Journal of Neuro-Ophthalmology, June 2014, Volume 34, Issue 2

Accessory Lateral Rectus in a Patient With Normal Ocular Motor Control

Although supernumerary extraocular muscles are common in monkeys and other species, they are relatively rare in humans and typically are noted in the context of childhood strabismus. We present a case of an incidentally found unilateral accessory lateral rectus muscle in a 51-year-old woman with normal ocular motor control. In this patient, the accessory lateral rectus was approximately 10% the size of a normally sized lateral rectus muscle. It originated from the orbital apex, traveled between the optic nerve and the lateral rectus and attached to the superolateral aspect of the globe. This unique case demonstrates that accessory lateral rectus in humans may have no impact on eye movement and ocular alignment.

Accessory Lateral Rectus in a Patient With Normal Ocular Motor Control Yaping Joyce Liao, MD, PhD, Jaclyn J. Hwang Abstract: Although supernumerary extraocular muscles are common in monkeys and other species, they are relatively rare in humans and typically are noted in the context of childhood strabismus. We present a case of an incidentally found unilateral accessory lateral rectus muscle in a 51- year-old woman with normal ocular motor control. In this patient, the accessory lateral rectus was approximately 10% the size of a normally sized lateral rectus muscle. It originated from the orbital apex, traveled between the optic nerve and the lateral rectus and attached to the supero-lateral aspect of the globe. This unique case demonstrates that accessory lateral rectus in humans may have no impact on eye movement and ocular alignment. Journal of Neuro-Ophthalmology 2014;34:153-154 doi: 10.1097/WNO.0000000000000109 © 2014 by North American Neuro-Ophthalmology Society A51-year-old woman with a pituitary tumor was referred for evaluation of visual function. She had no subjective vision complaints. Visual acuity was 20/20 in both eyes, with no relative afferent pupillary defect, normal color vision, and normal visual fields. Her anterior and posterior segment ex-aminations were unremarkable, and ductions, saccades, pur-suit, and ocular alignment in all directions of gaze were normal. Brain magnetic resonance imaging (MRI) revealed slight pituitary enlargement without optic chiasm compression. In the orbit, there was a small, linear structure located parallel and just medial to the right lateral rectus. It arose as a discrete entity from the orbital apex and attached to the globe superiorly and medially to the lateral rectus. It had the same signal intensity in all sequences as that of the other extraocular muscles. It was not consistent with a vascular structure, fibrous band, or tumor, but rather appeared as an accessory lateral rectus muscle (Fig. 1). We estimated the size of the accessory lateral rectus muscle using thresholding and quantification of the coronal MRI images with ImageJ (http://rsb.info.nih.gov/ij/), and it measured 11.3% that of the right and 11.7% that of the left lateral rectus muscles (1). Ultrasound was not performed to confirm whether it contracted like a muscle. Accessory lateral rectus muscle in humans is relatively rare and typically discovered when associated with childhood strabismus (2-5). It has been reported in one case of congen-ital third nerve palsy (2) and in a patient with orbital malfor-mation syndrome (6). As in other patients evaluated with CT (2,6,7) and MRI (3,8,9), the accessory lateral rectus in our patient appeared to run parallel and just medial to the nor-mally sized lateral rectus but was much smaller in size (6,8,9). FIG. 1. Magnetic resonance imaging of right accessory lateral rectus muscle (arrows) on unenhanced T1 axial image (A) and postcontrast T1 coronal scans with fat suppression (B-D). Department of Ophthalmology, Stanford University School of Med-icine, Stanford, CA. Y. J. Liao was supported by the Career Award in Biomedical Sciences from the Burroughs Wellcome Foundation and the Weston Havens Foundation Grant. The authors report no conflicts of interest. Address correspondence to Yaping J. Liao, MD, PhD, Department of Ophthalmology, Stanford University Medical Center, 2452 Watson Court, Palo Alto, CA 94303-5353; E-mail: [e-mail] Liao and Hwang: J Neuro-Ophthalmol 2014; 34: 153-154 153 Photo Essay Section Editor: Timothy J. McCulley, MD Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Human pathologic studies have shown that an accessory rectus muscle can originate discretely or in the same tendon as that of the lateral rectus. It may have a muscle belly or exist simply as a fibrous band. It can divide anteriorly and join different rectus muscles, not necessarily causing strabismus (3,9,10). The majority of cases of accessory lateral rectus are unilateral (8). There is 1 reported case of bilateral, asymmet-ric accessory lateral rectus muscles in a patient with Gorlin syndrome, a condition attributed to Ptch 1 mutation and disruption of sonic hedgehog signaling (6). Although supernumerary extraocular muscles have been associated with strabismus, our case provides evidence that the presence of accessory lateral rectus muscle does not necessarily impair ocular motility. Supernumerary extra-ocular muscles should be considered in cases of unusual strabismus patterns (5,9) or when strabismus surgery does not achieve the expected result, because release of accessory muscle is sometimes necessary as part of the surgical pro-cedure (2,6,7,11,12). Intraoperative forced duction testing should be performed to ensure that no other unsuspected anomalies or muscles are present. Although accessory lateral rectus is relatively rare in humans, it is a normal finding in monkeys. It is typically very small, inconsistently located, often superior to the optic nerve and medial to the lateral rectus (13,14). In monkeys, the accessory lateral rectus has been attributed to a developmental remnant of the retractor bulbi, which is innervated primarily by the sixth nerve but may also be innervated by the third nerve. Although the function of the accessory lateral rectus is unclear, it is not usually associated with strabismus in mon-keys (13), and, in fact, it has been proposed to be protective against esodeviation, which is rare in monkeys (14,15). Sim-ilarly in amphibians, the accessory lateral rectus is thought to protect the eye and to prevent ocular protrusion (10). REFERENCES 1. Demer JL, Kono R, Wright W. Magnetic resonance imaging of human extraocular muscles in convergence. J Neurophysiol. 2003;89:2072-2085. 2. Park CY, Oh SY. Accessory lateral rectus muscle in a patient with congenital third-nerve palsy. Am J Ophthalmol. 2003;136:355-356. 3. Khitri MR, Demer JL. Magnetic resonance imaging of tissues compatible with supernumerary extraocular muscles. Am J Ophthalmol. 2010;150:925-931. 4. Nussbaum M. Vergleichend-anatomische Beitrage zur Kenntnis der Augenmuskeln. Anat Anz. 1893;8:208-210. 5. von Ludinghausen M. Bilateral supernumerary rectus muscles of the orbit. Clin Anat. 1998;11:271-277. 6. Dobbs MD, Mawn LA, Donahue SP. Anomalous extraocular muscles with strabismus. AJNR Am J Neuroradiol. 2011;32: E167-E168. 7. Sa HS, Kyung SE, Oh SY. Extraocular muscle imaging in complex strabismus. Ophthalmic Surg Lasers Imaging. 2005;36:487-493. 8. Fichter N, von Arx G, Kirsch EC. Accessory lateral rectus muscle in Graves' orbitopathy: a case report. Clin Neuroradiol. [published ahead of print July 5, 2013] doi: 10.1007/s0002- 013-0233-y. 9. Lueder GT. Anomalous orbital structures resulting in unusual strabismus. Surv Ophthalmol. 2002;47:27-35. 10. von Ludinghausen M, Miura M, Wurzler N. Variations and anomalies of the human orbital muscles. Surg Radiol Anat. 1999;21:69-76. 11. Qi Y, Yu G, Wu Q, Cao WH, Fan YW. Accessory extraocular muscle-a case report and review. Zhonghua Yan Ke Za Zhi. 2011;47:1111-1116. 12. McNeer KW, Jampolsky A. Double elevator palsy caused by anomalous insertion of the inferior rectus. Am J Ophthalmol. 1965;59:317-319. 13. Narasimhan A, Tychsen L, Poukens V, Demer JL. Horizontal rectus muscle anatomy in naturally and artificially strabismic monkeys. Invest Ophthalmol Vis Sci. 2007;48:2576-2588. 14. Jampolsky A. Unequal Visual Inputs and Strabismus Management: A Comparison of Human and Animal Strabismus. St Louis, MO: Mosby; 1978. 15. Boothe RG, Quick MW, Joosse MV, Abbas MA, Anderson DC. Accessory lateral rectus orbital geometry in normal and naturally strabismic monkeys. Invest Ophthalmol Vis Sci. 1990;31:1168-1174. 154 Liao and Hwang: J Neuro-Ophthalmol 2014; 34: 153-154 Photo Essay Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited.