Anomalous Orbital Structures Resulting in Unusual Strabismus

SURVEY OF OPHTHALMOLOGY VOLUME 47 • NUMBER 1 • JANUARY–FEBRUARY 2002

MAJOR REVIEW

Anomalous Orbital Structures Resulting in Unusual Strabismus Gregg T. Lueder, MD Departments of Ophthalmology and Visual Sciences and Pediatrics, Washington University Medical Center and St. Louis Children’s Hospital, St. Louis, Missouri, USA Abstract. Anomalous orbital structures are a rare cause of strabismus. These structures attach to the globe and produce a mechanical restriction, resulting in incomitant motility disorders. Three types of anomalous structures have been described. The first arise from the extraocular muscles themselves and insert in abnormal locations. The second are fibrous bands located beneath the rectus muscles. The third are discrete anomalous muscles that originate in the posterior orbit and insert in abnormal locations on the globe. These structures have been associated with unusual patterns of strabismus. Clinical findings that suggest the presence of anomalous orbital structures include globe retraction not associated with Duane retraction syndrome, very large vertical strabismus, and an elevation deficit that is worse in abduction. When looking for anomalous orbital structures in patients with atypical strabismus, imaging studies should be considered. (Surv Ophthalmol 47:27–35, 2002. © 2002 by Elsevier Science Inc. All rights reserved.) Key words. extraocular muscle • fibrosis • orbit • restrictive myopathy • retractor bulbi • strabismus

nous bundle extending from the external border of the inferior rectus muscle to the lateral rectus muscle, and which fused with the bulk of the lateral rectus muscle.”10

I. Background A. HISTORICAL REVIEW

Anomalous orbital structures that attach to the globe and cause restriction of motility have been reported as a rare cause of unusual strabismus during the last century. The earliest cases were discovered at autopsy, and no clinical information regarding motility abnormalities was included. Nussbaum first reported this anomaly in 1893 in a patient with an accessory muscle that arose from the lateral rectus muscle and divided into three heads. One of the heads rejoined the lateral rectus muscle, one attached to the inferior rectus muscle, and the third attached to the superior rectus muscle.15 One year later, LeDouble described a patient as follows: “We have found in two eyes of the same subject a volumi-

B. EMBRYOLOGY OF THE EXTRAOCULAR MUSCLES

The extraocular muscles arise from mesodermal tissue in the orbit, which begins to differentiate into early myoblasts approximately 5 weeks after conception and acquires the characteristics of mature muscle by approximately 14 weeks.21 All six extraocular muscles develop simultaneously along their entire length.22 The mesenchymal tissue that condenses to form the muscles is separated into inferior and superior complexes. The superior rectus and superior oblique muscles develop from the superior complex, and the inferior rectus and inferior oblique muscles 27

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develop from the inferior complex. The medial and lateral rectus muscles develop from both complexes. The muscles continue to mature postnatally, particularly during the first 3–6 months of life.16 During development, embryonic isoforms of myosin are preserved at the distal and proximal ends of the extra-ocular muscles, which may contribute to their ability to adapt to changes in level of innervation postnatally.16 The molecular biology of human extraocular muscle development is not well understood. Rare inherited forms of congenital strabismus, such as congenital fibrosis of the extraocular muscles (CFEOM) types 1, 2, and 3, have been linked to chromosomes 12, 11, and 16 respectively.5 Although the extraocular muscles in CFEOM 1 (the most common form of this disorder) are markedly abnormal, Engle et al have shown that the muscle fibrosis most likely results from an abnormality of innervation of the muscle, rather than a primary myopathic process.6 In an autopsy study of a patient with CFEOM, Engle et al described the absence of the superior division of the oculomotor nerve and its alpha motor neurons, in addition to abnormalities of the muscles innervated by this nerve (the levator palpebrae superioris and superior rectus muscles). They concluded that the abnormalities resulted from maldevelopment of the extraocular muscle lower motor neuron system.6 Similarly, absence of the motor neurons in the abducens nucleus has been described in patients with Duane retraction syndrome, indicating that this disorder is also primarily neuropathic rather than myopathic.9,13 A major advance in the understanding of ocular motility has been Demer et al’s recent description of a pulley system in the orbit.4 The pulleys are connective tissue sleeves through which the extraocular muscles pass prior to inserting on the globe. Heterotopic locations of the pulleys have been associated with incomitant strabismus.3 The embryology of the pulleys has not been described, but it is likely that they arise from the same mesenchymal anlage from which the extraocular muscles develop. It is reasonable to assume that these pulleys contribute to the strabismus that occurs in patients with anomalous orbital muscles, but this remains to be elucidated. An interesting hypothesis regarding the origin of some anomalous orbital muscles is that they represent an atavistic retractor bulbi muscle.10,15,25 The retractor bulbi muscle is found in amphibians, some reptiles, and lower mammals. It originates at the orbital apex and travels within the muscle cone, either as a continuous sheet or as separate muscle bundles, to insert on the posterior surface of the globe.25 It is innervated by the sixth cranial nerve. It functions in a protective capacity by pulling the globe directly posteriorly into the orbit. It has been postulated that

some anomalous orbital structures may represent vestigial remnants of this retractor bulbi. Von Lüdinghausen described a patient with bilateral anomalous muscles that originated at the annulus of Zinn, traveled beneath the lateral rectus muscles, and attached to the inferior and lateral rectus muscles.24 He suggested that this anomalous structure was more likely a result of an early disturbance of the extraocular muscle anlage, rather than a vestigial retractor bulbi muscle. His two arguments to support this point were the following: 1) the retractor bulbi should lie in close approximation to the optic nerve, which it did not in the patient he described, and 2) the retractor bulbi should be innervated by the sixth cranial nerve, although in his patient the structure was innervated by the third cranial nerve.

II. Types of Anomalous Orbital Structures A. STRUCTURES ARISING FROM THE EXTRAOCULAR MUSCLES THEMSELVES AND INSERTING IN ABNORMAL LOCATIONS

The first type of anomalous orbital structure is characterized by structures that originate from the rectus muscles and insert in abnormal locations. Apple reported a patient with a congenital abduction deficit who was found to have a 4-mm wide muscular band that inserted posterior to the normal medial rectus muscle insertion and extended posteriorly 1.5 cm, where it fused with the internal surface of the medial rectus muscle.1 He described a second patient with a similar accessory muscle band, but it is unclear whether this originated from the medial rectus muscle itself or more posteriorly in the orbit. Scobee reported a child with marked esotropia who was found to have an accessory muscle that arose from the inferior border of the medial rectus muscle and inserted inferiorly and 4 mm posterior to the normal medial rectus muscle insertion.20 McNeer and Jampolsky reported an adult with a large left hypotropia and restricted elevation on forced-duction testing.12 This did not improve intraoperatively after disinsertion of the left inferior rectus muscle, and further exploration revealed a muscular structure arising from the inner surface of the inferior rectus muscle and inserting posteriorly onto the sclera (Fig. 1.). This was released and forced ductions were freed. B. FIBROUS BANDS LOCATED BENEATH THE RECTUS MUSCLES

The second type of anomalous structure consists of tough fibrous bands that are found beneath the rectus muscles but which are distinct and separate from the muscles. Salus described two 19-year-old

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gical attempts to sever the fibrotic structure in the right eye were not successful because the structure could not be safely separated from the optic nerve. Mühlendyck’s second patient was a 29-year-old with limitation of elevation of the left eye and globe retraction in upgaze. A CT scan revealed a structure beneath the optic nerve. Intraoperatively, a tough fibrous band was found posteriorly beneath the inferior rectus muscle. C. DISCRETE ANOMALOUS STRUCTURES ARISING IN THE POSTERIOR ORBIT

Fig. 1. Anomalous muscle tissue (small arrow) arising from undersurface of left inferior rectus muscle (large arrow) and inserting on sclera posterior to normal insertion (surgeon’s view). (Reprinted from McNeer and Jampolsky12 with permission of Elsevier Science.)

patients with globe retraction who were found to have fibrotic tissue bands beneath the medial rectus muscles.18 Haase described a 14-year-old patient with a very large left hypertropia (greater than 30 degrees) and exotropia (Fig. 2). During surgery, the patient was found to have a fibrotic muscular structure that inserted on the globe beneath the superior rectus muscle 14 mm from the limbus (Fig. 3). Mühlendyck et al reported two patients with abnormal fibrotic bands.14 The first was a 13-year-old with a complete absence of abduction and severe limitation of adduction and vertical extraocular movements in the right eye, with marked enophthalmos and globe retraction in attempted abduction. In the left eye there was mild globe retraction in upgaze. Computed tomography (CT) scans revealed a structure inserting on the globe, near the superomedial aspect of the optic nerve in the right eye, and a structure beneath the optic nerve that inserted on the posterior globe in the left eye. Sur-

The third type of anomalous orbital structure is characterized by discrete muscles that originate in the posterior orbit and insert in abnormal locations on the globe or extraocular muscles. In some reports the precise origins of the accessory muscles could not be ascertained, because imaging studies were not available and the posterior extent of the structures could not be visualized at the time of surgery. Fleischer first reported this anomaly in 1907.7 He described the autopsy findings in a neonate with multiple congenital anomalies, including microphthalmos and a retinal coloboma. An accessory muscle was present that originated at the orbital apex and inserted on the inferior portion of the optic nerve, 4 mm posterior to the lamina cribosa. A branch of the third cranial nerve appeared to be innervating this structure. Two forms of these anomalous structures that originate in the posterior orbit and insert in abnormal locations have been reported. The first, like Fleischer’s case, are single discrete structures that originate in the posterior orbit and insert directly on the sclera. The second are structures that arise at the annulus of Zinn, course laterally, and divide into structures that insert on the recti muscles. This latter form was reported by Whitnall in a 50-year-old male with bilateral anomalous structures: “In each case the muscle arose by a tendinous origin from the annulus of Zinn . . . at the apex of the orbit, and passing forwards within the cone formed by the four recti muscles of the globe, divided in the left orbit into two and in the right orbit into four muscular slips, which fused with the fleshy bellies of the recti muscles about halfway along their inner . . . sur-

Fig. 2. Fourteen-year-old patient with exotropia and a very large left hypertropia and marked limitation of left eye motility. (Reprinted from Haase8 with permission of Georg Thieme Verlag.)

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Fig. 3. Anomalous band of fibrous tissue beneath left superior rectus muscle (surgeon’s view). Forceps reflect the superior rectus muscle at the bottom of the photograph. Arrows indicate the insertion of the anomalous fibrous band. A muscle hook retracts the superior oblique tendon. (Reprinted from Haase8 with permission of Georg Thieme Verlag.)

faces.”25 (Fig. 4). Von Lüdinghausen reported similar structures in a postmortem description of the orbits of an 84-year-old male.24 The patient had no history of strabismus, but he was found to have accessory extraocular muscles bilaterally. In the right eye a sheet of muscular tissue originated at the annulus of Zinn and extended between the optic nerve and lateral rectus muscle. The structure split, with one end extending to insert in a 4-mm band on the superior rectus muscle and the other end attaching to the anterior end of the inferior rectus muscle. A similar structure was found in the left eye, originating at the annulus of Zinn and attaching to the inferior and superior rectus muscles. Branches of the inferior division of the third cranial nerve entered the anomalous muscular structure in both eyes.

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The second form of this anomaly, in which single muscular structures arise in the posterior orbit and insert on the sclera, has been reported more frequently. Posey described a patient with large-angle esotropia, who was found to have an accessory muscle inferior to the lateral rectus muscle (Fig. 5). The muscle inserted onto the globe by a short tendon, and coursed obliquely inferotemporally “until lost in the tissues at the equator of the globe.”17 Because the patient was esotropic, it is likely that this structure had no effect on the strabismus, and it was discovered only incidentally during surgery. Chaum and Hatton reported a similar anomalous muscle in a 65year-old woman with congenital nystagmus who underwent a scleral buckling procedure for repair of a retinal detachment. She was found to have a muscular structure in the inferotemporal quadrant that inserted just beneath the lateral rectus muscle in the right eye, and a tendinous structure that coursed superotemporally (Fig. 6).2 Scobee described a similar structure adjacent to the superior rectus muscle in a child with hypertropia.20 The patient had a muscular structure 4 mm lateral to the superior rectus muscle and 2 mm anterior to the equator (Fig. 7). The structure was not tracked posteriorly, so it is not certain whether it arose separately from the orbital apex or from the superior rectus muscle itself. Three patients with limited elevation due to discrete anomalous structures have been reported. Valmaggia et al described a 6-year-old child with an elevation deficit that worsened in abduction. Imaging studies revealed an abnormal structure that originated in the posterior orbit and inserted on the globe posteriorly beneath the optic nerve (Fig. 8).23 A child with similar motility findings (i.e., an elevation deficit that worsened in adduction) was described by Lueder et al.11 A CT scan revealed a 2-mm muscular structure that appeared to arise from the posterior orbit (Fig. 9). During surgery, a muscular structure was found to be attached to the sclera 18 Fig. 4. “The retractor bulbi muscle in the right orbit. The lateral rectus muscle has been cut from its two heads of origin and turned forwards, thus exposing the tendon of the retractor bulbi, with its four muscular slips passing to the four recti muscles. The third and fourth nerves passed between the sheath of the optic nerve and the tendon of the retractor bulbi, the latter separating these nerves from the sixth.” (Original description and figure reprinted from Whitnall25.)

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associated with encroachment of fibrous tissues into muscle fascicles (Fig. 11).”11 These findings confirm that the structures were muscular in origin. It is unclear, however, whether the structures were primarily fibrotic, or whether they became fibrotic over time, perhaps due to limited movement. It is also possible that the fibrosis was a secondary effect of abnormal innervation of the muscle tissue.

IV. Imaging of Anomalous Orbital Structures

Fig. 5. Anomalous muscular structure inserting beneath right lateral rectus muscle. (Reprinted from Posey17.)

mm from the limbus in the inferomedial quadrant (Fig. 10). Savino et al described a 10-year-old girl with limited elevation who was found, with use of echography and CT studies, to have a structure that arose at the orbital apex and inserted on the posterior globe inferomedially.19 The structure was presumed to be muscular, based on its acoustic features and changes in thickness with dynamic ultrasonography.

III. Histopathology of Anomalous Orbital Structures Histopathologic findings have been reported in two patients with accessory extraocular muscles. In Fleischer’s report published in 1907, the accessory structure was found to have fascicles of extraocular muscle.7 Histopathology of the distal portion of the structure in Lueder et al’s patient revealed “a paucicellular strip of pale fibrous tissue with small fascicles of mature skeletal muscle at one end. Focally, atrophy and loss of individual muscle fibers were

Imaging studies have been reported in only a few patients with anomalous orbital structures. Mühlendyck et al reported CT results in two patients.14 The first patient, who had significant restriction of extraocular movements in all directions, had no abnormalities on her initial CT scan. Repeat studies with a higher resolution scanner later revealed a structure inserting near the supero-medial aspect of the optic nerve. The second patient, who had limited upgaze, had a structure that inserted beneath the optic nerve. The patients reported by Valmaggia et al, Lueder et al, and Savino et al had structures that inserted on the posterior inferior portion of the globe, one on the lateral side and two on the medial side.11,19,23 The magnetic resonance imaging (MRI) and CT findings in Valmaggia et al’s (Fig. 8) and Savino et al’s patients showed the origin of the anomalous structure in the posterior orbit. The CT scan in Lueder et al’s case suggested a posterior origin, although the entire length of the structure was not visualized (Fig. 9). The structure in Savino et al’s patient was presumed to be muscular, based on features that were similar to the normal extraocular muscles on MRI, CT, and A-scan ultrasonography, as well as changes in shape with extraocular movement documented by dynamic ultrasonography.19 Abnormal structures in the orbit are rare, and careful communication with the radiologist may help

Fig. 6. Intraoperative photograph of right eye (left), demonstrating normal lateral rectus muscle with retraction suture beneath insertion, accessory lateral rectus muscle coursing inferotemporally (arrowhead), and tendinous structure coursing superotemporally (arrow). An artist’s rendering of the anatomy is on the right. (Reprinted from Chaum and Hatton2 with permission of Slack.)

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Fig. 7. Diagram of right eye as viewed from above, demonstrating an anomalous muscular structure inserting lateral and posterior to the superior rectus muscle. (Reprinted from Scobee20 with permission of Elsevier Science.)

in their identification. In the case of Lueder et al, the abnormal structure on the computed tomography scan was originally interpreted as a large orbital blood vessel. Reevaluation of the study in light of the clinical history and computer reconstruction of sagittal images suggested that an anomalous orbital structure was the most likely explanation for the imaging findings. If MRI is performed, the use of surface coil techniques may aid in the differentiation of orbital soft tissue structures.

V. Clinical Findings Suggesting Possible Anomalous Orbital Structures When evaluating patients with incomitant strabismus, one looks initially for motility abnormalities that are characteristic of a specific clinical entity (e.g., an elevation deficiency that is present only in adduction is typical of Brown syndrome). If the pattern of a restrictive strabismus is atypical, the differential diagnosis broadens to include orbital trauma, orbital masses (e.g., tumor), thyroid ophthalmopathy, and intrinsic extraocular muscle abnormalities. The presence of an accessory orbital structure should also be included in the differential diagnosis of patients with atypical restrictive strabismus. Several unusual patterns of ocular motility may suggest the presence of such a structure. The first unusual ocular-motility pattern is globe retraction. This may occur if a restrictive structure limits globe mobility. Globe retraction in adduction

Fig. 8. Top: coronal T1-weighted magnetic resonance image demonstrating anomalous structure (arrow) inferolateral to left optic nerve. Middle: sagittal T1-weighted image demonstrating anomalous structure (large arrow) inserting on posterior globe inferior to the optic nerve. Bottom: axial postgadolinium T1-weighted image with fat saturation demonstrating that the anomalous structure (arrow) has the same intensity as the lateral rectus muscle (L). (Reprinted from Valmaggia et al23 with permission of Elsevier Science.)

is typical of Duane retraction syndrome (DRS). Although accessory muscle bands have been reported beneath the medial rectus muscle in patients with clinical findings consistent with DRS,1,18 the paucity of these reports in this relatively common form of strabismus suggests that such abnormal structures are rare in patients with DRS. Globe retraction in other fields of gaze, however, is quite unusual, and has been reported with abnormal orbital structures.

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Fig. 9. Bottom: coronal computed tomographic scan demonstrates anomalous structure (arrowhead) inferomedial to left optic nerve (arrow). Top: computerized sagittal reconstruction demonstrates structure (arrowhead) inserting on inferior globe beneath optic nerve (arrow). (Reprinted from Lueder et al11 with permission of Mosby.)

Fig. 10. Operative photograph, left eye, surgeon’s view. The anomalous muscular structure (arrow) is exposed and stretched between two muscle hooks. The black retraction suture is beneath the inferior rectus muscle insertion (not visible), and the eye is being displaced superotemporally, giving a view of the inferomedial quadrant. (Reprinted from Lueder et al11 with permission of Mosby.)

evation deficit is usually symmetric in abduction and adduction). One of the patients reported by Mühlendyck et al had globe retraction in upgaze, and the second had globe retraction in abduction.14 A second atypical ocular motility abnormality associated with accessory orbital structures is the presence of a very large vertical strabismus. The patient reported by Haase had a hypertropia of greater than 30 due to a fibrotic band of tissue beneath the superior rectus muscle (Fig. 2).8 McNeer and Jampolsky’s patient had a 60-prism diopter hypotropia resulting from accessory muscle tissue beneath the inferior rectus muscle.12 A third unusual pattern of strabismus associated with an accessory extraocular structure is an elevation deficit that worsens in abduction (Fig. 12).11,23 This strabismus pattern is distinctly unusual in children, and distinguishes it from more common forms of vertical strabismus, such as Brown syndrome (in which the elevation deficit is greatest in adduction), and monocular elevation deficiency (in which the el-

VI. Treatment Not all anomalous orbital structures result in strabismus. Some of these structures appear to have been found in patients incidentally, either at autopsy24 or during surgery for a problem unrelated to the anomalous structure.2,17 Therefore, it is important to correlate the motility findings with the location of the anomalous orbital structure. If the structure produces a clinical effect, it should be one of restriction, limiting motility in the direction away from the anomalous structure. The first step in treating patients with strabismus due to anomalous orbital structures is recognition of the underlying anatomic abnormality. Orbital imaging studies (MRI or CT) should be considered for patients with the unusual ocular motility patterns noted above. The presence of an anomalous orbital structure is often not obvious, in part due to the rarity of this problem. The patients described by Scobee and McNeer and Jampolsky had undergone two and

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LUEDER Fig. 11. “Dense, paucicellular fibrous tissue (left) was predominant component of resected tissue. Skeletal muscle fibers in small fascicles are present on right. (Hematoxylin and eosin stain, original magnification  100). (Legend and figure reprinted from Lueder et al11 with permission of Mosby.)

three surgeries, respectively, before the anomalous structures were identified.12,20 The strabismus produced by anomalous orbital structures is typically restrictive. Intraoperative forced ductions are useful in these patients, as they are almost always restricted. A clue to the presence of an anomalous muscle is the persistence of restriction after disinsertion of a rectus muscle. In the patient described by McNeer and Jampolsky, the persistence of marked restriction to elevation after disinsertion of the inferior rectus muscle led to additional exploration, and to the discovery of the accessory structure inserting posteriorly on the sclera.12 In many patients, disinsertion of the anomalous orbital structure alone has produced good outcomes.1,20 Others have reported improvement, but not complete resolution of restriction, with disinsertion.1,11 Salus’s patients had no improvement.18 In the patients reported by Haase and McNeer and Jampolsky, recession of the adjacent rectus muscles was performed in addition to disinsertion of the anomalous orbital structures, and both patients had good outcomes.8,12 Mühlendyck reported a patient who initially had a good result after severing of a restrictive band and recession of the inferior rectus muscle. The patient’s symptoms recurred after two months,

however, and CT scans revealed probable reattachment of the anomalous structure to the globe.14

VII. Conclusion Anomalous orbital structures are a rare cause of strabismus. They should be considered in patients with atypical restrictive strabismus. Recognition of these structures may be difficult, given their rarity. The presence of specific unusual motility patterns and orbital imaging may assist in diagnosis. Release of the restrictive structures may improve motility in some patients.

Method of Literature Search MEDLINE and Ovid were used to search the medical literature from 1966 to the present. Other sources included references in identified articles and textbooks. Key words used were extraocular muscles, anomalous, accessory, and retractor bulbi. Four German and one French article were translated.

References 1. Apple C: Congenital abducens paralysis. Am J Ophthalmol 22:169–73, 1939 2. Chaum E, Hatton MP: Congenital nystagmus associated with multiple congenital anomalies of the extraocular muscles. J Pediatr Ophthalmol Strabismus 36:155–7, 1999

Fig. 12. In upgaze, the patient shows an elevation deficit of the left eye and exotropia. (Reprinted from Valmaggia et al23 with permission of Elsevier Science.)

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ANOMALOUS ORBITAL STRUCTURES 3. Clark RA, Miller JM, Demer JL: Location and stability of rectus muscle pulleys. Invest Ophthalmol Vis Sci 38:227–40, 1997 4. Demer JL, Miller JM, Poukens V, et al: Evidence for fibromuscular pulleys of the recti extraocular muscles. Invest Ophthalmol Vis Sci 36:1125–36, 1995 5. Engle E: A genetic approach to congenital extraocular muscle disorders. J Child Neurol 14:34–7, 1999 6. Engle EC, Goumnerov BC, McKeown CA, et al: Oculomotor nerve and muscle abnormalities in congenital fibrosis of the extraocular muscles. Ann Neurol 41:314–25, 1997 7. Fleischer: Musculus retractor bulbi und drittes lid bei einer menschlichen missbildung. Anat Anz 30:465-70, 1907 8. Haase W: Operative therapie bei einem fall mit doppeltem m. rectus superior. Klin Monatsbl Augenh 158:709-13, 1971 9. Hotchkiss MG, Miller NR, Clark AW, Green WR: Bilateral Duane’s retraction syndrome. A clinical-pathologic case report. Arch Ophthalmol 98:870–4, 1980 10. LeDouble: Variations des muscles de l’oeil, des paupieres et du sourcil dans l’espece humaine. Arch D’Opthalmol 14: 218-39, 1894 11. Lueder GT, Dunbar JA, Soltau JB, et al: Vertical strabismus resulting from an anomalous extraocular muscle. J Am Assoc Pediatr Ophthalmol Strabismus 2:126–8, 1998 12. McNeer KW, Jampolsky A: Double elevator palsy caused by anomalous insertion of the inferior rectus. Am J Ophthalmol 59:317–9, 1965 13. Miller NR, Kiel SM, Green WR, Clark AW: Unilateral Duane’s retraction syndrome (Type 1). Arch Ophthalmol 100:1468–72, 1982 14. Mühlendyck H, Markakis E, Helwig AT: Abnormal retraction syndrome due to persistent retractor bulbi. Transactions of the 19th meeting of the European Strabismological Association, Crete 169–73, 1991 15. Nussbaum M: Vergleichend-anatomische beiträge zur kenntnis der augenmuskeln. Anat Anz 8:208-10, 1893 16. Porter JD, Baker RS, Ragusa RJ, Brueckner JK: Extraocular muscles: basic and clinical aspects of structure and function. Surv Ophthalmol 39:451–84, 1995 17. Posey WC: Concerning some gross structural anomalies of the eye and its adnexa. Trans Am Acad Ophthalmol Otolaryngol 28:243–56, 1923 18. Salus R: Ueber die angeborene retraktion des augapfels. Klin Monatsbl Augenh 92:79-85, 1934 19. Savino G, D’Ambrosio A, Tamburrelli C, et al: Restrictive

20. 21. 22. 23. 24. 25.

limitation of sursumduction caused by an anomalous muscular structure. Ophthalmologica 212:424–8, 1998 Scobee RG: Anatomic factors in the etiology of heterotropia. Am J Ophthalmol 31:781–95, 1948 Sevel D: A reappraisal of the origin of the human extraocular muscles. Ophthalmology 88:1330–8, 1981 Sevel D: Extraocular muscles; their development and peculiarities, in Gorman CA (ed): The Eye and Orbit in Thyroid Disease. New York, Raven Press, 1984, pp 33–41 Valmaggia C, Zaunbauer W, Gottlob I: Elevation deficit caused by accessory extraocular muscle. Am J Ophthalmol 121:444–5, 1996 Von Lüdinghausen M: Bilateral supernumerary rectus muscles of the orbit. Clin Anat 11:271–7, 1998 Whitnall SE: An instance of the retractor bulbi muscle in man. J Anat Physiol 46:36–40, 1911

The author gratefully acknowledges the assistance of Joern Soltau, MD, and Alex Levin, MD, in translation of original articles. The author has no commercial or proprietary interest in any product or concept discussed in this article. Reprint address: Gregg T. Lueder, MD, Department of Ophthalmology and Visual Sciences, Washington University Medical Center, Box 8096, 660 S. Euclid Ave, St. Louis, MO 63110.

Outline I. Background A. Historical review B. Embryology of the extraocular muscles II. Types of anomalous orbital structures A. Structures arising from extraocular muscles and inserting in abnormal locations B. Fibrous bands located beneath rectus muscles C. Discrete anomalous structures arising in the posterior orbit III. Histopathology of anomalous orbital structures IV. Imaging of anomalous orbital structures V. Clinical findings suggesting possible anomalous orbital structures VI. Treatment VIII. Conclusion