Acquired and congenital displacements of the paths of the eye muscles (pulley shifts and pulley heterotopias) are newly recognized causes of strabismus. Downward displacement of the lateral rectus muscles causes acquired esotropias including the “sagging eye syndrome” of aging, the “heavy eye syndrome” seen with high myopia, the “acquired distance esotropia” or “divergence insufficiency” seen in patients with axial length above 23.5 mm, and acquired V-pattern esotropia with extorsion. Displacements of other muscles may also cause exotropia, hypertropia, torsion, and other unexpected complex strabismus patterns. The surgical correction of pulley abnormalities and associated eye muscle displacement is a field yet in its infancy, and procedures are still evolving. Early results of surgeries to correct displaced muscles have been promising.
Procedures already in use include equatorial myopexy, lateral to superior rectus myopexy (with either nonabsorbable sutures, a loop, or the use of a silicone band), pulley sleeve fusion, muscle transpositions, and direct repair of traumatic pulley ruptures and displacements. Partial angled recessions and resections of rectus muscles can also have small corrective effects on muscle pulley shifts.
Adding to the complexity is a new concept that underlying collagen weakness may be associated with muscle displacement in some patients, who may not respond the same to surgery as patients with stronger collagen and healing. The surgeon needs to adjust the surgical plan to compensate for weak collagen in treating muscle displacements.
19 Displaced Muscles (Pulley Heterotopias)
The anatomy of the orbital connective tissue was first detailed by Koornneef in 1977, 1 but little attention was paid to these tissues in strabismus teaching until the recent large body of work identifying the orbital “pulleys” 2 , 3 , 4 , 5 , 6 , 7 (Chapter 4). It was during initial attempts at developing a computer model of the oculomotor system in the 1970s that Robinson et al discovered major flaws in the classic assumptions. Plugging the then-understood anatomy and physiology of the extraocular muscles (EOMs) into his model predicted that the globe would flip and point backward toward the orbital apex. He realized that the assumptions were incorrect and launched the investigations that inspired the anatomical and functional studies of the pulleys (see foreword by Dr. Demer).
We have been armed with a wealth of new anatomical and functional studies of the pulleys but are only just beginning to incorporate these principles into surgical practice. Writing this chapter and its companion chapter, “Pulley Surgeries” (Chapter 30), is like trying to pin down a moving target due to the rapid evolution of techniques. Dramatic responses to seemingly small procedures are already being achieved, and other new approaches will undoubtedly follow.
19.2 Pulley Abnormalities and Strabismus
Strabismus due to congenital orbital anomalies has long been known and is now known to be associated with pulley displacements. 8 , 9 , 10 Acquired orbital anomalies due to trauma may also disrupt the pulleys 11 as well as the muscles themselves (Section 19.10 Flap Tear and Pulley Trauma). Other disorders that may disrupt the orbital connective tissue “scaffolding” include sinus and orbital surgery, chronic sinusitis with orbital floor collapse, inflammation, infection, and neoplasia.
Less well studied, but potentially just as important, are systemic collagen disorders. As the pulleys are primarily connective tissue structures, genetically weak collagen may lead to muscle displacements and strabismus 10 (Chapter 5). Aging and chronic illness cause acquired weakening of collagen, 12 , 13 which has been shown to lead to esotropia (sagging eye syndrome [SES], see below).
As with any medical problem, and particularly strabismus, much of the needed information begins with the history (Chapter 2). Specific questions to ask regarding muscle displacements would include general questions about collagen, such as unusual flexibility or stiffness, frequent sprains and orthopaedic abnormalities. Was myopia present? Prior cataract or refractive surgery may have corrected myopia, so the patient should be specifically questioned about this, which is then confirmed by axial length measurement and a search for muscle displacement. History of blunt facial trauma should be questioned to rule out flap tear and occult orbital fracture, which are often associated with pulley disruption.
Facial inspection may alert the examiner to the presence of orbital anomaly with or without orbital rotation. Compare the lateral and medial canthal positions of the eyelids. Abnormal eyelid rotation can provide a clue about underlying orbital rotation (Fig. 19‑1). 12 Look for orbital asymmetry, due to congenital or traumatic etiology.
During the complete strabismus evaluation, special attention should be paid to the objective assessment of torsion on fundus examination (Chapter 11 Torsion). Pulley heterotopia may mimic oblique dysfunction with profoundly abnormal torsion. 10 , 12 Version testing will often reveal reduced duction of the globe into the field(s) of action of the displaced muscle(s). Version testing beginning from different positions of gaze (i.e., comparing elevation in adduction, beginning first with adduction before elevation, and then beginning with elevation before adduction) can distinguish between commutative and noncommutative eye movement. Noncommutative movement is suggestive of pulley dysfunction and can aid in distinguishing a pulley abnormality from true restriction. 14
22.214.171.124 Supine Measurement of Alignment
Imaging can detect pulley instability in different gaze directions, but what if the pulley shift has a gravitational component? Alignment is measured with the patient’s head erect, but imaging is performed in the supine position. Author IL has begun to recheck alignment in the supine position in patients with suspected pulley instability. This has only been performed in a few patients, so results are preliminary, but a substantial difference in alignment measured in the supine position as compared to erect alignment was observed in some patients.
19.3.3 The Role of Imaging in Evaluating Patients with Suspected Muscle Displacement
Magnetic resonance imaging (MRI) studies of the extraocular muscles has had a crucial role in elucidating the role of muscle displacement in the heavy eye syndrome (HES), 15 and SES. 12 , 13 However, some of these studies, especially those by Demer et al, were done using MRI techniques that are not readily available worldwide. Furthermore, MRI studies are expensive and require a long waiting time in some countries. Hence two questions arise:
Should we do imaging of all patients with suspected muscle displacement?
If so, what image modality is preferred—computed tomography (CT) or MRI?
In our experience, imaging of these patients rarely changed our surgical approach and technique. When evaluating a patient with large strabismus fixus and elongated globes (axial length around 30 mm), the diagnosis is apparent. Moreover, during surgery, the displacement of the muscles is very obvious. In these cases, imaging did not contribute to our treatment plan, and therefore we ultimately stopped imaging such patients.
Imaging may play a role, however, in the more subtle cases, where there is mild esotropia for distance only, and slight abduction deficit. In these cases, the downward displacement of the lateral rectus (LR) may be seen, confirming the diagnosis. Imaging can also rule out other causes for abduction deficit, such as sixth nerve paresis due to an intracranial lesion. We have found that good-quality CT with thin slices can be as effective as MRI in evaluating the muscle path.
Displacement of the vertical rectus muscles is difficult to diagnose without high-quality MRI with coronal sections. MRI is also very useful to sort pulley heterotopia from oblique dysfunction. MRI sometimes shows a small superior oblique (SO) belly, indicating SO palsy or aplasia.
Another important examination that should be done in patients with suspected LR displacement and distance esotropia is axial length measurement. In their study, which compared axial length of patients with esotropia due to muscle displacement (acquired distance esotropia) with that of other patients with esotropia and patients awaiting cataract surgery, Morad et al showed that all patients with muscle displacement had an axial length of above 23.5 mm, averaging 25.05 mm. In contrast, the control patients with esotropia or cataract had an average axial length of 22.4 to 22.9 mm. They concluded that axial length below 23.5 mm makes this diagnosis questionable. 16 Similarly, Demer et al reported that patients with SES had an average axial length of 24.1 mm in one study and 32.0 mm in another. 13 , 17
19.3.4 Intraoperative Evaluation
Diagnosis of muscle/pulley displacements may not be definitive until inspection is performed under anesthesia. Forced duction and force generation testing can help distinguish between restrictive strabismus and gaze limitation due to malpositioned muscle (just as it would help to identify a palsied muscle) (Chapter 18 Cranial Nerve Palsies). When oblique dysfunction is suspected but normal torsional forced duction is present, this suggests the presence of pulley displacement. The converse does not hold true. Abnormal torsional forced duction is frequently present with muscle displacement and resolves with restoration of the muscle path(s). The exaggerated traction test 18 of the oblique muscles should not be impacted by pulley displacements, and this can help distinguish between true oblique dysfunction and pulley heterotopia.
Prior to the conjunctival incision, the muscles can often be viewed through the conjunctiva, and their posterior paths traced by viewing the ciliary vessels and the muscle fibers’ posterior positions (Video 19.1).
Two small fornix incisions are sufficient to inspect all four rectus muscles: an inferonasal incision for the inferior rectus (IR)and medial rectus (MR) muscles, and a superotemporal incision for the LR and superior rectus (SR) muscles. One must not dissect the intermuscular septum or capsule; just place the hook and relax traction, which can displace the muscle belly. The Desmarres retractor is used to allow viewing of the muscle path (Fig. 19‑2, Video 19.2).
19.4 Eye Muscle Displacements—Esotropias
19.4.1 Heavy Eye Syndrome
For years, HES was an enigma in strabismology. Patients with this condition, all of whom were highly myopic, suffered from acquired esotropia and limited abduction. Strabismus tended to increase dramatically over time, until in some cases the eyes were stuck in an esotropic position with absent abduction, hence the term strabismus fixus (Fig. 19‑3).
In some cases associated hypotropia of one eye was seen, leading to the term heavy eye, as if the heavy weight of the large globe simply caused it to drop down with time. 19 , 20 , 21 , 22 Conventional recession or resection procedures of the horizontal rectus muscles yielded disappointing results. 19 , 23 Many theories attempted to elucidate the etiology for this phenomenon. It was postulated that the increased weight of the eyeball or forward movement of the center of mass of the globe resulted in a drop of its anterior half, hence the term heavy eye. 24 , 25 Parks felt that the MR muscles were acting like fibrous bands that prevented abduction in these patients. 26 Duke-Elder suggested structural changes in the extraocular muscles as well as a shortening of the optic nerve as the source of the problem. 27 Others postulated that the large globe compressed the LR against the orbital wall, causing ischemia and LR muscle atrophy, 20 , 28 or myositis. 29 The demonstration of amyloid in histopathology studies of LR tissue samples led others to believe that HES was a congenital myopathy. 30 Since conventional MR recession surgery often resulted in recurrence of the esotropia, 31 more extreme surgical procedures were suggested, such as MR tenotomies, 20 or disinsertion and myectomy of each MR muscle, combined with resection and advancement of the LR muscles. 32 While these severe measures could reposition the eyes in primary position, full ductions were never restored.
A breakthrough in understanding the etiology of this entity was achieved by Krzizok et al, who evaluated magnetic resonance scans of the orbits of these patients. 15 , 33 They demonstrated that the large globe is dislocated temporally and superiorly between the lateral and superior rectus muscles, displacing the LR downward and the SR medially (Fig. 19‑4, Fig. 19‑5). They suggested that alignment could be achieved by performing a large recession of the MR and anteropositioning of the LR to its physiologic meridian by suturing it to the sclera with a nonabsorbable suture. 33 Hayashi et al later suggested that a partial Jensen procedure would restore alignment in these patients, 21 and Yokoyama was the first to suggest performing LR to SR myopexy 34 (Chapter 30 Pulley Surgeries).
Since first described in early 2000, the treatment of esotropia in patients with extreme myopia has become widely popular utilizing different methods to restore the original path of the lateral and superior rectus in these patients, such as loop myopexy, silicone band, and even rectus muscle transplantation. 35 36 37 The authors have begun to achieve similar success with SR to LR pulley sleeve fusion, suturing into the pulley sleeves alone. This restores the muscle path without disrupting the muscle tissue itself (Fig. 19‑6, Chapter 30 Pulley Surgeries).
19.4.2 Sagging Eye Syndrome
Another breakthrough in our understanding of the etiology of esotropia due to muscle displacement was achieved when Demer and Rutar published their paper on SES in 2009. 12 , 13 They described a group of elderly patients who suffered from esotropia greater for distance than near that was caused by downward displacement of the LR. This, however, was not caused by a large globe (axial length in these patients was 24 mm on average, similar to their nonstrabismic control group), but was due to degeneration and rupture of a specific band that connects the superior and lateral rectus muscles (Fig. 19‑7). This band tends to atrophy over time, and its rupture allows downward displacement of the LR (Fig. 19‑8), causing esotropia as well as cyclovertical deviation due to limited supraduction of the affected eye. In a later study, Demer and Tan elucidated the difference between HES and SES. 17 In HES the major finding is prolapse of the globe temporally and upward between the SR and the LR, causing downward displacement on the LR and nasal displacement of the SR; but in SES the main problem is rupture of the SR–LR band causing downward sagging of the LR without nasal shift of the SR (Fig. 19‑7). According to Demer et al, this distinction is important for the surgical treatment of both conditions: while SR–LR myopexy is the preferred treatment for HES, augmented MR recession will be enough for the treatment of SES. (Clark recommends LR myopexy for SES. 38 , 39 , 40
19.4.3 Divergence Insufficiency
As first described by Duane in 1896, divergence insufficiency is an acquired form of concomitant esotropia characterized by deviation greater for distance than near. 41 Although this condition was reported to be associated with neurologic problems, mainly in children, 42 in adults it is usually a benign phenomenon, which was lately referred to as “age-related distance esotropia.” 43 In contrast to acute acquired comitant esotropia, which usually is greater for near and appears in all ages, the natural history of these patients is characterized by diplopia for distance, at first usually occasionally, which later deteriorates to constant diplopia for distance and near. As noted by Webb and Lee, most patients were myopic and therefore were initially treated with prisms. However, as the esotropia increased over time, many required surgical treatment. 44 , 45
Conventional treatment options for divergence paralysis esotropia were LR resection as suggested by Stager et al, or MR recession as suggested by Chaudhuri et al, Repka et al, and Mittelman. 46 , 47 , 48 , 49 Although both methods achieved good results, it should be noted that 10% of the patients who had LR resection in Stager’s study still had undercorrection and required prisms to avoid diplopia. MR recession, on the other hand, was reported to cause overcorrection for most authors.
In their 2017 study, Morad et al postulated that age-related distance esotropia is also caused by LR downward displacement, with or without nasal shift of the SR. 50 They presented a series of patients with distinct clinical features: acquired esotropia greater for distance, which is constantly progressing; diplopia mainly for distance; mild abduction deficit; and moderate myopia (average 5.5 diopters). Interestingly, 77% were women. The authors treated all patients with the same operation regardless of the size of the deviation: bilateral SR–LR myopexy with unilateral MR recession on an adjustable suture, with the final recession ranging from 0 mm to 4 mm. The surgical technique for SR–LR myopexy will be described in detail in Chapter 30 Pulley Surgeries. In short, this surgical technique involves placing a nonabsorbable suture over half of the muscle belly of both the superior and lateral rectus muscles, and then tying both muscles together. This approach eliminated diplopia in all cases. In two cases in which diplopia recurred after 6 to 12 months, recession of the second MR resulted in cure. Later, the same authors compared the axial length of a similar group of patients, with axial length of patients with “conventional” esotropia, as well as with a random sample of patients with no strabismus, awaiting cataract surgery. 16 While the average axial length of the conventional esotropia and cataract groups was similar (average 22.4–22.9 mm), the axial length of the distance esotropia group was significantly greater (25.05 mm). In fact, except for one patient with axial length of 23.5 mm, all patients had axial lengths greater than 24.0 mm.
LR equatorial myopexy for SES/age-related distance esotropia without involving the SR was proposed by Clark (Fig. 19‑9). 10 , 38 , 39 , 40 He presented a series of patients who underwent this procedure, with promising success. 38 Kowal also reported good results with this method. 51 One complication of this technique, which has been observed by Dr. Kowal as well as this chapter’s coauthor (IL), was a new effective posterior insertion of the muscle created by an aggressive healing response at the site(s) of the myopexy.(L. Kowal, personal communication). This complication caused decreased muscle action due to a new effective posterior (pseudorecessed) muscle insertion rather than the expected improvement of muscle action due to restoration of muscle path. Correction required removal of the myopexy sutures and small resection of the LR instead. A recent adjustment of technique by this coauthor has been made to only incorporate the orbital/external layers of the muscle into the myopexy suture, which may help to prevent this complication.
19.4.4 Congenital and Juvenile Acquired LR–SR Band Deficiency
Patients have recently been identified by author IL with a clinical appearance similar to heavy eye and sagging eye syndromes, without associated high myopia, large axial length, or aging. Some present with V-pattern, extorsion, and pseudo-SO palsy. Others have a more comitant esotropia without extorsion. Both types have greater esodeviation at distance than near, and inferiorly displaced LR pulleys. Those without V-pattern and extorsion seem to have larger nasal displacement of the SR than those with V-pattern and extorsion. Intorsion created by the nasal shift of the SR may serve to cancel out the extorsion caused by the inferior shift of the displaced LR (Chapter 11 Torsion). In all these patients the LR–SR band (Fig. 19‑7, Chapter 4) is deficient or absent. Several had convincing histories of congenital esotropia, possibly due to congenital abnormality of the LR–SR band. LR sag has also been recently reported by Clark et al as a cause of recurrent esotropia in children. 52
A marked decline in the incidence of congenital esotropia has been observed in recent years. If some of these cases had been due to LR-SR band insufficiency, could the high frequency of forceps-assisted childbirth in the mid-twentieth century have contributed to esotropia by damaging the LR-SR band in these infants?
126.96.36.199 Acquired LR–SR Band Deficiency—Possible Collagen Abnormality
Some patients with acquired esotropia have been identified as having LR–SR band deficiency. In several of these, the development and progression of esotropia was documented by author IL, and as the esotropia progressed, so did the fundus extorsion. The common features of these cases were histories of unusual musculoskeletal flexibility, and in one child, a family history of joint hypermobility syndrome. 53 It is postulated that underlying collagen abnormalities led to pulley instability in these acquired cases. These patients responded well to LR myopexy or LR–SR pulley sleeve fusion procedures, but these approaches are new and evolving. Long-term results are not yet available (Chapter 30 Pulley Surgeries).
19.4.5 Traumatic LR–SR Band Rupture
Acquired isolated LR displacement without myopia or identifiable collagen weakness has also been observed. This may have resulted from minor blunt trauma in several patients, in whom it was unilateral (Section 19.10.2 Intermuscular Band Trauma, Section 19.14.1 Case 1).
19.4.6 Esotropia Due to Displaced Muscles—Summary
Downward displacement of the LR with or without nasal shift of the SR is a common cause of acquired esotropia. Clinical presentation varies from extreme cases, in which a large globe dislocates upward and temporally between these muscles causing strabismus fixus, with large esotropia and very limited abduction, to mild cases in which downward sagging of the LR alone causes esotropia and diplopia only for distance.
Common features for all these cases are esotropia greater for distance than near, mild abduction deficit, and deficiency of the SR–LR band. Many of these eyes also have high axial lengths.
The main goal of treatment is to restore the original muscle path. If both the SR and LR are displaced, then SR–LR myopexy or SR–LR pulley sleeve fusion should be performed. If the LR is the only muscle sagging downward, then equatorial myopexy may be enough. The surgeon may choose to put one MR on an adjustable suture in case further recession will be needed. This may be due to MR contracture that developed over time.
19.4.7 Collagen and Surgery
The surgical plan should take into account the strength of the patient’s collagen. If collagen is weak, then sutures may not hold in tissue, tightened layers may stretch out again, and the repair will fail. These issues plague all surgical procedures that attempt to deal with the consequences of weak collagen, such as in uterine and bladder prolapse, incisional abdominal hernia, and joint hypermobility (Chapter 5). 53 , 54 , 55 In some patients with weak collagen and displaced muscle(s), combining both myopexy and pulley sleeve fusion adds support to the repair, and seems to improve postoperative alignment stability. This has been successfully performed in a few patients with weak collagen.