Anatomical Considerations During Surgery

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 cause injury to the nerve.

Cranial nerve IV is outside the muscle cone and is 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; the nerve can be damaged by surgery in this area. 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. These nerves and the inferior oblique muscle can be injured by an inferotemporal retrobulbar block as well.

Surgery 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, a second or third belly is not identified, the action of the muscle may not be sufficiently altered and additional surgery may be required as a result.

The NFVB 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 decreases 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 intermuscular septum between rectus muscles and especially between the rectus and oblique muscles can help locate a lost muscle during surgery. 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, for example, the inferior oblique muscle from 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 have to be disconnected during recessions and resections of either of these 2 rectus muscles.

The medial rectus muscle 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 (see Fig 3-8).

The superior rectus muscle is loosely bound to the levator palpebrae superioris muscle. The eyelid may be pulled downward following 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 3-8).

The blood supply to the EOMs provides almost all of the temporal half of the anterior segment circulation and the majority 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 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. As this area is the site of most eye muscle surgery, especially recession procedures, scleral perforation is always a risk during eye muscle surgery. The surgeon can minimize this risk by

using spatulated needles with swaged sutures

working with a clean, dry, and blood-free surgical field using loupe magnification or the operating microscope

Chapter 14 discusses these procedures and complications in greater detail.