From the Department of Ophthalmology, Soonchunhyang University Cheonan Hospital, Soonchunhyang University College of Medicine, Cheonan, Republic of Korea.
The authors have no financial or proprietary interest in the materials presented herein.
Address correspondence to So Young Kim, MD, Department of Ophthalmology, Soonchunhyang University Cheonan Hospital, 23-20 Bongmyungdong, Cheonan, 330-721, Republic of Korea.
Congenital anomalies of the extraocular muscles are a rare cause of strabismus and most reported cases have resulted in incomitant strabismus, which resembles paralytic strabismus. We report a case that, to our knowledge, represents the first instance of anomalous insertion of the medial rectus muscle presenting as comitant esotropia.
An 8-year-old girl who had been diagnosed as having esotropia at 8 months of age came to our clinic for operative treatment. After she was diagnosed as having esotropia, she had no follow-up examination or treatment because she seemed to have no serious ophthalmic or cosmetic problems. However, as she grew, her parents felt that her cosmesis suffered.
At initial examination, her best-corrected visual acuity was 20/20 in both eyes. Cycloplegic refraction was −2.0 diopters (D) in the right eye and −0.5 D in the left eye. The alternate prism cover test revealed that the patient had 25 prism diopters (PD) of comitant esotropia at distance and near, with and without correction. No abnormal findings were noted on duction and version tests. The patient had no significant medical or neurological history. Thorough medical and neurological examinations, including brain imaging, revealed no abnormal findings. We planned to perform bilateral medial rectus muscle recession.
Introperative forced duction testing revealed no abnormal findings. We performed a 4.5-mm left medial rectus muscle recession as planned. However, we found that the insertion of the right medial rectus muscle was abnormal. The superior half of the muscle fibers fanned out in the anterior direction and broadly inserted on the sclera 1.5 mm from the limbus (Figs. 1 and 2). We performed a 4.5-mm recession of the inferior half of the muscle fibers and measured the distance from the limbus to the newly created insertion. We then recessed the superior half of the muscle fibers the measured amount from the limbus, next to the inferior half of the muscle fibers, and sutured them together. After 1 year of follow-up, the patient showed 4 PD of esotropia at both near and distance.
Figure 1. Exposure of the Right Medial Rectus Muscle. Note that the Superior Half of the Insertion of the Right Medial Rectus Muscle Fans Out in the Anterior Direction and Inserts onto the Sclera 1.5 mm Posterior to the Limbus.
Figure 2. Disinsertion of the Insertion of the Right Medial Rectus Muscle. The Dotted Lines on the Schematic Drawings Represent the Insertion of the Right Medial Rectus Muscle.
Anomalous orbital structures, including fibrous bands, accessory muscles, and developmental anomalies of the extraocular muscles, are rare causes of strabismus.1 These structures usually cause mechanical restriction and result in incomitant strabismus resembling paralytic strabismus.
Isolated anomalous extraocular muscles without craniofacial syndromes or other systemic anomalies are rare. Moreover, anomalies of the medial rectus muscle are extremely rare: only six cases have been reported. Four of these cases have been accompanied by congenital absence of the inferior rectus muscle.2–5 Therefore, only two cases of isolated anomalies of the medial rectus muscles have been reported.6,7 Both of the reported cases have been accompanied by exotropia, in contrast to our case. To the best of our knowledge, our report represents the first case of an isolated anomalous medial rectus muscle with no systemic anomalies in a patient with nonrefractive comitant esotropia.
Embryologically, the extraocular muscles arise from mesodermal tissue in the orbit. The mesenchymal tissue 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 develop from the inferior complex. The medial and lateral rectus muscles develop from both complexes.8 We believe minor problems during embryogenesis might lead to abnormal medial rectus muscles.
It is not clear that the anomalous muscle was the direct cause of esotropia in our case. In many experimental studies, normal excursion of the globe and coordinated eye movements have been affected insignificantly or not at all after transposition of the various extraocular muscles.9 Transposition is not likely to have a major effect on ocular alignment unless a severe anomaly, such as total absence of a muscle, is present.10
Strabismus surgeons should be aware of the possibility that anomalous muscles, although uncommon, can cause unusual types of strabismus. Clinicians should be prepared to modify their surgical plans in such situations.
- Lueder GT. Anomalous orbital structures resulting in unusual strabismus. Surv Ophthalmol. 2002;47:27–35. doi:10.1016/S0039-6257(01)00285-5 [CrossRef]
- Asano K, Takahashi K. A case of congenital absence of the inferior rectus muscle. Jpn Rev Clin Ophthalmol. 1985;79:1709–1711.
- Harada M, Shinozuka K. A case of congenital absence of inferior rectus muscle. Jpn Rev Clin Ophthalmol. 1959;53:1123–114.
- Hirotsuji I. A case of congenital absence of the inferior rectus muscle accompanied by anomalous insertion of the medial rectus muscle. Jpn Rev Cin Ophthalmol. 1961;55:1211–1213.
- Imai S, Sonoda H, Sakai T. A case of congenital absence of both inferior rectus muscles. Jpn J Clin Ophthalmol. 1983;37:1443–1446.
- Sundaram V, Chen SD, Colley S, Hundal K, Elston J. Bifid medial rectus muscle insertion associated with intermittent distance exotropia. Arch Ophthalmol. 2005;123:1453. doi:10.1001/archopht.123.10.1453 [CrossRef]
- Okano M, Matsuo T, Konishi H, Hasebe S, Tadokoro Y, Ohtsuki H. Anomalous posterior insertion of medial rectus muscle simulating congenital oculomotor palsy. Jpn J Ophthalmol. 1990;34:275–279.
- Porter JD, Baker RS, Ragusa RJ, Brueckner JK. Extraocular muscles: basic and clinical aspects of structure and function. Surv Ophthalmol. 1995;39:451–484. doi:10.1016/S0039-6257(05)80055-4 [CrossRef]
- von Noorden GK. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. St. Louis: Mosby; 2002:67.
- Duranoglu Y, Gözkaya O. Bifid lateral rectus: a case report. Strabismus. 2005;13:89–92. doi:10.1080/09273970590945074 [CrossRef]