Bilateral muscular slips between superior and inferior rectus muscles: case report with discussion on classification of accessory rectus muscles within the orbit

Abstract

Accessory rectus muscles have rarely been reported as muscular 'bands' or 'slips' originating from the common tendinous ring (annulus of Zinn) and inserting in atypical location. This group of muscles is innervated by the inferior branch of the oculomotor nerve, lies on lateral side of the optic nerve and inserts in rectus muscles. Since there are only few descriptions of such unusual findings in the medical literature, the anatomical data on accessory rectus muscles is limited. Furthermore, existing reports vary in terms of studied objects (cadavers or living subjects), medical history (absence or presence of ocular movement disorders or eye movement abnormalities) and details of anatomical description. This report complements earlier publications and provides complete anatomical description of the accessory rectus muscle observed bilaterally during the dissection of a 68-year-old male cadaver with no eye movement abnormalities reported in the medical history. The accessory rectus muscle was divided into two 'slips' or 'heads'-superior and inferior-running in the sagittal plane (laterally to the optic nerve and the main trunk of the ophthalmic artery) and attached to the superior and inferior rectus muscles. Noticeable thickening of both superior and inferior rectus muscles at the insertion point of the accessory muscle heads was observed only in the sagittal plane. On both sides, the inferior head of the accessory rectus muscle was innervated by one of sub-branches derived from the inferior branch of the oculomotor nerve. No sub-branches to the superior head were macroscopically observed during the dissection. The classification, embryological background and clinical relevance of this variation have been discussed.

Keywords: Anatomic variation; Extraocular muscles; Inferior rectus muscle; Orbit; Superior rectus muscle.

Fig. 1

Atypical muscular slips between superior and inferior rectus muscles. Lateral view. The atypical muscle (marked by black asterisk) originates at the common tendinous ring (origin marked by black arrowhead) and passes laterally to the optic nerve (II) and the ophthalmic artery (OA). a Dissection of the specimen harvested from the right orbit. The lateral rectus muscle has been removed to expose the accessory muscle. b Dissection of the specimen harvested from the left orbit. One of sub-branches derived from the inferior branch of the oculomotor nerve (IB) innervates inferior slip of the accessory muscle (this small branch has been marked by white arrowhead). IR inferior rectus muscle, IO inferior oblique muscle, NIO nerve to inferior oblique, SR superior rectus muscle, III oculomotor nerve

Fig. 2

Two slips (heads) of the accessory muscle fusing with the superior and inferior rectus muscles. Sagittal section of the specimen harvested from the right orbit. Lateral view. Thickening of both superior and inferior rectus muscles at the insertion point of the accessory muscle slips has been clearly visualized. Black asterisk marks the accessory muscle, black arrowhead marks the origin of the accessory muscle located at the common tendinous ring, II optic nerve, III oculomotor nerve, IR inferior rectus muscle, SR superior rectus muscle, OA ophthalmic artery

Fig. 3

Histological observations on the right accessory rectus muscle. a Part of the superior head of the right accessory rectus muscle. Fibers of striated skeletal muscle have been visualized. H&E stain, ×10 objective. b Sample fibers of striated skeletal muscle obtained from the superior head of the right accessory rectus muscle. The striations of skeletal muscle tissue have been visualized. H&E stain, ×40 objective. c Part of inferior head of the right accessory rectus muscle shown near its origin from the inferior rectus muscle. Fibers of striated skeletal muscle as well as cross sections of small nerves (marked by white arrowheads) have been visualized. H&E stain, ×10 objective. d Sample fibers of striated skeletal muscle obtained from the inferior head of the right accessory rectus muscle. The striations of skeletal muscle tissue have been visualized. H&E stain, ×40 objective. An anterior, Po posterior

Fig. 4

Histological observations on the left accessory rectus muscle. a Part of the superior head of the left accessory rectus muscle (marked by grey arrowheads) shown at its origin from superior rectus muscle (SR). H&E stain, ×2 objective. Fibers of striated skeletal muscle and cross-section of small nerve (marked by white arrowhead) have been visualized. b Part of the tendon with visible bundles of collagen fibers (tissue sample taken from the origin of the left accessory rectus muscle). H&E stain, ×40 objective. c Part of inferior head of the left accessory rectus muscle (marked by black arrowheads) shown at its origin from the inferior rectus muscle (IR). H&E stain, ×2 objective. Fibers of striated skeletal muscle and cross section of small nerve (marked by white arrowhead) have been visualized. d Sample fibers of striated skeletal muscle obtained from the inferior head of the left accessory rectus muscle. The striations of skeletal muscle tissue have been visualized. H&E stain, ×40 objective. An anterior, Po posterior

Fig. 5

Different variants of the supernumerary orbital muscles reported in the literature. Lateral view. For ease of comparison and increased transparency, the same side has been presented on all schemes. Terminology applied by von Lüdinghausen et al. [17] has been taken into account. a Anatomical variation described in this report—an accessory muscle observed on 68-year-old cadaver with no eye movement abnormalities reported in the medical history. The accessory muscle is divided into two delicate slips (heads): superior (marked by black asterisk)—forming muscular bridge connected to the superior rectus muscle (SR); and inferior—corresponding to accessory rectus muscle (AR) and attached in the anterior half of the inferior rectus muscle (IR). b Supernumerary orbital muscle (AR) with a broad muscular bridge (marked by black asterisk) to the SR and attachment to the anterior part of the IR. The accessory muscle was well-separated from the IR. c Supernumerary orbital muscle (AR) with a thin muscular bridge (marked by black asterisk) to the SR and close attachment to the anterior part of the IR. Variants b and c were described by von Lüdinghausen et al. [17] on the adult cadaver with no problems with mobility of the eyeball in the medical history. II optic nerve

Fig. 6

Schematic drawings simulating MRI or CT coronal scans demonstrating spatial organization of muscular structures within the orbit in the event of presence of accessory (supernumerary) rectus muscles or muscular bands between superior and inferior rectus muscles. The drawings have been prepared on the basis of comparison of different MRI scans presented by Khitri and Dremer [7] and Kightlinger at al. [8]. a Complete muscular bridge seen between temporal edges of superior and inferior rectus muscles (marked by grey arrowheads). On drawing (b) only fragments of certain heads of the supernumerary rectus were captured (grey arrowheads). IR inferior rectus muscle, LR lateral rectus muscle, LPS levator palpebrae superioris muscle, MR medial rectus muscle, SR superior rectus, SO superior oblique muscle, II optic nerve