Ординатура / Офтальмология / Английские материалы / Handbook of Pediatric Strabismus and Amblyopia_Wright, Spiegel, Thompson_2006

EXTRAOCULAR MUSCLE FASCIA

A smooth white connective tissue, Tenon’s capsule, underlies the conjunctiva and envelops the globe and extraocular muscles. This delicate membrane partitions the orbital contents, isolating the globe and extraocular muscles from the surrounding orbital fat. Another fascial structure interconnected with Tenon’s capsule is the muscle sleeve or extraocular muscle pulley, which suspends the extraocular muscles.

Muscle Pulley (Muscle Sleeve)

Each of the rectus muscles passes through a pulley system consisting of a sleeve or ring of collagen, elastic, and smooth muscle fibers. Previously, this structure was termed muscle sleeve. The medial rectus muscle pulley has the most fibroelastic tissue and smooth muscle. Muscle pulleys connect to the orbital layer (OL) of the rectus muscle, to the orbital wall, to adjacent extraocular muscles, and to Tenon’s capsule.10 The pulley or sleeve extends for approximately 10 mm from the equator of the globe anteriorly to approximately 6 mm from the muscle insertion. During strabismus surgery, one can see these bands as they connect the surrounding muscle sleeve or pulley to the OL of the rectus muscle. Similar to the trochlea and superior oblique tendon, the pulleys guide the rectus muscles to their insertion point. In contrast to the superior oblique muscle, which changes direction after passing through the trochlea, rectus muscle pulleys keep the muscle in line with their anatomic origin. Demer has suggested that in secondary gaze positions the extraocular muscle path is “discretely inflected by the pulley.”6 Demer et al. also hypothesized that OL muscle fibers insert into the pulley system and actively influence pulley position and the mechanics of ocular rotation.11,28

Tenon’s Capsule

Tenon’s capsule is a collagen-elastic tissue that is a continuous membrane surrounding the eye and extraocular muscles.22 This membrane separates surrounding orbital fat from the globe and extraocular muscles. The elastic nature of Tenon’s capsule allows free rotation of the globe and unrestricted muscle relaxation and contraction. For clinical and surgical purposes, it is useful to subdivide it into the following categories:

1.Intermuscular septum

2.Anterior Tenon’s capsule

3.Posterior Tenon’s capsule

4.Check ligaments

5.Muscle sleeve (see Pulley System, earlier)

INTERMUSCULAR SEPTUM

This thin tissue lies sandwiched between the conjunctiva and sclera, spanning between the rectus muscles (Fig. 2-19).30,40 During strabismus surgery, intermuscular septum can be identified as the white membrane on each side of the rectus muscles. When elevated with muscle hooks, the intermuscular septum takes on the appearance of the wings of a manta ray (Fig. 2-20).45 The intermuscular septum can be safely incised during strabismus surgery, as it is not a barrier to orbital fat.

ANTERIOR TENON’S CAPSULE

This tissue is the subconjunctival membrane anterior to the muscle insertions. It proceeds forward with the intermuscular septum and fuses with the conjunctiva at 2 to 3 mm posterior to the corneal limbus (Figs. 2-18, 2-20). When suturing a muscle during strabismus surgery, it is important to dissect anterior Tenon’s capsule off the tendon insertion to avoid the complica-

SR

IMS

IMS

Anterior

Tenon's Capsule

FIGURE 2-19. Anterior ocular fascia. Intermuscular septum (IMS) is the connective tissue that spans between the rectus muscles underneath the conjunctiva. Anterior Tenon’s is that tissue anterior to the rectus muscle insertions; it fuses with the conjunctiva 3 mm posterior to the limbus.

A

B

FIGURE 2-20A,B. (A) Lateral rectus muscle with intermuscular septum and check ligaments. Check ligaments overlie the rectus muscle and connect the muscle to the overlying conjunctiva. Intermuscular septum is seen on either side of the lateral rectus muscle, spanning between the superior and inferior rectus muscles. (B) Photograph shows the Jameson hook under the lateral rectus muscle and Desmarres retractor pulling the conjunctiva posteriorly. (Figure published with permission of J.B. Lippincott Co. from Wright KW. Color Atlas of Ophthalmic Surgery: Strabismus. Philadelphia: Lippincott, 1991.)

Anterior Tenon's capsule

Medial rectus Anterior ciliary muscle artery

FIGURE 2-21. Anterior Tenon’s capsule is the white tissue retracted anteriorly with a small Steven’s hook (bottom left hook). During strabismus surgery, it is important to remove the anterior Tenon’s capsule to visualize the muscle tendon for suturing. Note the anterior ciliary vessels on the tendon insertion.

tion of a slipped muscle (Fig. 2-21). If anterior Tenon’s capsule is left on the tendon, the surgeon may inadvertently suture and secure anterior Tenon’s capsule, missing all or part of the tendon. The unsuspecting surgeon then disinserts the unsutured tendon and allows the muscle to slip posteriorly while anterior Tenon’s capsule is placed at the intended recession site.31 A slipped muscle is a frequent cause of unexpected overcorrection after recession procedures, as it often goes unrecognized at the time of surgery. Remember that some slipped muscles involve only part of the muscle and can present as a mild overcorrection with relatively good muscle function.48

POSTERIOR TENON’S CAPSULE

This tissue lines the posterior globe and functions to separate orbital fat from the sclera (Fig. 2-22). Just anterior to the equator of the eye, the four rectus muscles penetrate Tenon’s capsule and become surrounded by intraand extraconal orbital fat. At this juncture, Tenon’s capsule unites with the capsule of the rectus muscle to form a muscle pulley or muscle sleeve (see Muscle Pulley, earlier). The muscle sleeve is an important surgical

Fused anterior Tenon's and conjunctiva

Intraconal

fat

Medial rectus muscle

FIGURE 2-22. Drawing of a rectus muscle showing fascial relationships. Note that the muscle penetrates posterior Tenon’s capsule; the capsule at this point forms a muscle sleeve or muscle pulley. Intraconal and extraconal fat are isolated from the globe by Tenon’s capsule.

landmark when looking for a slipped or lost rectus muscle. A lost muscle is a rectus muscle that has become completely detached from the globe because of trauma or a surgical mistake.32,48 Once lost, the muscle will slip posteriorly within the muscle sleeve to be surrounded by intraand extraorbital fat. To find a lost muscle, first find the muscle sleeve located between the intraand extraconal fat; then, carefully follow the sleeve to retrieve the muscle. When looking for a lost medial rectus muscle, avoid the tendency to follow the sclera posteriorly, as this leads to the optic nerve. An important complication of attempted retrieval of a lost medial rectus muscle is inadvertent transection of the optic nerve that is enshrouded in postoperative scar tissue.

Together, posterior Tenon’s capsule, anterior Tenon’s capsule, and the muscle sleeve are very important structures as they are the barrier that keeps orbital fat from the globe and extraocular muscles. If posterior Tenon’s capsule or muscle sleeve is traumatically or surgically violated, fat adherence can occur because orbital fat prolapses through the torn Tenon’s capsule and scars to the sclera or an extraocular muscle (Fig. 2-23). The scarring of orbital fat produces a restrictive scar, which extends from the periosteum to the eyeball. As the scar

contracts over weeks to several months, the scar pulls the eye, producing a restrictive strabismus associated with limitation of eye movements. Fat adherence can occur as a complication of almost any extraocular surgery (e.g., strabismus surgery, retina surgery) or periocular trauma.31,49 Extreme care must be taken when operating in the area of orbital fat, which starts 10 mm posterior to the limbus. Once fat adherence occurs, it is almost impossible to correct. Surgically induced fat adherence can usually be avoided if the surgeon carefully dissects close to muscle belly or sclera, thus preserving the integrity of the overlying posterior Tenon’s capsule and muscle sleeve.

CHECK LIGAMENTS

These are fine falciform webs that overlie the rectus muscles and join the muscle capsule with overlying bulbar conjunctiva

A

B

FIGURE 2-23A,B. Diagram modified after Parks and published in Ophthalmology by Wright (1986)49 shows the pathophysiology of the fat adherence syndrome. (A) Normal anatomy with orbital bone, periorbita, extraconal fat, muscle, and intermuscular septum. Note that the fat is isolated from muscle and sclera by intact Tenon’s capsule and intermuscular septum. (B) Violation of Tenon’s capsule with fat adherence to the globe and muscle (to right).

50% from the anterior ciliary arteries.44 The conjunctival vessels also contribute to anterior segment circulation.14 Anterior ciliary arteries and the conjunctival vessels merge at the limbus to form the episcleral limbal plexus.27 These vessels in turn connect with the major arterial circle of the iris, which is also fed by the two long posterior ciliary arteries. The superior rectus, inferior rectus, and medial rectus muscles have at least two anterior ciliary arteries and are major contributors to the anterior segment circulation.18 The lateral rectus has a single anterior ciliary artery and, of the four recti muscles, the lateral rectus probably provides the least in the way of anterior segment circulation.20,41 The oblique muscles do not have anterior ciliary arteries, and they do not contribute to the anterior segment circulation.

Iris angiograms can be used to assess anterior segment circulation in blue-eyed patients. Removal of a vertical rectus muscle will cause hypoperfusion in that area that relates to the vascular input.18 It is interesting that this hypoperfusion lasts only 1 to 2 months because its collateral circulation and vasodilatation will replenish the hypoperfused area.45 Additionally, infants and children do not typically show hypoperfusion even when multiple rectus muscles are removed. Removal of a rectus muscle during strabismus surgery will permanently interfere with vascular supply of the anterior ciliary arteries unless the surgery is performed specifically to maintain anterior segment circulation. Surgeries have been devised that attempt to maintain anterior segment circulation despite manipulations of the muscle position.24,45 Iris angiograms can be used to document anterior segment blood flow from the anterior ciliary arteries in nonhuman primates. A muscle-to-sclera plication developed by this author (Wright plication) is designed to tighten a rectus muscle but spare the anterior ciliary arteries. Instead of resecting the muscle, as is done in the standard muscle tightening procedure, the Wright plication folds the muscle, suturing muscle to sclera without disrupting the anterior ciliary vessels. Figure 2-25 shows an iris angiogram after inferior rectus muscle plication and surgical removal of the other three rectus muscles in a nonhuman primate. The iris angiogram demonstrates intact perfusion from the inferior rectus muscle and hypoperfusion superiorly because the arteries of the other three rectus muscles had been sacrificed on surgical removal.

Anterior segment ischemia can be a consequence of strabismus surgery, most often after a threeor four-muscle

FIGURE 2-25. Monkey fluorescein iris angiogram, early phase after Wright plication of the inferior rectus muscle and removal of the other three rectus muscles. Note the hypoperfusion superiorly (black area of iris) as the medial, lateral, and superior rectus muscles have been removed. The perfusion from the inferior rectus remains intact after the Wright plication because fluorescence is seen inferiorly (white vessels on iris).46

transposition procedure.35,42 This is a rare occurrence, as collateral circulation from the long posterior ciliary arteries can usually maintain adequate perfusion to the anterior segment even when three or four rectus muscles have been removed.36 Factors that predispose to anterior segment ischemia include arteriosclerosis, hyperviscosity of the blood, and scleral encircling elements such as 360° retinal buckles posteriorly, all of which can compromise the long posterior ciliary arteries. Older patients have a higher likelihood for developing anterior segment ischemia, whereas infants and children are generally protected from this condition.15 Anterior segment ischemia has even been reported after removing as few as two rectus muscles in high-risk patients.12,15 It is important to remember, however, that disruption of anterior ciliary arteries associated with strabismus surgery is permanent, and anterior segment ischemia can occur years or decades later, as the collateral circulation diminishes with age.34

PHYSIOLOGY OF OCULAR ROTATIONS

Donder’s and Listing’s Laws

Ocular movements are a result of contraction and relaxation of multiple muscle groups that act to rotate the eye around a fixed center of rotation. There are three axes that pass through the center of rotation, termed the axes of Fick (Fig. 2-26). The axes of Fick include the Z axis (vertical orientation) for horizontal rotation, the X axis (horizontal orientation) for vertical rotation, and the Y axis (oriented with the visual axis) for torsional rotation. Listing’s plane is a vertical plane that includes the X, Z, and oblique axes that pass through the center of the eye (Fig. 2-26). Listing’s law states that virtually all positions of gaze can be achieved by rotations around axes that lie on Listing’s plane. Donder’s law is related to Listing’s law and states that there is a specific orientation of the retina and cornea for every position of gaze. This corneal orientation is specific for each position of gaze regardless of the path the eye took to achieve that position of gaze. Figure 2-27 demonstrates Listing’s and Donder’s laws, showing the specific corneal orientations for ocular rotations around various axes on Listing’s plane. Note that when rotations

B

A

C

FIGURE 2-26A–C. The three axes of Fick allow horizontal rotation. (A) Vertical axis (Z axis): horizontal rotations. (B) Horizontal axis (X axis): vertical rotations. (C) Visual axis (Y axis): torsional rotations.