Strabismus Fixus is a progressive, large-angle esotropia and hypotropia with restricted ocular rotation that develops in patients with long-standing pathological high myopia. It is also known as “Heavy eye syndrome.” It is considered the most severe form of age-related divergence insufficiency.
High myopia is generally defined as a refractive error of -8.00D or more and an axial length of 27 mm or more. In most cases of fixed strabismus, the axial length reaches 30 mm or more. It occurs in association with congenital fibrosis, amyloidosis, high myopia, etc.
The age of onset is often middle-aged or older, and the course is progressive. There is a marked sex difference, with women accounting for about 90% of cases.
The global increase in the prevalence of myopia is also an important background. By 2050, the world’s myopic population is expected to increase to about 5 billion, and the high myopia population to about 1 billion7). In Japan, a survey of 22,379 people in their 60s and 70s reported that 6.96% had high myopia with an axial length of 26.0 mm or more in both eyes1), and the prevalence of high myopia (-7.90D) has increased from 0.2% to 1.6% over 30 years1).
It is more common in elderly women with high myopia. About 90% of patients are women, and the majority have an axial length of 30 mm or more. It typically develops after middle age and progresses gradually.
The eye is fixed in an adducted and infraducted position; in the most severe cases, it does not move at all. Mechanical restriction of movement is observed in abduction and supraduction. The traction test is positive in the abduction and supraduction directions.
There are two main phenotypes of fixed strabismus2).
Esotropia and Hypotropia Type
Eye position: Mainly inward and downward deviation. The most common phenotype.
Abduction limitation: Eye movement in the outward direction is significantly restricted.
Elevation limitation: Upward eye movement is also impaired.
Strabismus angle: In typical cases, esotropia reaches about 110 PD and hypotropia 14 PD2).
Exotropia and Hypotropia Type
Eye position: Mainly outward and downward deviation. A relatively rare phenotype.
Adduction limitation: Eye movement in the adduction direction is restricted.
Differences in pathogenesis: The patterns of downward displacement of the lateral rectus and nasal displacement of the superior rectus differ.
Associated findings: May be accompanied by a yellowish, soft fatty mass under the conjunctiva (fat prolapse)2).
When accompanied by subconjunctival fat prolapse, it is observed as a yellowish, soft mass under the superolateral conjunctiva2).
There are multiple causes of fixed strabismus, but high myopia is the most common.
The following classification of causes shows differences in pathogenesis and clinical features.
| Cause | Mechanism/Features |
|---|---|
| High myopia (most common) | Globe dislocation due to axial elongation |
| Congenital fibrosis | Loss of elasticity of the medial rectus muscle |
| Lateral rectus palsy | Secondary medial rectus contracture and fibrosis |
| Amyloidosis | Amyloid infiltration of extraocular muscles |
| Other | Mechanical factors, progressive fibrosis, myopathy, myositis |
High myopic strabismus fixus: The mean axial length in myopic strabismus fixus varies by report: 28.9±2.03 mm, 30.5±3 mm, and 32±5 mm 1). In many patients, myopia is -15 D or more and axial length exceeds 31 mm 2).
Not all patients with high myopia develop strabismus fixus. If axial elongation occurs in a uniform direction and dislocation of the eyeball superolaterally does not occur, strabismus fixus does not develop. The relationship between the direction of axial elongation and the muscle cone is key to pathogenesis.
Orbital MRI coronal sections are the most suitable diagnostic method. T1-weighted imaging (without fat suppression) is optimal, clearly evaluating extraocular muscle displacement and posterior globe dislocation.
Measurement of the dislocation angle (SR-LR angle) is important, reaching an average of 179.9±30.8° in fixed strabismus, which is significantly larger than the average of 102.9±6.8° in cases without fixed strabismus 4). In mild cases, MRI in multiple directions (forward gaze, right-downward, left-downward) is necessary.
Coronal CT can also evaluate the dislocation position of the posterior eyeball and the condition of extraconal fat 2).
Differentiation from the following diseases is important.
Coronal orbital MRI is most suitable. T1-weighted imaging (without fat suppression) evaluates extraocular muscle displacement and globe dislocation. In fixed strabismus, the dislocation angle between the superior rectus and lateral rectus averages 179.9±30.8°, significantly larger than in cases without fixed strabismus 4).
Surgery is the only effective treatment. Because the deviation is large and incomitant, conservative treatments such as prisms are generally not indicated.
This is the first-choice procedure. The muscle bellies of the superior rectus (SR) and lateral rectus (LR) are sutured and fixed 15 mm posterior to their insertions, aiming to return the dislocated posterior globe into the normal muscle cone. Standard suture material is 5-0 polyester.
Whether to add medial rectus recession to the Yokoyama procedure is debated. In cases with long-standing abduction limitation, medial rectus contracture may have occurred, so adding medial rectus recession may be recommended 1). However, some reports indicate no additional benefit of medial rectus recession compared to pure Yokoyama procedure, and others point out a risk of overcorrection 1).
Koiwa et al. (2024) reported a case of a 68-year-old woman with an extremely long axial length of 33.97 mm 1). Preoperatively, the right eye was fixed in adduction and infraduction, presenting with approximately 113Δ esotropia. MRI showed a dislocation angle of 181°. After Yokoyama procedure (suturing SR-LR 15 mm posterior to the insertion with 5-0 polyester suture) combined with 5 mm medial rectus recession, residual esotropia improved to about 20Δ, and abduction beyond midline became possible. Postoperative MRI showed improvement of the dislocation angle from 181° to 104°.
As an alternative to suture muscle fixation, loop muscle fixation using a silicone band is available 2). Compared to conventional suture muscle fixation, it has advantages of fewer complications such as globe perforation, muscle dehiscence, suture-related complications, and anterior segment ischemia 2)6).
Lalwani & Kekunnaya (2021) reported a 60-year-old male with bilateral high myopic fixed strabismus (axial length: right eye 34.21 mm, left eye 34.41 mm) complicated by fat prolapse 2). Surgery combining lateral rectus-superior rectus fixation with a silicone band (No. 240), medial rectus recession, and fat excision was performed. Postoperatively, abduction limitation improved from -4 to -1, and fat prolapse resolved 2).
In cases with a deep orbit or narrow palpebral fissure, suturing 15 mm posterior to the insertion may be difficult with the conventional method. A modified technique adding lateral canthotomy, lateral canthal ligament disinsertion, and superior conjunctival fornix incision has been reported to secure a wider surgical field 3).
Lee et al. (2021) performed the modified Yokoyama procedure on a 69-year-old female with a deep orbit and narrow palpebral fissure (Hertel values 11 and 12 mm) 3). In addition to lateral canthotomy, partial removal of the lateral orbital wall ensured an adequate surgical field. At 3 months postoperatively, good ocular alignment and improvement in extraocular muscle movement were achieved 3).
Surgery is the only effective treatment. Because the deviation is large and incomitant, conservative treatments such as prisms are generally not indicated. In addition to the difficulty of prism correction due to the angle of strabismus, the essence of treatment is to release the mechanical restriction of the eye, so no fundamental improvement can be expected without surgery.
The mechanism of high myopic strabismus fixus involves the following stepwise changes.
In highly myopic eyes without fixed strabismus, the average dislocation angle is 102.9±6.8°, whereas in fixed strabismus it is significantly larger at 179.9±30.8°4), indicating that the degree of dislocation is directly involved in the development of fixed strabismus.
General pathology: Histological findings of extraocular muscles show loss of myofibrils and fibrosis. In congenital fixed strabismus, congenital fibrosis of the medial rectus muscle is the main pathological finding; in cases secondary to lateral rectus palsy, secondary contracture and fibrosis of the medial rectus muscle are predominant.
Wabbels et al. (2021) reported real-world clinical data of the Yokoyama procedure in a large-scale multicenter analysis5). The addition of medial rectus recession was found to have no additional effect compared to the pure Yokoyama procedure, and the risk of overcorrection was also pointed out1). These results have deepened discussions on determining the indications for the surgical technique.
The global increase in the prevalence of myopia is predicted to lead to an increase in the number of patients with fixed strabismus1). With the high myopia population expected to reach approximately 1 billion by 20507), research on prevention and early intervention for fixed strabismus will become an important future issue.
The effectiveness of modified techniques for anatomically challenging cases such as deep orbits or narrow palpebral fissures has been reported3), and combining lateral canthotomy or partial removal of the lateral orbital wall may expand surgical indications. Additionally, ongoing studies are evaluating the combination of silicone band myopexy and fat resection for cases with fat prolapse2).
