Anatomical Considerations for Ophthalmic Procedures
The nerves to the rectus muscles and the superior oblique muscle enter the muscles approximately one-third of the distance from the origin to the insertion (or trochlea, in the case of the superior oblique muscle). Damaging these nerves during anterior surgery is unlikely but not impossible. An instrument thrust more than 26 mm posterior to a rectus muscle’s insertion may injure the nerve.
Cranial nerve IV is outside the muscle cone and is usually not affected by a retrobulbar block. However, any EOM could be reached by a retrobulbar needle and injured by injection of local anesthetic.
The nerve supplying the inferior oblique muscle enters the lateral portion of the muscle, where it crosses the inferior rectus muscle; surgery performed in this area can damage the nerve. Because parasympathetic fibers to the sphincter pupillae (for pupil constriction) and the ciliary muscle (for accommodation) accompany the nerve to the inferior oblique muscle, with a synapse in the ciliary ganglion, surgery in this area may also result in an enlarged pupil. In addition, an inferotemporal retrobulbar block can injure these nerves and the inferior oblique muscle.
An operation on the inferior oblique muscle requires careful inspection of the inferolateral quadrant to ensure that all bellies are identified. If, during a weakening or strengthening procedure, the presence of a second or third belly is not recognized, the action of the muscle may not be sufficiently altered, and additional surgery may be required.
The neurofibrovascular bundle along the lateral border of the inferior rectus muscle can become an ancillary insertion site for the inferior oblique muscle when the muscle is anteriorly or medially transposed. Anterior transposition of the inferior oblique creates an anti-elevation effect.
Maintaining the integrity of the muscle capsules during surgery reduces intraoperative bleeding and provides a smooth muscle surface with less risk of adhesion formation. If only the muscle capsule is sutured to the globe, the muscle can retract backward, causing a slipped muscle.
The surgeon can use the intermuscular septum (between the rectus muscles and especially the section between the rectus and oblique muscles) as a point of reference in locating a muscle that has been “lost” during surgery or as a result of trauma. Extensive dissections of the intermuscular septum are not necessary for rectus muscle recession surgery. However, during resection surgery, these connections should be severed to prevent unexpected consequences, such as the inferior oblique muscle being advanced with the lateral rectus muscle. Often, there are 2 frenula: one that connects the lateral rectus muscle to the underlying inferior oblique at its insertion and another that connects the superior rectus to the underlying superior oblique tendon. Usually, these must be disconnected during recessions and resections of either of these 2 rectus muscles.
The medial rectus is the only rectus muscle that does not have an oblique muscle running tangential to it. This makes surgery on the medial rectus less complicated but means that there is neither a point of reference if the surgeon becomes disoriented nor a point of attachment if the muscle is lost.
The inferior rectus muscle is distinctly bound to the lower eyelid by the fascial extension from its sheath. Recession, or weakening, of the inferior rectus muscle tends to widen the palpebral fissure and result in lower eyelid retraction. Resection, or strengthening, of the inferior rectus muscle tends to narrow the fissure by elevating the lower eyelid. Therefore, any alteration of the inferior rectus muscle may be associated with a change in the palpebral fissure (Fig 3-7).
The superior rectus muscle is loosely bound to the levator palpebrae superioris muscle. The eyelid may be pulled downward after resection of the superior rectus muscle, thus narrowing the palpebral fissure. In contrast, the eyelid is not usually retracted upward with small or moderate recessions. In hypotropia, a pseudoptosis may be present because the upper eyelid tends to follow the superior rectus muscle (see Fig 3-7).
Figure 3-7 Attachments of the upper and lower eyelids to the vertical rectus muscles.
(Modified with permission from Buckley EG, Freedman S, Shields MB, eds. Atlas of Ophthalmic Surgery, Vol III: Strabismus and Glaucoma. St Louis: Mosby-Year Book; 1995:15.)
The blood supply to the EOMs provides almost all of the temporal half of the anterior segment circulation and most of the nasal half of the anterior segment circulation, which also receives some blood from the long posterior ciliary artery. Therefore, simultaneous surgery on 3 rectus muscles may induce anterior segment ischemia, particularly in older or vasculopathic patients.
Whenever muscle surgery is performed, special care must be taken to avoid penetration of the Tenon capsule 10 mm or more posterior to the limbus. If the integrity of the Tenon capsule is violated posterior to this point, fatty tissue may prolapse through the capsule and form a restrictive adhesion to sclera, muscle, intermuscular septum, or conjunctiva, limiting ocular motility.
When surgery is performed near the vortex veins, accidental severing of a vein is possible. The procedures that present the greatest risk of damaging a vortex vein are recession or resection of the inferior rectus or superior rectus muscle, weakening of the inferior oblique muscle, and exposure of the superior oblique muscle tendon. Hemostasis can be achieved with cautery or with an absorbable hemostatic sponge.
The sclera is thinnest just posterior to the 4 rectus muscle insertions, an area that is the site of most eye muscle surgery, especially recession procedures. Thus, scleral perforation is always a risk during eye muscle surgery. See Chapter 14 for further discussion of EOM surgery.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.