Letters to the Editor
We read with interest the report of Liao and Hwang of an accessory lateral rectus muscle in a patient with normal ocular motility (1). Accessory extraocular muscles are rare anatomic anomalies. They may represent vestigial structures homologous to the retractor bulbi muscle in amphibians, reptiles, and lower mammals (2) or a disturbance in mesodermal development of extraocular muscles (2–4). We recently evaluated a patient with an accessory extraocular muscle with clinical and radiologic findings differing from the case of Liao and Hwang.
A 26-year-old woman was referred for evaluation of right enophthalmos worse with eye movement present for 3 years and worsening since ptosis surgery 1 year ago. There was associated headache but no other visual or neurologic complaints. Visual acuity was 20/25 right eye and 20/20, left eye. There was limited abduction of the right eye, and the patient reported diplopia in right gaze. The right eye was 4-mm enophthalmic in primary position, and the enophthalmos increased to 7 mm in right superolateral gaze (Fig. 1). The rest of the examination was normal.
Multidetector 2-mm axial noncontrast CT images were obtained with bone and soft tissue technique and coronal isovoxel reformatted images with the patient in primary position and right and left gaze (Fig. 2). Imaging sequences showed a thin elongated structure isodense to normal extraocular muscle, extending from the annulus of Zinn to the posterior globe, inferior, and medial to the optic nerve insertion. On right gaze, the eyes were disconjugate with limited abduction of the right eye. On left gaze, the eyes maintained a conjugate position and the accessory extraocular muscle approximated the nasal aspect of the optic sheath. Given the clinical and neuroimaging findings, the patient was diagnosed with an accessory extraocular muscle.
The prevalence of accessory extraocular muscles is unknown, but English-language PubMed search revealed 22 reported cases. Most accessory extraocular muscles arise from the orbital apex (14/22 cases) with others arising from an extraocular muscle. Half of that arise from the apex (7/14) insert onto the sclera of the posterior globe in varied locations: inferomedial (2 cases) (3,5), superolateral (2 cases) (1,6), inferior (1 case) (7), and inferolateral (2 cases) (4,8). Two patients had retraction of the globe (3,7), 3 had enophthalmos (4,6,9), and 1 was asymptomatic (1). In addition, all patients with accessory extraocular muscle attaching to the sclera had some component of restricted ocular motility, except the case reported by Liao and Hwang (1).
There is not always a clear anatomic correlation between clinical and neuroimaging findings, although such was the case in our patient. Her marked enophthalmos was the greatest in right superolateral gaze, opposite to the inferomedial insertion of the accessory extraocular muscle. In comparison, the other 2 cases of inferomedial insertion were associated with restricted elevation on abduction (5) and globe retraction with superior and inferior eye movement (3). In addition, the 2 cases of inferomedial insertion were associated with restricted elevation on abduction (5) and globe retraction with superior and inferior eye movement (3). In addition, the 2 cases of globe retraction associated with an accessory extraocular muscle showed differing radiology findings. One patient experienced globe retraction with adduction and the accessory muscle attached to the posterior sclera inferiorly (7). Retraction of the eye in the second patient occurred with vertical eye movements and the accessory muscle attached to inferomedial posterior sclera (3).
Strabismus is the most common indication for surgery and usually performed in patients with anterior scleral or direct extraocular muscle insertion of the accessory muscle. There are no reports of improvement in enophthalmos with eye muscle surgery.
1. Liao YJ, Hwang JJ. Accessory lateral rectus in a patient with normal ocular motor control. J Neuroophthalmol. 2014;34:153–154.
2. Lueder GT. Anomalous orbital structures resulting in unusual strabismus. Surv Ophthalmol. 2002;47:27–35.
3. Savino G, D'Ambrosio A, Tamburrelli C, Colosimo C, Dickmann A. Restrictive limitation of sursumduction caused by an anomalous muscular structure. Ophthalmologica. 1998;212:424–428.
4. Dobbs MD, Mawn LA, Donahue SP. Anomalous extraocular muscles with strabismus. AJNR Am J Neuroradiol. 2011;32:E167–E168.
5. Lueder GT, Dunbar JA, Soltau JB, Lee BC, McDermott M. Vertical strabismus resulting from an anomalous extraocular muscle. J AAPOS. 1998;2:126–128.
6. Krasny A, Lutz S, Gramsch C, Diepenbruck S, Schlamann M. Accessory eye muscle in a young boy with external ophthalmoplegia. Clin Anat. 2011;24:948–949.
7. Ozkan SB, Ozsunar Dayanir Y, Gokce Balci Y. Hypoplastic inferior rectus muscle in association with accessory extraocular muscle and globe retraction. J AAPOS. 2007;11:488–490.
8. Valmaggia C, Zaunbauer W, Gottlob I. Elevation deficit caused by accessory extraocular muscle. Am J Ophthalmol. 1996;121:444–445.
9. Merino P, de Liano PG, Ruiz Y, Franco G. Atypical restrictive strabismus secondary to an anomalous orbital structure: differential diagnosis. Strabismus. 2012;20:162–165.