From the Departments of Ophthalmology (Y-HL, S-HK, YJ) and Pathology (K-SS), Chungnam National University Hospital, Daejeon; and the Department of Ophthalmology (S-NL), Eulji University Hospital, Daejeon, Korea.
The authors have no financial or proprietary interest in the materials presented herein.
Address correspondence to Youngjoon Jo, MD, Department of Ophthalmology, Chungnam National University Hospital, 640, Daesa-dong, Daejeon, 301-721, Korea.
Patients with anatomical abnormalities of the extraocular muscles have been reported infrequently. Nussbaum described this situation for the first time in 1893. Since then, agenesis, hypoplasia, bifurcation, and reduplication have been reported as abnormalities of the lateral rectus muscle.1 We describe the case of a patient with exotropia who had a normal-appearing lateral rectus muscle that was attached in an abnormal posterior position and severely hypoplastic tendon-like tissue attached in the normal lateral rectus insertion site.
A 30-month-old boy presented with exotropia. Except for mild mental retardation in his medical history, the patient had no medical problems or history of surgery. His fixation and following movement were normal. He showed exotropia of 15 prism diopters (PD) on distant and near fixation and had no limitation or overaction of eye movement.
During the surgery, tendon tissues that were believed to be tendons of the lateral rectus muscles with hypoplasia (5.0 mm width) were found. The tendon tissue inserted at the normal point (6.5 mm from the corneal limbus) and followed the course of a normal lateral rectus muscle in both eyes. The initial surgeon (S-NL) performed a 4-mm recession of “the hypoplastic tendon” in the right eye but could not manipulate the left “hypoplastic tendon” due to its more severe hypoplasia. After the surgery, the exotropia decreased slightly to approximately 10 PD. Two years after the first operation, the patient presented at the second surgeon’s (Y-HL) clinic with 25 PD of exotropia and 8 PD of hypertropia in the left eye. Magnetic resonance imaging revealed both lateral rectus muscles with normal contours attached to the sclera posteriorly (Fig. 1). The second surgeon (Y-HL) performed a reoperation.
Figure 1. (A) A Coronal T1-Weighted Magnetic Resonance Image. (B1 to B3) an Axial Post-Gadolinium T1-Weighted Image Demonstrating Abnormal Posterior Insertion of the Lateral Rectus Muscle Without Abnormal Structures.
At the second surgery, definite tendon tissue that had been believed to be the hypoplastic tendon was not found at the first operation site in the right eye. Instead, loose fibrous tissue that was discriminated from the surrounding Tenon’s capsule was detected, and an excisional biopsy was performed. At 15.5 mm posterior to the limbus, a lateral rectus muscle with a normal shape was found in the right eye. At this position, no evidence of previous surgery, such as adhesion or scarring, was found (Fig. 2). A 2-mm recession and superior transposition of the right lateral rectus muscle was performed. The medial rectus muscle had a normal appearance and its insertion was located 5 mm behind the limbus. A 3-mm resection and superior transposition was performed on the medial rectus muscle.
Figure 2. An Ectopic Lateral Rectus Muscle (Arrows Indicate the Upper and Lower Border of the Lateral Rectus Muscle) of Normal Width and Appearance, Inserted 15.5 mm Posterior to the Limbus. No Evidence of a Previous Operation Was Observed at the Insertion Site.
In the left eye, the insertion of the lateral rectus muscle was discovered 15.0 mm posterior to the limbus. The shape of the left lateral rectus muscle was normal. Histologic findings of the excised tissue from the right eye showed a pattern of collagenous connective tissue arranged in parallel bundles (Fig. 3), similar to a tendon. Seven months after the second operation, the primary gaze showed exotropia of 8 PD and mild excessive depression in adduction. No limitations on eye movement were observed.
Figure 3. Histopathologic Findings of Resected Connective Tissue from the First Operation Site Showing Collagenous Fibers Arranged in a Parallel Bundle (Hematoxylin–Eosin Stain, Original Magnification ×400).
Although various types of anatomical abnormalities of extraocular muscles have been reported,1–7 a case similar to this one has not been previously described. Lueder1 classified the abnormality of extra-ocular muscles into three types, the first and second of which seem to have some similarity to this case.
The first type is characterized by structures that originate from the rectus muscles and insert in abnormal locations. Apple2 described 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 1.5 cm posteriorly, where it fused with the internal surface of the medial rectus muscle. Scobee3 also described a patient with marked esotropia who was found to have an accessory muscle that originated from the inferior border of the medial rectus muscle and attached 4 mm posterior to the normal insertion point.
The second type in Lueder’s1 classification is characterized by fibrotic bands that are found beneath the rectus muscle. Salus4 described a 19-year-old patient with globe retraction who was found to have fibrotic tissue bands beneath the medial rectus muscles. Haase5 described a patient with a large hypertropia and exotropia of the left eye who was found to have a fibrotic muscular structure that inserted on the globe beneath the superior rectus muscle, 14 mm from the limbus.
The third type is characterized by discrete muscles that originate in the posterior orbit and insert in abnormal locations on the globe or extraocular muscles.1
The current case showed a hypoplastic tendon structure with the normal course of a lateral rectus muscle at its normal insertion site and a lateral rectus muscle with a normal appearance at an abnormal posterior site in the right eye. The findings in the left eye are supposed to be similar to those on the right side but in a more severely affected form. We assume that this unique anatomical abnormality is a type of accessory lateral rectus muscle. The lateral rectus muscles in the normal position did not develop normally; instead, accessory lateral rectus muscles with ectopic insertion developed fully. Histologic findings indicated that the excised tissue was not identical to normal tendon tissue, although the observed parallel arrangement of collagen bundles was characteristic of tendon tissue. This tissue probably did not bear tensile force for a long time, so the compact arrangement of the collagen fibers was probably lost. We found no other abnormal structures in either orbit.
The patient initially exhibited exotropia but, contrary to the deviation angle, the lateral rectus muscles were found to be severely hypoplastic at the first surgery. Subsequent magnetic resonance imaging showed a normal shape for the lateral rectus muscles with a posterior insertion. If the status of extraocular muscles is not matched with the strabismus status or the motility pattern is unusual, consideration of positional abnormalities of the extraocular muscles is required. Orbital imaging was useful in our diagnosis.
- Lueder GT. Anomalous orbital structures resulting in unusual strabismus. Surv Ophthalmol. 2002;47:27–35. doi:10.1016/S0039-6257(01)00285-5 [CrossRef]
- Apple C. Congenital abducens paralysis. Am J Ophthalmol. 1939;22:169–173.
- Scobee RG. Anatomic factors in the etiology of heterotropia. Am J Ophthalmol. 1948;31:781–795.
- Salus R. Ueber die angeborene retraktion des augapfels. Klin Monatsbl Augenheilkd. 1934;92:79–85.
- Haase W. Surgical treatment in a case of double m. rectus superior [article in German]. Klin Monatsbl Augenheilkd. 1971;158:709–713.
- Valmaggia C, Zaunbauer W, Gottlob I. Elevation deficit caused by accessory extraocular muscle. Am J Ophthalmol. 1996;121:444–445.
- von Lüdinghausen M. Bilateral supernumerary rectus muscles of the orbit. Clin Anat. 1998;11:271–277. doi:10.1002/(SICI)1098-2353(1998)11:4<271::AID-CA8>3.0.CO;2-U [CrossRef]