5 reported four cases of the congenital fibrosis syndrome, which was exhibited in infancy as unilateral blepharoptosis, strabismus, limited ductions, globe displacement (enophthalmos and blepharoptosis), and decreased vision. Results of histopathologic examination and orbital imaging studies showed that there was extraocular muscle atrophy, anomaly, or absence and/or infiltration by fibrous tissue.
The divergent type of strabismus fixus is more unusual than the convergent type.13 It might or might not be accompanied by ptosis or generalized extraocular muscle fibrosis. Sharma et al.13 reported a case of divergent strabismus fixus with a normal vertical gaze and no ptosis, and surgical treatment was offered at the age of 61 years. There was a slight improvement in the head posture but none in the eye movements 2 months postoperatively, and early surgery was suggested. Awan14 reported craniofacial anomalies with the divergent type of strabismus fixus. The pathogenesis of this type of strabismus fixus remains poorly understood. Gillies et al.15 believed it to be a part of extraocular muscle fibrosis syndrome.
Unlike these cases, our three patients showed unilateral, non-progressive strabismus fixus since birth, with marked limitation of movements in all directions. However, strabismus might or might not present with the eyes in the primary position. Case 1 presented with esotropia, case 2 with exotropia and hypertropia, and case 3 with an almost normal primary position. Considering the fact that all of our cases were unilateral and non-progressive, the cause was not likely a systemic factor but rather a localized abnormality. For a better understanding of the cause of these cases, we performed further MRIs of the anatomy of the ocular motor nerves and their corresponding extraocular muscles. Interestingly, all three cases showed similar imaging results with an abnormally low signal of the muscle cone within the orbit. It was interpreted by the radiologist as typical of fibrosis and adhesion of posterior Tenon's capsules, which involved the superior rectus, inferior rectus, lateral rectus, medial rectus, and inferior oblique muscles and the globe. The lower eyelid scar in case 2 might be due to a prenatal inflammation and fiborsis in the orbital tissues.
Tenon's capsule is dense elastic and vascular fibrous connective tissue that surrounds the globe up to the cornea, which is divided into anterior and posterior portions at the point of penetration of the rectus muscles.16 Jones17 found that the anterior Tenon's capsule was already present in the embryo and the posterior Tenon's capsule formed at a later evolutional stage than the anterior part, which signified that both structures might have had different origins. In our cases, adherences among the extraocular muscles, posterior Tenon's capsule, and the globe limited the movements of the globe in all directions, although the muscles might be normal anatomically and functionally. Therefore, the clinical characteristic of the current case series was highly variable, depending on the severity of involvement of each extraocular muscle. Any type of strabismus could be present, even in a normal primary position as in case 3, not limited to the convergent or divergent form as reported previously. The so-called convergent or divergent form could be the result of adhesion rather than the cause.
Traditionally, strabismus fixus has been treated with surgery. As Hayashi and Maruo18 suggested, surgery should be offered early in cases with extreme degrees of deviation. Many surgical methods have been described as successful for this condition, although undercorrection and recurrence may occur postoperatively.19 Under optimal conditions, a small field of single binocular vision can be restored. In this report, the first two cases did undergo surgery for the deviations. However, both cases showed no postoperative improvement in the primary position and eye movements. On the basis of our experience and understanding, surgery was not suggested for case 3 because this specific condition is unlikely to benefit from traditional strabismus surgery.
There are some limitations to our study. This was a retrospective study with a relatively small sample size. However, this disease is rare, and it will hopefully be of benefit to have a better understanding of the clinical characteristics to offer a tailored treatment plan and have an informed prognosis.
Congenital monocular strabismus fixus is characterized by limited ocular motility in all directions with or without strabismus in the primary gaze and adherences among the extraocular muscles, posterior Tenon's capsule, and globe by imaging, and traditional strabismus surgery is ineffective. Given its rarity, recognition of this condition may be challenging. The synergy of detailed history, careful ocular examination, and MRI are essential in the diagnosis. Our study suggests that the presence of unusual ocular motility patterns and the typical findings on orbital imaging examination will be helpful in the diagnosis of congenital monocular strabismus fixus. Further studies are needed to guide its possible treatment. Restrictive motility problems cannot be solved without the release of adherences that cause the restriction. Therefore, continued investigations of pharmacological strategies that combat excessive adherence of extraocular muscles, Tenon's capsule, or orbital fat are anticipated to aid in the discovery of efficacious medical treatments for congenital monocular strabismus fixus and may have broader treatment implications for other restrictive problems related to strabismus surgery.